I recently obtained this Telefunken 5650MX set and cannot move the bass and treble controls, even with much force applied. The volume control works smoothly but the 2 controls below it seem frozen.

I notice that the treble and bass potentiometers are different from the volume pot? Is this normal and has anyone experienced this problem with this set?

Any help will be appreciated,

Frank

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Dear Mr. Van de Ven,

please always post your model-related qestions directly from the correct page (here: Opus Studio 5650 MX).
Threads created in such manner will be permanent connected with the model, this means your post won't get lost. Furthermore, in the "Talk" you will not get subject-specific answers. Thank you.
(Thread should be moved to the model page, my answer shall be deleted afterwards)

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Dear Frank,

the problem you are facing is quite common and not related to the specific model. I encountered the same problem on a Telefunken Opus 2650, but also on other radio sets and on several turntables.

It may be caused by hardened grease in the potentiometer bushing. The solution is a good spray for loosening rusted screws, such as the Wurth Rost Off. You should have a lot of patience, applying the spray fluid and giving it enough time to penetrate. You should repeat the said operation two or three times, gently tapping the locked shaft after each application.

Good luck!  

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Also be aware that some old radio tuning shafts had tiny retaining rings ("clips") at the base of the shaft. No amount of WD-40 or Wurth spray (or even heat and muscle) will help you until that retaining ring is removed (use extreme care!). Just something to keep in mind when the shaft seems impossibly tight and stubborn.

Jeff

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How about a tuning shaft that used to work smoothly but has become sticky?  The tuning capacitor on one of my DKE-38's was very easy to turn but lately it has gotten progressively harder and harder to move, to the point that it is essentially frozen in place now.  Even a vise-grip on the shaft can't move it.  (I removed it from the chassis for that).  A good dose of penetrating oil had no effect.  I'm pretty good with mechanical objects but this one is very puzzling.  Ominously enough, when I removed it from the chassis, a few flakes of mica fell out.  I blew these out with compressed air, but still no luck.  Is it even possible to find a replacement for this part?

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Hello Everybody,

I am working on Telefunken Opus 5650 MX. The left audio channel appears to be weaker than the right one. I suspect that one of the transistors in the left channel might have failed. I printed the available schematics for the radio. Unfortunately, the third portion of the schematics, which depicts the audio output stages, is of a very poor quality and it is very difficult to read. Does somebody have good quality schematics for this radio? I would highly appreciate a copy of it.

Thank you very much.

Andy

 

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Dear Andy Pinsky  
Please see the Schematic at the german Edition of this Radio  Opus 2650   ID3954.I think the Amplifier Part is most similar.
Please change at first all Elcos by the Amplifier Section,because this is mostly Reason for differently and poor Sound Quality.
Best Regards

Holger Ortmann

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Dear Holgen, 

Thank you very much for your assistance. 

I downloaded the schematics for the Telefunken Opus 2650, but it suffers from the same problem - the four voltages I need to measure in the audio amplifier section are very light and unreadable. 

Also, you suggested replacing Elcos. What are Elcos? Are you referring to electrolytic capacitors in the audio amplifier? 

Thank you again. 

Regards,

Andy

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Dear  Andy Pinsky

Question:Are in the bad Channel
A:very high Distortion or ?
B:Less distortion with extremly less Bass aund Treble ?

To Point A
Please measure and compare the Voltages between good and bad Channel.
by Transistors etc.

To Point B
In The Amplifier Section of Model Opus 2650 are Electrolytic Capacitors with lost Capacities very often!The Result:Less Bass aund Treble,and sometimes with lower Volume too.

The Electrolytic Capacitors are by Schematic Opus 2650
C556  100 
C 564 ,1564 

C 702 , 1702  

C 554 ,1554   
C 553 ,1553  
C 555 ,1555
C 552 ,1552   
C 551 ,1551  
C 236 ,1536  or 538,1538  
Elcos are not so good for High Frequencies.If its possible i prefer to change Electrolytics  in normal Condensers(MKT,MKP Condensers)or bipolar Elcos
Best regards
Holger Ortmann
 

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Dear Holger,

Thank you very much for this information.

I will get back to the radio in a few days – the beginning of the week will be busy.

There is no distortion in the left channel. It is just weaker than the right one (25 - 30 percent of the right) and it looks like a power transistor(s) might have failed.

I will get back with voltage measurements in a few days. 

Best regards,

Andy

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...You're right; the third section of the schematic is considerably harder to read than the other two.  And there appears to be a small 'strip' missing between the second and third sections.  But let me see if I can help you read it. 

...There are two supply voltages for the transistorized audio section.  The driver and output stages are powered by a -37 volt supply derived from bridge rectifier RC701, and the rest of the audio section is powered by a -20 volt (approx.) supply derived from bridge rectifier RC302.  You want to see -37 volts on the collectors of T701 and T303 on one side, and T1701 and T1303 on the other.  You want to see -19 volts on the collectors of T702 and T304 on one side, and T1702 and T1304 on the other side.  You DO NOT want to see -19 volts (or any DC voltage, for that matter) on the speaker connectors, which would indicate a shorted or leaky C702 or C1702.  You want to see the same -19 volts on the collectors of T302 and T1302, and -1.8 volts on the emitters.  The voltages shown on T552 and T1552 are -14.5 on the collectors and -5.5 on the emitters.  There aren't any base voltages shown; I'll presume that these are germanium transistors and there's only about 0.1-0.2 volt difference between the base and emitter. 

...At this point I'll say that, since you have said there is no distortion, it does not sound like something is wrong in the output (power) stage.  Output stage problems usually produce pretty severe distortion, and in this circuit, with everything being direct-coupled, an output transistor failure would probably become a 'chain-reaction' failure and take out one or more other transistors, plus a lot of those low-value resistors, with it.  (In other words, you'd probably be able to tell you had an output failure by the presence of several 'crispy critters' in the driver/output section.)  I'm more likely to concur with Herr Ortmann and say one or more of those 45+-year-old electrolytic capacitors may have dried out, which will result in partial or total loss of audio.  You'll see a lot more electrolytic capacitors used as coupling/DC blocking and emitter bypass capacitors in the audio circuits than you'd be used to seeing in tube radios, and unlike in tube radios, the failure mode is different.  You're less likely to see catastrophic failures because they are subjected to much lower voltages than in tube sets, but what typically happens is the sealing of the capacitor deteriorates over time, and the capacitor dries out.  Whan that happens, it 'opens' and loses its ability to pass a signal.  This is a common problem in older solid-state radios of all types and is a major culprit in 'low or no volume' issues.  The best way to find this would be to use an oscilloscope and start comparing signal levels between the two channels, starting at the bases of T205 and T1205, then the collectors, then the bases of T551 and T1551, then the collectors, etc., working toward the output stage.  Once you see a significant difference between the two channels, concentrate on that stage where you first see the difference, and suspect whatever electrolytics are there, especially if the voltages look reasonable...

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Dear Todd,

 

Thank you very much for this detailed post. I will report my results in a few days.

Best regards,

Andy

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Hello Everybody and Todd in particular,

I finally got back to the radio and made voltage measurements. In the interim the radio developed a host of problems. While I will describe them immediately since they may interrelate to the audio stages, I will deal with them later on when the audio stages are fixed.

This radio came to me without the front end tube ECC85 and I had to order it to make the radio electrically complete. While AM was working perfectly without ECC85, the SW1, SW2 and FM did not without it. The tube came yesterday and I installed it in the radio. The FM is working fine being full of stations but the radio has a trouble to keeping them tuned – I have to constantly retune for FM reception.

The first and most strangest problem the radio now has is that AM stopped to work properly. That is if and when I press the AM push button there will be no stations heard. There is static but no stations. If I press FM button first and let radio to play for couple minutes on FM and then switch to AM, the radio works just fine on AM. However, after several minutes (1 – 5 depending how long the radio was on FM) the volume starts to slowly decrease until it vanishes completely. To get AM back to work I have to repeat the cycle by switching radio to FM first and then switching it to AM.

The second problem, which is closely related to fixing the audio stages is that, the right channel, the channel that worked perfectly, developed a short. That is if and when I insert a fuse into the right channel slot, the dim bulb tester is instantly showing that the radio has a short. This leaves me with only left channel that has low volume problems. Fuse is not get destoyed, so the short is somewhere down the circuit.

These are the measurements from the left channel:

T1301

E –5.3

B –5.5

C –11.9 (should be -16.0)

 

T1302

E –1.2 (should be -1.8)

B –1.3

C –13.2 (should be -19.0)

 

T1303

E –14.3

B  0

C –26.3

T1304

E –14.0

B –13.0

C –0.1

 

T1702

C –14.2 (should be -19.0)

 

Capacitors C301, C302, and C303 measure:

C301 -41.0

C302 -31.1

C303 -23.2

 

 

It looks to me that T1304 went south and has to be replaced. I am not sure where these antique transistors could be obtained. I searched Allied Electronics and they did not have them. I am not sure that I have to immediately replace all electrolytics in the audio stages (16 in total) but I am considering doing so because transistor voltages are a bit low.

 

 

Please let me know what are your suggestions. Thank you very much.

Regards,

Andy

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...when it rains, it pours...

...First, the AM and SW should work without the ECC85.  That tube is only used in the FM tuner and has nothing to do with the other bands.  The AM and SW 'front end' duties are handled by the ECH81 tube.  It sounds like you're losing the oscillator function.  That and the instability of the FM tuner could be signs of the B+ for the tubes dropping (also, does your EM87 indicator tube lose brightness?), and that is a classic indicator that rectifier RC1 could be failing (I believe that's a selenium bridge rectifier).  If I read that part of the schematic correctly, you want to see 320V on the + terminal of RC1.  (Also, have you checked/changed C108, C109 and C110?)  What you can do is connect a voltmeter across the + and - terminals of RC1 with the radio off and cool, then turn the radio on and watch the voltage.  If it starts to drop rapidly, turn off the radio and see if RC1 is hot.  If so, it's probably time to replace RC1.  Either you'll need to rig up a bridge with four 1N4007s or use a packaged bridge rectifier (1000V 1A will probably be more than adequate), and you'll need to add a dropping resistor between the + terminal of the new rectifier and C108 to get the B+ right (245V at C109).  C108, C109 and C110 can (should) be replaced with 47µF 450V.

...Now...that audio section.  The supply that the fuse protects stops at T1302, so the problem is between T1302 and T1701/1702.  That 0V reading on the base of T1303 is most interesting; that should be somewhere near the same reading as on the base of T1304.  Now, because of that 'strip' that is missing between the two sections of the schematic, I cannot see what, if anything, is between RC1301and R1701 (which is where the base bias for T1303 and T1304 comes from).  But the fact that T1301 has no base voltage suggests to me that something has opened (such as one of those two resistors going to the cathode of RC1301, which I cannot identify because of that same missing part of the schematic), because it is derived from that -37V supply.  T1304 does sound like it has a problem, but because of the direct-coupled nature of this output circuit, it is probably necessary to check all five transistors from T1302 back, because any one of them could have a problem and be throwing everything else off...

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Hi Todd,

Thank you. I will provide a detailed reply later, but for now I would like to upload two PDF files of Opus 2650 MX which has the same schematics of the audio stages. The 2650 MX files are of a MUCH better quality and have pc boards which are very helpful for locating individual elements. Unfortunately, the posting program does not allow me to attach my files. I could send it to your email address. Or you can download them at raupenhaus dot de.

Andy

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...Thank you.  I have downloaded the info from Raupenhaus.  Very good diagrams, even if they are all in German (which is not a huge problem for me), and yes, they appear to match the 5650MX nearly perfectly, especially that audio section.  That missing section from the other schematic is important, because R1305 and R1317 were missing, R1316 and R1318 were not identifiable, and I wound up missing the connection from the junction of Dr1301/R1323 to the junction of C1309/R1315...

...Now I can see the circuit to the base of T1303 better.  That 0 volts you measured there is really bugging me, because unless something is open somewhere you should see pretty much the same voltage on the base of T1303 and the base of T1304.  I'm wondering at this point if something has happened to R1305 (trimpot).  Measure both sides of Gr1301 and see what's there for voltages, then look at R1305.  You'll probably see a very small difference across Gr1301, and whatever is there should also be on the base of T1303.  Also look at R1305 for signs of breakage, corrosion, etc.  Those trimpots can get a little 'brittle' over time...

...But...in the bigger picture, I'm inclined to think that something has happened to one or both of T1701 and T1702 (the power transistors, off the audio board) and that went back and caused 'collateral damage' to one or more of the previous transistors, and possibly to some of the other components around them...like I said before, a 'chain-reaction' failure.  You'll need to take T1701 and T1702 out and check them for shorts or leakage, and probably do the same with T1302, 1303 and 1304, and look at the resistors around them for blackening or discoloration or other signs of overheating.  That collector voltage on T1302 is independent of the -19 volts that should be on the collector of T1702 (it is derived straight from the -37 volt supply) and should measure -19 volts; something downstream of there is pulling it down.  (You're probably correct about a short in T1304, but that may be some of that 'collateral damage' I talked about; that is, it's not the only problem.  This is what makes a direct-coupled circuit like this one a pain to troubleshoot...) 

 

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Hello Everybody and Todd in particular,

I am completing a detailed answer to all points you made. Meanwhile, I would like to see what replacing of all transistors in the output stages would cost. Allied Electronics has AD149 at $15.00, but I cannot find any of the smaller ones - AC117, AC122, AC122/30, and AC175. Do you know what I can replace them with in North America?

Thank you very much and I will post my findings tomorrow morning.

 

Andy

 

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...Your best bet is probably to look for NTE replacements for these.  Being germanium transistors, and over 45 years old, the original types are likely going to be somewhat difficult to find, and consequently rather pricey.  (In fact, I'm rather surprised to see the AD149 still available...)  NTE subs should be easy enough to get in Canada...and possibly less expensive...

...Looking up NTE replacements (go here, then click on 'Component Cross Reference' at the top of the page) I find:

AD149 crosses to NTE104 or NTE121; they also offer the NTE121 in a 'matched pair' (NTE121MP) for push-pull or OTL output stages like this one, which I would recommend...two AD149s are going to cost you $30; I don't know right offhand what the NTE121MP costs, but it might be worth looking into...

AC117 crosses to NTE102A

AC122 and AC122-30 have several choices.  There are numerous variants of these, with some sort of colour coding, and depending on that, there are three choices: NTE100, NTE102A, or NTE158...best to look your particular variant up at the above site and pick out the right sub...

AC175 (A/B/P) crosses to NTE103A

 

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Hello Todd,

Thank you for the detailed reply. I will investigate prices, but checking NTE121MP revealed price of $46.00. I am not sure if this is per transistor or per pare. Most likely per transistor.

These are the problems the radio has as of this writing. They are in a descending order of importance.

1. Left audio channel has a short;

2. Right audio channel has a short;

3. AM is disappearing and has to be switched on through FM;

4. SW1 is deaf - no stations;

5. SW2 is deaf – no stations;

6. Mushy reception with a reduced volume on FM. Radio does not hold stations. This problem does not exist on AM.

I was running the radio through a dim bulb tester, consequently all voltages measured before were lower than recommended by the schematics by about 5 Volts. Without the dim bulb tester all voltages are slightly higher than recommended - by about 5 Volts. It does not look as if rectifiers delivering 37V and 20V need to be replaced immediately. This radio does not show signs of extensive use. It probably was in a storage for many years.

Since there are shorts is both the left and the right channel, I removed fuses (they were blown anyways) and connected a signal tracer to the input connector of the output amplifier. The preamplifier delivers a good, clear and strong signal to the output amplifier in both the left and the right channel. The radio currently plays through the signal tracer. The speaker connectors (I have to buy two DIN connectors) are showing about 0.03 VAC without fuses and about 2A when I measure current - that is why the fuses are blown.

The preamplifier does not show any signs of damage and all voltages for all six transistors T205 & T1205; T551 & T1551; and T552 & T1552 are right on.

All the resistors on the preamplifier and output amplifier boards look fine – there are no signs of decolouration and there no “critters”. I measured all resistors and the values were almost right on, inluding four variable resistors.

EM87 does not lose brightness. It always was very bright and it operates perfectly on AM opening and closing as required. It does not open and close on FM, it stays open all the time. The volume of stations on FM is lower than on AM. I will check voltages for ECC85 later after the audio problems are fixed. Checking voltages on plates of all tubes showed about 185 Volts. Again everything functions just fine on AM, with exception of vanishing (could indedd be the oscilator problem, which is a good sign that the required voltages are there.

Antique Radio forum has a discussion of fixing audio in another 5650MX radio that had symptoms very closely paralleled by the symptoms of my radio.

I checked resistance between cans of the AD149s and the heat sink. The AD149 transistors are sitting on mica pads and are isolated from the heat sink. I believe that the transistors of the left channel are on the top of the vertical line and the transistors of the right channel are on the bottom of the line. The resistance between the cans of the first, second and fourth transistor and the sink is more or less 100 Ohm. The resistance between the can of the third transistor and the sink shows 0 Ohm. Measuring resistance between the can of T1304 (it is also isolated from the sink) and the heat sink shows 0 Ohm. The previous owner complained about low volume problems and I think that someone has already tried to fix the audio. For all these reasons, and the shorts in both channel, I would like to replace all transistors in the output stage.  I would also like to replace all electrolytics in the output stage.

Andy 

 

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...The price for the NTE121MP will be for the pair.  The description given by NTE for the NTE121MP states that it is a 'matched pair' of NTE121s, selected so that their gains are within 10% of each other.  Obviously, however, at $46.00 for the NTE121MP, you're better off price-wise with the two AD149s.

...Properly checking the AD149s for shorts will involve disconnecting them from the circuit.  In-circuit testing of transistors is usually not recommended, because other components will usually throw off your measurements.  Also, don't rely solely on those mica sheets to isolate them from the heat sink.  There should be little plastic bushings on all the mounting screws to insulate them from the case of the transistors.  If one of those is missing or damaged, that will also cause you to read a short to the heat sink.  If someone has been in there previously, they could have left one of those plastic bushings out (it only takes one to be missing...). The best thing to do is going to be to disconnect the transistors and remove them from the heat sink, then test between the two pins and the case.  Your 2650MX schematic shows the basing of the AD149, looking at the underside.  With an ohmmeter, start by putting the - lead on the B pin, then put the + lead on the E pin and on the case.  You should read a fairly low resistance, but not 0 ohms.  Next, reverse the meter.  Put the + lead on the B pin, and the - lead on the E pin and then the case.  Ideally, you should not get any movement on the meter (or get an overrange indication; I'm used to doing this with an analog meter and you may be using a digital meter).  Finally, check between the case and the E pin in both directions.  If you get a low reading either way, and especially both ways, you have a problem (and I wouldn't be all that surprised if one or more of these indeed shows a C-E short; that is a common failure mode in output transistors).  You'll probably need to do a similar set of tests on the other transistors as well.  If you have a digital meter with a diode test function, you can try using that; B-E and B-C should show diode action (conduct one way but not the other), and C-E should show 'open'.

...The reception issues...If your B+ voltages are stable and not dropping over time, then I'd look at changing out the ECH81 for the AM/SW problem.  You can try monitoring the voltage on pin 8 of the ECH81; it's supposed to be 80 volts.  Monitor it over time and see if it drops when you start losing AM reception. 

...For the FM reception problem, you might also need to look at the detector stage (inside the dotted rectangle labelled Fi203).  There is a 10µF electrolytic (C IV) in there that could be throwing off the detector action and degrading the recovered audio if it has failed.  It will also affect the AFC circuit in the front end, which would explain the difficulty in keeping a station tuned in.  (There are two voltages derived from the detector that are fed back into the front end and end up on either side of Gr601, which is most likely an AFC diode.)  I don't know how much of a problem that is going to be to get at.  Very likely any kind of repair like this would have originally required replacing the entire Fi203, which would probably be next to impossible now unless you had a 'donor' unit to take one out of.  But it may be possible to very carefully remove and disassemble the Fi203 assembly and replace the capacitor...but the key words here are 'very carefully'...

...but we can only slay one dragon at a time; as you said, get the audio section squared away first, then worry about the other things...

 

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Good day Todd,

Thank you very much.

I checked first, second and fourth AD149s looking from the top. The third one is difficult to reach and I did not get to it. The first and the fourth transistors showed resistance of 6 KOhm in both directions when I measured resistance between E and C. I will be ordering power transistors tomorrow. I just have to decide whether to go with matched pairs of NTE121MP or with individual AD149 – both options are available form Allied Electronics. I also will be ordering other transistors for the output stage. Since I will be pulling the audio stage apart, I decided to recap the output and the preamplifier stages as well, and I will also be ordering electrolytic capacitors for both stages. I did not pull and measure other transistors in the output stage because it is a laborious task and I anyway decided to replace them since they were through some abuse before, and certainly since, I got the radio. I will check them when I will be replacing them.

I pulled ECH81 out of its socket and measured voltages on all pins. I got more or less 220VDC on pins 1, 6, and 8. I was surprised to see voltage so high on the pin 1. Meanwhile the radio stopped to act up on AM and now it operates perfectly without any need to switch between AM and FM. The FM problems however is still there. Thank you for the suggestion about detector. I will deal with it after I fixed the audio.

Please monitor this topic, as I will be back with updates after I finished with the audio.

Andy

 

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...You should have measured the voltages around the ECH81 with it still plugged into its socket.  Your readings were high because you were essentially measuring open circuit voltages.  Now, seeing that the AM is working better since you removed and reinserted the ECH81, I would suggest a careful cleaning of all the tube sockets.  The best way to do this would be to put a little contact cleaner on the pins of each tube and work the tube in, out and around its socket a few times.  Dirty or oxidized contacts in that ECC85 socket could also be wreaking havoc with your FM reception.  (Once upon a time I would have simply suggested spraying all the tube sockets with contact cleaner, but it seems that that can cause problems with certain types of tube sockets, either attacking the socket material or causing leakage between the pins.  The method of applying a little cleaner to the tube pins insures that the cleaner stays on the metal contacts where it belongs.) 

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Hi Todd,

Thank you for the reply.

I express myself poorly – the AM problem disappeared not because I removed ECH81. It disappeared by itself.

I will measure all voltage from underneath of the chassis later when I will be fixing FM and SW. It is difficult to reach and it requires removal of the chassis from the wooden board. I cleaned all contact immediately after I acquired the radio. All tubes and their pins were in a good shape and all but missing ECC85 were original.

I will report my results after I replaced the transistors and capacitors. They all were ordered today, but some are not in stoke and I will have to wait for some time for delivery.

Please monitor the subject, as things will get interesting both during repair of the audio and FM, SW. The problem with FM most likely is located in the FM converter itself. I am thinking to order a full complement of new tubes for this radio and a new ECC85 may cure the problem. Yet, I have a feeling that the problem is deeper. I am also thinking that the next step should be fixing SW which might cure the FM problem or at least prove that the FM problem is located in the converter.

Best,

Andy

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Tubes?

Did you recap this radio?

I hardly ever need to replace tubes, but find that I have to replace all the leaky capacitors!

The electrolytics and selenium rectifier are suspect too. 

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Hello Everybody and Todd in particular.

In the last few months I have been slowly bringing the radio to proper operating conditions. All electrolytics were replaced along with all tubes (some were very weak) which gave me a very nicely sounding radio indeed. I was almost ready to report a success story, when today the radio decided to act up.

The radio was humming on the background today, when suddenly the sound level went way up. There was no noise or smock, as far as I could see or sense. As the result the radio now does not have zero volume. Volume control continues to operate properly, but does not have zero point. I examined schematics of the radio, and I am inclined to replace the volume control, but before I would undertake this job, I would like to get some advice.

Thank you very much.

Andy

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...This means you have lost the 'ground' connection to the volume control.  In your case, you have lost BOTH ground connections.  Your volume control is in two sections, R579 and R1579, and is indicated on the schematic by the circled L (for 'Lautstärke'='volume').  You need to be concerned specifically with the two wires marked 'A' on the 'Regler Platte' ('control board').

...Compared to what most of us are used to, this is one wild and crazy circuit.  The transistorized audio section in this set has its own 'ground', derived from the + side of Gr302 and Gr701, and that 'audio ground' appears to be completely isolated from the main 'chassis ground'.  The only 'ground' interconnection between the two boards that make up the audio section ('Regler Platte' and 'NF Platte') appears to be the shielding of the leads coming off the volume control.  The schematic indicates a connector ('St 551') that links the two boards.  Apparently it is a female plug that connects to five pins on the 'NF Platte', I'll presume those to be 'd1' through 'd5', and it will have three wires on it.  Two of those wires will be shielded, and in between there will be a wire carrying -19V onto the 'Regler Platte'. 

...What I will suggest is: First, try moving 'St 551' around and see if that helps.  If so, a shot of contact cleaner on 'St 551' might be in order.  If not, connect one lead of an ohmmeter to the positive lead of C301, C302 or C303 (whichever is easiest to get to) and the other lead to those 'A' wires and check for continuity.  If you don't see any, you need to look at those shielded leads (see if the shielding has broken away from 'St 551' or the solder lugs on the 'Regler Platte' next to where the 'A' wires connect), and again, check soldering, particularly at those solder lugs and 'd2' and 'd4' on the 'NF Platte'. 

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Todd,

Thank you, I will attend to this over the weekend. I will post an update.

Thank you again.

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The temptation was too high...

"...What I will suggest is: First, try moving 'St 551' around and see if that helps.  If so, a shot of contact cleaner on 'St 551' might be in order.  If not, connect one lead of an ohmmeter to the positive lead of C301, C302 or C303 (whichever is easiest to get to) and the other lead to those 'A' wires and check for continuity.  If you don't see any, you need to look at those shielded leads (see if the shielding has broken away from 'St 551' or the solder lugs on the 'Regler Platte' next to where the 'A' wires connect), and again, check soldering, particularly at those solder lugs and 'd2' and 'd4' on the 'NF Platte'. "

1. I moved ‘St 551’ around and cleaned pins it sits on and the ‘St 551’ itself. It was strictly speaking unnecessary because the 'NF Platte' functions properly and produces a powerful signal in both channels.

2. I inspected ‘St 551’ and although all five solder joints were in perfect shape, I re-soldered them.

3. I measured continuity between pins ‘d1’ through ‘d5’ on the ‘NF Platte’ and the corresponding pins on the ‘Regler Platte’. I had continuity in all cases.

4. I connected one lead of my ohmmeter to the positive lead of C301 and the other lead of my ohmmeter to each of the ‘A’ leads of the resistors R1579 and R579. I had continuity in both cases.

5. Finally, I connected one lead of my ohmmeter to the pin ‘d1’ on the ‘NF Platte’ and the other lead of my ohmmeter to the point ‘E’ of the resistor R1579in the ‘Regler Platte’. At the point of ‘zero volume’ or full counter clockwise the ohmmeter measured approximately 48KOhm. At the point of the ‘full volume’ (fully clockwise) the ohmmeter measured 0.5Ohm.

6. I then I connected one lead of my ohmmeter to the pin ‘d5’ on the ‘NF Platte’ and the other lead of my ohmmeter to the point ‘E’ of the resistor R579in the ‘Regler Platte’. At the point of ‘zero volume’ or full counter clockwise the ohmmeter measured approximately 48KOhm. At the point of the ‘full volume’ (fully clockwise) the ohmmeter measured 0.5Ohm.

In other words, being measured from the “NF Platte” both variable resistors R1579 and R579 are functioning properly and changing resistance from 0.5 Ohm to 48 KOhm. However zero volume points are absent in both channels.

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...OK...let's try this:  Do the tests you did in 5. and 6. above, except:  Instead of probing the 'E' wires, probe the 'A' wires (or the + side of C301).  You should see the same behaviour, but reversed: the 48k reading should now be at the 'full volume' setting, and the 0.5 ohm reading should be at the 'zero volume' end.

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Yes, Sir.
That is exactly what I have 48 KOhm at ‘full volume’ and 4.5 Ohm at ‘zero volume’ when I measure from pins d1 and d5 to + side of C301.

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...OK.  That pretty much eliminates the volume control as the problem, and also the wiring.  Now it's head-scratching time...

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I would not eliminate volume control just yet. It looks that signal bypasses the volume control thus eliminating ‘zero volume’ point. Perhaps point ‘E’ in each resistor R1579 and R579 shortened to ‘S’. (What continues to disturb me is the fact that the problem is present in both channels.) I do not see any other way for the signal to bypass the volume control and to get to the ‘NF Platte’. I will explore this theory tomorrow.

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OK, did another check measuring resistance between points 'A' and 'E' of each resistor and in both cases the resistance was around 48KOh.

All voltages of the pre-amplifier ('Ragler Platte') measured exactly as shown in the schematics.

Still, no zero point volume in both channels. Strange. Signal still bypasses the volume control and gets to the 'NF Platte'

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