telefunken: Opus 55/TS, Selenium Rectifier.

Respected Members,

Restoring my Opus 55 TS the new magical eye illuminates lightly. The B + is 40 volt low the normal value. The Selenium rectifier is very old and I replaced its contained with four silicon diodes 1N4007 forming a bridge with four 4.7 nf capacitors in parallel to each diode.  To adjust the B+ value  I put a 220 ohms 10 watts resistors in series with the power transformer.  Now the B + is in normal value and the EM80 glow well. 

But during the first and half minute, after power on the radio, the B + rises to 340-360 volts and slowly reaches the normal value.  I understand that is a characteristic of the Silicon rectifiers and not in the selenium rectifiers. 
This abrupt increase of voltage can be dangerous for other components of the radio as power capacitors, output transformer or output valves?  It is necessary to control this abrupt increase of tension? 

Thanks in advance,
Bruno.

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Dear Mr. Gandolfo,

what You see is quite normal, and happens also with the selenium rectifier. The reason is, that when turning the radio on, all the tubes are cold, and they don't pull any current. Maybe this effect is bigger with silizium rectifier.

Therefore the voltage rises higher than normal, and when after a minute the tubes are warm, and pull the correct current, You reach the normal +B Voltage. This was discussed in German in the Forum before, but doesn't seem to be dangerous.

I Don't know your line Voltage, and don't know the Voltage setting. You can try to switch to 240 Volts, but You need to check for the correct Heater Voltage of 6.3 Volts AC. Please check this Voltage at one tube socket away from the Transformer. If You are above 6.0 Volts, it schould be OK.

Best Regards,

Henning Oelkers

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Dear Mr. Oelkers,

You always attending my electronic problems.. I am very thanked therefore.  I apologize if this point was tried in German language. Unfortunately I do not have access to that language.

I know that silicons rectifiers have an extremely low forward resisteance which means that heavy peak currents flow at initial switch-on and the DC output is very likely to be equivalent to the peak value of the AC input.  I think this can overload the HT line so much above the design ratings as to cause components or even valve failure.  Mr Suliemanagich comments that his choke was burn when he change the rectifier. 

If you are so kind to revise the diagram, establishes a voltaje for ECH81 pin 8, the triode plate, with voltage for UK and UKW function.  I understand that the triode oscillator should have 0 volatge in UKW.  The values 95 (105) are underlined.  Can have a special meaning or can be an error of the diagram? 

Thanks in advance,
Bruno.

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Dear Mr Gandolfo,

there is a little Text in the Diagram ( above the Bands Switch )

This Text says:

"Values in Brackets are measured with Instrument 50kOhm / V.
Values without Brackets are measured with Instrument 333 Ohm / Volt in the 300 Volt Range.
All Values are measured in FM Position.
Underlined Values are measured in AM Position"

hope this helps.

Best regards,

Henning Oelkers 

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Dear Mr. Suleimanagich,

Now my Opuss 55 is working with the precise B+. You have the same model but to obtain the same B+ your radio need a lower resistor. I think that the difference is originated on the differents currents that we have.

The output voltage in a transformer varies in inverse sense with the frequency of the current in the primary.  That is to say With 60 cycles as is in US the voltage will be lower that with 50 cycles that we have in Chile.  Then your model needs a resistor of smaller value to obtain the same B +. 
I believe that can be a reasonable explanation. 
Thanks by your comments. 
Bruno.

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I would definatly use your radio on the 240 VAC setting  if the power transformer still gives you about 6.3 volts for the tubes.  This way, your power transformer runs cooler  and the power resistor resistance needed on the bridge rectifier is lowered as well 

The choke was already toast when I received my Opus 55, a long with a defective selenium rectifier (that was about to die a smoking death), an open smoothing cap, and a barbequed power transformer that I had a replica custom made as a replacement.

In this radio, change ALL the paper and electrolytic caps, other wise you are asking for trouble (and stop worrying about the ECH81)!

If you want a little more sensitivity in this radio on AM and shortwave, replace the 0.0047mf caps on the EF89's to 0.0068mf ro even up to 0.010's!

Around the EABC80, there is a 10 meg resistor that I always replace in this radio too, along with the 1 and 1.5 meg resistors if they are out of range(check the 1 meg around the output transformer)!

If there is no FM, check the 470 K resistor that is used when the FM button is engaged.

I hope this helps.

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Dear Mr. Oelkers,

With your translation now all is very clear and all the voltages are in order. 
Thank you so much for the support. 

Greetings, Bruno.

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Omer,

Above you stated "If you want a little more sensitivity in this radio on AM and shortwave, replace the 0.0047mf caps on the EF89's to 0.0068mf ro even up to 0.010's!".  You excited my curiosity since I am always interested in ways to improve radio performance.  I looked at the schematic for the Opus 55T/S and could not find .0047 capacitors connected to the EF89's.  Have I missed them or are they on a different schematic?  What is the function of the capacitors you are mentiioning?  Are they cathode bypass?

Thanks for whatever clarification you will provide.

Paul

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You know what, you are absolutly right, I confused this model to the Opus 7 (which I'm restoring now).  The Opus 7 has the 0.0047mf caps on the EF89's, the 55  only has 0.0025mf caps (I would at least bump these up to 0.0047mf caps!).

Paul, what's your take on recapping these babes or, just leaving them in "crash and burn" mode"? 

Anything noteworthy in your restoration approach or experiance with these radios?

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Omer,

The .0025 uf capacitors that I see are plate supply (B+) bypass and screen grid bypass.  Are these the capacitors that you find increase radio sensitivity if you increase their value?  Is it the plate bypass or screen bypass or both that have this affect?  If this is true I don't see a problem with increasing their value significantly, like to .01 uf or .02 uf, since their function is to prevent signal from this stage from interfering (feeding back) to any other stage through the B+ bus.  If you or anyone has any thought of a problem with making these too large in value I would be very interested in your thoughts.

I very strongly recommend replacing all of the paper (non-ceramic or non-styrene) capacitors in these radios.  I have an encellent tester for capacitor evaluation (Sencore) and I find these capacitors are always leaky and way off value (something like 300% to 1000% of their marked value).  Actually I can state that I have never found one of these capacitors that had no leakage (many were very leaky with an equivilant leakage resistance less than 100K ohms) and I have never found one of these capacitors that was within 10% of its marked value.  The leakage will cause excess power supply current or tube bias errors when they are used for bypass, and the leakage will really screw up the AVC (automatic volume control) operation because AVC circuit impedances are very high.  Leakage current will also cause severe tube bias errors when they are used as coupling capacitors to the control grids of the audio amplifier, phase inverter, and output tubes, and can severely damage the output tube(s) and burn out the output transformer and/or cathode resistor.  Capacitors used in bypass or coupling circuits usually don't cause any performance problems when they increase in capacitance value; it's all about leakage current.

Most of these European radios have complex tone control and negative feedback circuits that use many capacitors to contour the sound quality.  That's the main reason they sound so good.  Capacitors that drift out of tolerance severely impact the performance of these precision circuits.  So in these circuits it's not usually the leakage that is a problem but the change in capacitance value.

I find that the styrene capacitors are OK.  These are the clear plastic capacitors where you can see the silver foil inside.  These capacitors are usually used in the critical RF and oscillator circuits and have a special feature that their capacitance value is very stable with temperature change.  Replacing these capacitors without using special temperature stable (NPO) capacitors can cause the radio to drift off frequency as it warms up.

In comparison to my capacitor test finding that I mentioned above, that I have never found a paper capacitor in one of these radios that passed test either for leakage or value, I always test the new yellow axial-lead capacitors before I install them.  I have never found one of these new capacitors that was outside its value tolerance and I only found one that had any measurable leakage.  The capacitor that had measurable leakage was a rather large value (I believe that it was a .22 uf) and at 630 volts the leakage current was very small (if I remember corrrectly I believe the equivalent leakage resistance was something like 400 megohms).

I don't have a lot of information on ceramic capacitors other than to report that I sometimes measure some low-level leakage current at a level that should not bother radio operation.  These capacitors are not usually used in high impedance circuits where small leakage current would cause a problem.  I believe that the leakage that I do measure is caused by moisture absorption through the ceramic.  (Bypass capacitors on the AVC bus and coupling capacitors connected to the control grids of audio amplifier, phase inverter, and output tubes must have no leakage.)

I find that electrolytic capacitors have held up much better than the paper capacitors.  My Sencore tester can evaluate them for value, dialectric dissipation, equivalent series resistance, and leakage current.  I usually find that they are within spec for capacitance value and ESR, and they often have somewhat high leakage current at full voltage if they are not exposed to much voltage in their circuit or if the radio has not been used for a long time.  This leakage current usually drops to an acceptable value when they are given a chance to re-form (test voltage is applied to the capacitor for several minutes).  However, I usually replace all of the electrolytic capacitors that are not in the chassis-mounted filter cans because this is reasonably easy and inexpensive to do since there are not many of them in a radio, and I understand that the new electrolytics are more durable because of advances in technology.

I always test the main filter capacitors and if they are within capacitance tolerance (-40% to +100%), have good ESR, and have OK leakage current values (meaning thay won't heat up on their own at the voltage level in the radio), I usually let them alone.  I have not yet gotten into restuffing filter capacitor cans, although I plan to try this.  I have done enough investigating to know that it often requires careful capacitor selection to find ones that will fit into the original can. However, I'm discussing radios that I have restored for my own collection, and I have not wanted to clutter up or obviously modify the underchassis apearance. If I was getting paid to restore these radios I would want to replace all of the filter capacitors to be certain that my repair work would be long-lasting. I certainly would not want to be responsible for burning out a customers power transformer. I doubt that a customer would ever see under the chassis, or know what they were looking at if they did.

Paul

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Gentlemen,

Who has been able to locate the specific Telefunken data for the component Dr1 (the filter choke). I believe several individuals have stated the original was defective, and it was replaced.

My main objective is to determine what value Telefunken had assigned to the choke. I have been unable to read the part number/data from the component; and have not seen the specifics on the data sheet.

Respectfully,
Robert Sarbell
I am aware of the approximate value, but it would be most appropriate to know precisely what Telefunken installed.

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Please take a look at at the provided attachment from a Sam's file for the Opus 6.  You can purchase a drop in replacement from,  http://www.die-wuestens.de/eindex.htm,

A 10 henry choke of around 100mA/330ohm would do.  Before replacing the choke, I would check to see if the original is actually open.

Good luck with that unit.

P.S. Did that transformer stop buzzing after injecting it with varnish?
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Hello Omer,

Thank you for the reply. . . . . . .I shall review what Jans offers. I have checked the original and it is definitely not open. Digikey also sells some "drop-in" DC filters which have the physical dimensions and they offer a range of ratings that are "close".

My power transformer has never had the buzzing problem that you describe. I did put a slight amount of the GC red varnish on a few of the plates, but it made no difference.

The OPUS 6 data from SAMS will lend support to identifying the proper choke.

Regards,
Robert

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