rca: Help please on RCA 2-62

Hello, I've been working on a rca 6-62 and ran into a snag that I can't figure out. Both the IF and RF 5th pin (cathode) voltage is high 1.8 V DC. I believe because of this, I can't pick up stations but the radio does have white noise. Any ideas? Very greatful for your help.

Thanks,

Bill

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Bill, that means cathode current is to high (if resistors are ok, but let's assume that). This is probably caused by a positive voltage leaking to the grids. Have a look at the fading control voltage, is that below zero? If not, check for bad insulation towards positive.

Regards, Steffen
 

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Hello Seffan,

Thanks for your responce. The resistors all seem ok but are a little low from age. I have checked the insulation on the wiring and found no shorts. I don't understand what you mean by checking the fading control voltage, could you explain that for me? 

thanks,

Bill

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Hi Bill,

did you get yourself the schematic? You can then easily identify a lead running from both grids via the filter coils to a resistor and then to the detector diode. This voltage will go negative with a strong station, but without something tuned in it should be near zero or slightly below.

Regards, Steffen
 

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Thanks Steffen,

I do have a schematic. The 12sq7 pin 4 is the diode pin I believe your talking about. when tested it  bounces right around 0 volts to slighty negative. Anything else I can check?

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yes, that's the diode where it starts. Then check what is on both sides of R7 and R8 and finally at the grid itself. If there is still no positive voltage, my assumption is bust and we'll have to look for other causes. There could be some allowance for an analog meter, and possibly this voltage is only valid if a station is present or the fading control clamped as you would do for aligning.

Did you check if the oscillator is working? 
 

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Hello Steffen,

I checked  both sides of R7 and found very little difference in voltage if any. the voltages fluctuated right around 1.7v.

R8, on pin 2 side, is .45v and on pin 3 side is .01v.

the grid is pin 2 and is .45v. 

I also checked pin 4 again and the voltage climbed a bit from the other day and reads around 1v but varies widely.

I checked the oscillator. One side tested 9.2 ohms and the other tested .7 ohms. Is there a better way to test oscillators?

I'm wondering if the phono switch is the problem???

Thanks for your help,

Bill

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Hi Bill,

I can't make sense out of your results. It seems you either have a different schematic or something is very strange. I was referring to the one here on RM.org. Let's try again:

The slight positive reading on the grid of the 12SQ7 indicates a somewhat low vacuum, but not that much that this stage has a problem. It will be somewhat sensitive due to the high value of the grid resistor. From what you write, your R8 seems to be this one.

Please get the schematic from RM so we can be sure to talk about the same items. R8 there is 2M2 in the fading control lead and R7 is 56k coming from the detector filter. If there is indeed 1.7 V at this R7 while zero on the diode pin (4), that would mean the filter coil or some solder connection is open. If that really is the case, you have the bug.

To check the oscillator: Please do a search on RM first. There are quite a few ways, one is using another radio to pick up the osc. frequency. Another easy one is indicated in the schematic: There should be at least -10 V present at the osc.'s grid. Use a decoupling resistor to measure there (100k or more at the pickup point), otherwise the oscillations will stop when you attach the test cable.

The phono switch does not seem to be an issue so far.

Good luck,

Steffen
 

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Hello Steffen,

Sorry for the confusion, I very well could have made an error in my readings.  I did down load the schematics from RM’s web site.  

Below I listed all the voltages from the 12sq7 pins 1-8 and also took some more readings on both sides of resistors 7and 8. I hope this clears things up, I’m at a loss.

1)     0                 5)   -.01

2)  -.4                 6)   55v ac

3)  -.01               7)   .3

4) -.09               8 )  0

Both resistors 7 and 8 tie into the phono switch at the same place. The voltage at that connection is slightly negative at -.1.  The other side of r7 is about the same slightly negative. The voltage on the other side of r8 is approximately 2 volts.

Thanks for helping,

Bill

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Hallo.

Der Katodenstrom der beiden 12SK7 beträgt 10,2mA

An den Widerständen  R1 und R4 muessen demnach 180Ohm /10,2 mA = 1,836 Volt

und an den 120Ohm/10,2 mA = 1,224Volt zu messen sein und nicht 2mal 0,7 Volt

Knoll    

Hello.

The cathode current of the two 12SK7 is 10,2mA
therefore must at the resistors R1 and R4 180Ohm / 10.2 mA = 1.836 volts and the 120 Ohm / 10.2 mA = 1,224Volt be measured and not 2 times 0.7 volts
Knoll

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Hello Hans,

that's right, I also found that 1.8 volts should be there without signal. I didn't elaborate as it was getting late (see post 6).

Hello Bill,

now your results at R7/8 make more sense. The positive voltage at one side of R8 is a bit doubtful, it should be near zero. If it indeed is 2 volts, then there would be even more at the cathodes. 1.8 volts is for zero at both sides of R8 and it's ok as Hans said.

The readings on the 12 SQ 7 are ok with two exceptions. 1: there can't be AC on the plate. This level would make you ears fall off with hum... 2: There should be some AC on one heater connection.

But let's look elsewhere. As you say, you have white noise. I take it you can judge if that is more than the audio stage would do. So I assume at least the major part of IF is working. Thus I would go for the osc next as in post 8.

Late again...

Steffen
 

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Hallo Steffen hallo Bill
Bill sagt er koenne etwas empfangen,  aber mit sehr viel  Rauschen.
Er hat zwar noch immer nicht den Oszillator getestet, was doch kein Problem mit dieser Methode ist.
Zum Rauschen, das Radio hat eine abgestimmte  Vorstufe 12SK7  Wenn  ein verrauschtes Signal ankommt, kann  man  davon ausgehen, dass der Oszillator schwingt aber vieleicht mit zu wenig Amplitude.  Deine 10Volt DC  sind O.K.
Wenn die Mischstufe nicht arbeitet, wird nicht alles an Rauschen in die ZF-Lage gemischt.
 Ich denke,der Fehler liegt im Antennenkreis mit Rahmenantenne,  Die Resonanzuebehoehung im Rahmenkreis fehlt  vielleicht.  Verbindungen heiss und kalt zum Drehko ofen ?.Damit wird  die Funktion  des Drehko  wirksam. Den  Kondensators C10 eventuell  ersetzen, der den Drehko an die Zentralmasse legt.
Mehr ist im Moment nicht zu sagen.Ich weis das sie vieles davon auch wissen. 
Knoll  

Hi Steffen hello Bill
Bill says he could receive something, but with a lot of noise.
Although it has still not tested the oscillator, which is not a problem with this method.
For noise, the radio has a C-tuned RF- Stage with 12SK7. When a noisy signal arrives, it can be assumed that the oscillator oscillates but maybe with not enough amplitude. Your 10 Volt DC are o.k. If the mixer tues not working, no Noise or everything, is mixed to noise in the IF position.

 I think the fault lies in the antenna circuit with the loop antenna, the resonance in the input circuit may be lacking. Conections < hot> and  for Drehko? .
The makes the function of the variable capacitor effectively. Ore  the capacitor C10  may replace, that sets the variable capacitor to the central mass.K.
More is not to say at the moment.
Knoll, only with   google translation
 

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Hello,

For some reason I can now only replay to my own post, not sure why 

I Spent some time reheating old solder joints and have some different readings from the 12sq7. By doing this, I can now pick up radio channels across the dial but only if I hold the tip of a screw driver on the 12SA7 grid, pin 8.  Any thoughts?

 12sq7

1) 0

2) -3

3) 0

4)  -.069 On a strong station this diode voltage goes down to -4v

5) 0

6) 43

7) -.1

8) 0

Also, R7 and R8 readings changed and are as follows

R7      -.1        -.1

R8      -.17      .063

Bill

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After heating the connections of the volume control, and replacing some weak resistors I was able to receive a strong radio station but still with a lot of white noise. I noticed, at this time, some critical voltages had changed, namely the detector diode. It went from 2.5 v dc to -4v dc. After reading Han's reply, I held a screw driver to the converter grid to substitute as an antenna and was able to receive stations normally.  By heating up the solder connections on the loop antenna, trimmer condenser and Oscillator coil the radio then started working normally.

It appears that not just one thing was wrong with this radio but a combination of many. For a simple radio it sure gave me a lot of head ake!!!

 Steffen and Han, Thank you for your Patients and help,

I still cannot reply to your posts If you can lend any advice please do

Bill

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Pooh - that would have taken me a long time to find out in remote mode. Very nice to hear you had success, even though I feel my part wasn't too big. Hans, I'm not that sure I'd have put my finger on the right spot. Your comment is a good guide indeed.

Bill, if you want to play some more, hook your meter to the detector diode and see the voltage go negative as you tune in a station. And you will see the 1.8 volts from post 1 decrease in the same instant.

Don't forget to enjoy now!

Steffen
 

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Hallo Steffen

 Keine Bescheidenheit.

Gegen 25 Jahre in Vaters Radioshop, dazu  32 Jahre als Entwickler von Unterhaltungselektronik, hast Du einen Rückstand an dem Du nicht Schuld bist.

Hello Steffen

No modesty. Counter 25 years in father's Radio Shop to then 32 years as a developer of consumer electronics, You have a residue on which you are not to blame.

Hans

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