Problem with Paramount v1.1

[Doc B]

You could try running some signal through the amp while you test various points in the circuit where that signal should be present, to see if you can locate where it is stopping. Set your DMM to AC volts and attach the black test lead to signal ground at point like that negative terminal of the bypass  capacitor that we had the typo about. With the amp on and some relatively loud and steady music signal going into the input jack check the center pin of the input jack, then the side of the .1uF coupling cap at the grid of the 300B, then the side of the parafeed coupling 3.3 uF cap that feeds the output transformer. You should see low level AC volts at the input jack that vary with the music, for example maybe a range of 0 to 1 volts. At the .1uF cap this should be a bit higher, maybe up to 20V. At the parafeed cap it should be even higher, maybe up to 150V, so be sure to set your meter to at least 200VAC for that measurement. It should go without saying that one needs to be very careful taking these measurements on a live amp. Make sure it is sitting in a stable position and keep one hand in your pocket while probing the test points. None of those points should have much DC voltage on them, but many nearby terminals will have.

If you can find where the signal stops we can narrow down the places you need to look to find the issue.

 

[Roger]

You could try running some signal through the amp while you test various points in the circuit where that signal should be present, to see if you can locate where it is stopping. Set your DMM to AC volts and attach the black test lead to signal ground at point like that negative terminal of the bypass  capacitor that we had the typo about. With the amp on and some relatively loud and steady music signal going into the input jack check the center pin of the input jack, then the side of the .1uF coupling cap at the grid of the 300B, then the side of the parafeed coupling 3.3 uF cap that feeds the output transformer. You should see low level AC volts at the input jack that vary with the music, for example maybe a range of 0 to 1 volts. At the .1uF cap this should be a bit higher, maybe up to 20V. At the parafeed cap it should be even higher, maybe up to 150V, so be sure to set your meter to at least 200VAC for that measurement. It should go without saying that one needs to be very careful taking these measurements on a live amp. Make sure it is sitting in a stable position and keep one hand in your pocket while probing the test points. None of those points should have much DC voltage on them, but many nearby terminals will have.

If you can find where the signal stops we can narrow down the places you need to look to find the issue.

I have a function generator and a scope. I just need a few BNC to RCA adaptors to use the test equipment to test the amps. I'll order some and do some measurements. Will take a few days though. Meanwhile, I have other amps I can use.
Roger

 

[Doc B]

Excellent, the generator and scope will make the job much easier.

 

[Roger]

Hmm, confusing.

I have done some AC measurements today with a Tektronix MSO2000 scope and a TTi function generator. I have done tests with various levels of sine waves, frequency was 1kHz.
With the function generator set to 2V p-p I measured 31.2V (62.4V p-p) at B4 (anode of 5670), the same at the OB side of the 300K resistor (which it should be) and the same at A3 (grid of 2A3).
When measuring at the output transformer side of the 3.3uF parafeed capacitor I get a disturbed 4.0 to 6.0V reading.
Measured the heater voltage between A1 and A4 to be 2.2V DC.
I then did some measurements with a higher AC voltage in. The gain in the 5670 again showed to be approx. 31x.
I got the same disturbed 4.0 to 6.0 V reading at the 3.3uF cap.

I resoldered all the connections on the four pin socket, the 22ohm resistors, the hum pot, the plate choke (PC2), the output transformer (OT1), the two cathode resistors and all the wires leading from these points to the terminal boards. No change.

I'm running out of ideas for what to do. Any ideas?
Roger

 

[Roger]

Did the same measurements with the other amp. Same 2V p-p, 1kHz sine wave in. Measured 31.4 V (62.8V p-p) out of the 5670, and 146V out of the 2A3. So everything is fine here for now with this amp.

I suspect a bad capacitor (parafeed) in the other amp. Comments?

Roger

(forgot to mention that I measured 10.6V AC at the output of the output transformer in the functioning amp)

 

[Grainger49]

Roger,

Doc suspects an open in the signal path.  You can give it a signal and trace where it disappears.  Since some will have a DC on the signal AC couple the input to your oscilloscope.  That way you only see the 1k Hz signal.

It comes in on the center conductor of the RCA jack.  Measure there then measure at pin 3 on the 5670.  The output of the 5670 is on pin 4.  For 2A3 that exact signal should appear at tube pin 3 on the 2A3.  And the output of the 2A3 is on pin 2.  That goes through the parafeed capacitor.  This signal should be seen on the output transformer Terminal 1 (red wire).  Of course the output of the transformer is taken away on a red wire too.  Since I don't know your speaker impedance I can't tell you which lug.  But that red wire goes to the red (+) lug of the speaker outputs.

Or you can start by touching up all the connections mentioned.  There will be several that I didn't mention in between but you should be tracing the wires between the connections/terminals as well.

 

[Roger]

Roger,

Doc suspects an open in the signal path.  You can give it a signal and trace where it disappears.  Since some will have a DC on the signal AC couple the input to your oscilloscope.  That way you only see the 1k Hz signal.

It comes in on the center conductor of the RCA jack.  Measure there then measure at pin 3 on the 5670.  The output of the 5670 is on pin 4.  For 2A3 that exact signal should appear at tube pin 3 on the 2A3.  And the output of the 2A3 is on pin 2.  That goes through the parafeed capacitor.  This signal should be seen on the output transformer Terminal 1 (red wire).  Of course the output of the transformer is taken away on a red wire too.  Since I don't know your speaker impedance I can't tell you which lug.  But that red wire goes to the red (+) lug of the speaker outputs.

Or you can start by touching up all the connections mentioned.  There will be several that I didn't mention in between but you should be tracing the wires between the connections/terminals as well.

Hi Grainger,

I'm not sure if you read my posts or not. I have just measured my amps with a function generator giving signal in to the amp, and I have done a bunch of measurements with a scope. Everything measures correctly up to and including the input to the 2A3 tube. On the parafeed cap I get crappy measurements.
The scope has been set to AC measurements (so DC is filtered out), and everything measures as expected up to the 3.3uF cap, as mentioned above. All connections to the four pin socket and to the end of the amp have been resoldered.
Roger

 

[Paul Joppa]

Of course a bad capacitor is a possibility - for instance, one with a broken connection inside the cap. If you have access to a capacitance meter you can check it (disconnect one side); otherwise swap it for th one in the good amp.

Another possibility is an error in wiring the output transformer secondary. If the secondary is shorted, that would present only the DC resistance to the 2A3, reducing the output substantially. You can disconnect the transformer from the parafeed cap to check.

 

[Roger]

Of course a bad capacitor is a possibility - for instance, one with a broken connection inside the cap. If you have access to a capacitance meter you can check it (disconnect one side); otherwise swap it for th one in the good amp.

Another possibility is an error in wiring the output transformer secondary. If the secondary is shorted, that would present only the DC resistance to the 2A3, reducing the output substantially. You can disconnect the transformer from the parafeed cap to check.

Hi Paul

Thanks for the advice. Checked the output transformer connections, and they are correct (16 ohm, my speakers are 18 ohm). My DMM can't measure capacitance, but when I measure it's resistance I can see that it charges from the battery in the DMM.
Roger

 

[Grainger49]

Roger,

Since I don't own a Paramount I skimmed over the voltage readings.  My bad!  The magnitude of the voltage is inconsequential.  I didn't follow your tracing the signal from input to output.  

The fact that the amp worked after you found an open trace/solder joint means nothing was defective to begin with.  This is a very good thing!

The quick and easy way to check if you have a bad parafeed cap is to take the one from the "good" amp and put it in the "bad" amp.  If that solves the problem look at the outer cover of the "bad" capacitor for damage.  These PP Film caps are very, very rarely bad on installation.  And changing a tube should never damage the cap.  

That is why I'm betting on a missed connection.  That is why my last suggestion was to touch up all the solder joints along the way.

 

[Roger]

Roger,

Since I don't own a Paramount I skimmed over the voltage readings.  My bad!  The magnitude of the voltage is inconsequential.  I didn't follow your tracing the signal from input to output.  

The fact that the amp worked after you found an open trace/solder joint means nothing was defective to begin with.  This is a very good thing!

The quick and easy way to check if you have a bad parafeed cap is to take the one from the "good" amp and put it in the "bad" amp.  If that solves the problem look at the outer cover of the "bad" capacitor for damage.  These PP Film caps are very, very rarely bad on installation.  And changing a tube should never damage the cap.  

That is why I'm betting on a missed connection.  That is why my last suggestion was to touch up all the solder joints along the way.

Thanks for the advice Grainger. I have already been over all the solder connections from where I found the problem up to the end of the amp. Everything has been resoldered. I have also checked for wrong connections, but I can't find any. So, I'm running out of options here. I can always try to change the caps, but I have never experienced a film cap being bad.
Roger

 

[Grainger49]

Then...

  .  .  .  The fact that the amp worked after you found an open trace/solder joint means nothing was defective to begin with.  This is a very good thing!

The quick and easy way to check if you have a bad parafeed cap is to take the one from the "good" amp and put it in the "bad" amp.  If that solves the problem look at the outer cover of the "bad" capacitor for damage.  These PP Film caps are very, very rarely bad on installation.  And changing a tube should never damage the cap.  .  .  . 

That is the only way I see to nail down the problem.  I've been kit building since 1965. 

 

[Roger]

Found the problem. A failed 2A3. A lot of trouble for a trivial problem. But, now everything is working fine, and I learned a lot about this kit during the troubleshooting.
Roger