[dconant]

It crapped out about 5 minutes after posting my last post. AVC is now at -2.8 volts.

Screen grids of 6A8 and 6K7 are at 70 vs 90 volts. Don't know what they were prior. Plate volts are 220 vs 246 on 6A8 and 215 vs 237 on 6K7. B+ off rectifier is 280 vs 314 off the rectifier. So B+ is low and not much better with a different rectifier tube.

I checked the Candohm after running for a while and the resistance are off being 500, 35, 150 VS 300, 27, 125 ohms. I have 350, 20, 120 ohm resistors. I'm going to try with these and see what happens.

[morzh]

So, we can safely exclude

1. the antenna coil (was not involved in the failure of the 2nd test).
2. the 1st IF can (the signal was going through it during he 1st test).
3. Tuning cap and its trimmer (same as in the #1, were not part of the failure).
4. AVC circuitry including the 6H6 (the voltage was good during the 1st test and did not drop)
5. IF amp, the 6K7 and the 2nd IF can.

The items left are:

1. 6A8 itself (you said you changed it, right?)
2. The oscillator circuitry (oscillator seemed to work but the whole mixer part is under suspicion as the failure involves the 6A8 being part of the signal path).
3. The oscillator's (the right side) of the band switch.
4. R1, C4, C1, possibly C6. R1 especially.
5. Any solder joint involved.






When you say your B+ is low, was it the same during the 1st test (where all was good?)

PS. For the heck of it, can you do the Test #2, and measure the Candohm's voltages (relative to GND, all three taps) when you start and repeat when it craps out?

[dconant]

Yes 6A8 has been swapped with at least 5 different tubes. I will replace parts in #4 one at a time and see what happens.

Mike I edited my last post but you may not have seen my edits. Please go look at that. I mentioned Candohm values.

[morzh]

Yes, 500 Ohm vs 300 Ohm is way out of tolerance, but if anything, it should result in 6F6 conducting less current and so not lowering B+ as much as it should. Which is opposite to B+ being lower than it should be.

Though not necessarily related to what we are troubleshooting,

1. Do you know for a fact both your 5Y4 rect tubes are good? Or at least one of them?
2. What is your AC IN voltage? (I cannot find what they did their measurements at, usually 115 AC). Also do check with voltmeter when set the volts by Variac: if not dititally displayed, the Volts on top of the Variac are for reference only and do not really reflect the real values.
3. Unless you use the old style meter with 1,000 Ohm/V impedance, the real volts you should be seeing on the DMM should be higher than those in the chart. So, if you use a DMM, yours are even lower than expected if referenced against the older measurements equipment which were used to make that chart.

[dconant]

AC in volts is 115. I put my best 5Y4 in which tests 64 where good starts at 55. B+ is 300 volts now. Candohm -2, -3.7, -21 volts.

I used DMM for AC volts at the connection inside of radio. For DC volts I use a VTVM.

[morzh]

Use DMM for everything for now. Makes it easier.

300V is better than 280V, obviously.

-21V is questionable. This voltage is used exclusively to bias the 6F6, and the bias at the Grid of it says -3V.
What is it when you measure it (try the DMM). Measure both the centertap (which is what you have as -21V at the leftmost point Candohm) and the Grid of the 6F6 output tube.

PS. I am sure this is unrelated but still makes sense to chart it out.
PPS. Yes, replace that Candohm as soon as you can.

[dconant]

Normal tone playing. Candohm is replaced with 120, 20, 350 5 watt resistors. Voltages on Candohm now -1.5, -2.5, -20. For the 6F6 tube grid is-18 volts, plate is 230 volts both to ground, grid is 0 volts to center tap. AVC voltage is -5.5. Perhaps my 350 ohm replacement for the 300 ohm section is too high? Best I have is 350 ohms.

[dconant]

If I look up the 6F6 data is see sometime like -16 volts or so on the grid depending how its wired. I don't know how it is wired. So I wonder if the grid voltage shown on the schematic is wrong. I have put a 2K resistor in parallel with the 350 ohm giving me 297 ohms total. The -21 is now -16.

I may have been wrong about the radio acting up this morning. I had the radio off for a while replacing the candohm and now the AVC is at -3.5 volts with 100mv going in. So I say the radio is running normal at this point.

[morzh]

350 Ohms is better than 500.

I am not sure how -16V develops.


Let's see.
Your three last K's voltage (like the detector, audio and output) are around 2V, which it is (you said in the beginning it was some 2.3V or so). 1.5V is even lower.

Now,, most current flows through this Candohm.

Let's say that 6A8 and 6K7 conduct 14mA together. That will make 125/27 Ohm junction sit at 1.75V (close'nuff).
Then this dictates the last three K's voltage (which is what you have, you said 1.5V or so).
Then (skip the diode, it barely conducts), the 6F5 and 6F6 together conduct 35mA together. Add to that the 14mA, getting 49mA total.
This will make the 27/350 ohms sit at +16V over the centertap, which with the minus sign and the -1.75 added to it will be -18.5V, which is close enough to what it was or what it is now.

And this is what the 6F6 grid is sitting at, -18 or so Volts.

PS. My currents are from the datasheet - they may be a bit different in your specific case.

[dconant]

So are the negative measurements ok?

[morzh]

Yes.

[dconant]

I did not like how the sig gen connection to 6A8 tube cap went yesterday so I am going to be doing it again today. I found my connection to the grid cap was not the best.

[morzh]

It has taken a while already, so might as well do it right.

[Chas]

Any metalized cap that has been installed with little or no gap in the soldered connection to the body is at risk for partial disconnection of the schoopage to the film. It will be intermittent and subject to malfunction with temperature or voltage surge.

Clean up the band switch with 91% alcohol dry completely, examine each contact tab and look for the tab to separate as the blade passes into the tab. Tease the tab if need be, apply a tiny drop of DeOxit to the blade do not flood the band switch with DeOxit.

GL

Chas

[dconant]

Chas all mica caps have been replaced and band switch has been removed and cleaned.

Tone from the signal generator played for over 12 hours yesterday with no problems. Today the signal generator is on the antenna terminals. We will see what happens so place your bets.