Assistance with signal path please?

[Randall]

SWR power module. No output. Rails are there. Signal gets to the base of Q1, and I scope the sine wave at about the same level at Q1's collector. At the emitter on R38 and R39 I get a smaller, squashed wave. I have to admit, I don't see what the signal path is here, or what exactly Q1 and Q2 are doing. The difference between the positive rail side and the negative rail side is also confusing me a bit.

Thanks for any light you can shed.

Workingman's 4004-2x10c Power Amp Rev D (2).pdf

[Jazz P Bass]

Q1 & Q2 are considered a 'Long Tailed Pair'.

You will have to look that up.

Go directly to Q3 & see what signal you have on the Collector.

[Randall]

There is no signal on the collector of Q3 as measured at R11.

[Dave H]

It's not stuck in mute is it? What is the DC 'tail' voltage on the C3 + terminal.

[Randall]

I measure 10.1 vdc at that point Dave H.

[MarkusBass]

I'm with Dave. It looks like the amp is muted. Have you looked at the muting circuit? Here is how it works:
-75V rail through R32 resistor and D8 diode cause -0.65V on base of the Q13 transistor. But rectified AC voltage (through D7 and R30) cause positive voltage on C16 and this voltage cause that Q13 starts conducting. This causes that Q14 also starts conducting. And this causes that on the collector of Q14 you should get about 70V. If you have there 10V only, I would check the DC voltage on C16 and AC voltage on J1. If they are OK, check what voltage you have on collector of the Q13 and Q14 transistors.

Mark

[Randall]

Here's what I get.

J1 = 60 vac
C16 = 12.4 vdc
Q13 collector = 43 vdc vdc (edited)
Q14 collector = 35.9 vdc

[Dave H]

J1 = 60 vac
C16 = 12.4 vdc
Q13 collector = 0.01 vdc
Q14 collector = 35.9 vdc

Q13 collector should be low but Q14 collector should be at 70V or more. It's either dead or something is holding the voltage down. Check C17 and C3 are not leaking. Where does the J11 slave connector go to? Could that be holding the voltage down. If the + of C3 is still at 10V then there is 25V across R8, that's 3.5W. R8 would be stinking hot and all the current (140mA!) would have to be flowing through C3 unless there's a short on the board. How hot is the can of Q14?

[MarkusBass]

Here's what I get.

J1 = 60 vac
C16 = 12.4 vdc

So at least AC input and C13 are OK.

Q13 collector = 0.01 vdc
Q14 collector = 35.9 vdc

I have some doubts whether your measurements are correct . Q13 may be shorted but before you start replacing it, please make sure that you measure the voltage on collector and not on emitter.
The voltage on Q14 seems to low but here I have also some doubts. If the voltage is 35V and the voltage on C3 is 10V, the voltage drop on R8 is 35V - 10V -0.65V = 24,35V. And the power it dissipates is 3.3W which is rather impossible (unless it is burnt and has much higher resistance than 180 Ohms). Please make the measurements once again. Also measure the voltage base-emitter of Q14 (or R34) and voltage drop on R7 and R8 resistors.

[Randall]

My bad, you are correct the collector of Q13 is very difficult to access. I now read 43 vdc at the junction of it and R33. Also, I now see that R8 has been replaced with a 56K, which would seem suspect. It doesn't get hot, though. The metal can Q13 is a few degrees above ambient. And there is nothing connected to the Slave connector J11.

[MarkusBass]

Also, I now see that R8 has been replaced with a 56K, which would seem suspect.

Are we talking about R8 that should be rather close to 180 Ohms?

[Randall]

Yes R8 is supposed to be 180 Ohms, so I'll need to replace that. I pulled the board and found Q13 open between C and E, but I don't understand what, if anything it has to do with R8, which once I pulled it seems to have been some smoke scraped off the board under it. So someone has been in here before me. D1 tests good in circuit as does D6.

[MarkusBass]

I pulled the board and found Q13 open between C and E, but I don't understand what, if anything it has to do with R8, which once I pulled it seems to have been some smoke scraped off the board under it.

Be creative . Open Q13 transistor caused that the amp was permanently in MUTE state. R8 forms a power supply rail for input transistors (switched on and off by the muting circuit). So Q13 and R8 are both in the same muting circuit - so they have a lot of in common. There could be some other failure (like e.g. shorted C3) that caused R8 burnt. Someone tried to fix it but he had no idea what he was doing. Other parts may have been replaced incorrectly too. I would check (with magnifying glass) which parts were replaced. This is usually visible. Especially, that the guy had no idea what he was doing so I'm sure that he didn't clean the PC board after soldering.

Mark

[Randall]

I have replaced C3 and C17 since I had them on hand, to rule them out. I will replace Q13 and R8 when parts arrive next week and report back.

Thank you for your help fellows.

[Randall]

One more thing, when the time comes to switch this back on, I wonder who thinks it will be OK to "fly" the board for a quick moment for testing purposes, or must it be installed to the heat sink?