Assistance with signal path please?

[Dave H]

I am still seeing 45v on collector of Q14. Is this a problem?

Having triple checked, the junction of R33 and the collector of Q13 measures 0.02 vdc. R33 measures 17.9K ohms in circuit. The junction of R33 and the base of Q14 measures 42 vdc. The amp is passing signal.

So, now I am even more confused.

Me too The schematic has the emitter of Q14 at 75V but its base measures 42V so it has 33V from B to E. The BE junction breaks down at about 7V. I think Q14 must be dead. Have you replaced it?

The amp will pass signal with 42V (45V?) on Q14 collector but it may clip on the positive peaks before full output.

Edit:
I've been trying to work out how Q14 collector can be sitting at 42V
The amp output is at 0V. If Q14 BE is open circuit and BC is shorted then the potential divider formed by R34 and (R7 + R9) will put Q14 at about 42V.

R7 + R9 = 3.17k
75V * 3.17/(3.17+2.7)) = 40.5V

[Randall]

I have not replaced Q14, nor have I removed it to measure it. I will do this next. Thanks for all the help on this, I do really appreciate it!

[Dave H]

Before you replace Q14 try scoping the max peak to peak output at J5 pin 1 without a load connected. If it's working correctly it should clip at close to the rail voltage i.e. > +/-70V p-p. If it can only do -70V, +45V then it is faulty.

[Randall]

Dave H, to clarify J5 pin 1, do you mean speaker out with no load? And this is with a sine wave at the input?

[Dave H]

Dave H, to clarify J5 pin 1, do you mean speaker out with no load? And this is with a sine wave at the input?

Yes and yes

[Randall]

with a sine at the input and no load, the positive side shape looks good, but only goes to about +10v. the negative side is squashed and wider than the positive side. Then after being on for about 30 seconds it jumps up to a nice symmetrical 84 p-p. My rails are about +/- 84vdc as well.

[Dave H]

...after being on for about 30 seconds it jumps up to a nice symmetrical 84 p-p. My rails are about +/- 84vdc as well.

Have you replaced Q14? and is 84 p-p at clipping or is it a clean sinewave? Turn the sig gen up until it is just clipping. It should clip at over +/-80V (160V p-p) with +/-84V rails. The 30 seconds could be the mute circuit but I'm not sure why it's taking that long. To check if it's the mute circuit short Q14 C to E to defeat the mute. It should work without a delay with mute disabled.

[Randall]

Q14 has not been replaced. 84 vp-p is clean sinewave. When turned up to clipping it is as you say, 160v p-p. With Q14 C- E shorted it comes up without delay. Am I chasing my tail at this point?

[The Dude]

That's a possibility. I don't normally look at .33V as an acceptable offset. Maybe SWR does?

Edit: On a side note, are we sure that there isn't a metering error or something giving you an incorrect reading measuring output offset? Maybe it's not as "bad" as we think it is. Low battery in the meter or something of the sort?

[Jazz P Bass]

I would try replacing Q14.

[Randall]

Good idea about the battery in my Fluke 77, it was 7.5v. With a fresh battery the offset measures 0.30 vdc at 0 on the master volume and 0.01 vdc full up on the master. It still has the 20 sec delay before it unmutes. Dare I connect a load?

[Jazz P Bass]

That turn on delay is way too long.

[Randall]

Ok, any ideas why that is or how to proceed? I'm not feeling secure to run a load at this point, but I don't know if I'm being too paranoid either. Is there a cap that isn't up to form that could be part of the problem?

[MarkusBass]

Ok, any ideas why that is or how to proceed? I'm not feeling secure to run a load at this point, but I don't know if I'm being too paranoid either. Is there a cap that isn't up to form that could be part of the problem?

The amp has turn-on delay circuit (Q13 and Q14 transistors). We already know that the circuit failed in the past and it was incorrectly fixed (R8 resistor = 56k). It seems to me that the circuit still works incorrectly. The circuit can be easily verified by measuring voltage on R8 (and how quick it rises after turning the amp on). It was also suggested many times to replace Q14 but you haven't done this (why?). Of course, before you start replacing Q14, I would measure the voltage on R8. For me, it seems that the voltage rises to slow and is to low. But, as I said, you can easily verify this.
Are other component values in the turn-on delay circuit correct? I mean: R30, R31, C16, R33. You know that the guy who was fixing the amp previously, had no idea what he was doing. So you can assume all kinds of other mistakes (like incorrect component values).

EDIT:
as Dave told you:

The 30 seconds could be the mute circuit but I'm not sure why it's taking that long. To check if it's the mute circuit short Q14 C to E to defeat the mute. It should work without a delay with mute disabled.

the delay circuit can be verified by shorting Q14 collector to emitter (I would desolder Q14 doing this). The reason that the circuit is so slow may be caused by failed one of the input transistors (Q1 and Q2). Also the voltage on the output indicates that there is something wrong with the input transistors. Have you check them?
BTW, I have one SWR amp at home, I could measure the voltage on output but at the moment I'm fixing other amps.

Mark

[Randall]

"I would try replacing Q14."

It should arrive pretty soon, and I will do just that.