Peavey Heritage VTX, no sound output

[Sowhat]

I got a Peavey Heritage VTX in for repair (no sound output). A little history, 2 years ago a tech for a small music store in San Diego examined and tore apart this amp and proclaimed that Q9 was shot and it should be replaced. The next day he died so 2 years later it appears on my bench, in pieces, to get fixed.

Schematic:
https://www.dropbox.com/s/k1h7wbifo2...x_sch.pdf?dl=0

I did a quick examination of Q9 and associated circuitry after slapping it back together good enough to power it on. Sure enough Q9 was not conducting, Q7 however looked to be doing it's job at about 74 VDC and I forget the milliamps on the emitter resistors but I remember it to be right about what is stated on the schematic. The emitter resistor for Q9 had no current going through it… looks like the dead tech was right.

I swapped in a pair of MJ15003's for a quick test and the swap changed nothing… mmmm??? Using my brand new whiz bang PIC based semiconductor identifier/checker which also does resistors and capacitors and ESR etc etc, oh my! That little dealie-bop told me that the original Peavey 6465 T0-3 NPN's were good and healthy, I believe it, how could a micro-controller based gizmo designed and programmed by someone unknown and soldered up by an old man with vision problems and shakey hands ever be wrong… still, I believe it.

anyway, let's get down to brass tacks…

I installed a matched known test quartet of 6L6GC's and brought the amp up with a variac while checking B+ and , speaker outputs hooked to a 4 ohm 400 watt dummy load, no signal input. Things looked cool and collected all the way up to 120VAC, all 4 heaters were emitting that soothing heater glow. Everything at that scale looks fine.

I checked the 0.522 and 1.07 VDC values in the base circuits of Q7 and Q9 and the Q7 circuit was pretty much what is stated on the schematic, (I wish I had my scratch sheet with me right now for the measured values) but let's say the 0.522 reference was 0.535 and the 1.07 reference was 1.09, that's pretty close right? Amazingly the troubled Q9 side was something like 0.486 and 1.08, a little low but still pretty close. I'm thinking that Q9 is not turning on, do I understand how a hybrid cathode controlled output stage works? Probably not, don't both sides need to match within a certain tolerance for the sides to be declared symmetrical, how much does that matter?

I decided to backup here and take a look at some of the things I should have looked at first, like I said, sometimes you never really know with me. The output transformer is GOOD, The power rectifiers are GOOD, so is the filter caps, the wires etc etc… The heaters got 6.3VAC, The B+ is right around 502VDC and looks relatively clean, The +15 volt regulated source puts out +15.001VDC, the regulated -15VDC is missing, Ah HA!!!!

I know that none of the fuses made it to me on their 2 year odyssey in the store room so I installed new proper fuses. R174 the 150 ohm 5 watter was open. I replace that and checked the current through the resistor and there was about 80ma, that should be fine, no wonder back when I injected the pre-stages the waveforms looked kind of funny. Those are all right as rain now, all the way up to the HV path through the output transformer primary, I got about 500 VDC in all the right places. I restored the -15VDC, so, power supply GOOD.

This had no effect on the primary complaint of no sound output as I figured it wouldn't , but it had to be taken care of, so back to the output stage.

I checked the sockets, pins, traces, solder connections, wire/connector integrity and just about anything else I can think of but this non conducting Q9 still persists… and Q9 and Q7 have been swapped and tested a few times, both with that PIC thingy and with my multimeter old style, and they both agree, R155 and R159 have been monitored closely and no change, within tolerance, whatever that really means.

Any suggestions?

[Enzo]

Your link requires me to register at the site, and I don't do that. here is an accessible file:
http://bmamps.com/Schematics/Peavey/...tx-sch+pcb.pdf

Have we discussed this elsewhere? It looks familiar.

So Q9 doesn't conduct? But it has proper half a volt on the base? I don't worry if it is more or less close. Read the notes lower right on the schematic - ALWAYS read schematic printed notes - it tells you what they consider acceptable range of the pertinent voltages. So do we have no voltage dropped across R159, and it measures about 3 ohms? Or do we have the 30mv across the resistor? And maybe zero volts on the cathode? What is your reason for the not conducting diagnosis. Not saying you are wrong, just want to know how it was determined.

You don't really need to understand how cathode drive works to troubleshoot this. It pays to know about the amp of course, but this little corner is just a transistor controlling current. That is what they do. The base needs to turn the thing on, and the emitter needs that 3 ohm path to ground, but all the current through the transistor comes through the tube/tubes.

I can't accept "all the right places". Do you have 500v more or less at both plate and screen grid of both tubes on the Q9 side, I mean right at pins 3 and 4, no remote readings? Is ther +15 at pin 5 of each tube? Is there continuity from pin 8 of the tubes to the case of the transistor? No inferences, direct readings point to point. The drive transistors are on th main board, but the tubes are on their own board, there is a six wire cable connecting the two boards. There could be a broken wire, broken connection, cracked solder.

do I understand how a hybrid cathode controlled output stage works? Probably not, don't both sides need to match within a certain tolerance for the sides to be declared symmetrical, how much does that matter?

It matters as much as matching the tubes matters, which is to say not a lot, but it is desirable nonetheless. Your issue is that one side is not running at all apparently, not an issue of balance.

How does it work? The current through the tube is controlled by the relationship between grid and cathode. The more negative the grid, the less curent flows. But that means negative with respect to the cathode. A +75 cathode and a +15 grid are 60v apart, grid negative relative the cathode. To the tube that is no different from zero volts on the cathode and -60 on the grid.

usually we hold the cathode steady and move the grid up and down, but it is just as valid to hold the grid steady and move the cathode up and down. The tube only knows the difference between grid and cathode, not either one and ground. So you can inject a signal either way. And like cathode bias, the voltage across the tube is reduced by the amount on the cathode.

[Sowhat]

Your link requires me to register at the site, and I don't do that. here is an accessible file:
http://bmamps.com/Schematics/Peavey/...tx-sch+pcb.pdf

I have not figured out how to attach pdf's to this forum... maybe I can't, sorry about that.

I don't think I've ever posted about a Peavey Heritage before, Someone else has but their problem did not apply to my current situation.

Screens and plates are all hot at slightly over 500 volts for all four power tubes... I even put in some tube extenders I can probe to see if it's getting to the actual tube pins and not just the socket and, yes it is. All the circuitry from the tube pin, through the output board and through the connector to the main board tests good including the path from the cathodes to collectors connections.... at least that's what my instrumentation tells me.

Yeah, there's no drop on R159 which measures 3 ohms on the button, and no 74 or so volts on the collector of Q9. It's like the emitter collector path in Q9 is blown open or R159 is open or something, but it is not... bizarre! Q7's Vk(idle) is 1 or so volts under the stated minimum but that should not be too evil at this point. I figured the approximately correct value rule of real world electronics applies.

Let me get back to you one this. Tomorrow morning I will recheck everything once again, maybe I suffered a senior moment during the previous check, afterall reality is in no way obliged to conform to my notion of how things should be and sometimes appear.

[Enzo]

Note I didn't attach a pdf either, I just posted a link. No big, its all cool. But you can post pdfs directly as well, we can figure that out later.

Guys so often write in asking about some part or other, and I like to preach that it ain't about the parts, it's about the circuit. A cracked solder joint is the same as an open component as far as the circuit is concerned. So that is why I ask for things like tube pin to transistor case or emitter pin to chassis. That tests not only the part, but also the connections and circuit paths. So for example R159 itself might be fine, but have a cracked trace beteeen it and the emitter pin. or something similar.

I will believe your transistors are OK, you swapped them out for no change as well as tested them. I'll believe your ohm meter. If you get the half volt at the transistor bases, then the base ought to work if the rest of the circuit is intact.

I don't recall, but we haven't overlooked making sure the heaters are glowing in those two tubes?

Stick with it. And remember that you have the Q7 side sitting there working, so some quick A/B readings comparing point by point the two sides might inform.

[nashvillebill]

I'm following this with great interest, I've got a Heritage VTX on the bench now, waiting for Q7/Q9 to arrive. I posted on that one maybe a week and a half ago, it's likely the post Enzo recalls. (Enzo, if I'm ever up your way, I'll treat you to a steak dinner, you have been so helpful to everyone)

[Jon Snell]

This is a very simple circuit but until understood, it may just as well be a brick wall.
Take one side at a time; Q7/9 is excited with a small voltage on the base from the +15v supply firstly via R149 then secondly R147. Assume the 6L6s are conducting some current, the voltage across R155/159 will increase until the driver transistors conduct and regulate the Vbe on the output transistors. Forming a Norton Mirror. If the 6L6s don't conduct, neither will the power transistors.
Check the grid voltage is +15V on all valves.
Check the DC voltage on the base of each power transistor.
Check for leakage on the driver transistors. If Q8 is leaky that will stop Q9 from conducting.
Check the voltage on each supply regulator diode pack; cold end of R149 and R147.
Check the quiescent current increases when a 4k7 resistor is placed between +15 (pin 8 of U9) and Tp1.
Hope that helps to explain the way this works.

[g1]

the primary complaint of no sound output

I realize this thing needs to get fixed and Q9, V3, & V4 need to get conducting, but still, none of this would explain no sound would it?

[Dave H]

Yeah, there's no drop on R159 which measures 3 ohms on the button, and no 74 or so volts on the collector of Q9. It's like the emitter collector path in Q9 is blown open or R159 is open or something, but it is not... bizarre!

You're right. It doesn't add up. If Q9 is open circuit the voltage on its collector would be pulled higher than 75V by the tube. Are you sure the collector is connected to the tube cathodes? If it is and you measure 0V on the collector the tube has +15V on its grid and 0V on its cathode. How could that be?

You don't need to register with DropBox to download the schematic, just cross out the registration screen and go to Direct Download.

To upload a pdf click on the 'paper clip' above the message box, click 'Add Files', browse to the file on your PC, click 'Upload' then click 'Insert Inline' below the Attachements box then click 'Done'. It's easier done than said.

EDIT:
Are C60-C63 reverse polarity? The op-amp outputs (cap +ve) will be at 0V and the cap -ve terminals are at +1V.

[Sowhat]

OK guys, the plot thickened and thinned and got real obscure and then got crystal clear all pretty much with no intervention from me, well, almost.

This morning I started out fresh and started checking the continuity and voltages of everything concerned... let's see, it went something like this: With tubes out I got 524 volts on all 4 plate pin sockets, got about the same on all 4 screen grid socket pins. I got +15VDC on all the grids and got 0 ohms from the cathodes to the respective Q7 & Q9 transistors. That all looks perfect as far as I can see.

I slapped the test quartet back in and powered up, Q7 had about 75 volts on it and .... drum roll please ... Q9 had about the same!!! Pardon my French but WTF!!!
There must be a flakey pin connection on the power output board even though I looked and looked till my eyes watered. I'm just going to suck all the solder from the pins and melt some new fresh stuff in there, afterall, there's nothing really there but sockets anyway.

I shot some sine into the input and still, virtually nothing at the output and I got that sinking feeling that I just missed the obvious again. I sheepishly took my plastic poker and poked the switch contacts on the power amp in jack and voilla, output galore! It's the old send/receive dirty contact routine.

So, let's review, the dead tech was dead wrong about his diagnosis, there was nothing blown in the output loop, it was just a flakey socket connection. The regulated -15VDC missing was indeed a problem and I don't see why that resistor opened up, but it did so there something to ponder. Other than that, the amp works great and my total parts cost for this venture was 95 cents for the opened R174. The cost to my sanity however is more incalculable, but then again, that's the price we pay for this fun sometimes.

[Sowhat]

One last thing, here's a shot of the crime scene...

[Sowhat]

I realize this thing needs to get fixed and Q9, V3, & V4 need to get conducting, but still, none of this would explain no sound would it?

Good point, but considering that even though this is a cathode drive circuit, it's still operating in class AB push pull as far as I know, so, maybe 1/2 of the output wave would be there but certainly not the entire 360 degrees. You'll see the lack of throughput was due to something completely unrelated in my summation post in this thread.

[Sowhat]

To upload a pdf click on the 'paper clip' above the message box, click 'Add Files', browse to the file on your PC, click 'Upload' then click 'Insert Inline' below the Attachements box then click 'Done'. It's easier done than said.

Thank's Dave, I'll give this a whirl next time.

[g1]

it's still operating in class AB push pull as far as I know, so, maybe 1/2 of the output wave would be there but certainly not the entire 360 degrees. You'll see the lack of throughput was due to something completely unrelated in my summation post in this thread.

That was what I expected. If the only fault was the loss of half the output, there would have still been some sound. So I figured there must still be something else as you said there was no sound at all.
Glad it turned out to be fairly simple issues and you got it fixed.