Peavey 5150 - Preamp and Poweramp seem fine, but NO SOUND

[greenridr]

Hey everybody-

I just posted this over in the FJA forums, but figured I'd also post here in the hopes of reaching a wider audience. I've already learned a lot by browsing through reams of Ampage threads, so any further advice is much appreciated!

Here's my story- I picked up a "as is" 5150 for $99 bucks, knowing that I'd have my work cut out for me (but secretly praying for nothing more than a cold solder at the power input or a blown fuse). I was told that it wouldn't power up, so I was surprised that all lights powered up in Standby and On mode. Of course, it produced no sound. Upon initial examination, here's what I found (after swapping ALL TUBES with known functioning ones):

-All power/channel lights power up
-All tubes light up/glow
-No sound coming out of Effects send
-No sound produced when plugging another preamp into Effects return
-Speakers are wired and sound fine when hooked up to another power amp.

I opened it up, and took out the preamp PCB. There were a number of cracked solder joints, a few discontinuities in the circuit board, and evidence of tampering with before. I cleaned up every dodgy connection I could find, and moved on, disconnecting every quick connect and ribbon between the boards, and tested them for continuity, no problems. Looked at the small boards for the efffects loop and speaker outs, cleaned the jacks, saw no problems. All fuses tested fine.

I put it back together, and now the preamp is generating a signal when I plug headphones into the effects send. Sounds as expected, and I noted that the presence and resonance controls have no effect at this point (which, if I understand their placement correctly, is to be expected). However, the power amp is still not producing any sound through the speakers. Nothing, Zilch, dead quiet. (measured resistance across the resonance and presence knobs seemed fine, too)

The power tubes appear fine. The glow faintly in standby, and more vigorously when turned on. I measured the voltage on both tubes from pin 3 to ground, and registered right about 520 in STANDBY, right about 496 in ON mode. I also checked the screen grid resistor values, both measure at right around 100 Ohms.

It appears to me, then, that both preamp and power amp are functioning within their expected parameters, but there's some sort of disconnect between the power amp and the speaker output that I haven't been able to trace. Does any one have any ideas as to what might be the problem here, or suggestions as to where I should go from here in terms of isolating the problem?

*Note- I am not an expert on tube amps, but have worked on quite a few, and have a reasonable knowledge of circuitry and electronics. I am aware of the risks involved, and take considerable precautions whenever I am inside an amplifier. IF this problem gets me in over my head, I will take/send it to a tech. In the meantime, I'm hoping I can get to the bottom of it.

thanks again!

[Enzo]

Find Q7, a J174, and see if it is shorted.

Note, pramp tube order across the row is V1-2-5-3-4. Wirh amp running, verify both plate AND screen voltages on the power tubes. Not enough to find B+ on pin 3 of 6L6, must also be on pin 4.

Now V4 is the phase inverter. Touch pin 7 with a meter probe or a small screwdriver. Does that make a noise come out the speaker? If so, the PA is probably OK. Now try that at pin 2 of V3. ANy noise? For that matter touch pin 5 of power tubes.

And there is always signal tracing. Apply a good steady signal at the FX return. Sine wave is fine, I just use a feed from the shop stereo. Now use a scope or AC meter and follow the signal from the jack through V3, V4, and on to the power tubes.

[greenridr]

Enzo!

Thanks for taking the time to reply, most of what I know about this amp I've learned from reading your old posts, so I was hoping you might chime in.

I had to head out of town for a few days, but I managed to tap a meter probe on pin 5 of the powertubes, and lo and behold, I heard the popping through the speakers. An encouraging sign, and I will now go back and check the other components that you mention. Will post back in a few days when I've had the chance.

Thanks again!

[greenridr]

OK-

-Screen voltages: measured on pin 4 of the power tubes measured at 500 and 496 volts.

-Q7: I went to look at Q7, but noticed it's not a J174 as you mention, but is labeled as follows: J/202Δ/V430AO. The schematic labels Q7 as a J231. Is the test I'm looking for resistance across pins 1 & 3 with the amp on, or continuity across 1 & 3 with the amp OFF?

-Touching pin 5 of the power tubes: caused crackling in the speakers. Are the other pin tests on the preamp tubes required, or is this adequate to deem the power amp section as functioning?

Next step will be to stop guessing and trace a signal as you recommend.

[Enzo]

Exactly what amp do you have? Q7 is clearly labeled a J174 in both the 5150 and 5150-2 schematics. The only 5150 schematic I can find with Q7 called out as a J231 is the 5150 Combo. The 5150 COmbo is a very different amp from the 5150 or 5150-2. DO NOT try to ise the 5150 Combo schematic if you have a 5150 head. Once we know what you have, we can be more specific.

Q7 is a 5150 is the "clamp" transistor - a muting transistor. In the 5150 Combo, Q7 is a series pass element for turning off the reverb. In a 5150, it can kill the signal , since it is a mute. The amp will work with it removed, so removing it is a valid test. In the Combo, Q7 will not create the problem described. In the Combo CR9 and Q12 make the clamp.

Though it is only one possibility, I suspected the clamp in whatever you have.

Pin 5 noise tells us only that the power tubes amplify, at least the one you touched. It tells us nothing about the phase invereter stage. My suggestion was for a 5150, of you have the COmbo, then test the PI there in similar facshion.

[greenridr]

Wow, sorry I forgot to mention that it is the COMBO version. I know that's an important distinction, and am surprised I neglected to mention it. I'm also looking at the combo schematic from Peavey.

Can C9 and Q12 be removed from a combo, or is that going to open up a whole new can of worms?

[Enzo]

OK, now that I know that, forget CR9 and Q12 for the moment. I was not complete. In the COmbo, those two do MAKE the clamp, but it is Q1 and Q4 that do the work. See Q1 upper right? When ON, it grounds off the signal path on its way down to the FX send buffer. If it shorts or gets stuck off at its gate, no sound. Q4 does the same job on the route to the reverb. Q4 can't silence the main amp directly, but since CLAMP is wired to both gates, a short to ground on Q4 gate will wind up also grounding Q1. SO in that manner it can affect the sound.

Remove Q1, see if the amp wakes up.

Q1 is still a J174. That is a P-channel JFET. Lots of other P-ch JFETs would work fine, after all this is simply used as a shunt. But watch out, there is no standard leg arrangement. I always use an ohm meter before installing a JFET. Two legs will show a low resistance betwen. The one leg not connected to the others is the gate. Stick the legs in the holes accordingly.

Obviously other things can be wrong. Just because I spend a lot of time on Q1 doesn't make it the culprit. If plugging the guitar into the FX return results in no sound, then Q1 is probably safe. V12534 is still the tube order. V4 still the PI. Pin 5 of the 6L6s made noise so move back to the PI and touch pin 2. OK? Then move back to pin 2 of V3.

[greenridr]

OK-

I took the preamp PCB out again today, and gave it a thorough once over, and noticed a break in the PCB trace leading to the blue and white quick connect to the power amp PCB. I jumped the break, and voila, the amp is back in business!

However, the reverb does not seem to function. I've swapped it out for a functional tank, and no deal. The amp amplifies the spring shake if the tank is tapped, but the guitar signal itself is not picking up any reverb. Any likely culprits?

getting closer...

[Enzo]

Same thing we were starting to do with the main signal - follow the signal path and see where it ends. Look at the schematic. We know ther is signal at the top of R31 - at least schematically - because it passes on through the amp to the speaker. So apply a steady signal to the amp and find it at R31, then the other end of R31, (is the bottom end there shorted to ground by a stuck Q4?), then the base of Q9 (top of R67), then colector Q9, then base Q6, then the bases of Q8,Q10, then R58, then other end of R58, then the connector post J8. SOmewher along that you must be losing signal.

And speaking of broken traces, power off and measure continuity between J8 and J9. You should be seeing the resistance of the INPUT end of the reverb pan. That tests the cables and connections to the pan.

And from R31 to the driver circuit, the signal path has to cross two board boundaries, so look for broken jumper wires.

[greenridr]

Finally got some time to get back to the amp project here. I ran a signal from a looper into the input, and proceeded to measure the following values:

R31 - 42/42 mV
Q9 - 1.145/1.145/18.4 V (Source/Gate/Drain, I think)
Q6 - 18.4/18.4/18.6 V
Q10 - 17.37/17.39/24.96 V
R58 - 22.78/14.58 V
J8- 14.48 V
J9 - 4.4 mV

Tested for continuity across J8/J9 - tested open, resistance at 350 ohms

Do these values seem reasonable? I have to admit that interpreting them is beyond my understanding. Could it just be the reverb tank itself? I've tried it with a functional one from another amp, so it seems unlikely.

[Enzo]

You leave me with questions.

First the reverb pan. You tried a known good one from elsewhere into this amp? And it doesn;t work? OK, but did you try this pan into that other amp. The test showed there is more wrong than just the pan, but that doesnlt mean th pan isn't ALSO bad. But don't do that just yet, I have more concerns.

Your voltages. It is important to specify if they are AC or DC voltages. And in the case of DC, also polarity. DC will be power, AC will be signal. SO if that 42mv at R31 is DCV, then that is normal, but tells us nothing. If it is ACV, then that is a darn weak signal, but then so would the signal going on through the amp be. POlarity matters, because if I have +24v on both sides of a transistor, it means something totally different from +24v on one side and -24v on the other. Can't just say 24v and 24v.

Q9 is a bipolar, so has no gate, it has collector, base, and emitter leads. Only the FETs have source drain, and gate. And those numbers look like DCV to me. I am assuming all these voltages are DC. Q8, Q10 work together inthis push pull driver stage, they move together. Q6 is what tells them where to go. Looks to me like Q6 has pulled the whole output up to +18v.

And that has put that 22v on R58. Is that +22v? At the other end of R58 it is down to 14v. I'd bet that if you unplugged the reverb pan, that 14v would come up to the 22v from the other end. I suspect the circuit is trying to send +22v down to the reverb pan. ANd that is why I dont want you connecting the good pan into the amp right away. You might damage the good pan. But do test the old pan in another amp.

Just make sure when doing this pan swapping that they are both the same type pan. A 4EB2C1B won't work where a 4AB2C1B belongs and vice versa.

COntinuity J8/9 - tested open, 350 ohms. What does that mean? Either it is open, or it has 350 ohms resistance. CAn't be both.

When I suggested following the path through that circuit looking for voltages, I had in mind signal voltages - that is AC volts. The test signal going through the amp should show up there. The DC readings are revealing, and before moving on, you need to cure the DC on the reverb output J8. Only once you get about zero VDC there, should you connect the pan and hunt signal.

[greenridr]

Enzo-

Thanks again for the invaluable information. I'll have a chance to test everything on AC tomorrow night, and will report back.

Regarding the pin 8/9 test- I tested it on my multimeter's beeping continuity test setting, and got no beep, so I set it to test resistance, and it turned up the 350 ohms value.

Also- Both reverb pans function in another amp. The one in the 5150 is stamped 4EB2C1B. I assumed that was the stock tank, but maybe not?

[greenridr]

And now a new issue!

As I was testing values across Q10, I noticed a wisp of smoke, and resistors R56 and 65 (both 47 ohm) were shorting and turning black. Could a short in Q10 cause an excessive draw through these resisitors?

[Amp Kat]

Sounds like you may have shorted across thetranny with the meter leads so yes I would change Q-10 and the two 47 ohmers especially if Q10 reads shorted. It's possible the resistors still read 47 ohms even if they are black it just depends on how hot they got but I wouldn't risk it.

[John G]

Hi.
When you have got it all up and going, it may pay you to swap out C57 and its PSU filter cap mates as I have just spend several hours trying to trace the dreaded noisy/switching pops/nothing wants to work properly problem that people have been experiencing with these amps for some years................I ended up with replacing C57
Yes! C57 can by itself cause all these problems ???bizzar.
By the by the brand of the cap is IC (Illinois Capacitor??)and I have had to change many of these caps in Fender Blues Deluxes and Devilles as well.
Thanks for letting me interupt.
John