Peavey VTX Classic troubleshooting help

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GFT58
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Peavey VTX Classic troubleshooting help

Post by GFT58 » Wed Feb 03, 2021 11:39 am

My VTX Classic was on the lead channel and started sounding odd and then shutdown. I found the fuse on the mains (3A 250V) blown and replaced it. The amp powered up but after taking it off standby it produced a loud hum. I searched through the forum and the schematics then checked the bias current at pin 5 on both tube sockets, resistors 112/113, and 139/140. The current readings are consistently 16.56V (-16.99V on R140) across the board at these points. My understanding is the the values should be 15V per the schematic.

From what I've read so far this would cause the power tubes to red plate because it's receiving to high of a bias feed.
I'm a complete newbie at this so what the next step?

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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Thu Feb 04, 2021 8:06 pm

Nope The bias really comes in the cathodes. Think about this: if I have +15 on my grid and +65 on my cathode, I effectively have -50 on the grid. In that the grid is 50v less positive than the cathode. Remember, this is just a guitar amp, those voltages are just ball park figures.

LOok at the notes on the schematic lower right corner. Specifically the Vk and Vi numbers.

A loud hum? First guess a failed power tube, next up one of the cathode transistors failed.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Fri Feb 05, 2021 12:52 pm

Enzo! Thank you so much for the reply. I've just read thru many of your threads here you're a master.
Here the readings I've taken:

Vi - V1 = 0.4 mV, V2 = 29.8mV
Vk - V1 = 6.8V, V2 = 83V

This would indicate Q6 has failed true?

Pin 2 = 6.3VAC
Pin3/4 = 433V
Pin 5 = 17.1V
Pins 2/7 = 6.3VAC

C3 = (+) 433/216
C4= (+) 216/0
C6 = (+) 28.2/0
C7 = (+) 0/-28.2V

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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Fri Feb 05, 2021 2:38 pm

Yes, Q6 is certainly suspect. Did we try swapping places with the 6L6s? Just to make sure it isn't a bad tube instead. Just takes a few seconds.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Fri Feb 05, 2021 3:30 pm

Yes I did multiple times with different tubes. Each time the tube in V1 got very red and hot quickly.

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Enzo
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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Fri Feb 05, 2021 8:02 pm

The stock part, Peavey number 5884, is an MJE15028. A 120v part, to me an MJE15030 would be better, a 150v part. And frankly, in my shop I stock the MJE15032, a 250v part. I use them for these and for the similar Music Man amps.

Replace in pair. In other words when you replace one side, replace the other side too.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sat Feb 06, 2021 7:44 am

Got it! Thanks for the parts info. Enzo, I can't thank you enough for your help, I wish I could return the favor.

I do have 2 more questions if you don't mind.

(1) It would appear someone has worked on the amp in the past and has used either a glue stick or some kind of epoxy to secure a few components. How would you go about removing this material without damaging the PCB board?

(2) I read on a another forum where someone had recommended taking out the spike suppression diodes on the tube sockets pins 3/7. Do you agree with that?

Enjoy your weekend and stay safe!
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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Sun Feb 07, 2021 12:42 am

Why do you want to remove the glue? Many manufacturers goop their boards to hold larger components still. This helps prevent broken solder. Yours looks like hot-melt glue, the other likely type is silicone sealer which is rubbery.

Easiest way to get rid of hot melt is to spray it with freeze=spray, which makes it brittle and you can shatter it and break it away with a small screwdriver.

And again why would we want to remove the safety diodes? yes, it will run fine without them. Just like your car will run fine if you take out the seat belts and air bags.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sun Feb 07, 2021 8:01 am

Just seemed suspicious that certain components were gooped and other similar ones weren't, the caps on the power board, resistors near the heat sink.

Thanks for the advice.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Thu Feb 11, 2021 10:43 am

I replaced Q6 and Q7 with MJE15030G transistors, powered up the amp and F2 immediately blew. Replaced the fuse, powered up, F2 did not blow, and then took some readings:

Vi - V1 = 3mV
V2 = 28mV
Vk - V1 = 88.4
V2 = 89.8

Plugged a guitar in no loud hum but no sound. I took some more readings and came back and retook Vi/Vk. Vk at V1 was now 102.4. Soon afterwards F1 blew. I replaced F1, powered up, and checked Vi/Vk again. Readings were back to what the initial readings were. So I monitored Vk at V1 and saw a quick rise in voltage to over a 100 after 10 min.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Fri Feb 12, 2021 9:07 pm

I've done some more research on other forums and found out some more readings to take.

Q6 leg = 0.02
Q7 leg = 0.48

Junction of R119/C39 junction = 1.92 (R119 15v leg = 16.8V), R116/CR28 junction = 0.64, R127 = 1.27, R121/CR32 junction= 1.3, R136/CR37 junction = 1.05.

The Q6 leg and R116/CR28 junction look to be low. Enzo I've read one of your post that Q4 cannot be tested in the circuit, true? The other concern I have is with the HT voltage. The print references 525V and I'm at 434V. Any help would be appreciated.
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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Sat Feb 13, 2021 3:01 am

What does "Q6 leg" mean? Q6 has three legs. And I am guessing that is a DC voltage?

CR39,40,41 are just there to set a voltage. Each diode drops about 0.63v, so three in series sets a 1.9v standard. SO ut doesn't matter that your +15v coming through R119 is 15v or 16v or 17v, the three diodes will still just drop the 1.9v, as you found with yours.

Same deal with CR32,33, but there is only two of them, so 1.2v.


Look at Q4, it has a 47 ohm resistor across two of its legs. That will confuse any meter you test it with. WHen I replace Q6,7 I generally will spend the extra few cents to replace Q4,5 as well, as they are closely connected to the larger ones and may suffer damage when the big ones burn up.

If your B+ is 100v low, then either it has lost filtration or something is loading it down, like a particularly hot power tube.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sat Feb 13, 2021 8:22 am

The Q6/Q7 measurements were taken as referred to in the attachment. If you were facing the front panel they were taken on the left left leg. At the time I ordered parts I hadn't considered replacing Q4/Q5. Not a good move. Thanks for the reply Enzo.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sat Feb 13, 2021 3:44 pm

I have a new set of tubes but I'm hesitant putting them in until I get a functioning preamp section.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sat Feb 13, 2021 7:16 pm

So looking for replacement parts for Q4/5 I can find MPS6530 and 2N3904 both from Mouser. The 2N3904 was suggested from another forum. I don't have a clue what the difference is between the two of them.

Enzo the readings I took from Q6/7 left legs were indeed in DC as you speculated.

I did run the preamp out jack into another amp and that seems to work just fine.

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