Peavey VTX Classic troubleshooting help

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GFT58
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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sun Feb 14, 2021 8:34 am

I took some more readings on the AC side of things on the power board. Seems I'm short of voltage there too.
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GFT58
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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Mon Feb 15, 2021 3:25 pm

It looks worse and worse for this amp as I wait for parts. F1 blew yesterday, I replaced it and so far it has not blown again. My heater readings are down to 5.7VAC, where they were a consistent 6.3VAC. My DMM had a low battery so I posted some erroneous readings yesterday, I've re-posted today. To me is this looking like a failing PT? Anyone else have opinions?
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Enzo
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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Mon Feb 15, 2021 4:42 pm

OK, with F1 removed and the standby switch OFF, do you still read 100v low on the HV AC winding?

Go to RG Keen's Geofex.com web site and under tech tips (I think) find his simple transformer tester. Knock one together and test your transformer.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Mon Feb 15, 2021 6:22 pm

Readings were still 100V low with F1 removed and standby switch OFF. I'll check the website thanks Enzo, or taking it to the curb.

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Re: Peavey VTX Classic troubleshooting help

Post by rogue777 » Mon Feb 15, 2021 7:26 pm

How did you get 183VAC (from center tap to each xformer end) and you only get 300VAC end to end.
Shouldn't it be more like 366VAC?

You made two measurements at the same test points using two ranges on your DVM. 183 VAC (200 ACV range) and 149 VAC (500 ACV range).
Only one can be right. I think your DVM 500V range is reading off (broke).
Last edited by rogue777 on Wed Feb 17, 2021 11:20 am, edited 2 times in total.

GFT58
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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Mon Feb 15, 2021 7:54 pm

I had switched scales on the DMM. When I'm on the 200V range the readings are 183, I switched to read both taps to the higher range (500V) I got 303 but forgot to go back to the individual taps and retake. The individual taps are 151 on the higher range. Why the difference? I have no idea.

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Enzo
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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Mon Feb 15, 2021 9:53 pm

He SHOULD be getting different voltage readings, but if the winding has a shorted turn between the two halves of the winding, the results could occur.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Tue Feb 16, 2021 12:02 pm

No place but online to get parts so.... in the interim .. Enzo, I found these tests from GEOFEX to meggar the PT. I am a little confused on how perform these steps and want to do them correctly.
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Enzo
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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Tue Feb 16, 2021 2:39 pm

Um...

Go back to Geofex.com. On the left click Tech Tips

On the tech tips page on the right see Tube AMp Tips, click the second one: Transformer Short Tester.

It is a super simple test you make with a 9v battery and a neon bulb.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Tue Feb 16, 2021 7:28 pm

So planning for a PT replacement I start searching for Peavey P/N# 705-18718 and 705-18348 and turned up zilch. What would you recommend if an OEM PT is not available Enzo?

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Re: Peavey VTX Classic troubleshooting help

Post by Enzo » Tue Feb 16, 2021 10:54 pm

The amp is 40 years old, so unlikely Peavey still has them, but you won't know until you ask them directly.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Fri Mar 05, 2021 6:01 pm

I've took a different DMM and rechecked the transformer readings and wah la they're where they should be. So I changed out Q4 and Q5 and turned it on.
I could hear that there was noise coming from the speakers so I grabbed a guitar and low and behold I've got sound, albeit a little fizzy sounding.

So I checked Vi at V1 = 88.4, and V2 = 87.5, Vk at V1 = 3.4 V2 = 22.8. I figured I need to change R126, so I put in a new set of tubes, replaced R126, turn it on and the readings were the same.

Replaced Q6 again and the readings were Vi at V1 = 91.3, and V2 = 90.0 , Vk at V1 = 3.2 V2 = 3.2. I'm baffled, defective parts?.

Enzo, how can you check the new parts for Q4/Q5 and Q6/Q7 is just a continuity check?

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sat Mar 06, 2021 7:27 pm

I found some info on using diode function of DMM to test parts out of circuit.

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Sun Mar 07, 2021 2:08 pm

So I went ahead and pulled Q4/5 out and checked them and they tested fine. Then I pulled r123/124/128/129 and the all tested fine. So, at this point I'm at a loss for where to go next. Anybody have any ideas?

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Re: Peavey VTX Classic troubleshooting help

Post by GFT58 » Mon Mar 08, 2021 1:10 pm

So I checked continuity between points from Q6/7 working my way back to Q4/5. It was either a bad solder joint and or broken trace between the emitter of Q6/7 and R126/130. I ran some jumpers and hooked everything back up then checked all voltages on the tube pins, Vi and Vk, and the power board. All were within the expected ranges. Plugged a guitar in and got sound.

So my question is how do I go about fixing a broken trace? I did check continuity from underneath and it was sporadic, so I reflowed some solder and then got no continuity between points. I am I better off making permanent jumpers up top? That seems to me to be the easiest solution. Curious what others have done.

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