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Author Topic: Peavey Classic VT Blowing Fuses  (Read 10957 times)

ChewyNasalPrize

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Peavey Classic VT Blowing Fuses
« on: October 08, 2014, 01:12:48 PM »
This was working great not long ago. I had it serviced and several caps replaced but recently, I turned it on and there was a loud hum for about a second before the fuse blew. Replaced the fuse twice with the same results. Any thoughts on what might be the culprit? Have multimeter. Thanks- Chews.

g1

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Re: Peavey Classic VT Blowing Fuses
« Reply #1 on: October 08, 2014, 01:27:05 PM »
  Try with power tubes removed.

Roly

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Re: Peavey Classic VT Blowing Fuses
« Reply #2 on: October 09, 2014, 03:54:43 AM »
No more fuses!  Limiting lamp naow.


Perhaps not all the suspect caps were replaced.

This is fairly typical of a valve amp that has been in storage for a long time, and the electrolytic caps in the power supply can de-form or lose their electrolytic layer and go low resistance.

Depending on how bad they are they can sometimes be re-formed by energising via a low wattage (20-40W) limiting lamp for a while.  If they don't show a drop in current within a few minutes then replace.

Should also check the main rectifier diodes.
If you say theory and practice don't agree you haven't applied enough theory.

ChewyNasalPrize

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Re: Peavey Classic VT Blowing Fuses
« Reply #3 on: October 16, 2014, 11:56:32 AM »
Ok- I built a limiter lamp and plugged the Peaver into it. I have a 15 watt bulb in it right now. Don't have a 20 to 40 handy- figured a 15 would be ok for now.

When I turn on the amp the bulb comes on probably its full brightness. The fuse does not blow. The amp doesn't look like it is coming to life though- no glowing tubes- no power light.

I've been letting it set for several minutes like this- not quite sure how to test the capacitors or where the rectifier diodes are. Doesn't look like any of the capacitors in the power section have been replaced. Several on the preamp board have been replaced however.

10-15 minutes passed and light still at full brightness and not life in amp. Turned off, turned on a few times... nothing.


g1

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Re: Peavey Classic VT Blowing Fuses
« Reply #4 on: October 16, 2014, 12:14:31 PM »
  Did you take out the power tubes yet?

ChewyNasalPrize

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Re: Peavey Classic VT Blowing Fuses
« Reply #5 on: October 16, 2014, 02:32:46 PM »
Yes- turned it on without the power tubes in and still shorts out somewhere.

DrGonz78

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Re: Peavey Classic VT Blowing Fuses
« Reply #6 on: October 16, 2014, 03:29:44 PM »
I would want to increase the light bulb to 40 watts. Check to see if the bulb goes dim with tubes removed and a higher wattage bulb.

Also Roly had suggested to test the rectifier diodes. If your multimeter has a diode check setting this can be easily done. Set meter to diode setting, check black meter on Cathode and red meter on Anode. You will sometimes see a combined reading of two diodes in series if the diodes are still in circuit. To get a more accurate test on the diodes in series you will have to lift one leg on the component.

Also, to point out that there is High voltage on this amp and it will be best to learn to drain the caps. Do you know how? Just want to make sure none of those 350v caps have a charge while you are touching anything in the amp. Use voltmeter, connect black probe to ground, to test either side of those 350v filter caps to confirm if there is a stored charge. If there is a charge you need to be able to know how to drain the caps.

Here is the schematic for all to share here and a picture of testing a diode. Good luck!

EDIT: Also could be a shorted filter cap too. At least test the diodes to clear them from the equation.
« Last Edit: October 16, 2014, 03:35:29 PM by DrGonz78 »
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ChewyNasalPrize

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Re: Peavey Classic VT Blowing Fuses
« Reply #7 on: October 16, 2014, 05:23:20 PM »
Ok- I discharged caps per instructional video that said hook one end of wire to chassis and the other to pin 1 of a tube socket. I guess it worked- I didn't touch the caps to find out.

Then I tested the diodes with my multimeter- it indeed has a diode testing feature. The ones at the front (top in the pic) all tested roughly between 7k and 10k. I tested the ones at the back and they all tested very close to each other at around 5 M. I guess this means they are OK?

I'll need to stop at the store and pick up a 40 watt bulb- I have plenty of 60 watters laying around if one of those might work.

ChewyNasalPrize

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Re: Peavey Classic VT Blowing Fuses
« Reply #8 on: October 16, 2014, 06:25:26 PM »
Found a 40 watt bulb-

I turned on the amp and I guess it was probably half the brightness as it should be.

Turned off and removed tubes and turned back on- still seemed to be about the same brightness.

Turned off and replaced tubes and turned back on- light bulb got WAY dim-- almost barely on and for a brief moment the power indicator lamp on the front of the amp came on -- but then went out. Bulb still very dim.

??

DrGonz78

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Re: Peavey Classic VT Blowing Fuses
« Reply #9 on: October 17, 2014, 02:34:04 AM »
From what I am reading it seems that the amp is not pulling excessive current. The power indicator will also be dim as the voltage will be lower with the light bulb limiter. It would seem that you measured the diodes with ohms setting on the meter and not diode check, still a valid way to test. It can be confusing measuring diodes in circuit, but you can also check to see if any are shorted. If we see resistance across them then they are not shorted.

As far as not touching the caps to find out if they were discharged... Use your meter still to confirm that the voltage has been drained. There are ways you can even use your meter to monitor, in real time, the voltage being drained and all you need are clip on leads or something.

Still something is causing the amp to blow fuses... The 60 watt bulb would have been fine too. What I notice is that a 40 watt bulb will glow brighter than a 60 or 100 watt bulb on a working amp.

Read this on diode checking too...
http://www.allaboutcircuits.com/vol_3/chpt_3/2.html

I am a bit stumped as to what the problem is at the very moment. I would double check those diodes and run the amp at 60 watt bulb too. Let's also see what anyone else here says in addition to my comments. BTW do you have a speaker or load plugged in on the output?

Edit: Just to point out that when I wrote "If we see resistance across them then they are not shorted" that I mean high resistance. A shorted diode will have very low resistance both ways like a piece of wire. Just thought I better make my statement hold true there that's all.
« Last Edit: October 17, 2014, 02:53:19 AM by DrGonz78 »
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ChewyNasalPrize

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Re: Peavey Classic VT Blowing Fuses
« Reply #10 on: October 17, 2014, 10:25:39 AM »
I did not have a load hooked up to it. I plugged it into an 8 ohm cab and switched to a 60 watt bulb. Turned it on and the bulb got very dim and the power indicator lamp on the amp came on and stayed on. Very faint glow in the power tubes too.

I just rechecked the front diodes (this time using the correct diode testing settings) and they all read right at .495 +/- .001 or .002. The rear diodes are all measuring around .52.


g1

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Re: Peavey Classic VT Blowing Fuses
« Reply #11 on: October 17, 2014, 11:14:16 AM »
 The dim bulb indicates the short is not present right now.
It's possible one of your power tubes has an intermittent short/failure.

ChewyNasalPrize

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Re: Peavey Classic VT Blowing Fuses
« Reply #12 on: October 17, 2014, 02:46:54 PM »
ARITE- I think I found the problem.

I plugged the amp into a regular outlet and sure enough it fired up like normal. I gave the tubes a bit of a wiggle just to make sure they were seated good and then "POP" goes the fuse.

I put another fuse in and plugged it back into the limiter and when I wiggled the tube closest to the power transformer, it would crackle. On closer inspection and probing with a wooden skewer, a little black diode bridging two of the pins was sparking when I poked it and shorting out. I unplugged everything and took a close look at it and there is a crack running almost the length of this little guy.

Guess I'm off to the electronics store!  :dbtu:

Edit to add: This diode is marked "2873." Are they readily available? Should I use something else/better? I'm thinking I should replace the one in the other socket as well just for good measure?
« Last Edit: October 17, 2014, 02:59:43 PM by ChewyNasalPrize »

g1

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Re: Peavey Classic VT Blowing Fuses
« Reply #13 on: October 17, 2014, 03:55:10 PM »
  Those are SR-2873 diodes, rated for 2000V.  Probably no longer available except through peavey.
  Some people use 3 1N4007's in series instead.

Enzo

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Re: Peavey Classic VT Blowing Fuses
« Reply #14 on: October 17, 2014, 07:16:22 PM »
PV would have them.  There is nothing absolute about their value, so if I run out of the ones PV uses, I just grab a couple R3000 from my Fender bins.  All those types do the same thing.