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Old 22nd February 2013, 01:43 AM   #61
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I have a feeling that the filter caps should be replaced, and so should the rectifier diodes along with CR49 and CR50. Basically rebuild the power supply. Regardless, I'm going to be doing recap.
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Old 22nd February 2013, 01:45 AM   #62
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-3Vdc on that rail sound like a shorted or leaky diode.

You can lift one end of it to check it but DO NOT turn the amp on while it is out of the circuit.
As this will over voltage the opamps and they won't like that at all and will blow them
!!!

jer
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Old 22nd February 2013, 01:48 AM   #63
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No need to replace the rectifiers unless they are bad.
Filter Caps, Yes, This is a good idea for an amp with this age.

jer
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Old 22nd February 2013, 01:50 AM   #64
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CR50 measures.025V when out of circuit on diode test. Its .025 EITHER polarity. So sounds like thats a problem. Should read open in one direction.

CR49 (out of circuit) measures infinite in one direction, and .739V in the other

Last edited by ouimetnick; 22nd February 2013 at 01:54 AM.
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Old 22nd February 2013, 01:54 AM   #65
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Yep. it sure looks like it is a bad diode

Shorted!!!

Replacing it would be the simplest and cheapest way to go or you can mod it and put a pair of 3 terminal regulators in it as well.

A LM7815 for +15V and a LM7915 for -15V.



jer

Last edited by geraldfryjr; 22nd February 2013 at 02:01 AM.
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Old 22nd February 2013, 02:05 AM   #66
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What confuses me, is where did the AC come from? AC goes through a rectifier CR45-CR48, and this takes the AC and spits out DC. Then this DC goes into the filter caps C92 and C93, then it goes through someother caps and resistors, and lastly the diodes CR49 and CR50. But the AC was supposedly converted into DC back at CR45-CR49. So why did I measure AC ripple?

And why does my meter go crazy when measuring AC ripple on both the preamp's filter caps AND the amps's 4700mfd 63V caps?

Also, CR49 and CR50 are 40102 diodes (well SZG-40102) 15V, 1W 5% ZENER diodes. Peavey states that a 1N4744 is a sub. So I'll just buy some of these when I purchase some caps..

1N4744A_T50R Fairchild Semiconductor | Mouser

I'll also stock up on some OP amps for this and for my other devices with 4558 OP amps, as it seems from what I hear, the TL082CP is superior than the 4558, and there is no issues (except in rare circumstances) replacing 4558s with TL082CPs.

EDIT: The diodes have 1N4744 printed on them, so I wouldn't be subbing. It appears that SZG-40102 is a Peavey house part number

Last edited by ouimetnick; 22nd February 2013 at 02:11 AM.
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Old 22nd February 2013, 02:23 AM   #67
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The A.C. comes from the increased ripple voltage on that line do to the excessive current draw from the short diode.
Basically it was feeding the -15V rail with -3V half sine wave pulses!!

Your meter was going nuts becuase it is an autoranging type and it didn't know what it was looking at.
A couple of times it did stabilize and that was the ripple voltage.

Use you should use no less than 1 watt types as specified more is good.
But a 1/2 watt zener would just blow out and short due to due to being over the rated current that flows through it while it is clamping at 15v.

jer
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Old 22nd February 2013, 02:32 AM   #68
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I love the TL082/72 series and still use them today!!

There are better ones these days but all they do is cost more.
Some say that they can hear the difference and that is okay.
They sound fine to me.
They were the state of the art that every one had to beat back then.
I have been using them since they first came out in like 1979 or so.
I used to make some very very high gain fuzz boxes with them and they were dead quite when the guitar was turned off.
Now you can get them for about 30 cents a piece and radio shack still has them if I ever need one in a pinch!

jer
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Old 22nd February 2013, 02:38 AM   #69
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When I turn my amp up to play it,it sound like I am at the beach the hiss is so bad!!!
I can imagine how quiet that thing must must be with some TL072/82 in it!!!

jer
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Old 22nd February 2013, 02:50 AM   #70
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It seems the TL072 is actually better than the TL082

So I will replace them all with this

TL072CPE4 Texas Instruments | Mouser
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