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Old 17th August 2019, 02:49 AM   #1
CapnDenny1 is offline CapnDenny1  United States
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Default Peavey 400

Ihave a Peavey Musician that has a 400 series power amp. 8 power transistors, 200 watts. It has me stumped. I made some progress, but have been stuck for a while. My customeris tired of waiting.

Here is where I am at. If I power it up slowly it seems to be ok. Above about 75 vac I have to really slow or else it starts shorting out. I have a light bulb in series, so it doesn’t damage anything as long asI shut it down. I can smell a hot part. Butcan’t see anything burning. That only happens if I leave it shorted.

I have raised it slowly up to 120vac, and it seems stable. But if I leave the voltage at 120 and turn power off and wait, then turn it all on at once my bulb. May flash or start to toggle.

I have been thru the transistors, and have checked all the smaller ones. The power transistors are new. I was trying to troubleshootwithoutthe bulb in series, soI went thru a few powertransistors.

My next guess is to just replace the electrolytic caps. I guess it could be a film cap? Ihaven’t seen them fail on a ss amp before. Tube amps yes, pretty common.

Any ideas?
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Old 17th August 2019, 07:02 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Check the Red/Yellow and Orange/White wires from the mains transformer. If they have a bad connection to ground, anything could happen.
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Old 17th August 2019, 07:28 AM   #3
dotneck335 is offline dotneck335  United States
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I think I would separate out the power feed to the power amplifier section and see if the same thing happens. Film caps RARELY fail.
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Old 17th August 2019, 01:26 PM   #4
CapnDenny1 is offline CapnDenny1  United States
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Separate the power feed from what? I don’t understand.
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Old 17th August 2019, 02:15 PM   #5
turk 182 is offline turk 182  Canada
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Quote:
Originally Posted by CapnDenny1 View Post
May flash or start to toggle.

from previous experience, this condition would have me replacing the supply line caps.


and on older Peavey models be wary of replacement component spec's in some cases uprating a transistor (as in wider bandwidth, more gain) can result in instability.

Last edited by turk 182; 17th August 2019 at 02:25 PM.
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Old 17th August 2019, 03:35 PM   #6
JMFahey is offline JMFahey  Argentina
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Sorry but none of the above, including "guessing" capacitors with no reason behind that.
Basic problem is that many amps "wake up stupid" and take a few seconds to stabilize; bulb stops them halfway .

You may have 2 separate problems.
1) even if amplifier is fully repaired, many do not like these 3 together:

* turned ON,
* with a speaker load connected, and
* through a bulb limiter

2) So first disconnect speaker or any load, and turn amp ON .

Bulb should flash, then trickle down to red or dark orange and stay there.

Measure DC at speaker out, you should have less than 100mV.

Measure rails, they should be present but reduced because of the bulb limiter.
If normal is, say, +/-40V, you might find , say, +/-30, that´s normal.

3) then reconnect speaker, play some Music at low power, say 1W or so.
IF everything looks fine, repeat turn on and measuring sequence, again starting without speaker, but without the bulb limiter.
It "should" work.

4) if not, then amp is far from fixed, it may be unstable, oscillate and burn power transistors or they may be fakes, in which case you´ll need to keep troubleshooting and get good ones.

Please do the tests in that order and post results.
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Last edited by JMFahey; 17th August 2019 at 03:39 PM.
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Old 17th August 2019, 11:51 PM   #7
CapnDenny1 is offline CapnDenny1  United States
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As long as my line voltage is below 80 vac, I can flip the power on, lamp comes on for a second or less and goes out.

If I slowly increase the voltage, like 1 or 2 volts every 10 seconds, I can get to 120 vac.

If I then turn it off, let everything discharge and turne poweron, it flashes then sometimes it flickers about 1 Hz, orjust comes on solid.

I put a dummy load on but it made no difference.

I ordered new electrolytic caps. They are due regardless.

I am thinking if the bias is just suddenly jumping way up. It seems like the output is sstable in terms of voltage, but suddenl it draws too much current. I know from experience, if I remove the limiter, I will blow transistors.
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Old 18th August 2019, 08:49 AM   #8
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Do some measurements. Bias voltage, Symmetry of power supply etc etc etc then you will know what the issue is.
Absolutely no point in guessing with random component changes.
Unfortunately for the experienced engineers within our forum, it is not easy to second guess your cause. Hands on is required.
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Old 19th August 2019, 02:34 PM   #9
CapnDenny1 is offline CapnDenny1  United States
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I started making some measurements. After an hour of making some readings I realized I had put the power transistors in the wrong place. I meant to put the driver and the first one of the 3 parallel transistors. But I was off by one, so I had 2 of the parallel transistors on one side with a driver, and the other side only the driver. So all my measurements were pretty useless in that configuration.

So I moved one of them so I had the driver and one of the main power transistors. After that it seemed to work. It would power up. The series bulb would flash but then get dim, and it seemed to be working. I injected a signal into the power amp and it seemed to be amplifying OK. I even tried setting it at 120vac and just flipping the on switch. All OK.

Now this was all great, but in the back of my head was a voice that said, but what did I do to fix it? I've been here before, and then WHAM! But being hopeful and really sick of working on this amp, I ignored that voice.

I went ahead and put the remaining 4 transistors in so I had the entire bank of 8 power transistors. And it still seemed as if all was well.

So i went for broke and hooked the preamp back up to the power supply/power amp board. It supplies power to the preamp. I injected a signal to the guitar input jack, and it didn't seem right, but I could see signal making it from the input all the way to the speaker output. I still had no load on the speaker jack. I noticed the limiter bulb was glowing brighter, not bright but slightly brighter. And I couldn't get full voltage out of the amp. So I reasoned that the bulb was dropping some of the voltage I needed? So I removed the bulb.

When I powered it up my variable AC Power Source I have from work to work from home 2 days per week (I borrowed it for this amp.) went into fault protect. I had the current limit set to 0.2A. So i reasoned that without the blub limiter the amps in rush was above the 0.2 amps limit. The bulb would flash briefly pretty bright when I still had it in circuit. All logical and reasonable, right? Right?...

So I cranked the current limit up to 2A, or 5A, I don't remember. Then I powered it up, and I actually saw the fuse flash from the back side of the fuse holder in the amp. It blew a 5A fuse. So i put a 2A fuse in there, and put the bulb back in, but I was pretty sure I knew what had happened. That little voice was now screaming, "I told you so, you just never listen!" I powered it back up with the bulb and the bulb just burned really nice and bright. Even at lower voltages it was on bright and solid.

So I was so disgusted with it all, I just left it on the bench and walked away. I will probably tear into it tonight. Hopefully I didn't blow all the transistors. I think any more troubleshooting and I will just use a single. As long as I keep the power level down, it should be OK.

So there is still something in there that goes with rapid power on.
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Old 19th August 2019, 02:37 PM   #10
CapnDenny1 is offline CapnDenny1  United States
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In case anyone wants to see the schematic. I thought I had posted this?

Click the image to open in full size.peavey 400 Musician pa by Dennis Kelley, on Flickr
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