Peavey Renown 400 Issues

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Please don;t get into wholesale op amp swapping. FIX THE AMP FIRST. then if you want to "upgrade" op amps, do it later. The problems in the amp are not due to one type op pamp not being "as good as" some other type.


If your channel has a problem, it might be because one of the TL082s or 4558s is bad, but not because of the type op amp it is. 4580 might be lower in noise than 4558, but your problem is not just an overall higher noise level, you have a specific noise.

5532 are low noise op amp, but they do draw about twice the power supply current as the others. Not an issue with a couple of them, but if you tried to replace a whole mixer full of 4558 with 5532, you might run out of power supply.


Find the problem, shotgun is not effective in all cases. What if you replace evey darn part in it and eventually find that the problem is an open trace?

No scope? Make a signal tracer out of some other amp. COnnect a probe to the input of some amp, add a series cap to block any DC. Use it to probe various points in the circuit and listen to what is there.
 
Some times you can find a bad cap by poking at it with say a tooth pick or a Plastic rod of some sort.

I found the bad Jfet when I had some cold spray as erratic semiconductors are sensitive to temperature changes.
A little heat from a nearby soldering iron can affect it as well.

Check the diodes in the footswitching circuits for any leaky ones as this can cause this type of noise as well.
They must show conductivity only in one direction if they show a high resistance in the reversed bias state then it is leaky and should be replaced.

The same goes for checking the Bipolar switching transistor as well.
No leaky ones!!

This would be the first things to check rather than swapping out a bunch of parts that may very well be good.

jer :)
 
See my problem with this, is I need to break it into sections.

I can see the schematic and figure out what is the preamp, and what is the power amp section, but thats it.

Can someone circle the foot switch section?

I'll grab some freeze spray tomorrow. I have a soldering iron handy, and a plastic spudger (stick) so I will try to find the issue that way.

I have a little Class D amp I'll use as a signal tracer. I assume the outer ring in the rca connector (ground) goes to the amp's chassis, and the center (hot) will be for probing. What value cap should I use to block the DC? 2mfd? Thats what my Hafler amp uses on it's input to block any incoming DC, but I would rather not use the Hafler.
 
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Enzo is right!!

If you have another small battery operated amp you can use this to signal trace with.
Just be sure that you ground the input every time before you make a new connection to a test point.
This will discharge any voltages that may be stored in the input cap because if it happens to be one from a test of a higher voltage part of a circuit it can blow out the next semiconductor that you are trying to check with it.

Also using your probing tool pick at the parts from the PCB side of thing if you can.
With the age of this amp it wouldn't surprise me if it is just a cold or corroded solder joint on a component lead.

I know that this will be very difficult with this chassis but DO NOT power up this chassis without some kind of heatsink on the output devices.

Even while they are idiling there is a significant amount of bias current running through them and they will heat up very quickly and destroy themselves without a heatsink to dissipate the idle heat.
Especially if you found a spot and it starts to crackle or set the amp into oscillation.
When I got my very first one of these when they came out, The molex connector was bad and I blew out to speakers because the amp was intermittently oscillate at an ultrasonic frequency that I could hear.
The shop couldn't figure it out as it was under warranty but I did.
I used to work on T.V.'s back then in a shop.

jer :)
 
I'll test the stuff from the component side. Or should I push around with a nylon stick to try to locate a cold joint? I went over the solder side with my bare eyes, and didn't see any cold or broken joints. Maybe I should reflow all of the joints?

I'll also try the freeze spray and tip of a soldering iron to try to see if its a thermal issue.

If that fails and I trace it, what value cap should I use on the "signal tracer" probe? 2mdf?
 
Here you go !!
The Reverb,Normal and Lead mute sections.

Jer :)
 

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Yes do try both sides of the board and use a good magnifying glass as well!!!
Many (just about 80%) of the TV's I fixed back in the 80's and 90's were faulty by bad solder joints and cracked PCB traces!!!

Sometimes you can just lightly touch them and others you have to pick at them pretty hard in order to find them !!! :(

Ahhhh the joy's of troubleshooting !!!! He,he,he,he

jer :)
 
With a volt meter you can check the voltages on the gates of the Jfet's and if you see any erratic voltages on one of them while you either have the stage switched on or switched off then you may have found where the noise source is coming from.
Check both states for each one.

jer :)
 
Well folks in my home are going to bed, so I suppose this will wait until tomorrow. :(

Can't be causing noise.

I will let you know tomorrow afternoon or night what I find.

Now I'm going to note all the cap values, and the transistors, and make a parts list. I always recap a unit. After 25 or so years, the electrolytics dry up, and need replacing anyways. I need some 4558s for my parts drawer. And mind as well figure out what transistors I need just incase.
 
I have a Renown 400 as well, and was an amplifier repair technician in a former career. You're getting good advice here, and are hopefully on your way to fixing this. If it were mine and I was having this problem, the tool I'd reach for is a scope. I know you said you don't have one, but consider borrowing or even buying one. Good analog scopes are easy pickings on eBay, as people have been dumping them for the last 3 decades to buy fancy digital scopes. A good, tested, working, dual-channel analog scope can be had for around $100.

My rule of thumb with any sort of noise was "If I can hear it, I should be able to see it [with a scope]." -I'm expecting you to have a bad ground connection somewhere, or a bad opamp. You shouldn't have any problem with the TL082 or TL072 replacing a 4558, I've replaced plenty of 4558's with Jfet opamps. As already mentioned, the TL082/72 has a faster slew rate and should have a higher input impedance. These are good things in an opamp. I've seen ONE schematic, that was for a MusicMan amp that specifically indicated not to replace its 4558's with Jfet opamps, although I'd bet it would work just fine if I did it. In any case, if you had a scope, tracing down this noise problem would be dead simple. I'd actually go buy one before I'd shotgun a bunch of parts. (Bonus, you'll have a working amp AND an oscilloscope when you're done!)

Good luck, I'll be looking for how this resolves.
 
About the only substitution problem I recall was some marginally unstable Marshall Valvestate amp, and I had replaced a bad 1458 with a 4558. APparently the 4558 has a higher open loop figure or something, so it made the amp more unstable than the older 1458 type. Someone at Matshall told me I really ought to use actual 1458s in that amp. So to this dayt I have a few of those in my drawer.

A scope is the best tool of course, but for chasing noise, even a signal tracer works well, and that is something one can make quickly and simply.
 
Well I reflowed all solder joints. And touched my soldering iron to all the transistors in the preamp section. No change.

I also replaced the 5 22mfd 25V electrolytics for reliability. I wasn't suspecting them, and indeed, it isn't.


One odd thing. When I turn the post gain knob on the LEAD channel, and turn it to the 12:00 position, the hum decreases. But it is still humming.
 
As quoted from post 1# "It isn't a grounding issue, a pot issue, or a solder joint issue."

It very well could be one of these problems and they are very hard to trace.

I some how missed the Humming part, Have you swapped out any of the opamps yet?

How are the +/- 15V supply's.
Is this noise only happening on the lead channel?

jer :)
 
Check CR6 and CR7 for leakage as they are connected to the preamp's output opamp and are reversed biased to the the +/- 15v supply.
Same with CR14 and CR15 connected to the input of opamp U6.
There are several diodes like this in the muting stages as well.
Since this is a preamp problem ignore the ones in the poweramp stage.

jer :)
 
Noise and hum occur regardless of anything plugged it, and occur even when all the controls are turned all the way down.

So it doesn't matter what channel. :p

I will check them diodes tomorrow. I'll also check the +/- 15V supply. Also will check the outputs of each OP amp. Just to make sure, I'm setting my meter for DC, and am checking for DC offset on these output pins 1 and 7. Other annoying thing, is the turn on/off thumps are really violent. I know thumps are normal, but these actually cause the speakers to violently bottom out.
 
U5B is the only opamp stage that is between all of the post volume controls and the power amp.
It is the one with the diodes CR6&7 connected to the output.
Why they did this I do not know.
Of all of the opamp circuits I have built through the years, I have never had to do this for any reason what so ever.

I am guessing it is for in case the stage does go bad and bottom out to one of the rails and this would cause a loud thump as well.
and the diodes would limit the size of the pulse by going into conduction mode just like the ones in a power amp would do for a speaker.
These make a lot of sense if you are hearing hum as well.
I have blown opamps before and it is a good indication if hum is coming out of it!!!

My amp pops and clicks when I turn it on and off as well (sometimes) but no thumps.
Does it do this if you plug a open cable to the power amp in and turn it on and off?

If not then I would suspect that something may be wrong with U5B then.
U5A is the presence filter and last opamp stage of the lead channel.

Yes, do check the DCoffset outputs of the opamps they should all be close to 0V.
If one is not then this would cause the violent thump as well!!!

jer :)
 
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