Peavey Renown 400 Issues - Page 4 - diyAudio
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Old 19th February 2013, 02:45 AM   #31
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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.
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Old 19th February 2013, 02:47 AM   #32
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Okay, Good luck and I will check back!!!

jer
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Old 21st February 2013, 12:13 AM   #33
Bugsi is offline Bugsi  United States
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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.
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Old 21st February 2013, 12:42 AM   #34
Enzo is offline Enzo  United States
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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.
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Old 21st February 2013, 01:16 AM   #35
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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.
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Old 21st February 2013, 01:50 AM   #36
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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
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Old 21st February 2013, 02:04 AM   #37
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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
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Old 21st February 2013, 02:28 AM   #38
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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.

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.
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Old 21st February 2013, 03:27 AM   #39
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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

Last edited by geraldfryjr; 21st February 2013 at 03:34 AM.
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Old 21st February 2013, 04:05 AM   #40
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I will give it a shot tomorrow or Friday. I will take the measurements, and the other fancy things. As for the diodes you mentioned, I'll check them as well. I think I have some 1N4148 diodes left if needed. I will keep you posted. Thanks!
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