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Old 19th February 2013, 01:40 AM   #21
Enzo is offline Enzo  United States
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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.
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Old 19th February 2013, 01:43 AM   #22
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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
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Old 19th February 2013, 01:47 AM   #23
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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.

Last edited by ouimetnick; 19th February 2013 at 01:53 AM.
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Old 19th February 2013, 02:03 AM   #24
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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
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Old 19th February 2013, 02:04 AM   #25
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Yes, give me a moment and I will high lite the sections for you.
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Old 19th February 2013, 02:09 AM   #26
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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?
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Old 19th February 2013, 02:18 AM   #27
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Here you go !!
The Reverb,Normal and Lead mute sections.

Jer
Attached Images
File Type: jpg Renown Revreb Mute.jpg (59.7 KB, 27 views)
File Type: jpg Renown Normal Mute.jpg (160.7 KB, 23 views)
File Type: jpg Renown Lead Mute.jpg (98.9 KB, 23 views)
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Old 19th February 2013, 02:21 AM   #28
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I'm going to start poking, and use an old soldering iron with a bad tip and lightly for a second touch each semiconductor in the preamp section.
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Old 19th February 2013, 02:23 AM   #29
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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
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Old 19th February 2013, 02:29 AM   #30
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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
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