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Now just how do I use this new Oscilliscope to find a problem?

Bravo, Dingman! They can't ever take away experience or education. Just a little beyond me right now, I'm just trying to determine where this problem develops in the amp. MBZ had some tips that were more along the lines I was thinking, to start at the output and work backwards. I need to learn how certain circuits affect the signal, I guess.

Thanks, Conrad, I do have the manuals (service manual too) and have been working with them to learn how to manipulate this tool.
 
Here's my predicament - because the oscillation develops and changes quickly as the volume control is raised, I only have a tiny window to see it before I have to shut the amp down for protection. The bias and offset voltages just take off, so I turn it off. I've seen this on the DMM, both channels. At low power the amp sounds great. I'm trying to find what draws the current. Audio board and power supply board were recapped.
 
tusntuk2 said:
Here's my predicament - because the oscillation develops and changes quickly as the volume control is raised, I only have a tiny window to see it before I have to shut the amp down for protection. The bias and offset voltages just take off, so I turn it off. I've seen this on the DMM, both channels. At low power the amp sounds great. I'm trying to find what draws the current. Audio board and power supply board were recapped.
Click to expand...
Both channels-----> power supply. Are you using a DBT ?
 
Thanks, Avionic. Yes, using DBT which quickly dims. I can go back to AC and start the amp, which plays fine at low level, but after warm up it shows the problem as I crank up the volume. Shutting down volume reduces the speed and voltage of the problem, but I usually just turn it off to be safe. I will concentrate on the PS more, I recapped that board too. When vol is down the PS voltages measure very close to specs on SM, +/- rails seem OK, bias is spot on or low, offset is under 10mv. As I crank it up something starts pulling current and the speakers start jumping.
 
avionic said:
Does it trip the protect circuit relay ? Do you even get a relay click shortly after power up ?
Click to expand...

No, it does not trip relay, relay closes after 2-3 secs on powerup. Amp starts mormally, only shows problem after warming up and turning up volume control 9or bass)
 
I could be your DBT causing the problem. As you increase the volume. The circuit is demanding more power which is being bottle necked at the dim bulb.
 
FYI, I have also replaced diodes Dm7 with 1N5244B zener, flyback Dm5, bias trans's Qm7,8 (KSA992) and bias pots, and voltage regulator Qk1 with FJP13009H2TU. My new concern is that I upped the main filter caps to 10k - maybe I should revert to 7500?
 
When I started this project the relay diode was bad, which left the relay open. Once the diode was changed everything worked, or so I thought, as it played fine. Then I recapped the audio board and power supply, trimmers and bias trans's. After firing it back up again I started seeing this apparent oscillation. That's when I started hunting a cause, and changed other diodes and transistors involved in supplying power. I'm not an engineer, I just read the SM and go from there. I do not know if the problem preceded my work, I did not test (play) the amp very much before reccaping. My bad, but here we are.
 
tusntuk2 said:
My new concern is that I upped the main filter caps to 10k - maybe I should revert to 7500?
Click to expand...
That won't cause you any problems. Unless you have a bad solder connection or improperly installed.Or your replacement cap was defective.
 
Agreed on the 10K caps.
Hmmm. Almost sounds like a transistor breaking down and then, ya feedback-oscillation. Nah, scratch that.

I'm sure you've triple checked parts you replaced to make sure correctly installed.

I forget or can't find it - what was the original symptom and after the first repair - did this runway problem come up right after that?
 
tusntuk2 said:
The bias and offset voltages just take off,
Click to expand...
Bias is quiescent current or "at rest" ie idle current. Its adjusted with volume at min. As soon as you start increasing the volume control. Your no longer at idle. So yes your bias is going to change as the amplifier starts to work. DC offset will drift +/- as well.
 
Thanks, Avionic, I figured the bias would increase or fluctuate with volume, I would expect it to need to compensate or control voltages for the output stage, right? I mean, without knowing how it works, isn't the bias current meant to balance something so the output doesn't go crazy high voltage? (my attempt to understand transistor theory) If the bias gets too high, the output transistors heat up faster, which I take to mean more current. The DMM readings showed low to near spec bias as the amp heats up, say, the first ten minutes. Changing the Vol during warm up shows minor fluctuations, like maybe 1-10ma , and Offset showed rises to maybe 1v (if memory is working). Once the amp was warm, turning up the Vol showed hundreds of ma's of bias and voltages to 10v or more in the offset, which quickly reduced with turning down vol or shutting down amp.

I use a pair of tiny computer speakers (8ohm) on the workbench. When turning the vol up I could see the apparent oscillation as it maxed the speaker back and forth, first a low freq (slow speaker travel) and then getting faster with more volume.
 
Dingman, original problem was open relay. Replaced the flyback diode with help from this forum. Relay worked correctly after that, and still does. I played the amp for a few songs while i tested controls and switches for noise and functions. Everything worked, and of course I needed a little deoxit, but I did not play the amp hard. Since the project was to recap the amp, I did that, recapping the power supply and the audio board. Not the FM tuner, not the preamp (there's a chip). DBT dimmed right away after this work, relay even clicked in. Plugged it into the wall and it played fine in tiny test speakers. Then I noticed the tiny cones flapping in the breeze, slowly at first, then faster with more bass or more vol. Sure I was overlaoding the tiny speakers, but this was different, and visible and audible as "wup-wup-wup" on top of the music.

When cold the voltage readings on the PS board are very close to spec. Rails are 39.3v for a spec of 39v for example. 14.1 for spec 14. I figured OK. I have re-checked all the caps I installed for polarity (there are a couple of non-pol's). There could still be a culprit lurking!

Now I started reading the SM and guessing a bit, with the help of ideas in AK, of course. A suspect transistor was the 1419 voltage regulator, so I changed that. Likewise a zener diode Dm7 at the end of the rail. I changed the bias trans'. In each case I tried testing the old part with a DMM and one of those cheap chinese semi testers Everything tested OK. Amp runs same as before with the replacements, voltages same, with tighter control on the bias v. BUT - did not fix the problem.

Dr Audio here suggested a scope would be need to find the offender, so that's when I got a scope and started learning it. Now I need to learn how to apply it to the amp, just how to use this tool in a logical way to find where or when the amp suddenly draws the current. I believe in this forum, and I know we can find the answer!
 
Thanks NAD80, it seems to be a nice scope, very simple to run and very basic in operation. Like me. Figured it was a couple of years newer than the KR-5600, so it ought to match up pretty good. I was blessed with extra probes, all the manuals, and more in the deal, and it's sharp and clean. Hope 60mhz is enough for vintage receivers and tuners, etc. I'm not doing anything digital.
 
tusntuk2 said:
Once the amp was warm, turning up the Vol showed hundreds of ma's of bias and voltages to 10v or more in the offset,
Click to expand...
I don't like either of the 100's ma or 10Vdc. Is this with audio playing and with spkrs connected?? 10Vdc would trip the protection relay. I'll find the SM.
 
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