I've had this Superscope R-340B receiver for years and I've never been able to figure out this weird issue. It distorts the top half of the sinewave on one channel. The other channel might not be 100% either but it is hard to tell. It is the power amp section, and it only does it with a load. The outputs are new, the transistors are new, the capacitors are new. Sometimes it seems to be a bit better or a bit worse, but again it is hard to be sure what is going on.
The power amp is the same as a Marantz 2215B
Here are some pictures of the o-scope. The second picture is when it is pushed a little bit harder.
Does anyone here have any ideas?
The power amp is the same as a Marantz 2215B
Here are some pictures of the o-scope. The second picture is when it is pushed a little bit harder.
Does anyone here have any ideas?
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You got oscillation. Did you replace *all* of the capacitors or just the electrolytics? Suspect parts would be C711, C713, C715, C717, C719, not necessarily in that order. If you replaced them, double check the values. Be sure R739 is not open.
Check R739 and R740, if bad these can result in oscillation.
They are commonly damaged during high power amplifier testing.
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R740 tests okay. I replaced the electrolytics, not the ceramics. I ruled out C710 and 719 though by replacing 710 and swapping C719 and 720 channel to channel)
I've never seen ceramics fail but I've never seen such a stability issue either.
I've never seen ceramics fail but I've never seen such a stability issue either.
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But are they Correct Spec?
If they are faster OR slower, the NFB will like to oscillate. It is not so uncommon for it to be OK at small current but not at large current (loaded).
MJE15032/MJE15033
I've used them in countless other gear with no issues.
MJE243 and MJE253 for H704-708-710
I forget if I was having this issue before changing those parts. Maybe it was why I changed them. Also I changed both channels. The issue should be equal in both channels if those parts were the issue, right?
I'll check R741/742
I got caught out with a 1980 Maplin 225WRMS amp.
I bought it off ebay with output transistor missing.
So I bought them in, fitted them and they oscillated badly.
I had to increase VAS capacitor a little to stop them oscillating.
I can only guess the gain was a bit more or a wider bandwidth was the problem with the new transistors.
Is there a VAS capacitor in the amp in question?
I had this exact issue in a sansui amp and the issue was the stability cap and resistor that went from the output to ground. C719/720, R739/740 in this amp. I replaced the caps and resistors and one channel is working good now, the other still has the issue.
VAS capacitor is C8 and 33pf, I had to increase it to stop oscillation on the output. It doesn't have a Zobel network on the output.
I had a similar problem with a more modern amp I designed.
That required 10R resistors between drivers and output transistors to stop oscillation.
Beat me to it !
Their value is a compromise.
Not so small you get oscillation.
Not so large it reduces bandwidth.
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you also got both lead and lag compensation in this amp, total 4 comp caps for stereo. And comp caps on the input transistors.
If that amp has ANY electrolytics with YELLOW sleeves, yank 'em out and replace them.
They're notorious for going bad.
Been there, done that.
They're notorious for going bad.
Been there, done that.
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Joined 2009
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I changed C714 to 100pf and that seems to have done it. I'm wondering if the original cap was faulty or if I actually had to bump the value. I'd swapped some caps channel to channel but I don't think that one. And the other channel works fine with the same parts and original values. Any negative effects possible from bumping that value?
It is the value not the part, I swapped the part from the other channel and the issue was there again. I had to know. This cap is at 100pf in both channels now.
This is a big win, I've been stuck on this unit almost since the start of my involvement with audio repair.
This is a big win, I've been stuck on this unit almost since the start of my involvement with audio repair.