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Amp HF distortion - only with covers on?
Amp HF distortion - only with covers on?
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Old 11th June 2021, 09:48 AM   #1
mainframe99 is offline mainframe99  New Zealand
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Default Amp HF distortion - only with covers on?

This MOSFET AB amp - PMF2150B - measures fine with the top and bottom covers off, within the limitations of my gear anyway. (theres a small DC offset issue, 6mV one channel 40mV the other, but I'm pretty sure I can fix that by replacing the input differential transistors with better gain matched pairs).

BUT When the top and bottom covers are back on, the sine wave on the scope goes really thick and fuzzy, while still visually resembling the fundamental, and on the basic (8bit?) FFT my scope has there is a huge spray of noise around 20kHZ and 220kHZ, and a way high noise floor. I can literally remove the distortion while unscrewing the covers. This is compared with a well functioning Yamaha MX1 AB amp I have here.

Hopefully it is something simple - the service manual under high frequency distortion suggests a few things, one being heatsink coupling capacitors - I checked the heatsink coupling capacitors and they measured 0.01uf where as the schematic says 0.1uf, could this be the problem? Or is it just lack of apparent EMI/RFI filtering on the Perreaux amp power supply vs the MX1? I have both schematics here and can post if need.

20210611_172806.jpg
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Old 11th June 2021, 10:38 AM   #2
MarcelvdG is offline MarcelvdG  Netherlands
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It's hard to make any more specific comments without a schematic, but it seems that your amplifier is oscillating. If it's at the edge of oscillating, just about anything can change its behaviour between seemingly normal and grossly distorting.

Chances are that those capacitors you wrote about are meant to keep the amplifier stable, so it could very well be related to their way too low value. It could also be something else.

Last edited by MarcelvdG; 11th June 2021 at 10:53 AM.
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Old 11th June 2021, 10:57 AM   #3
duncan2 is offline duncan2  United Kingdom
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Yes its oscillating due HF pick up seen it many times .

Is it properly earthed ?
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Old 12th June 2021, 11:32 PM   #4
mainframe99 is offline mainframe99  New Zealand
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After deep cleaning, (and a crash course in electronics and amplifiers) I've replaced the heatsink/rail coupling capacitors (C23 in the attached schematic) and it seems* to have sorted the problem. Pinched some good 0.1uf 100V caps out of another old amp until the replacements arrive.

The guy I bought it off was the original owner 35 years of oscillating much - d*mn it even runs cooler now too Wrong spec capacitors from the factory? It didn't look like it had ever been worked on, but I've not much experience to say.

*The HF hash/oscillation was pretty random and hard to replicate without the covers on, but for now it seems to be holding will have to keep randomly checking with the scope over the next few weeks.

img-1-hHMAdwyr3Q.jpg
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Old 13th June 2021, 12:04 AM   #5
duncan2 is offline duncan2  United Kingdom
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Considering the amount of Fets in your amplifier it doesn't take much to cause oscillation .

If it happens again SLIGHTLY ! increase the values of C 8 & C9 compensation capacitors.--remember -SLIGHTLY .
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Old 13th June 2021, 12:53 AM   #6
jaycee is offline jaycee  United Kingdom
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MOSFETs can be buggers for this sort of oscillation. It might be worth you renewing the small (too small IMO) rail decoupling electrolytics. The schematic seems to suggest those are 10uF.. personally i would use 100uF-470uF
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Old 13th June 2021, 02:41 AM   #7
mainframe99 is offline mainframe99  New Zealand
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Thanks for those tips guys, noted.

For clarification, the PCB cleaning did not solve the problem, although it was needed. C23 , the fix, was replaced days after.
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Old 13th June 2021, 07:00 AM   #8
Mark Tillotson is online now Mark Tillotson
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Quote:
Originally Posted by jaycee View Post
MOSFETs can be buggers for this sort of oscillation. It might be worth you renewing the small (too small IMO) rail decoupling electrolytics. The schematic seems to suggest those are 10uF.. personally i would use 100uF-470uF

Yes, and make sure there are some fast decoupling caps as well (MLCC chip capacitors are ideal, but any ceramics are better than none when it comes to HF oscillation) - MOSFETs are unforgivingly fast.

So 100uF electrolytic + 220nF ceramic perhaps? (And check the voltage rating of any ceramics!) The existing schematic shows two each of 10uF and 100nF, which is good, but increasing the values a bit can't hurt and will enhance stability and allow for electrolytics deteriorating over time.


Another observation is that 220 ohm gate-stopper resistors might be a bit small, increasing these a bit may help stabilize the MOSFETs too.

Last edited by Mark Tillotson; 13th June 2021 at 07:03 AM.
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Old 18th June 2021, 11:09 AM   #9
mainframe99 is offline mainframe99  New Zealand
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Just so I'm clear, you guys refer to C15-C22?

I've replaced Q1 and Q2 with some MPSA92's and the dc offset is now within +-4mV each channel, do you think it is worth replacing Q3,Q4 NPN'MPSA42's and Q5 PNP seeing as they are the same transistors and likely would have degraded also, or just leave them as factory?
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