• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

2385 Phono stage oscillation

Rick churay

Rick churay

New Member
I'm having an interesting issue with the phono stage of a 2385 receiver I'm working on for someone. When I received it, the complaint was that the phono stage would cut out when playing. Separately, I recapped all the boards and replaced many black legged transistors throughout. When I send a tone through the Aux and look at the signal from the speaker terminals, it's rock solid. When I put a 5mVrms into the phono section, the amplifier goes into oscillation (see attached video). If I don't reduce the volume immediately, the lights will start to flicker. If I disconnect the preamp going input into the power amp and look at just the phono signal from the preamp, again, it floats quite a bit but generally looks good and I can turn the preamp up to full volume with no issues. Since the phono amp generally seems to be working good and the amp works fine when played through Aux, I'm at a bit of a loss to find out what's causing the oscillation.

I mentioned above that the output of the phono board floats a bit. At low frequencies (<100Hz, it's pretty stable. At higher frequencies (>5 kHz), if seems to float quite a bit more. Not sure if the floating of the higher frequencies have anything to do with the oscillation. btw, I've also replaced all of the ceramic capacitors on the off chance that one of them went bad but as expected, no change.

If anyone has any thoughts on what area I might look at, I would appreciate any suggestions.

For reference, channel #1 is yellow and is the output from the preamp, into the power amp. Channel #2 is purple and is the output at the speaker terminals.

Oscilloscope video
 
Register to hide this ad
If this Phono board is anything like the 2325 I addressed that by placing a 20pf to 30pf between collector and base of the first two transistors from the input. If you replaced the transistors they may need a different value to avoid oscillation, these can sometimes be put in on a case by case basis.
 
Thanks for the suggestion. 1) I looked at the 2325 phono section and it is completely different. The 2385 is unusual form other's I've seen in that it has a differential pair at the input. 2) The oscillation was present before I did any capacitor or transistor replacement. Thanks
 
I would look at the positive and negative power supplies to the phono preamp with a scope, and see if the sag in power supply created by the power amp load on each cycle of the oscillation, is getting back into one of the phono preamp, due to boor regulation or poor filtering of one of the phono preamp power supplies. This could setup a low frequency oscillation like you are seeing.
 
If you slow the sweep down, you should be able to determine the frequency of the low frequency oscillation. In spectrum mode, I don't think the Siglent can go to that low a frequency, but might be worth a try. That might help isolate if it is 60Hz/120Hz or other frequency oscillation. You might also be able to isolate the phono amp stage that is causing the problem by scoping each stage of the P400 board to determine where the oscillation starts.
 
Last edited:
From the scope video it looked like it was a very low frequency oscillation. Something less than 10Hz. I bet it is showing up on one (or both) of the the phono preamp supplies (even before it gets really bad). They should be rather immune from fluctuations in the rest of the unit, due to the tight regulation on those supplies. If the regulation is defective (not isolating the fluctuations), it could easily setup a low frequency oscillation.
 
AE Bob said:
I would look at the positive and negative power supplies to the phono preamp with a scope, and see if the sag in power supply created by the power amp load on each cycle of the oscillation, is getting back into one of the phono preamp, due to boor regulation or poor filtering of one of the phono preamp power supplies. This could setup a low frequency oscillation like you are seeing.
Click to expand...
I just checked the +/- from the regulated supply during the event and it was stable....no fluctuation.
 
AE Bob said:
From the scope video it looked like it was a very low frequency oscillation. Something less than 10Hz. I bet it is showing up on one (or both) of the the phono preamp supplies (even before it gets really bad). They should be rather immune from fluctuations in the rest of the unit, due to the tight regulation on those supplies. If the regulation is defective (not isolating the fluctuations), it could easily setup a low frequency oscillation.
Click to expand...
To add a little more information, I've attached a clip of the output from the phono amp alone. Actually, I disconnected the input into the power amp and measured there so actually, it is the output from the phono amp and the preamp.. See clip below.

IMG_0484.MOV

2385 phono amp
audiokarma.org
 
I thought I should follow up on this topic since many of you were kind enough to offer suggestions. I never did find out specifically why I was getting the oscillation when trying to measure the phono output at the speaker terminals with my scope. I did determine that the phono section played just fine when hooked up to my turntable. I could measure the output at the tape out jacks to check the RIAA curve and everything was fine. The curve was pretty good.
Funny thing…..I now have a 2215b on the bench and when I tried to measure the phono output at the speaker terminals, I got the same oscillation. Based on this I think I can say the issue is with my measurement approach and not the amplifier. Possibly a ground issue or something else but since I see it on two different units, I think I can conclude the amplifiers are fine.
 
You must log in or register to reply here.
Back
Top Bottom