Rotel RA-812

[002]

Hi all,
I have a Rotel RA-812 that I am trying to fix up. It works and plays fine but has a very annoying issue whereby activating the three separate 'Filter' toggles for Subsonic, Low and High on the front of the unit causes a loud popping sound through the speakers. I've attached a schematic excerpt below.
Notes so far:

• I have nothing connected other than a single pair of speakers.
• Volume control does not have any effect on how loud the pops are, but I have it set to minimum.
• All three toggles cause a loud pop - they are slightly different but all *loud*.
• Subsonic is unique in that if I set the Low filter to 'In', toggling the Subsonic filter on or off does not cause any pop. As soon as I toggle the Low filter off again, Subsonic will pop again when toggled on or off. High and Low always pop regardless.
• Switching the speakers to off with the control on the front hides the noise, but I still see the VU meters jump up like they would do if the speakers were on.

Any insights in to what might be causing this?
TIA.

 

[002]

I just realised I forgot to state the issue in my thread title. If a mod could kindly change it to 'Rotel RA-812 - Speaker popping issue' or something to that effect, that would be appreciated.

 

[century tek]

I'd probably be looking at some of those coupling capacitors.

 

[mbz]

Both VU meters deflect, so issue with both channels.
Reads a bit like some dc voltage bleeding through coupling caps C509, 510, 511, 512, 529, 530.

With Hi Filter in, does switching the low cause a pop?

 

[mbz]

I'd probably be looking at some of those coupling capacitors.

Thinking the Vdc is coming back from C529, 530 since Low Filter/Subsonic behaviour

 

[002]

Thanks both. @mbz it was exactly those caps (C529, 530) you mentioned.

Most of the electrolytics in the unit are grey Elnas which still measure OK but I was suspicious of the lime green Sanyos because most of the 1uF's I was measuring were reading around 6uF. I did not think to check C529 / 530 because they were a good distance away from the filter controls and I supposed they were for tone control. I replaced the other coupling caps for good measure.

Problem solved!

 

[Ray Gianelli]

Thanks both. @mbz it was exactly those caps (C529, 530) you mentioned.

Most of the electrolytics in the unit are grey Elnas which still measure OK but I was suspicious of the lime green Sanyos because most of the 1uF's I was measuring were reading around 6uF. I did not think to check C529 / 530 because they were a good distance away from the filter controls and I supposed they were for tone control. I replaced the other coupling caps for good measure.

Problem solved!

Most capacitor testers don't check for leakage, which is most likely what the issue here was.

 

[002]

Most capacitor testers don't check for leakage, which is most likely what the issue here was.

Yep you are right. Leakage of the vintage lime Sanyo versus a brand new Wurth:

 

[Ray Gianelli]

Yep you are right. Leakage of the vintage lime Sanyo versus a brand new Wurth:

Those Sencore Z meters are one of the few that measure leakage. Really nice units but still really expensive.