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Revox B780 right channel issue

GPR

GPR

New Member
Hi all,
I pulled out my old Revox B780 receiver out of storage recently. It been sitting in there for 10 years. Plugged it in, powered it up and predictably it blew its Rifa safety capacitor on startup in a cloud of smoke. Undeterred, I chose to give it a good makeover, took it apart, changed all the electrolytic capacitors that I could find and gave it a good clean. love the thing, it’s a real solid piece of stereo gear.

The unit powers up fine, all systems work but I now have only a slight hiss on the right channel. Both power amp boards work fine, I swopped them around to isolate the cause and it’s still the right channel that’s hissing. The pre-amp has both channels working fine (tested using headset plugged into the pre-amp output at the front) the speaker safety relays engage fine as well. The main headset output at the front has the right channel hissing, consistent with the speaker output.

it seems that I’m loosing the right channel signal somewhere between the pre-amp and the right hand power amp. My electrics/electronics knowledge is basic at best and I operate on limited gear to test (multimeter). I’m currently at the end of my limited wits, any pointers on where to look next would be highly appreciated. Thanks folks..
 
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Why did you change the electrolytics? My ancient A720 preceiver plays just fine with the originals. And did you test before you replaced anything? Except the Rifa?

Often, with recapping errors are introduced which complicate fault finding. Search for dry joints and solder blobs shorting out pads. And get the manual.
 
I do have the manual. Thanks for the reply. I changed the Rifa and reading up on some of the restoration threads here, chose to go further with the most common caps to be replaced. While doing so, I looked out for bad joints and tested the new cap connections. As said, all working fine apart from the right channel final output. Pre-amp signal is fine on both channels. I’m using a radio input (antenna input) as a source for testing.
 
thanks PE. Ok, checked again. Right channel actually has content (sound and not just hissing) but it’s on a very low level, like 95% L to 5% R. It has not lost gain. Disconnecting the L channel speaker and turning up the Volume brings it out.
 
Sorry for that. Not sure if I understand you correctly. My understanding of gain was that it reacts to volume knob input changes. It does but the start baseline for L and R is very asymmetric. I did not change any caps in the front (pre-amp, digital display, controls for volume and balance etc) is there a main trimmer set for L-R channel distribution?
 
So. Your preamp is OK. Headphones are the same as speakers. Let's follow the signal then. You start at the audio connection unit 1.780.145. The audio exiting to the preamplifier PCB 1.780.205 is called ML and MR. This has no noise in it. Then it goes through the preamp PCB and is available on PREAMP OUTPUT jack. Still OK there you say. Then it goes back to the audio connection unit as PREL and PRER. It then exits as PWRL and PWRR and goes to the output amp 1.780.105 and goes to the speakers. The phones are parallel to the speakers so everything you hear on the speakers is on the phones too. I'm at a loss where your noise enters. There is nothing between PREAMP OUT and the output amp.
 
Note.... the silkscreen for the boards may be incorrect. Check your work on the recap.

I do not remember the specific spots but I do recall this being an issue.
 
Good information on the routing. Ok. I’ll trace that and report back if I find anything. Thanks a lot both.
 
Now that I look at it you may want to check the switch S1 on the audio connection unit.

You have sound out of the preamp connection on the front of the receiver and it's missing at the amp output and headphone output.

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Good point. Thanks. I’ll have a look at that. Will test and feed back if I find anything.
 
...and the S1 switch it was. Dust must have accumulated in there. Cleaned it up a little and voila, both channels work! Nice. Thank you both a lot! I would have turned in circles without you. Makes my day
 
Thanks again folks! Really appreciate you spending time to help. Regards from Singapore. Be well and stay safe.
 
This is exactly what makes this place stand out; useful, use it now info. My B780 started acting up. Left channel would occasionally fade and I noticed that balance was weird, with the center stage changing and the balance control seemed to only work sometimes. A few hours later I was even more confused as like the OP, everything in the preamp was fine. Did the search, found this and it is exactly what was wrong. That damn DIN switch was oxidized/intermittent depending. And I have never even used that connection, which is for inserting a device between the pre & power amps like NR, equalizer, etc. Thank you!
 
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