I am currently recapping (electrolytic only to start with) my Revox B261 FM tuner. I have completed the power supply, the MCU board, the audio board (schematic and PCB layout attached) and the stereo decoder board. Next is the demodulator (1 cap only). I will do the rest gradually.
I have some annoying instability on the headphone output (other variable output sounds fine). There is distortion that seems to be triggered by the output relay connecting the headphones (I'm using Grado SR80 and Sennheiser PX100). It also seems to settle when warming up. When I place my fingers under the PCB and brush them over the traces/joints the distortion will suddenly reduce and the sound suddenly improves (after lots of hum and the occasional whistle). I have checked the following
1/ All resistor values OK
2/ All ceramic caps OK. All electrolytic capacitors replaced (most of them have tested bad on a BK Precision 879B LCR meter).
3/ PN junctions of the transistors check out OK.
4/ Continuity checks done and solder joints reflowed.
I have NOT checked the components on the headphone PCB yet. That is my next job.
I'm just wondering if replacing the LM301 op amps would yield some improvement in performance and stability (OPA132 or NE5534A). I don't want to replace the ceramic caps unless needed to aid stability.
I've not injected any signals into the board or looked at the output on my scope yet, so maybe I should try this next. Any suggestions for any other next steps? Thanks.
I have some annoying instability on the headphone output (other variable output sounds fine). There is distortion that seems to be triggered by the output relay connecting the headphones (I'm using Grado SR80 and Sennheiser PX100). It also seems to settle when warming up. When I place my fingers under the PCB and brush them over the traces/joints the distortion will suddenly reduce and the sound suddenly improves (after lots of hum and the occasional whistle). I have checked the following
1/ All resistor values OK
2/ All ceramic caps OK. All electrolytic capacitors replaced (most of them have tested bad on a BK Precision 879B LCR meter).
3/ PN junctions of the transistors check out OK.
4/ Continuity checks done and solder joints reflowed.
I have NOT checked the components on the headphone PCB yet. That is my next job.
I'm just wondering if replacing the LM301 op amps would yield some improvement in performance and stability (OPA132 or NE5534A). I don't want to replace the ceramic caps unless needed to aid stability.
I've not injected any signals into the board or looked at the output on my scope yet, so maybe I should try this next. Any suggestions for any other next steps? Thanks.
Somebody will say the '301 opamp is ancient.
However this one is working feed-forward which was way ahead of its time.
There will not be an exact drop-in for this circuit.
I am sure it was not sick when new and I rather doubt the '301s have blown-up. Maybe study the fault before pot-shoting?
However this one is working feed-forward which was way ahead of its time.
There will not be an exact drop-in for this circuit.
I am sure it was not sick when new and I rather doubt the '301s have blown-up. Maybe study the fault before pot-shoting?
Brushing your finger over the tracks to locate instability is an old "trick " that still works , my body measures ( approx --varying ) 0.33nF .
As this is seems to be related to the relay ( your words ) then check the contact connections for pickup.
As this is seems to be related to the relay ( your words ) then check the contact connections for pickup.
Just a late update on this. It turns out that the instability was caused by a the relay in the audio PCB. The contacts in the relay had degraded and the audio signal going through the relay contacts was intermittent. I have made a custom PCB to replace the OEM relay as detailed here.
Audio PCB relay PCB for Revox B261 tuner
It works really well and the audio is now rock solid.
I have a few PCB's left if anyone wants to do the same modification. I can share the design data if needed. Please contact me by PM. Thanks.
Audio PCB relay PCB for Revox B261 tuner
It works really well and the audio is now rock solid.
I have a few PCB's left if anyone wants to do the same modification. I can share the design data if needed. Please contact me by PM. Thanks.
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