Revox B251 - Bringing it back to life

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stevecole

Member
Joined 2008
#1
I've been given one of these to check over, after many years of storage. The owner is missing the 2-pin mains lead, so he'd like a standard IEC inlet fitted. Having popped the lid off, I was surprised to find that it has a SMPS, rather than the usual linear PSU. Because of this, I'm a bit nervous about powering it up. I'm thinking of removing the PSU 'block' and testing it separately, just in case it damages the rest of the amp, but I know some of these PSU's have a 'minimum load' requirement too, so I'm wary of just doing that without checking. According to the SM, there are a lot of separate rails, some of which are switched by the control system, so I'd guess this PSU is unique to the amp, so I can't risk it going up in smoke. I'd therefore welcome any help/suggestions before going forward. Cheers, Steve C
 
#4
Some Revox with digital control don't like to be started on a DBT. I owned a B285 and it did not want to be tested with DBT and the 251 may share that characteristic. If the 251 is similar, adason is correct that you can unplug thepsu. Very modular.
 
#6
I decided to be cautious & pulled the PSU out before attempting anything else. Found one of the switching transistors to be shorted, so replaced both, along with the incoming reservoir capacitor and a number of Rifa branded metallised paper capacitors that were showing signs of failure. Took a deep breath and powered the PSU up, separate from the amp, to find all rails present & correct, with some relief. Reassembled it back into the amp chassis only to have it blow the mains fuse immediately, with BOTH switching transistors in the PSU now short-circuit. Not wishing to repeat this, I took a closer look at the service manual and discovered whoever had been in there before me had transposed the red & blue flying leads that bring the ±56vdc from the PSU onto the power amp! This puts a dead-short on the psu, since the power amps have what would normally be a reverse-biased diode across their power rails - ouch! No wonder that transistor was shorted when I received it. I'd taken photos before unplugging everything and just put it back the same way, foolishly assuming that was how it was meant to be - a hard lesson indeed.

Cutting a long story short, after installing another pair of PSU transistors, doing some basic checks for obvious things such as failed output transistors, and a few other minor fixes here & there, the amp powered up and produced audio for a short while. I left it running, driving a pair of cheap headphones (these are driven from the main output), only to have the Left channel lose output after about 5 minutes, with both fuses on that side of the power amp blown. The output stage has 3 paralleled transistors per 'half', and at first it looked like all had failed, but it turned out that only one of each group had gone.

There seems to be no other damage that I can find, so I'm at a loss as to what caused this, and what to do next. I will of course replace the full set of output transistors in the faulty channel, rather than just the dead ones, but how do I test it? I don't have a bench power supply capable of delivering the required voltages, which would give some control & protection, so I'm stuck with using the internal PSU. Would it be better to install some series resistors between the PSU & the amp, to restrict the current if something else is wrong, or else fit lower rated fuses to cut the current off quickly?

My fear with this amp is that I'm going to be chasing my tail, with another fault showing up as each weak link shows itself. Is the end result likely to be worth it? It is after all a 40 year old design.
 
#8
Have a good look at all solder joints. Check all electrolytic caps in the area.
Inspect resistors. After that let the power amp run just with the remaining
pairs of power devices for a test. Using big 10 .. 30 ohms resistors in place
of the fuses is a good idea.
 
#9
I have Revox-B251 service manual, I will show its power supply & power output stage etc circuit. It seem to be used a switching power supply.

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#10
The owner of the amp gave me the go-ahead to continue, so I fitted a full set of replacement output transistors. Couldn't source the original types at a reasonable price, so substituted FJA4210 & 4310 instead. Using a borrowed bench PSU, I ran the power amps up individually, and all seemed OK, no excessive current or heating being noted. The inbuilt protection seemed to prevent getting any output at the speaker terminals, maybe because the PSU was only ±35v and I was running one channel at a time, although not sure TBH. Decided to be brave & try it with the internal PSU, and again all good, but now with actual output from both channels, initially testing from the headphone outlets, and then into a dummy load. Set the DC operating points as per the SM and left it on, and again still all good.

My dummy load has a low level output for monitoring on headphones while the amp is doing real work, and the audio sounded fine, and the output waveforms look fine on the 'scope, right up to clipping. I am however surprised at how hot it gets. While running at 30 - 50W, both channels, I can barely keep my hand on the heatsink & the bracket on the bias control PCB, and can hear the liquid coolant bubbling in the heatpipe. Does anyone know whether this is normal?
 
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