Help please - Revox B285 - thin/weak sound

[jetwuk1]

Hello all,

Hoping for some help/advice please. I have a Revox B285 receiver/amplifier. Usual story; it's been sitting in a loft for 10 years ish unused and I've recently dug it out to see if it works. It powers on fine and seems to function fine apart from the following:

Clicking/flicking sound when increasing the volume (this is done electronically via volume buttons). This isn't a massive problem but it is a bit annoying.

The main problem is the lack of volume. When cranked up to full volume the sound is thin and lacking power. It can be run on full volume and the sound can be heard clearly but it has no weight to it and is nothing like the full 90W per channel it used to pump out. It almost sounds like it's playing through headphones if that makes sense.

This amplifier has a 'volume top' function that limits the maximum output volume. I've used this function to try and raise the output volume to no effect. The volume remains thin and weak.

After a bit of research on potential causes, I've replaced all the electrolytic capacitors on the power amp board, but this has had no effect. I plan to move on to the pre-amp board next, but thought it was worth asking for advice before just spending more time and money replacing caps.

Can anyone help with suggestions on this please?

Many thanks

James.

[Radio Wrangler]

If it's producing sound out of both speakers, then there probably isn't anything terribly wrong with it.

Some of the capacitors inside it are 'electrolytic' types where one of the electrodes is a sort of electrically conductive jelly. This is water-based and tends to dry up over years as water vapour escapes past and through seals.

This reduces capacitoance and that reduces bass leading to 'thin' sound. Much of the perception of loudness comes from the bass register and so you feel it's quieter.

Modern parts are easily available, cheap, and of a lot better quality than old ones.

So I think it's most probably fixable. Rocket science isn't involved. Going through all the electrolytics to find which ones are degraded is a fair amount of work. Just changing the lot of them is an alternative worth considering. It saves on diagnosis time, but more get changed and it may increase the risk of making errors if not done with care.

I restored an ex-BBC Revox B261 tuner for myself and found virtually every electrolytic capacitor in it had dried to some extent, so the lot did get replaced. Be careful because the printed circuit boards Revox made in this era are somewhat fragile, so tracks and pads are prone to fall off.

The hifi world seems to have a fetish about 'recapping' but in one of these it's worthwhile. Don't get suckered into magic audiophile capacitors. Well made ordinary ones from a reputable manufacturer, bought from a proper distributor should be fine. It's what it's had from new. Fairy dust doesn't exist, but honest to goodness reliability is quite real. Getting parts from a min line distributor like Farnell or RS is the main way of avoiding fakes. Yes people even stoop to faking small low-cost capacitors!

David

[jetwuk1]

Hi David,

Thanks so much for the reply, it gives me confidence to carry on the route to replacing the capacitors.

I 100% agree with your comment on the PCB fragility. About 50% of the capacitors I've replaced so far have resulted in having to bridge broken tracks, which is frustrating and time consuming. I'm now used to the process and am able to replace with less issues, but it's still a frightening process!

I've ordered replacement caps from RS and am sticking to Panasonic and Nichicon brands depending on availability of the values i need, the price of these items is (relatively) cheap so it doesn't make sense to me to go cheaper still. Yes I've read lots about 'better sounding' caps etc.. and I'll leave people to have their own opinion on those, for me I just want a reliable trusted brand to get this amp working again. I cannot fathom why people would make 'fake' caps, i guess some must order in big enough quantities to make it worthwhile!

As a side note, I suppose I'm pretty typical of new people posting on here in that I have a basic cheap digital multimeter. Is there a way to test the capacitors on the board with this type of equipment? or do i need a better multimeter and do they have to be off the board to be tested?

Many thanks again for the reply.

James.

[Ted Kendall]

Originally Posted by Radio Wrangler

Be careful because the printed circuit boards Revox made in this era are somewhat fragile, so tracks and pads are prone to fall off.

Quite so - I think solder braid is a bettter bet on these than the ubiquitous sucker-pop. I had some success reworking my car's instrument board with a bismuth solder which lowered the melting point of the existing joints considerably, but I haven't tried it on leaded joints. Experience, anybody?

Regarding capacitor sound, at small signal levels a properly designed circuit should have no signal voltage of any significance across it anyway - the distortion caused by dropping more than about 80mV is easily measurable and practically independent of manufacturer.

[Radio Wrangler]

Peak instruments make a number of little hand-held instruments. One is a capacitor ESR meter which is ideal for checking electrolytic capacitors. There are cheaper ones around as well as kits, but the Peak ESR meter is definitely good. One of those 'How did I survive without one?' tools.

For smaller capacitor values you'd need an LCR meter to check their C value, but the real trouble is high voltage leakage in valve circuits, so that would need a high voltage insulation tester.

David

[jetwuk1]

Thanks again David, those hand helds look great, but given the amount of use I'm likely to get out of it it's probably overkill for me.

I'll get my capacitor list sorted out and thug out replacements. From the looks of it some of the caps have been replaced previously, there seem to be a mix of different makes on the board.

cheers

[nicam49]

Hi, you can get a multifunction tester for around £15, just search TC1 multifunction tester on eBay.

[jetwuk1]

Ok, I'll have a look in to those. Thanks.