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Revox B251 Integrated Amp No Output and More

Before I dive in blind, here are the symptoms:

This amp, which had worked nicely at our summer house (seasonal use) for 4 years died one evening - slowly going quiet over about 30 minutes (I was dr owsing at the time and did not notice the problem until about the final 2 m inutes). No heat, no smoke, no fan operation, no smell. I turned it off for an hour thinking a part may have overheated. When I turned it back on, it promptly blew the main PS fuse. This is a solid-state 100WPC amp built as w ith many things Revox like a tank. LCD display, capacitance controls, and s o forth. It is also IR remote-capable.

I removed the unit from service and brought it home. Last night, after abou t a 18 months, I decided I would look it over. I replaced the fuse, and bro ught it up slowly on the variac. The first time, it drew about 45 watts, qu iescent, no activity, no display, no pilot light. When I activated the on/o ff switch, the display went wonky - random pixels, random words, but no act ivity. No controls, including on/off responded after that point, but the dr aw went up to about 65 watts. I shut the unit down and held the on/off swit ch for 40 seconds (hey, it sometimes works with computers and other devices containing computers). This time, when I applied power and reached about 1

20V, the fan came on, the display showed other random stuff, but still no a ctivity.

At this point, I decided that without direction, I would likely do more har m than good at random.

Thoughts? Suggestions? I am searching for the manual from HiFi engine, but I am also hoping that someone out there may have had some experience with t hese beasts.

Peter Wieck Melrose Park, PA

Reply to
pfjw
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Cold DVM-D of the main devices show up any disparities?

Reply to
N_Cook

It will take me all of 30 minutes to get to the PS and its caps. Almost a complete dis assembly is required. Clearly you have not worked on Revox devices.... ;-)>.

I am looking for some direct experience with this unit before I dive in cold. But, as always and given my history, as with many here, I suspect the main filter caps.

This unit has a huge mother-board, with the remaining boards plugged in in layers. It is also liquid-cooled with a switching P/S.

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Peter Wieck Melrose Park, PA

Reply to
pfjw

OK, got to the p/s, it ain't nohow easy.

Total of six (6) 220 @ 63V radial caps, and four (4) 220 @ 100V radial caps. In situ, one of the 100V and one of the 63V looked bad. My first pull of the 63V units had an ESR of 1.8, and read 114uf. All will go, both voltages.

But, I still welcome any suggestions or advice!

Peter Wieck Melrose Park, PA

Reply to
pfjw

Hi Peter,

I have NOT worked on a B251, but I have work on several B750's and other Revox products. I have often seen small 'bead' style tantalum caps short in all sorts of circuits. While I'm not a fan of wholesale replacement of capacitors, I'd ohm-meter them if accessible and change as needed. I'm not a huge fan of tantalum caps in power supplies, they have very low leakage, but are very intolerant of over voltage or reverse voltages.

Regards, Tim Schwartz

Reply to
Tim Schwartz

On Saturday, February 6, 2016 at 7:40:45 AM UTC-5, Tim Schwartz wrote: I'm not a huge fan of tantalum caps in power supplies, they

So true. They actually make a fair zener if a one shot crowbar is needed..

Reply to
John-Del

..

As I noted to Tim directly, I observed no tantalum-type caps visible anywhe re in this unit. The B251 is about a generation newer than the B750, so I e xpect this might be the reason. I don't like them much either, but I am usi ng some lately doing TIP mods on a series of Dynaco 120 amps that I tripped over. Three original configuration unit right down to the original 3055 ou tputs... That one of them actually works on both channels is remarkable. An yway, at a finished cost of less than $50/unit, all in including first-cost , they come out as not bad at all, and pretty rugged. Nothing like OEM in o ther words.

Peter Wieck Melrose Park, PA

Reply to
pfjw

I figured I would report back on this beast.

I did get the power-supply back and stable. I also got the power amps back. What I did not get back was the preamp left channel. During the cap replac ement process, I discovered no less than four (out of 28) cold solder joint s, including a resistor that I simple pulled off the board leaving a neat l ittle hole in each of its pads.

By the "Wiggle Method", I got the right channel to go intermittent as well. Suggesting that the cold solders are not confined to the power supply.

Which gets me to wondering if I should simply cut my losses at this point o r spend another few hour chasing down phantom intermittents.

Thoughts?

Peter Wieck Melrose Park, PA

Reply to
pfjw

Peter,

We used to sell these 30 years ago. They repeatedly had shorted power amps and overheated solder joints because they ran the biases way too high. I didn't take the time to modify the design because we thankfully sold so few of them. How one could be functional now since the boards were crispy after 2 years of normal operation is mind boggling. Probably the worse product Revox ever made. Chuck

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Reply to
Chuck

Perhaps. I would be inclined to agree that this series of components is not anywhere near the standards of their predecessors - but it does sound reas onably well. Considering what I paid for it (US$75) and that I got a good f ive years out of it makes it a reasonably cost-effective piece. I am trying to save it from landfill at this point, and the time - as long as progress continues - is therapy more than anything else.

Peter Wieck Melrose Park, PA

Reply to
pfjw

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