Revox B251 Integrated Amp No Output and More

[pfjw@aol.com]

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 drowsing at the time and did not notice the problem until about the final 2 minutes). 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 with many things Revox like a tank. LCD display, capacitance controls, and so forth. It is also IR remote-capable.

I removed the unit from service and brought it home. Last night, after about a 18 months, I decided I would look it over. I replaced the fuse, and brought it up slowly on the variac. The first time, it drew about 45 watts, quiescent, no activity, no display, no pilot light. When I activated the on/off switch, the display went wonky - random pixels, random words, but no activity. No controls, including on/off responded after that point, but the draw went up to about 65 watts. I shut the unit down and held the on/off switch for 40 seconds (hey, it sometimes works with computers and other devices containing computers). This time, when I applied power and reached about 120V, the fan came on, the display showed other random stuff, but still no activity.

At this point, I decided that without direction, I would likely do more harm 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 these beasts.

Peter Wieck
Melrose Park, PA

 

[pfjw@aol.com]

On Friday, February 5, 2016 at 9:11:47 AM UTC-5, N_Cook wrote:

> Cold DVM-D of the main devices show up any disparities?

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.

http://www.revox-gebraucht.de/assets/Uploads/ReVox-B251-neu-Innen-+-Back-003.jpg

Peter Wieck
Melrose Park, PA

 

[pfjw@aol.com]

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

 

[N_Cook]

On 05/02/2016 13:58, pfjw@aol.com wrote:

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 drowsing at the time and did not notice the problem until about the final 2 minutes). 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 with many things Revox like a tank. LCD display, capacitance controls, and so forth. It is also IR remote-capable.

I removed the unit from service and brought it home. Last night, after about a 18 months, I decided I would look it over. I replaced the fuse, and brought it up slowly on the variac. The first time, it drew about 45 watts, quiescent, no activity, no display, no pilot light. When I activated the on/off switch, the display went wonky - random pixels, random words, but no activity. No controls, including on/off responded after that point, but the draw went up to about 65 watts. I shut the unit down and held the on/off switch for 40 seconds (hey, it sometimes works with computers and other devices containing computers). This time, when I applied power and reached about 120V, the fan came on, the display showed other random stuff, but still no activity.

At this point, I decided that without direction, I would likely do more harm 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 these beasts.

Peter Wieck
Melrose Park, PA

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

 

[Trevor Wilson]

On 6/02/2016 12:58 AM, pfjw@aol.com wrote:

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 drowsing at the time and did not notice the problem
until about the final 2 minutes). 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 with many things
Revox like a tank. LCD display, capacitance controls, and so forth.
It is also IR remote-capable.

I removed the unit from service and brought it home. Last night,
after about a 18 months, I decided I would look it over. I replaced
the fuse, and brought it up slowly on the variac. The first time, it
drew about 45 watts, quiescent, no activity, no display, no pilot
light. When I activated the on/off switch, the display went wonky -
random pixels, random words, but no activity. No controls, including
on/off responded after that point, but the draw went up to about 65
watts. I shut the unit down and held the on/off switch for 40 seconds
(hey, it sometimes works with computers and other devices containing
computers). This time, when I applied power and reached about 120V,
the fan came on, the display showed other random stuff, but still no
activity.

At this point, I decided that without direction, I would likely do
more harm 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 these beasts.

Peter Wieck Melrose Park, PA

**I repaired one years ago. I have the full manual (paper version)
somewhere, if you really need some of it, I may be able to scan and
post. I do recall that it is a bastard of a thing to fix. Mine had (if I
recall correctly) a failed output stage.

--
Trevor Wilson
www.rageaudio.com.au

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[Tim Schwartz]

On 2/5/2016 8:58 AM, pfjw@aol.com wrote:

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 drowsing at the time and did not notice the problem until about the final 2 minutes). 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 with many things Revox like a tank. LCD display, capacitance controls, and so forth. It is also IR remote-capable.

I removed the unit from service and brought it home. Last night, after about a 18 months, I decided I would look it over. I replaced the fuse, and brought it up slowly on the variac. The first time, it drew about 45 watts, quiescent, no activity, no display, no pilot light. When I activated the on/off switch, the display went wonky - random pixels, random words, but no activity. No controls, including on/off responded after that point, but the draw went up to about 65 watts. I shut the unit down and held the on/off switch for 40 seconds (hey, it sometimes works with computers and other devices containing computers). This time, when I applied power and reached about 120V, the fan came on, the display showed other random stuff, but still no activity.

At this point, I decided that without direction, I would likely do more harm 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 these beasts.

Peter Wieck
Melrose Park, PA

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

 

[John-Del]

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

have very low leakage, but are very intolerant of over voltage or
reverse voltages.

Regards,
Tim Schwartz

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

 

[pfjw@aol.com]

On Saturday, February 6, 2016 at 3:57:48 PM UTC-5, John-Del wrote:

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
have very low leakage, but are very intolerant of over voltage or
reverse voltages.

Regards,
Tim Schwartz

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

As I noted to Tim directly, I observed no tantalum-type caps visible anywhere in this unit. The B251 is about a generation newer than the B750, so I expect this might be the reason. I don't like them much either, but I am using 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 outputs... That one of them actually works on both channels is remarkable. Anyway, 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 other words.

Peter Wieck
Melrose Park, PA

 

[pfjw@aol.com]

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 replacement process, I discovered no less than four (out of 28) cold solder joints, including a resistor that I simple pulled off the board leaving a neat little 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 or spend another few hour chasing down phantom intermittents.

Thoughts?

Peter Wieck
Melrose Park, PA

 

[Chuck]

On Wed, 10 Feb 2016 09:35:07 -0800 (PST), "pfjw@aol.com"
<pfjw@aol.com> wrote:

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 replacement process, I discovered no less than four (out of 28) cold solder joints, including a resistor that I simple pulled off the board leaving a neat little 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 or spend another few hour chasing down phantom intermittents.

Thoughts?

Peter Wieck
Melrose Park, PA
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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[pfjw@aol.com]

On Thursday, February 11, 2016 at 10:11:27 AM UTC-5, Chuck wrote:


> Probably the worse product Revox ever made. 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 reasonably well. Considering what I paid for it (US$75) and that I got a good five 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