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Revox b285 repair

K

kisshu

New Member
Revox b285 repair question

I turn on the b285 and it sounds fine for a few minutes then i get a lot of distortion in both channels L & R with any input- radio or cd; also the hotest part is just behind the tranformer on the control module at the first or second voltage regulator - the amp section itself stays cool. Does anyone have experience with what would likely cause this and what to test for?
 
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I think i can manage to replace the caps and some transitors but i need help on where to order replacements please?
 
Try www.mouser.com or www.digikey.com. Stick with the same uF values, but you can increase the volt values a notch or two if you have to. You should only need to replace the electrolytic caps. I like Nichicon brand, but Panasonics are also good.

Note that the big caps on the amp board and the power supply board have an odd 4 pin mounting which is no longer made as far as I can tell. So don't mess with those if you don't have to.

Also, Revox copper traces are fragile - it's too easy to mess up some while recapping.


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need expert advice

thanks bob - now i've discovered that some repairs were made - and entire section of ceramic and regular resitors were removed and only the ceramics were replaced with wrong values - brand -tru ohm 1250.

my amp board is 1.728.320-11 so i don't know what the values were - the service manual i have is for -00 and shows alterations 1-4.

where can i ask an expert?

thanks
 
You might want to purchase/download a schematic.

Unless this is a regular occurrence on that model, I don't think anybody will know off the top of their head. Especially if there's evidence that it's been f*cked with.

It does sound as though a power supply is failing. This would explain "distorted both channels". ReVox did use some sucky bridges and the dreaded Frako caps, but these would only be lucky guesses if replacing those solved the problem.
 
i have the service manual - i think its the same as the ftp i cant download it there. I asked studer if there is only one version.
 
replacement caps

I have no idea how to order capacitors i have only 4 desciptives in the service manual:
10 uf, -20%, >10v, electrolitic
etc
but the order forms have all these other attributes - ripple, impedance, ers, style blah blah. operating temperature.

so what do i do?

also anyone know where i could take it for repairs?
 
Revox almost always used standard industry parts. Ordinary capacitors, resistors and regulators will be fine as replacements. 85°C will work fine - but 105°C will possibly give a longer life. I'm not aware of any major revisions in the B285 history. One point of trouble in that amplifier is the TO-220 transistors that preceed the power output stages - these do get very hot in the earlier models and extra heatsinks help a lot.
(see my web page for my own method of heatsinking http://web.eject.co.za/s8nspawn/hifi/ak/b285_225_215.htm)

If somebody has been messing inside it and changed resistors, they might also have set the output idle current wrong. The service manual explains very simply how to do that correctly. Probably a good idea to replace the 2 trimmers that set idle current - they can go open-circuit (fortunately this failure is a 'safe' one, the idle current then goes to zero, rather than too high, but distortion increases)
 
I am located in Ivoryton, CT. I would prefer to pay someone to look it over and diagnos
it before i try to replace anything. anyone nearby?

Thanks for the tips steerpike2: It has 3 aluminum fins added to the back end transistors.
the idle current: I get a reading on dc and it fluctuates i think but i only have point probes. plus i have the heat sink off and it gets quite warm after a few minutes. I don't have extender board i would like to know where to get a ribbon to extend the amp section please?

i really think someone with experience should look at it.

thanks
 
kisshu said:
I am located in Ivoryton, CT. I would prefer to pay someone to look it over and diagnos
it before i try to replace anything. anyone nearby?

Thanks for the tips steerpike2: It has 3 aluminum fins added to the back end transistors.
the idle current: I get a reading on dc and it fluctuates i think but i only have point probes. plus i have the heat sink off and it gets quite warm after a few minutes. I don't have extender board i would like to know where to get a ribbon to extend the amp section please?
Click to expand...

The extender ribbon you can make up using a computer IDE cable (older ones preferably, not the 100MHz & up versions) and a multiway pin header plugged into one end of the ribbon cable.

Idle current should be set when it is warm, but not hot. I don't think you need the amplifier to be disassembled to set both channel idle currents. Just the top cover off is enough - but you need proper test probes for sure.
 
Was hoping you were on the west coast so our resident Revox guru Tinman could be an option. I don't know any referrals for Revox in your area, sorry.

John
 
I was trying to get a bias reading and got a spark off the copper clamp on the outer power amp not it reads dc message.

If tinman has a 115 volt b285 i could mail the controler and amp board for evaluation.
 
Send him a PM... but I doubt he will be too enthusiastic about getting brought into the middle of a DIY project that's already blown up something in the process of taking a simple measurement. Professional techs much prefer to have the whole device in front of them, and nobody else's hands in there messing around before, during and especially after. That's what I had in mind when I asked about location.

John
 
That is one of - if not THE most annoying things that you can do to yourself: break one part of something while trying to fix another!
But the B285 is a very well made machine and worth getting fixed up.

You may need to pay up for new power transistors, and here it is worth checking the credentials of whoever the parts come from: there are a lot of pirate/fake power transistors around, and it's not easy to identify real from fake, until the fake ones blow up and destroy everything else they are connected to. Generally, buy them only from a reputable dealer who can guarantee their chain of supply is secure.
 
I guarantee that IF Marc (Tinman) takes it on & fixes it, it will be done correctly and for far less in the long run, including shipping, rather than sending it to JMTechArts. They are very expensive, and will just fix the "problem". I bought my own B285 from someone that had it "fully serviced" by them, and it worked for 3 days after they got it back, then dead as a doorknob, and though the repair "may be covered, but it might be totally unrelated", shipping both ways was not. I got it for a decent price from him, did a total recap and replaced the power transformer and diodes that the failed filter cap had taken out. It is relatively easy to replace the large filter caps with a new equal, only need to drill 2 small holes in the PCB. Works perfect, though the B285 always run a little warm. One cool thing about the B285, is that the standard MM phono card has the traces for the MC amp built in. So all I had to do was buy the components, and follow the schematic and now mine is MM/MC switchable, and works great. You always have to replace all the Frako, Philips, and Rifa caps.
 
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