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Bryston / PMC repairs - amp blowing an internal fuse at power on

S

starbuck

pfm Member
I have a 3 channel Bryston power amp which is currently blowing one of the internal fuses upon powering on and I am trying to decide what to do with the amp regarding the fault, specifically do I get it repaired or sell it as is in the faulty state. The other two channels are working fine and it sounds great as a 2 channel power amp but I was hoping to use this for the left/centre/right front speakers in a 5.1 system. I have contacted PMC about the amp who have confirmed they would be prepared to undertake a repair, and they have quoted a perfectly reasonable cost of £70 for an hour of diagnosis which would then be deducted from the total repair cost if the repair is undertaken. My amp is well out of the 20 year warranty so a paid for repair is to be expected - I am not wanting a freebie - but it would be handy to get a rough idea if at all possible, of what the likely problem is and what their potential charge could be. I also want to know how long a piece of string is :).

I've attached a few photos below where you will hopefully be able to see the charred/scorched area between the fuse holders for channels 1 and 2, as well as some general photos showing the internals. There is nothing visibly apparent on the boards other than the scorched area between the fuse holders, though I am aware that problems aren't usually visible. I don't have any history of the amp so no clue as to what happened to cause the scorching near the fuses, but I do know that the insides were under a carpet of fluff (see last photo of it pre-cleaning using a soft brush and hoover). Does anyone have any ideas of repair costs at PMC for Bryston stuff, or any guesses as to what could have caused the scorching and subsequent blowing of the fast blow fuses?

PB120005 by Chris To Go, on Flickr

PB120008 by Chris To Go, on Flickr

PB120006 by Chris To Go, on Flickr

PB120007 by Chris To Go, on Flickr

PB120001 by Chris To Go, on Flickr

PB120004 by Chris To Go, on Flickr

20221031_160120 by Chris To Go, on Flickr
 
That's quite the blanket of fluff! :eek:

First - if fast-blow the correct specification of fuse?
Second - if its the same one everytime, and happens right at switch on - I think it is likely the diode rectifier bridge associated with that fuse. A single blown diode within the bridge can do this, and not uncommon in other amplifiers - it's where I start looking, for such issues.

Suitable replacements are cheap parts - the cost will be in the disassembly & diagnosis.
 
Two bridges can be seen on the 5th picture PB120001. If you have a diode test on your meter and the two flying leads are disconnected from the bridge it should be possible to test for a failed half of the bridge, probably the one on the right. Tricky detaching those bridges from the boards for replacement though.

John
 
Can't help with diagnosis but I've used PMC for two B60R repairs, two seperate amps towards then end of their warranty, and the service has been excellent. I'd just send it to them and see what they quote.
 
john.luckins said:
Two bridges can be seen on the 5th picture PB120001. If you have a diode test on your meter and the two flying leads are disconnected from the bridge it should be possible to test for a failed half of the bridge, probably the one on the right. Tricky detaching those bridges from the boards for replacement though.

John
Click to expand...
Three - there's one by each pair of reservoir caps.
Totally agree with the rest of your post - it will take a high-power iron, solder sucker & braid, to shift them.
 
martin clark said:
That's quite the blanket of fluff! :eek:

First - if fast-blow the correct specification of fuse?
Second - if its the same one everytime, and happens right at switch on - I think it is likely the diode rectifier bridge associated with that fuse. A single blown diode within the bridge can do this, and not uncommon in other amplifiers - it's where I start looking, for such issues.

Suitable replacements are cheap parts - the cost will be in the disassembly & diagnosis.
Click to expand...

Thanks for the replies. In answer to the question re. fuses, having double checked I am now not sure if I am using the correct types. After removing what I assumed was years worth of fluff and dust with the vacuum cleaner and finding that upon power on, channel 2 was dead, I simply replaced the blown fuse on a like for like basis as all three internal fuses were the same rating below the grub: fast blow 4A/250V. Having now more closely looked at the circuit board where Bryston were kind enough to label the fuse ratings near the fuse holders, and also double checking against the circuit diagram I found online (https://elektrotanya.com/bryston_3b_4b_5b_7b_8b_9b_14b_sch.pdf/download.html), it seems that they should be 2.5A/250V fast blow types. I will source some 2.5A/250V fast blow fuses and insert them in place of the 4A ones I have been testing it with so the ratings are correct.

Not sure if this is relevant to anything but something else I noticed is that with the fuses in place for channel 1 and channel 3, and either leaving channel 2 fuse holder empty or with a blown fuse in situ, the power lights for all 3 channels are illuminated on the front panel. I would have expected that only the two lights would be on for the working channels 1 and 3.

Thanks for all the advice on what could be wrong. I am leaning towards a PMC repair when I get the chance to take it over to them, it is an impressive sounding amp on the 2 working channels and would serve me well if it is fully working. I am not compentent to attempt the repair myself and would rather not do anything which could then jeopardise me being able to get PMC to look at it.
 
Another component that shorts when it fails are the tantalum capacitors.
If it was my amplifier, I would replace them if they are more than 10 years old no matter if they are in the fuse's circuit or not.
 
Bumping this thread to make any DIYers aware that the amp is now for sale in the classified room here on pinkfish. I don't have the skills to do the repair myself nor the means for a PMC repair unfortunately, so have reluctantly put it up for sale.
 
Bumping this to ask if anyone can advise what likely differences other than fuse ratings there would be on the main power supply board of the amp, for an amp supplied to the US market (so 110/120v)? The online link to the circuit diagram I found is here https://elektrotanya.com/bryston_3b_4b_5b_7b_8b_9b_14b_sch.pdf/download.html . There is a seller in the US who is selling the complete board which was pulled from an amplifier with a single faulty output board and I was wondering if this could potentially be a drop in replacement or if it would require significant changes (besides the fuses)? I have a relative in the US who could assist with purchase and transportation to keep costs down but it wouldn't make any sense if the board requires major work for it to be used in my 230v amp.
 
@starbuck,

On the PCB next the fuse holders it reads "ORANGE/120V" above "WHITE/230V", and the wires are orange. If this is a 120V market version then plugged directly into UK mains could certainly account for an arc across, especially so, were the dust bunnies a bit damp.

Craig
 
Thanks for the reply. I had noticed that before but assumed that given that all three channels have the same orangle cable connection rather than the white one as stated on the board - and the other two channels work fine without any signs of distress after many hours of powering on and replaying music-, that the colour coding is incorrect rather than the wiring being actually wrong (if you see what I mean). I notice from the transformers that are being sold by the same US based seller that also has the main power supply board, that their transformer output wiring has a couple of pairs of wires running to a single spade connector and I don't think the transformers in my 230V amp have the same pairing of wires: would that be because one is wired in series and the other in parallel? I would need to open the amp back up and double check the wiring, though, as I can't quite see it all in my photos.
 
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