Shorted rectifier & blown internal fuses in HH VX-150 amplifier

[Old Sparky]

Replacing a faulty on/off switch in one of the above amplifiers the other day, and getting no sound, I checked further and found that the BR82 bridge rectifier was short-circuit between both it's AC terminals, and from them to it's positive terminal, as a result of which the two 6.3A(T) fuses in series with the mains TX secondary had failed o/c.  This amp. uses 2 x 2SK105S and 2 x 2SJ162 MOSFETS in each channel, and a preliminary cold check shows no obvious fault in these.  With the faulty rectifier removed, there is no s/c across the PSU o/ps.  Not being familiar with these amps, idoes anyone have any idea what may have caused this failure?  The amp was brought to me to have a faulty mains on/off switch replaced.  Said switch was o/c on one pole.

[Murphyv310]

Hi.
Was the switch actually faulty? If so it's likely the contacts were arcing at some point which "may" have caused the rectifier problem or even an issue down stream. Replace the rectifier and fuses then suck it and see.

[Old Sparky]

That's what I intend to do, Trevor. I've ordered a replacement rectifier and fuses, and will, as you put it, 'suck it and see'. The switch was definetly faulty , with one pole o/c, even after repeatedly operating it, so that, too, will be replaced.

[Murphyv310]

Good stuff.

[Mark Hennessy]

Lamp limiter!

[Old Sparky]

I should have used one, Mark. Until now I've only used mine on vintage valve radios.

[ppppenguin]

Lamp limiters can be very useful with solid state kit. Though not good to use them if the kit has a switchmode PSU.

Within solid state kit you can use lamps or resistors to limit the current to a potentially troubled high power circuit. I've done this while diagnosing line output troubles on a TV. Not yet done it with an amplifier but a few ohms between the power rail and the output stage can save an expensive blow up.

[Murphyv310]

Some amps don't like running on low power, whether this amp is one I wouldn't like to say.
If the 6.3 amp fuses have blown due to a fault in the amp rather than arcing contacts in the switch etc the damage is already done. I'd most likely try lower rated fuses initially like 3.15 amp or even 2.5 amp, if all is well Its unlikely even the lower rated fuses will blow unless you turn the wick up!!!

[Mark Hennessy]

I dread to think how many of these the BBC bought. Hundreds, certainly. Standard Hitachi app-note fare, so easy to understand and repair (with the right techniques). No DC offset detection/protection, so can take out bass drivers when they fail (have photos somewhere). I don't touch them these days.

[Murphyv310]

Hi Mark.
Used to do lots of amps but you know its now nearly 14 years since I shut my shop for the last time, definitely getting rusty these days and recent things are befuddling my brain. The amp in question I just don't remember. There was one H&H that used to give a horrible DC offset when the mains was below around 180 volts if I remember correctly.