Musical Fidelity P170 Blown Transistors

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Evilbat

Member
Joined 2009
ianallenphotography.co.uk
Hi Andrew,

Everything was fine for about half an hour until I started playing Spectre on DVD. The crackling is almost continuous on the right channel now. It's pointing to a capacitor I believe. One of our engineers at work suggested that it could be the plates of one of the PSU caps. It's similar to listening to an amp when you've switched power off and the PSU caps are discharging whilst still powering the amp.

Some new caps won't go amiss anyway as most have been in there for about 20 years.

Ian
 
dragonweed

dragonweed

Banned
Joined 2010
Just a sideleine... Not that I want to discourage anyone from repairing that amp, but some time ago I kept repairing one for days, with the typical "blown FET-s" problem.
After the third set finally I gave up, because -despite measuring, checking all parts, the wiring, PCB connections,- it still contunued to blow the output FET-s, well :gnasher: I must admit I did not use a bulb limiter (biig mistake).
With those old and rather unsophisticated op-amps (LM 318- bad news!) it broke into oscillation after a while every time I tried it, so I scrapped it for parts.....
 
jaycee

jaycee

Member
Joined 2004
One thing I will note on the MF amps that used TO-3 package devices - MF often soldered a wire directly to the transistor case for the collector/source connection. Do not do this - use an appropriately insulated spade/ring terminal under one of the mounting screws.
 
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trean27

Member
Joined 2014
nicotronics.no
Hi, reading this post, and I have a question about the p170 i have. I have changed the power resistor for the voltage divider with 2 3.3k resistors at 7w but stil the transistor and the resistors getting very hot. I changed all caps and measured the transistors. but first, it getting hot and second, i get distorsion fast. with a signal more than 400mV it starting to distort. I changed to mpsa42 and 92 but i did not match them, but could that cause this distordtion. ? it was not distorting when i got it... but got very very hot. and the lm318 is sensitive, that it has to be fittet right and well secured I learned... and got experienced
 
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