Popping Fuses

[Red45]

Hi Folks,nice to find a decent forum.

I havnt dabbled for some years now and I have got a bit green.

Been using an old sansui amp (AU4900) with my pc to play music etc.
Unfortunately it has decided to blow 7 amp fuses in the power supply (only in negative side) between transformer and rectifiers.

As its an old amp I am suspecting a duff smoothing capacitor,I was getting 'farting' (apologies but it s the best description i can think of!) through the one speaker prior to a very distorted sound.

Can anyone confirm my diagnosis or point me in another direction?

thanks eddie

 

[TillEulenspiegel]

Normally if a cap is shot , you'll get a loud hum. It's possible your bridge has a shorted diode and is leaking AC thru to the cap which will still filter a bit but presents a short to the AC output of the power transformer. The way I check is to look for AC leakage is to use a scope , but you can probably get a reading on a VOM . Put it on the AC scale and see if you get any reading. Caveat is even a low level of AC past the supply should produce some hum, and will produce effects on both channels. It sounds more like adriver or output stage prob. Is the amp discrete or IC outputs?

 

[Nigel Goodwin]

Most common cause is the output stage going short circuit, although the bridge rectifier is possible as well - the PSU capacitors are fairly rare.

If the amp has discrete output stages check the transistors with a meter set to an ohms range - as it's a stereo amplifier you can compare one side to the other.

 

[Red45]

Thanks for that.

I wasnt getting a hum at all just a spluttering sort of noise,a sort of cross between crackle and hum.

It is a tranny output,dc coupled,from the '80's.(sansui au4900).

I'll put a meter across the output trannies and chassis (NO POWER!) and come back.

eddie

 

[TillEulenspiegel]

Ya I know that noise , it's called "Motor-boating", it usually means an open diode or filtercap. Again tho you would experience this on both channels UNLESS each channel has it's own power supply, which is seen in some hi power amplifiers.

edit to add: most amps like this are complimentry symmetry.. un hook the load and see if you have DC on the outputs.

 

[Red45]

Interesting!

The bridge circuit is made up of paired/parallel diodes for each of the four 'arms',two of them appear to be short circuited, i.e zilch resistance in both directions.

Output devices appear ok ,putting meter across the two visible pins (To3 type,D188A and D545A) gives 200 ohms in one direction and 50 in the other.

I will try replacing the on board bridge circuit with a rectifier pack and take it from there.

Happy days

eddie