cambridge wont stabilize

[homertooties]

Hi all
Hope someone can help
currently had problems with cambridge audio a3i .
kept blowing 2sa on left output,i have replaced all components in left side except resistors i did replace emitter ones though.replaced diodes all transistors and now the amp does not blow trannies anymore.
Probaly because i have wired a light bulb in series with live side of transformer.
There are 4 leds in board for 2 for neg rail and 2 for pos rail,trying to reset bias as new output stage and bias pots plus all surrounding bias parts on both channels.
Power up the amp the leds will not light unless the pots are pretty much fully clockwise ,leave a few seconds the led on the rail that feeds the original problem tranny 2sa goes out ,35 volts dc gets on left speaker output and protection relay clicks off,turn pot down a bit then relay will click back on ,then off again .
What is goin on i wish i new?
All output drivers and output stage are still okay,could it be anything to do with psu caps diodes or transformer,i am myhted as the right channel has never had a problem.
Also left channel gets vey warm right channel stays luke warm i also changed all electrolytics in both channels
cheers

[anatech]

Hi homertooties,
Neat problem. There is still something wrong obviously. MAke sure you got everything in the right way 'round. Correct polarities, that kind of thing.

Do you have a diagram so we can look at it?

-Chris

[agrobler]

Had one few years ago, they had thermal runaway prob.s due to wrong LED spec.s - especially if beefed up PSU caps and low impedance speakers... Apparently you had to get different value LED's - dunno which, can phone around and ask if you need...

Can also just contact Cambridge for info... at that point I had it done, didn't know anything about amps... still don't know much more - except for chips...

BTW bigger caps and some bypass MKP's did wonders for high freq.s - although must say our power is very "dirty"

[anatech]

Hi Steve,
Would it be possible to take a picture of the chassis from the top down? I am wondering if Q9 is on the PCB or the heatsink. I fixed a few before that were thermally unstable and was able to cure this forever by drilling a couple holes in the heatsink and mounting the bias transistors in the holes. I extended the leads to the transistor. That solved all the issues. The picture will refresh my memory.

-Chris (emailed to Steve also)

[anatech]

Hi Steve,
Those led's form part of a constant current source. They should light no matter what the bias current is. The fact that they do not light means that Q7 and Q10 are off. If one is defective or installed incorrectly, the diff pair will force the other off. Also, they will not light if Q1 and Q2 are off, or there is a problem with the current mirror formed by Q5 and Q6.

At this point, I would check to see that there are no solder bridges between any leads, also that the parts are in correctly. An incorrect part in Q7's position would cause trouble. If you didn't get the exact replacement part, stick the old one back in. It's unlikely to be damaged unless R12 and R13 are also burnt. Check for open traces while you are at it.

Don't ignore D2 and D3. If they go leaky or short, they could blow an output (look at D2 in particular). I've seen those go a couple times in other amps.

-Chris

[homertooties]

Hi thanks everyone for replies but i give up on this amp.
verything i test reads okay.

I am going to remove power amp get a smaller toroidal to power the op amps and use the amp as a preamp connecting the preamp output to the tape out put taking the phonos out of circuit first of course....cheers

[mastertech]

check resistors with an ohmmemter and the amp switched off and
unpluged from mains look for higher values or open ones

good luck