replacing amp transistors

[elaar]

Hi,
Sorry i posted this at the end of a very long thread so it wasn't really seen, so thought i'd start a thread

I have 2 cambridge audio A5i amps, these amps uses Sanken sap15 darlington pair transistors, 1 of which is dead (every leg seems to be shorted).

My question is, if i need to replace 1 sap15, is it better to get a complete set with the same batch number? i've noticed that there is some difference between the readings between the different saps in the two different players (things like checking diode readings which are currently identical to the other saps in that player, but slightly different to the other amp's.

I assume it will also mean i will probably have to adjust the variable resistors to get the idling current to as close to 40mA as possible, anything else that needs to be done if the new transistors are slightly different to the other channel?

Thanks,
Andy

[AndrewT]

Hi,
you may need to reset the output offset (DCv at output with no load).

If you do decide to buy a batch to replace all, then within the batch you could try matching the pairs so that differences are minimised.

[elaar]

Hi Andrew, thanks for the reply.

I probably will get a batch of them while i'm at it, but it looks like i'm going to have to read up a little as I have no idea what the output offset is, let alone measure it and adjust

Cheers,
Andy

[anatech]

Hi Andy,
Oh my! Do you know anyone that can help you and show you a few things?

It's the small things that will get you! Try to feel how tight the mounting screws are. Clean the heatsinks, the mounting surfaces must be spotless. Don't use insulators in poor condition, use fresh grease or "plastic compound" insultors.

-Chris

[elaar]

Hi Andrew

I have made amplifiers before, but they have all been pretty simple chip amps, so I've done alot of readig on power supplies, decoupling, snubbers etc.. I should also be fine with the "small things", i have a new tube of decent thermal grease and even some good mylar insulators.

It's about time i look into the world of solid state amplifiers anyway

Thanks again for the help
Andy

[cpemma]

Originally posted by elaar
...it looks like i'm going to have to read up a little as I have no idea what the output offset is, let alone measure it and adjust

That's the easy one, it's the DC voltage across the speaker terminals at zero input and should be zero.

The methods are given here though for idle current I'd stick to the maker's recommendation; commercial amps are unlikely to have the heatsinking to cope with the high levels he advocates.

[elaar]

Hi, thanks for the reply cpemma.

That's a very helpful tutorial. Without access to the appropriate cambridge audio service manual I wouldn't like to hazard a guess to the original quiescent currents, each channel uses a pair of SAP15s, would anyone like to guess a likely value?

It's a shame the channels aren't completely seperated with different bridges because at least then 1 channel would still be working and i could take readings from that.

Thanks,
Andy

[cpemma]

Originally posted by elaar
Without access to the appropriate cambridge audio service manual I wouldn't like to hazard a guess to the original quiescent currents...

I'd start a new thread with a more amp-specific title, someone may have the manual info.

[chewrock]

Yes,

AndrewT is very correct. Matching them for hFE (which for Darlingtons can be in the 1000 - 5000 or even higher range) is very important on many levels.

Class B elimination of 2nd order harmonic distortion can't occur unless the matching is very close.

And the VOS (voltage offset when the inputs are shorted) is also a big concern. Even a relatively small difference in hFE can cause the output to very strongly favor either the positive or negative side of the waveform. That's seriously bad for tone quality, and can even damage or burn out your woofers (the tweeters are protected from VOS / DC offset by the crossover capacitors).

Hope this helps.

[Tony]

from my experience with bipolar amps, you don't just replace the output transistors that went dead, you have the get the other dead parts too. otherwise, you may see smoke once you power up.

patience and presence of mind is key to a succcesfull repair.