audiolab 8000P - one channel dead - transistor SC2911 looks toasted - (newbie)

[wantanlp12]

Hi - I'm new to this forum, so please be gentle.

Audiolab 8000P, with one channel not working. Very loud crackle and pop from the bad channel.

I've looked inside, and see that one transistor (?) SC2911 looks to be toasted - scorched around the solder, and a bit loose on the board. Not good, I think....

I had the same problem a few years ago, possibly on the other channel, and it looks like another one was replace by hi-fi shop technician. Not a pretty job, but did work for a while.

Question:
- is this likely to be a source of the failure?
- if so, where can I buy some replacements (in the UK)? (nikko-electronics.co.uk comes up on google.)
- any other advice?

It's an old box now, so can't justify spending too much on it - also skint

I don't want to just bin it, as it sounds great when it is working.

Thanks

[eggontoast]

Originally Posted by wantanlp12

I've looked inside, and see that one transistor (?) SC2911

Thanks

Are you sure you have the correct part number, what is the component reference on the PCB ?

[indianajo]

Usually something takes out a transistor, they tend to be pretty long lived on their own. In a very old home installed box, it is not as likely the speaker wiring was shorted as it would be in a bar band amp. More likely on old amps, the fan stuck, or the electrolytic caps the semiconductor drives got dry and leaky. These are aluminum cans with minus in balls pointing at one lead, or plus pointing to the other. They can also look like peanut M&M's, the tantalum ones. Before replacing the one burned transistor, check the other semiconductors driving it that they aren't shorted or open with the diode test (diodes) or double diode test (BJ transistors) or the "open drain to source with gate shorted to source" test for MOSFET's. Resistors near the blown up parts should be checked, and low voltage rated caps (under the power supply voltage). The low voltage film caps look like rice grains. Then you can buy all the blown up stuff in one order, and purchase all the over 20 year old electrolytic caps at once, instead of letting them blow up things one at a time. Don't forget to buy heat sink grease if you change anything on a heat sink, and possibly a new insulating washer. If the blown transistor is an output transistor, you need to run the amp into a dummy load as you test it to make sure that any faulty DC doesn't blow the speaker. It is also possible a leaky over age non-polar electrolytic cap in the speaker caused too little impedance and blew the O.T. in the first place. If you don't want the pleasure of learning how to do this, then adding all these parts cost up gives you an estimate of whether to do the replacement-20 year re-up, or buy a new amp and give this one away. I use newark a lot, their UK affiate is farnell.com. They have the expected life of electrolytic caps in the selector table, I try to buy e-caps with life over 2000 hours. Some people like doing this repair thing and buy the short life caps from distributors that have the very best price.
I don't worry too much about matching up part numbers on BJT's. I tend to use MJ stuff in the same package with Vce about double the power supply voltage in TO3 and TO220 (tab) and MPS parts in TO92.I do change all the O.T.'s on one side to similar parts if I change one. If your amp has multiple O.T.'s and emitter resistors lower than 0.5 ohm you have to check the matching of the O.T currents after you install them. If mismatched change out the odd one. Pros use a curve tracer for this matching but I use the amp, slower but cheaper. I do have a 2 trace scope, very handy. Idle current (bias) through the E.R's is also important to be checked and adjusted if necessary: 40 ma is kind of an upper bound, 20 is more usual. Once any output DC is gone you can hook up the speakers.

[wantanlp12]

Thanks for the replies, folks. Indianajo - I'll re-read your comments and pick out the bits I understand (I'm not too clued up on electronics.)

In the meantime, here are a few pics of the board. Maybe even a quick visual may give a clue?

[indianajo]

MPSA92 sounds like a legit motorola/on semi number, and ztx753 sounds like a legit diodes inc number. The middle picture looks more like 2sC2911, try that number in datasheetcatalog.com. If they don't have it, maybe NTE has a cross reference. This is probably pretty old, the transistor on the left looks like TO202 package, which is EBC usually. Newer TO220 parts are usually BCE and require twisting the leads around if you are going to use them. In the lower right of the third picture is an electrolytic cap. No visual substitutes for pulling the semiconductors near the blown up one and checking them with the meter tests. The DVM tests don't really prove a transistor good they just prove them bad. I've had double diode okay output transistors blow up later. But, better testers are expensive.
For basics on meter use etc., Electronic Devices, the Electron Flow Version by Thomas Floyd is a useful text I picked up at Goodwill for $2. Retread from the local trade school.

[jaycee]

2SA1209/2SC2911 are Sanyo devices and are commonly used as VAS transistors.

[Jessica1988]

I have learned a lot of practical things, thank you!

[Ian Finch]

VAS transistors, as here and identified in Jaycee's post above, run with low fixed current in the 6-20mA region, but with high potential, they can still run quite hot. Check how hot in the good channel, with signal and up moderate loud but without speakers. If this one is hot, it might be wise to fit small (tiny) heatsinks to lower the temp and overheating risk that this may well be, considering the repeated failures.

Replace both transistors in the bad channel first, of course, but do check voltages and resistor values, comparing the channels before you do. Sketch it and label voltages on each pin of the Txs, so you know what changes to expect. Fit Txs (the right way) Fit
sinks and go. Recheck voltages and temp. of these in each channel. If different by more than 5%, stop!

[Mooly]

Parts... CPC,
CPC - Over 100, 000 products from one of the worlds leading distributors of electronic and related products. | CPC

Transistors so far mentioned all in stock.

The only way you stand a hope of fixing this is to come up with the correct circuit so we can advise and also to power the amp via a "bulb tester" to prevent further damage while working on it.

[wantanlp12]

Thanks again, everyone

It will take me some time to digest this stuff, but I will certainly look into comparing the 'good' channel vs the 'bad' channel. I think comparing the temperatures, and then (very) basic testing of the 4 individual transistors will be my first line of investigation.

I'll send an update on what I find.