Hello,

Can you suggest a good replacement for Q29 on digikey, i will buy a few just incase the left chanel needs replacing also

Perhaps it is an idea to have some databooks available.
PDFDrive has some:
https://www.pdfdrive.com/search?q=transistor+data&pagecount=&pubyear=&searchin=&em=

Bertus

 

@KeepItSimpleStupid

Here are the results of the readings you asked for,
No signal input

Q23B to Q28E = 0.633vdc
Q40B to Q36E = 0.918vdc
Q23B to Q40B = 1.566vdc

Pete

 

Here is Q29 ,

This thing needs a good clean when were finished

 

Is this B,E,C pinout the same regardless if the To-3 is NPN or PNP , i assumed the B and E would be reversed ?

 

Think i have answered my own question here,

 

Hello,

You might have noticed that the B and E pins are not in the middle of the housing.
Perhaps this page might help:
http://www.bustedgear.com/faq_transistor_substitution.html
Always check the datasheet for the pin setting.

Bertus

 

If am right that means the To32,s on the positive side of the schematic have to have 0.6vdc on the base to be forward bias from collector to emitter, and the To32,s on the negative side have to have --0.6vdc on the base to be forwatd bias from the emitter to collector?

 

Hello,

You might have noticed that the B and E pins are not in the middle of the housing.
Perhaps this page might help:
http://www.bustedgear.com/faq_transistor_substitution.html
Always check the datasheet for the pin setting.

Bertus

Yes i did notice that , but its hard to see on the data sheet which side the pins are on

 

@bertus ,

Can youtell me how i know what way round the transister is from this data sheet,

Pete

 

Hello,

Did you see this in the datasheet?

Bertus

 

**Q23B to Q28E = 0.633vdc
Q40B to Q36E = 0.918vdc
Q23B to Q40B = 1.566vdc

Thanks.

The ** is out of place. I'm not going to randomly ask for readings. I'm going to let you think for a bit.
Chase down the B-E junctions as though they are diodes. Start from one end. Compare to the other side.

You have that suffix issue I asked about. Without that issue I can't suggest a part. I did use www.mouser.com.

www.newark.com has all 3 variants of the BC546. That will substitute for th 546, 547, 548, 549 and 550.
You just have to get the suffix right. You have to figure out what variants (suffix) were used where.

There is likely a transistor or some part not behaving between Q23B to Q28E = 0.633vdc

One guy may hog all of the current and then bang eventually.

I'm suspecting that if Q29 was replaced, it was replaced with a higher gain that was there before.

www.mouser.com, www.digikey.com and www.newark.com are the major US suppliers. newark has ties to Farnell in the uk.
I also went out on a limb and searched another supplier: https://www.avnet.com/shop/us/search/bc546#categoryId=3074457345616680555&

Your not limited to digikey.

Futureelectronics has too high minimums.

There is https://www.findchips.com/search/bc546 too that will look at lots of places.

Another https://www.onlinecomponents.com/en/keywordsearch?text=bc546 obscure place. I ordered a bunch of power supplies from them recently.

I suspect you may have missed the A, B or C varients. You have ROHS and lead forming variants too.

In general the class AB amplifier has.
pre-amp/phase splitter
Voltage gain and current gain stages
A bias voltage temperature compensated regulator that keeps the "AMP" slightly conductive at the zero crossing.
Various protection circuits.

If you don't fix ALL of the bad parts, catastrophic failure occurs. Not matching transistors that are supposed to be matched might take a while for failure to occur. Not adjusting the bias regulator properly might take 20 minutes to occur.

 

You have that suffix issue I asked about. Without that issue I can't suggest a part

The suffix is C, i posted a photo of Q29 in post #203, however i think this has been replaced

 

The leads are too oxidized, I think for being replaced.

Are all of the BC* parts suffix "C", then?

Newark,com may be your best choice for parts then: https://www.newark.com/on-semicondu...pn-65v-0-1a-150deg-c-0/dp/46AC0503?ost=bc546c

 

The leads are too oxidized, I think for being replaced.

Just its different to the one on the left chanel, possibly i replaced the left chanel at some point

 

There are many BC549C available on ebay. It is probably worth getting some that way so you can make progress. When you have identifies all the faulty components you could order them all at once from a reputable supplier and install them.
What small NPN transistors do you have in stock. We may be able to identify one of them that will work well enough to make some progress.
What is the history of the amplifier from when it failed until now including components that have been replaced ?
This is to try to guess what the original fault might have been and what extra faults may have been introduced.

Les.

 

There are many BC549C available on ebay. It is probably worth getting some that way so you can make progress.

The 546 is the better part. Higher voltage.

This:

**Q23B to Q28E = 0.633vdc
Q40B to Q36E = 0.918vdc
Q23B to Q40B = 1.566vdc

Suggests that there is something wrong and it could be probing. In effect, the first two lines should be about the same voltage. The 0.633 "suggests" that one of the transistors is shorted or you probed wrong.

It's almost like three B-E drops and then three 0.6V voltage sources in parallel. i.e 0.6+(0.6||0.6||0.6) = 0.8 which is "close" to 0.9V. If one of the latter 0.6V drops were zero, you would end up with 0.6V.

"0.6V voltage sources in parallel" Not really, but close enough. A 0.6V source with a low value resistor is a current source,so, 3 current sources in parallel. A voltage source in series with a resistor can be transposed into a current source with a resistor in parallel.

That's where my thoughts are.

If there were 0.9 and 0.9 so the natural sum was 1.8V and then the regulator was putting it at 1.566V you have partial conduction giving you class AB.

The thermal feedback is essential. The suggested way I did my amp was pretty cool. two plastic diodes were essentialy press fitted and then one was laying on top of the heat sink.

 

Just to confirm my estimate on the emitter collector voltage on Q29 I built up the circuit around Q29 using the nearest transistor I had (BC547) I used a current of 2.5 mA (The same as in the amplifier.) The base emitter voltage was 0.668 volts. The emitter collector voltage was 4.2 volts. 6 time 0.668 is 4.08 . The difference is probably base current trough the 15 K resistor.
The TS could try doing this with the transistor from the amplifier to confirm that it is faulty.

Les.

 

I don't think it is faulty.

 

What small NPN transistors do you have in stock.

I dont have any bc series transistors, just some mpsa 49 and 92, i will order some cheap bc549c off ebay and come back tuesday, i am away again this weekend,

Thank you all for your input so far

 

Is this worth buying ??