Diagnose a cheapo amp?

[Valvehead]

Sorry missed those----

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1)np to ground, pp measures:
Q1 q2 base= 0v or only a few millivolts (< 100mv)


q1 base is 12v , q2 base is 22v




q1 q2 emitters= -550 to -650 mv .


Q1 emit is 1.6v , q2 emit is -35mv




q1 q2 collectors=+21v .


Q1 collector 0.95v . Q2 collector -0.65v













2) np to +21v, pp measures:
Q4 base= -600mv
q4 collector= around -21v

q4 base is -22v

q4 collector is 0v


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[J M Fahey]

Hi valvehead.
Pity you hadn't measured those before, it would have saved a lot of guessing.
Don't worry, we can go on with a different approach now.
This amp is far worse than I had thought, as you see clearly those voltages ar *way* off compared to what's expected, but of course it can be repaired.
You'll have to replace all transistors at once, it's not worth replacing one .... measuring .... replacing another ... etc.
Anyway those are cheap transistors.
Only problem is that they are Japanese, maybe trying to get the exact *original* one raises the price, but I should use generic ones.
I will suggest what I would use there, all available off the shelf for me, problem is that we use mostly European transistors in Argentina (Siemens, Philips, etc.) and Texas ones.
Maybe somebody can suggest types readily available there or some shop specialized in Japanese transistors at good price.
Q7/8: 2SD313=TIP31B or C Pinout matches.
Q1/2/3/5 2SC1815 Mouser has them
Q4/6 2SA1015 Mouser has BC556 , pinout may not be the same, check datasheets.
Personally I find them marginally rated as drivers, I suggest somewhat beefier there.
Q5: BC639. It's an 80V 1000 mA part, compared to the original's puny 150mA
Q6: BC640. Same considerations.
These are my preferred TO92 drivers for amps up to 40W, nothing readily available even approaches them.
1)Mouser has all of them, you will spend less $3 on the entire set.
Personally I would but 10 of each and have them handy, to justify mail cost.
Also order micas, shoulder washers and grease.
2)Watch pinouts, on substitutions (except on TIPs) you may have to bend legs a little to fit PCB holes.
Be careful to desolder with a good solder pump, solder wick for final cleanout is good, given date and price that amp must have phenolic boards, not epoxy glass.
3) When everything replaced, check pinouts 10 times and power the amp with a small series bulb (25W), and no speaker.
Check for no DC on output and hook the speaker.
4) I almost forgot: replace R28/D2/D3, they may be cooked.
5) I'm sure now it will work properly, just work relaxec and carefully.
As you see, repairing it is worth it, parts cost is nil, problem is time spent until finding the problem.
Good luck.
PS: obviously modern amps have it easier, just replace the TDA something.
As Enzo said, when a kid shows up at the shop with little money and a practice amp, many times it is still worth repairing his beloved amp if you spend no more than 10/15 minutes actual bench time on it, and you gain a future loyal client.

[Enzo]

Asian transistor types are easily sourced in the USA. SOme are in stock at Mouser, Digikey, but they will have far greater selection of those at houses like MCM. Also B&D Enterprises has a lot of Asian types.

That 2SA1015 is a common and pretty nice type. Nice companion to the 2SC1815. I stock drawers of both for general use.


The ASian TO92 types are ECB across the front, while US types like the 2Nxxxx and MPSAxxx are EBC. And I can never remember how the BC series runs. You can always find electrical equvalents, but I try to stay with Asians for Asians so I don;t have to deal with different pin order.

[Valvehead]

Im waiting on the owner to ok the $ for parts.

I found all the trans. locally / NTE stuff. looks like about $10 for all of them.

[Valvehead]

Thanks again ! I got the amp fixed.


I replaced the 8 transistors for $12 . Amp works perfect now.

[J M Fahey]

GOOOOOOOOODDDDDDD!!
Congratulations, you did it.
Problem on power amps is that, opposite to tubes, all parts are direct coupled, and *one* device, either shorted or open, throws *all* other voltages way off.
The beauty of chipamps is that you replace all active parts at once, but discrete ones can be repaired, of course.