23-04-2012, 12:48 PM
(23-04-2012, 11:34 AM)Joe Wrote: I was using PP9s when I blew those transistors. I wasn't sure if my PSU may be the cause of the trouble and it can only manage an output of about 15V.
OK, so good batteries are obviously "beefy" enough. Actually, modern PP9s seem to have an impressive capacity and internal impedance...
Quote:I've re-examined the corpses and found T3 (which is an AC127) and T5 are short circuit and T4 open.
I've just checked your serial number (27684), and see that it's quite a bit earlier than my 2 (43867 and 45834). So that's obviously a change that was made in the lifetime of the model - I see another data collecting exercise coming on
Quote:I can get the quiescent current down to 12mA and it work at low to moderate volume with things cool. Turning it (the volume) up higher caused a machine-gun type noise (motor-boating?) which the ammeter needle spikes up in time with it.
Check the 5-pin connector. Also, I think I'd change the remaining electrolytic capacitors now. If only to return the set to "as new" condition so we can start to apply logic.
Quote:I am wondering if there'd be any point in changing T2 so that all four would be silicon ones.
No, none whatsoever...
Silicon vs germanium questions need to be put into the context of the circuit you find them in. Often, no changes are needed when you swap them - sometime it's really important. Here, you've changed the output pair, and that requires a change in bias voltage, but as T3 provides the bias voltage, and T3 is a "Vbe Multiplier", it's automatically already in the right ballpark.
Quote:As to what was initially wrong, it seemed blooming loud. To listen to it the volume knob only needed to be just clicked on which is what lead me to think that there was something set wrong which was causing over-loading.
This is a common fault with old volume potentiometers - basically, the connection between the end of the track and the external connection goes high-resistance.
Don't worry about "overloading" - if you drive the amp into clipping it won't sound nice, but nothing will break. Once built into a well-designed circuit, transistors are not as fragile as valve aficionados would have you believe. They get a bit grumpy if you overload them, but wouldn't you?
