transistor substitutes>?

kepeb said:

vr2 to max resistance
check the voltages across each diode, ok.
and look for a 3v drop across the output stage.. could you clarify where i am to probe to do that?  :-[
also I only have my series lamp btw no variac  :

Click to expand...

OK transistor 101...  For NPN transistors when biased up, there will be approx .5V from base to emitter (+.5V for NPN, -.5V for PNP).

Since the actual output voltage could be anywhere, these measurements need to be relative to a given circuit node. Measuring wrt the final output, the final NPN (top half) power device's base will be approx .5V higher than the final output, the PNP base, will be -.5V. The several power devices are in parallel so should all measure similarly.

The next stage (drivers)  have their emitters tied to the final stage base, so their base will be up (and down) another diode drop, so roughly +1V at base of Q116, -1V at base of Q117 (wrt same final output node). The pre-drivers pick up another diode drop, so roughly +/- 1.5V at bases of Q112 and Q115, this +/- 1.5 adds up to the roughly 3V total I mentioned. VR 2 actually adjusts this nominal 3V for a precise idle current in the output power devices, turning them just very slightly on. If you turn them on too hard, they fight each other, get very hot, and may fail.

JR

kepeb said:

...maybe just see what the min order/postage is.

I guess I only need 1 or 2 of each for this repair.
i'll see if they have anything else i need

Click to expand...

I couldn't catch them on the phone, but I reached them by email.
There's no minimum order amount. You can order just one transistor if you like.
They only ship via 'Collisimo suivi' which is the fast option of the French postal service. Tracking number, and a signature for delivery. 18.66 euros for UK tho, so you'd better order something else...

Let me know if you need more help...

Axel

wow,
cheers alex, thats a great help.
i've marked the site i will need a bigger order to justify the postage so i'll wait till i need more.
gonna go over what JR and PRR suggested soon as i get time then i'll know.

doubt i'll be using Alesis tho they replied but they want me to send the amp in, i cant imagine the repair will be much less than half the cost of a new one.
i doubt the post will be paid for either. turns out it is a good time to learn how to fix them