Ashdown EvoII 500: two amps, two issues

[Alex R]

I attach a schematic for the power amp of the Ashdown EvoII 500.

I have two of these amps here.

One is stuck to the +ve rail, and all the semiconductors beep nicely on the diode tester.

The other has marked crossover distortion. I haven't tested anything on it yet.

Any timesaving suggestions about where to look first would be welcome from people who can understand the schematic better than I can (not entirely sure I see what TR5 and TR20 do, for instance).

Keen to learn, as always...

[Enzo]

TR5,20 are limiters. The current through the outputs causes a voltage drop in the ballast resistors like R33. The more current, the greater the voltage. Thatn voltage is sampled off through R28 to the base of TR20. If the voltage gets high enough, TR20 will turn on and try to short the drive signal to TR19. That serves to limit the drive it can provide. It is a protective circuit. Lift the diodes D1,D6 and the circuit is effectively removed.

You have coupled pairs of transistors at TR2,14,8,9. You better be real careful not to confuse yourself checking those in circuit. In each case there are juntions of one in parallel with its mate.

Open resistors can cause your sumptom as can missing powr supply voltages.

Crossover? SHorted TR10?

[Alex R]

Thanks Enzo, as always. I'll look into all that and get back to you.

[Alex R]

Okay, er, mixed results.

Amp 1, the one with the 'stuck rail', had already had a wideranging rebuild from me after a major short episode. New outputs and drivers, several burned tracks remade, etc. Turns out it was 'stuck' because I had cracked the jumper wire that ties the main caps to ground! It also had a shorted T2 which showed up after I took off T3 and T16 to test them as advised. I brought it up slow on the variac and all was well. I ran it into a load for a while, played through it... and suddenly it blew a fuse with no warning, no rise in current visible on the ammeter, no smoke. There's no sign of heat anywhere and the heatsinks were cool - but most of the outputs are shorted again. Oh boy. That one can rest awhile, and I'll get a price on a new board, I reckon too many potentially strained components on there (or else I made another mistake rebuilding it, there's always that possibility and the form doesn't look good I admit).

Amp 2, the one that showed 'crossover distortion' on initial testing, proves to have a good power amp when it is fed with a clean signal. The crossover-type notches are on the preamp output. They are there whatever you do with the preamp controls, all of them. They are quite marked and look just like power amp crossover except maybe even more sharply defined. Any ideas before I mess this one up too? Preamp schemo attached.

[Enzo]

OK, so feed it a nice signal and trace that through the stages to see where it gets distorted. Both power rails OK in the preamp?

Are ALL the preamp outputs affected or just some?

[Alex R]

Er, well, the 'crossover distortion' suddenly disappeared. Artefact of test conditions I guess. It was odd - it was 'there' on quite a few opamp outputs later in the circuit, but not there on earlier ones, and I was happily tracing away when suddenly they were all ok. It really did look just like crossover dist. Little notches in the wave as it crossed the 0v line, maybe a little offset one from another. Put a new speaker on it and it sounded fine. Ah well... I'll try not to waste too much more of your time, Enzo!