ETI 5000 problem

[Pulse-R]

sandyK
my understanding of the 5000's driver stage, is that the DC feedback will serve to correct the output's DC offset by biasing the transistors on/off more or less to maintain zero output. It seemed that way when I was testing, IIRC it was Q4 stuck 'on' causing the problem - on power-up, the output would swing down near V- and then, over about 0.5 seconds, the output would return to zero. with the driver stages stuck hard, there was only very small amount of 'swing' left, so the signal was easily distorted. Maybe I just imagined it that way, but my theory turned out well in practice.

[AlexRWard]

Hi SandyK,
Yes, my recollection is that negative feedback is used to set the overall DC gain to 1. So it would make sense that if major DC drift occurs the feedback might reduce the available swing to near zero. Good heavens (I was going to say 'bugger' but that is not polite) it has been 20 years since I built the thing - no wonder my recollections are bad. Anyway, I have faith (?) that replacing all the low level transistors should fix it.
Thanks,
Alex

[sandyK]

Pulse-R and AlexRWard
Hey guys, my recollection is hazy too ! Mine was built in 1981. (?)
I only asked the question because of the wording "and the dc offset corrected it back to zero," I wrongly jumped to the conclusion that yoy may have been talking about the 6000, which had a small DC offset corrector PCB fitted.
If you are interested in the small modification mentioned in that other thread, and discussed by Suzy, send me an email and I will reply with an underneath photo attached. It only involves cutting a single track and connecting 2 series connected 9V zeners across it. A 18V 1W zener would also be suitable.
SandyK

[Pulse-R]

interesting you mention the mods, and I had no idea at the time -
this one had the zener diodes, and extra capacitors on the output transistors... hmmm, I guess the old man wasn't too slow in the olden days


Might have to look into the SuzyJ amp there, might be another 'must-do' to add to the list.

[sandyK]

Pulse-R
In my case, the mod improved sound quality as it reduced the uneven loads on both halves of the previous stage. The mod stopped one half of the differential pair from getting markedly hotter than the other side . This meant that their VBEs were now similar, and less unbalance of the previous stage. The side where the zener was inserted, previously had more voltage dissipated in it's transistor, and thus due to being hotter, a lower VBE than the other side.
The mod was discussed with David Tilbrook at Jaycar's Gore Hill store and approved by him.
SandyK