Headscratching time. On first receipt tried the amp out as owner had
been using 1 channel only , after having blown the other by shorting it.
Both LV ac fuses ok but one SAP15 p-type blown.
Had to cross-over power Darlington terminals as mirror footprint from
the usual BCE orientation.
Cleared out and replaced with that fudge for the SAP and other blown
1/8W Rs and tried out at 50% mains.
Variac meter swung right over to the right and both LV fuses blew.
Replaced fuses and tried at 20% mains, half amp or so of mains draw.
Diconnect mains and main +ve and main -ve rail measure 0 ohm to ground
for 10 or 20 seconds and then goes high impedance.
Tried at 10% mains and sometimes +ve rail would be at zero ohms for 20
seconds or so and sometimes -ve rail like that, on repeating this game.
Cannot see anything in the way of a crowbar SCR anywhere or relay and
circuit looks much as the A500 that is out there , including overlay
numberings.
There is conductive brown glue to the main caps but that measured 2 meg
or so minimum, with close DVM probes.
All output devices and my fudge cross-compare DVM-D cold fashion, fine
still.
When I say 0 ohms I do mean that , well 0.1 ohm or less resistance.
Unless anyone has an idea what is going, the next will be to remove my
fudge and power up again.

Thats enough of this malarkey for today.
About 10% mains for 20 seconds or so , the only thing warming up on IR
thermometer was the 2 fuses, so something big presumably taking the
power. That with about .2 amp mains current draw, 5V ac over secondaries
and +/-1.8V on the DC rails

Nucase Kook wrote:


Thats enough of this malarkey for today.
About 10% mains for 20 seconds or so , the only thing warming up on IR
thermometer was the 2 fuses, so something big presumably taking the
power. That with about .2 amp mains current draw, 5V ac over secondaries
and +/-1.8V on the DC rails



** Bet you have revered the positions of the PNP and NPN Darlingtons - causing the internal C-E diodes to conduct soon a power is applied.



..... Phil

I'd wired in the train of 5 Shottky SMD diodes , the wrong way round so
no biasing, doh!

On 02/11/2016 13:36, N_Cook wrote:
I'd wired in the train of 5 Shottky SMD diodes , the wrong way round so
no biasing, doh!


So far good output level match and sinuosity with no load ,with 1KHz
input at 60% mains ,and reasonable biasing DC levels match between L and
R channels with original preset setting. Will add 8R loads and repeat
these tests and adjust bias while still at 60% mains ,interim, before
going up to 100% mains

On 03/11/2016 09:46, N_Cook wrote:
On 02/11/2016 13:36, N_Cook wrote:
I'd wired in the train of 5 Shottky SMD diodes , the wrong way round so
no biasing, doh!


So far good output level match and sinuosity with no load ,with 1KHz
input at 60% mains ,and reasonable biasing DC levels match between L and
R channels with original preset setting. Will add 8R loads and repeat
these tests and adjust bias while still at 60% mains ,interim, before
going up to 100% mains

Required 6 diodes of the SM" 2 in 1 pack BAT54S type I used, not 5, to
get the right bias zone for the preset, 100% mains and stable wiht load,
just quality of sound check to go