Idle Current Question: Toshiba SA-7100

[motorcityguy]

Ok, I'm stumped.... Service manual for my Toshiba SA-7100 talks about taking measurements at FOUR pairs of points, but offers only TWO locations for adjustment. See photo, attached, of the instructions.

It also doesn't call out the test conditions (presumably speakers off, volume down, and warmed up for 5-10 minutes?), nor does it clarify which adjusting pot is for which measurement.

Before I start tinkering, I thought I'd see if anyone could offer some additional clarification. Thanks!

 

[mbz]

The amp has 4 power transistors and 4 emitter resistors per channel.
The "extra" measurements are across each emitter resistor pair.

The MM measurement points are,
1 & 3 adjust R535 for 8.8 -> 22mV
then
2 & 4 adjust R535 for 8.8 -> 22mV

5 & 7 adjust R536 for 8.8 -> 22mV
then
6 & 8 adjust R536 for 8.8 -> 22mV

I would use minigrabbers and a connection block.
Wouldn't use a dummy load, volume at minimum, allow 5 min warmup.

 

[motorcityguy]

Ok, I get it... So, one adjustment for two measurements.

Hopefully each pair of power transistors is performing similarly enough that a single adjustment allows BOTH to fall within the spec. I'll give it a shot, and report back. THANKS!

 

[avionic]

Hopefully each pair of power transistors is performing similarly enough that a single adjustment allows BOTH to fall within the spec.

I don't think you will have any trouble with that spread.

8.8 -> 22mV

 

[motorcityguy]

The amp has 4 power transistors and 4 emitter resistors per channel.
The "extra" measurements are across each emitter resistor pair.

The MM measurement points are,
1 & 3 adjust R535 for 8.8 -> 22mV
then
2 & 4 adjust R535 for 8.8 -> 22mV

5 & 7 adjust R536 for 8.8 -> 22mV
then
6 & 8 adjust R536 for 8.8 -> 22mV

I would use minigrabbers and a connection block.
Wouldn't use a dummy load, volume at minimum, allow 5 min warmup.

Here's the initial measurements (no adjustments made). Good news is that both pairs of measurements are very close, and are within (or close to) the specs.

1 and 3: 9.0mV
2 and 4: 8.4mV

5 and 7: 9.4mV
6 and 8: 11.2mV

So, new question... When I adjust, should I adjust to min, max, or median values of the 8.8 - 22mV spec?

 

[mbz]

I don't know the amp but I would adjust for median, say 15mV, I'm guessing the engineers would have said 15mV +/-7mV
The bias will change (drop) slightly as the amp heats up, maybe you could go a little higher, 15-20mV?

 

[motorcityguy]

Ok, with a tiny bit of tweaking, I was able to set them very close to the mid-point of the spec (8.8mV - 22mV):

1 and 3: 15.1mV
2 and 4: 14.0mV

5 and 7: 13.9mV
6 and 8: 16.1mV

DC offset was non-adjustable, but measured pretty decent at: 4.2mV (R channel) and 27.5mV (L channel).