1. The DC offset is definitely really there. There is a thick wire running to each output channel on MAIN(1/2) from MAIN(3), these are wires I cannot disconnect (they are crimped at both ends to either board) and joked early on about splicing spade connectors into. The offset of about -6.5v is present on all of them except the black one, which is the ground reference for the board. Schematic indicates that all of these should be 0.0v.
Are these wires the channel outputs (before the protection/muting relays)?
Yes, I believe so. They are on the schematic on page 138, between MAIN(3) and MAIN(1/2), labeled W1004(A/B), W1005(A/B), etc, and their color is denoted as well.
2. MAIN(2) has a small 3 pin connector labeled W1003 that has I_PRT_R, -LB_R, and +LB_R. These are supposed to be +55.6v, -53.8v, and +55.7v respectively. The values I get when checking are +57.4, -6.85, and -1.87.
-LB_R,+LB_R (& -LB_L,+LB_L) are power rails to the power amp driver & as noted, they are completely out of spec e.g. -6.85V in lieu of -53.8V, -1.87V in lieu of +55.7V.
With the driver having such out of wack power supply rails, the channel power amplifiers won't be working in any sort of linear operating region.
The DC offset of -6.5V on the channel outputs (assumed from above) is more likely fortuitous voltage getting to the base of the output darlington transistors (which do have the proper supply voltages on their collectors).
-LB_R,+LB_R (& -LB_L,+LB_L) come from the +LB, -LB regulators Q1009, Q1010, Q1096 etc on Main(3). Do you now have better access to this area for a revisit/probing?
Q1009 is pretty easy to get to, 1096 & 1097 are more of a challenge, but I was able to get most of the readings. Enough I think. MAIN(3) is still mounted to the chassis, taking it out requires removing VIDEO(4) because with it in place, MAIN(3) cannot be slid inward far enough for all the output banana jacks to clear the chassis. VIDEO(4) is one of the power supply boards with a direct connection to the transformer, so positioning it while keeping it plugged in to everything will be tricky.
On pages 116 you can see what I have access to.
If so, could you do a voltage measurement of Q1096's collector? Could you also measure the voltage on Q1009's collector? (see attachment)
I took measurements of 8 of the 9 pins for Q1009, Q1096, and Q1097. Two of them are hardwired together (1096'&1097's emitters, 1009's emitter to 1097's collector) so I treated those as a single measurement. The traces are pretty fat and look undamaged, but I'll recheck these individually where possible if we have a reason to do so.
I've been harping on a bit (sorry) but there's something really fishy going on i.e. these regulators are turned off/faulty for some reason.
By all means, harp on it. Rechecking and rerechecking is prudent given my inexperience and potential for mistakes. Chassis ground was no good for these checks the last time, so rechecking was a requirement anyway. I did check them all and the results are below, but I think I need to retest. In the current configuration, AMP_LMT is floating because that board isn't connected. The schematic indicates there should be 3.1v present there, so perhaps I should retest with that pin tied to the makeshift 3.3v rail I'm using to control the power supply relay? I think many of these readings may be suspect because of this, and Q1096's gate doesn't have a pulldown or pullup on it to prevent it from floating in this situation.
| Base | Emitter | Collector |
Q1009 | 57.6v | -1.92v | 57.6v |
Q1096 | -0.25v | -0.13v | 57.6v |
Q1097 | --- | -0.13v | -1.92v |
EDIT:I went ahead and retested as mentioned above, with AMP_LMT tied to the 3.3v rail, and the results are more sane. Kind of embarrased neither of us thought of this the last time, oops!
| Base | Emitter | Collector |
Q1009 | 57.5v | 0.0v | 59.95v |
Q1096 | 3.3v* | 1.9v | 57.0v |
I did not check Q1097 because I can't easily access the base signal anywhere, and the other two are tied directly to already tested pins on the other two.
This seems almost normal. With the AMP_LMT signal addressed I am going to revisit checking the individual channels on the amplifier boards.
EDIT2:So the inputs to the transistors in the amplifiers are floating as well. W1042 and W1043 have signals for each channel coming from the disconnected FUNCTION(1) board which feed into the transistor gates on MAIN(1) and MAIN(2). I am going to tie these to the shared ground from MAIN(3) before testing the voltages, as the schematic indicates they should all be at 0v.