Sound degradation problem on the right side! - SOLVED -

[Franz]

When I turn the console on (I have an Atomic Instrument supply) the sound is heard well from both monitors. After 5 minutes, the sound on the right begins to degrade and decrease in volume until it almost completely disappears. Has this happened to any of you? Which center section card should be checked? Opamp? Some problems with the LF347N? Please help!

[Matt Sartori]

I'd say check where you signal degrades (or dies) first:

1-pre or post VCA Insert? 82E26 card

if you still get it post then

2-look at 82E25 card

[Franz]

Pre is OK, Post is OK.

Maybe the problem is on the monitor out?

[Chris Simon]

Perhaps. You'll have to trace the signal through the '25, '24 and '22 cards to find the fault. Does the RF quad meter indicate signal?

[Franz]

Yes, the RF quad meter works. I tried with a bantam - xlr cable from "PRE VCA INS" on patchbay to an external speaker and it works, then I also tried on "POST VCA INS" and it works perfectly. I tried then on the same patch row "RF" and the signal degrades... How can I pick up the signal from the '25, '24 and '22 cards to find the fault?

Thanks.

[Chris Simon]

That seems to point to the '25 card being the issue. Do you have the extension cards that let you run the master section cards outside the raised master section? If so, I'd put the '25 card on the jig and look for problems.

[Chris Simon]

On closer examination, if the RF meter works but RF audio doesn't, then the first place to look would be '24 card.

[Franz]

Unfortunately I only have the extender card for channels... Just for info I have done the complete recapping of the entire center section, I just finished. (Obviously the problem was already there before and unfortunately it didn't go away with the recapping). What could I test? Some LF347? Some 5534? Damn...

[Chris Simon]

Do you have an oscilloscope?  You could probe points on the 651 motherboard when the centre section is raised.

[Franz]

Yes! Which points I've to test and how? Thanks!

[Chris Simon]

Alright then. Patch the board oscillator into a channel. Pan it up the centre and bring up the level so that the quad meters indicate 0VU. Keep the monitor level low. With the centre section raised, start probing points with the scope. Probe pin 28 of the LF '26 card to get a good display on the scope, and then move to pin 28 of the RF '26 card. If good, then your RF '26 card is good. Next move to the '25 card, pins 53 (LF) and 50 (RF). Still good? Move to pins 32,33 (LF) and 40,41(RF). If your right Main Output patch point signal is bad, as you indicated before, then the pin 40/41 signal will look bad on the scope, and your fault will be on the '25 card.

[Franz]

So kind of you, thanks! I'll let you know

[Franz]

All the points you indicated work perfectly, I found that the signal does not reach pin 17 of the card '22... what should I check? Opamp? BC212LB? (capacitors are new), J112, J175 transistors?

[Chris Simon]

Is the signal present on pin 6 of the '22 card? That's the input R of the Mini LS system. Probably not, so work backwards from there. Pin 3 on the '24 card, is that O.K.? If it's not there, the fault could be in the '24 card.  You know if the signal gets to the RF Quad meter, the signal gets into the '24 card O.K. (on pin 24), but if it doesn't come out, the fault's on that card. Lot's of things can go wrong with that card, J112's, quad op amps, and all the logic.

[Franz]

Ok, tomorrow (I'm in Italy and now it's 9:30 PM) I'll start to work backwards. Maybe it's the '24 card... Do you think the "logic" or "analogue" section? Anyway tomorrow I will try the points you have indicated to me. Thanks again.