82E32 Cue Mix Bus card Overlay mistake?

[walrus]

I've just finished re-capping the centre section of our 4K desk, ending up with the 2 x 32 cards as the last ones. The component overlay of the card seems to be wrong, as it shows C15 and C16 (and the otherequivalents), what appears to be, the wrong way round.
As marked on the pcb, the caps negatives are together and connected to the 1MOhm R29 which goes to the POSITIVE rail, i.e. the caps are given a reversed bias!

I can't be sure now (very poor memory!) but I think the original caps were put in the wrong way too. The circuit diagram shows it should be the positives together, just like they are on all other boards in the desk that use this circuitry.

Has anybody else noticed this? Sintech?

[sintech]

Hi, here's the two 032 cards I have at hand, the one with the orange caps is from my 2 channel 4000 test console, the other a spare.

The spare has non-polarised electrolytic caps, but both show the shape.

[walrus]

If you look at the one with the orange caps, the second pair of caps from the right have their positives as per the overlay on the board. But if you turn the board over, you'll see that the cap negatives are joined together and have the 1M resistor goin to the positive.
Surely that is wrong as the caps will be reversed biased??

[sintech]

I think it's Ying-Yang.., one pair of caps is on the positive phase of the cross-coupled output, the other set on the negative.

[sintech]

The 1m's run from positive to negative on each of the three output stages. The schematic here is old.. and it looks on that, the 1m goes between + side of the caps a +20v.

[walrus]

On all of my schematics, the 1M goes to the positive rail, and all of the caps have the positives connected together as one would expect when you bias the centre point to the +20v rail. If there is some technical reason to connect the negatives of the caps together on the opposite polarity output, then surely they should be biased to the -20v rail?

[sintech]

In this G series schematic, the 1M are gone, with a 4k7 between phases and non-polarised output caps.

Looks like the orange cap E series has this arrangement also. (Resistor wise)

[walrus]

Yes, the E series card has the 4k7 resistor. But the fact remains that the caps are being reversed biased by putting the "sniff" of positive volts (as it was described on the training course 20+ years ago!) on the joined negatives!
I suppose the simplest way round this is to upgrade the cards to G+ circuitry......

[kilmister]

Obviously there's a mistake in the pcb's silk screening.

G style bypassing with bipolar caps is good choice also. In our console we had to do re-capping to all 242 eq's that uses same kinda bypassing. All the 292 with bipolars are just fine... just saying.

-Paavo

[walrus]

Obviously there's a mistake in the pcb's silk screening.

Indeed, but if you look at Andys picture of his card, with the original SSL installed caps, you'll see that they have actually installed the caps *according* to the silk screening i.e. wrong!!

[quote)
G style bypassing with bipolar caps is good choice also. In our console we had to do re-capping to all 242 eq's that uses same kinda bypassing. All the 292 with bipolars are just fine... just saying.

-Paavo
[/quote]
All the other cards in our desk I have replaced caps on a like for like basis. Well, apart from the channel VCA cards, which I have updated to the latest schematic I have.

[kilmister]

I still think it's an error. And cause silk screen markers are reversed, capacitor has gone reversed as well when pcb has been populated. If schemo is showing that bias voltage should be between positive sides of the caps I bet that's the way to go then.

-Paavo

[sintech]

Hi, doing some re-capping of 32 cards today.

Paavo, well spotted.. the lower card screen printing is wrong for C15/16 (on all three amps) and the top card correct.




The test sticker is from 1987, all other numbers on the cards are identical to the correct