10.79V on the +18 rail on section '20 and '21 - SOLVED -

[Franz]

Hi guys, I need your help. I own all the extender cards for all the cards in the central section and today suddenly the keys "flip", "vca to mon", "record", "replay", "mix", "cue" "omni" are dead. I tested everything on the cards and everything seems to be ok except for a strange fact: on the +18 rail on the back I detected a voltage of 10.79V on '20 and '21, from '22 on it is perfectly 18.50. This has never happened to me before. Could you help me? Thank you all!

Photo in attachment.

[amillar]

So IIRC this rails are big thick busses on the 651 PCB? In which case it has to be a tiny crack in the track between the 21 and 22 card - probably at one of the connectors. Soldering a piece of wire between these two connectors  along where the PCB track runs will fix that, make sure to superglue it to the PCB so it doesn't risk getting ripped off when the 651 is hoisted up and down.

But double and triple check before doing so (I haven't got a 651 PCB to look at) - as I say, IIRC  it's very obvious where these tracks run, and if the voltmeter reads one voltage at one point on the track and another voltage on another there must be crack there (OR the voltmeter isn't making a proper connection!)

Cracked PCB tracks are very common on all old electronics, and can confuse you for hours...

Good luck,

Andy

[Franz]

Hi amillar, first of all thanks for the quick response! You are an invaluable resource. As far as the voltmeter is concerned, I have tried it thoroughly several times and unfortunately the problem is there. Before doing this I changed many ICs like T4 and some BC212LB like TR7 on '21 card. I just have to try to connect the two tracks with a cable as you suggest. I'll let you know in a few hours. Thank you very much.

[Franz]

Hey amillar, you were right! I made a bridge and everything is working again. Thanks for the tip. What I'm wondering is how this could happen out of the blue. At first glance the track looked OK...

[amillar]

Oh, it just does! Anyone who's worked on (PCB based) valve gear knows this one very well: heat's the most common thing to just keep gently moving the PCB until eventually the copper cracks, but it can happen to any PCB that gets flexed over time. And the thicker the track the less flexible it is so more likely to crack. I'm guessing this desk was built in about 1986? So 34 years isn't doing too badly!

I might be tempted to strengthen the other power rails around there, if that track's been strained around there then probably the other busses will be. (That's not based on experience of old 651s particularly, just my general years of mucking around trying to get old electronics going...)

Really glad that's fixed it,

Cheers,

Andy

[Franz]

If it weren't for you I'd still be changing good ICs and components and tearing out my hair...