api 650A PSU

[pucho812]

I have 4 on a clients custom console for the +/-16V rails. 2 out of the four hum which has me thinking it needs a recap considering it's never been done before. Faulty filter caps I suspect. How do I get inside to do it.

I have removed what I think  are all the possible screws and yet no access...

 

[Walrus]

It's the screws marked in the attached pictures, then the case just pulls apart with a bit of wiggling.
The small caps. on the pcb will be the ones at fault, although I change the larger one too. The 2 big reservoir caps may need changing but not as often as the small ones!

 

[Walrus]

Second pic...

 

[Walrus]

Undo these 2 to fold out pcb and heatsink.....

 

[pucho812]

Thank you so much.  I wasn't 100% sure and I didn't want to damage it or bend any metal out of place. I could swear it looks much more difficult then that or maybe I just think so because I never  have opened one up. Your pictures make it  real clear. I owe you.

 

[Walrus]

You're very welcome.

Careful with the wires to the heatsink, as the insulation becomes very brittle over time and can flake off.
Had to completely rewire one psu that had had a burnout.....

 

[jsteiger]

Ha! I sent you the same exact pics this morning Pucho. I didn't use red arrows though, I use my big fat fingers!  ;D

Have fun with it.

 

[pucho812]

thanks Jeff for the pics. Thanks again everybody.  Jeff I owe you a beer. Hunting down hum in an old API. You guys are awesome. I'm going to have to send you guys some cool stuff. PM address's please.

 

[fazer]

Pucho  the caps on the PCB should be replaced first.  Usually those are the ones that go bad.  The Big 6800uf caps have held up in my 1977 console.  Also I will get problems with the output pass transistors (on heatsinks)  where one is bad but the others are ok.  Also As Walrus mentioned sometimes the wires get brittle and cooked.

The supply uses crowbar protection.  When they overvoltage, (cause is usually the 100uf cap on the pcb) the crowbar protection shorts the output and the supply sets there and cookes.  I don't remember loosing a fuse when this happens.  The thing gets hot and cooks the caps and wires. 

Just my 2cents. 

 

[pucho812]

well  starting big and working my way down, today I moved the psu's around there are a total of 8 X 650A's powring this 40 channel and 40 monitor beast. 4 feed the +/- 16VDC, 1 for 5V, 1 for 24VDC, 2 for lamps.
I readjusted all the supplies as per the connections in the rack mount chassis for them and fired it up. There was no change meaning not only was the hum still there, it was happening in same spot as before. According to the old techs labeling it is happening on the input side of the console. I pulled out all 40 input strips and eq's and the hum is still there. Even more interesting is how if I have nothing assigned to the 2mix it's obviously going into the monitors as well. I can adjust the monitor pot and watch the hum increase in volume as I increase the monitor level. Console passes signal and so forth just has huge hum in the back ground. I am thinking that maybe a console ground has broken somewhere as the guys who engineer at the studio had been moving some wiring around. 

 

[Walrus]

(Grandmother-egg mode) Have you measured the ripple on each supply just to make sure? I've had one in the past which worked fine as a lamp supply, but hummed badly when swapped into a 16v slot.
But earthing does sound like a possibility.

Studio 2 here has slightly more supplies......

 

[pucho812]

edited to make sense- wow nice. We got 2 racks units with 4 supplies. I will check for ripple today. But so far regardless of which 650a"s are on, I only get them hum when I switch on the input side which has me thinking elsewhere other then the PSU's


part II. console is now hum free. I labed each PSU and went letter by letter with each PSU on the +/-16. I got it hum free on the audio side but still have 2 noisy 650a's. running the lamps now.

here is a photo. Working out some bugs now. For example the monitor side only feeds the right side of the stereo buss as well as other random faults. But it's running well enough for today. Amazing how this board has been left to be such disarray. 

Thanks for all the help everybody. I love this place. 8)