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Old 20th February 2006, 02:53 AM   #1
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Default recapping

I'd like to do a little repair on a late seventies Altec 710 receiver that I picked up used not very long ago. It's been in use for four months or so, and I've gotten very attached to its sound. It's in mint condition and really doesn't show any need, visually, of attention inside or out.

The right channel started hissing, scratching, and popping recently.
It starts out mild and builds in about five minutes to threatening levels. Starts acting up again within five seconds of restart.

I poked around inside with a sharp nylon tipped prod while running to see if nudging any components would affect anything. Went through each board to no avail. Swapped output transistors - still right channel. Swapped power capacitors - still right channel. Monkeyed around with all the switches and knobs while listening - no luck. No bad solder points that I can observe. I snugged down all of the wire-wraps as well (shouldn't all these just be soldered to be on the safe side? About 1/3 of them already are.)

Any Ideas? I suppose I might as well start with some degree of recapping. I'm new to the idea. Is it simply a matter of telling some place like Digi-key or Mouser what values I need and waiting by the mailbox? Or is it more troublesome than that with a "vintage" amp like this. I'd be fine with upgrading to newer equivalents if possible.

Thanks for any help!
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Old 20th February 2006, 12:54 PM   #2
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If it takes several minutes to start acting up it is probably a thermal time constant at work. When you restart the heat hasn't had time to dissipate so it makes the noise again immediately.

Start cold and check the heatsink temperature and electrolytic cap temperatures every 30 seconds or so for several minutes. Bad caps will often run hot and the can will show signs of swelling. Extra hot semiconductors will make noise. Check the temperature of each device on the heatsink.

Also check for DC on the output of the amp- don't use you favorite speakers for this- connect a resistor to the amp output and check. Check it cold and check again after the amp has heated up.

The idea about soldering wire wrapped connections is a good one. In fact, many old equipment problems can be solved by resoldering connections. Tracking down bad solder joints in old equipment can be very time consuming. Wholesale resoldering is often a more productive.

Replacing caps is as easy as ordering them and waiting for the package to arrive. Remove the old ones one at a time and replace with the new ones as you go. New electrolytic caps are almost always smaller than old ones so you shouldn't have any trouble fitting them to the PCB. Order a couple extra of each value in case you mess one up. They are cheap.

I_F
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Old 20th February 2006, 01:23 PM   #3
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Thank you I_forgot

... it's a shame Abbott and Costello neglected to mention your play on the field that day.

I did notice the size difference in caps. I looked em up at partsexpress to see if it was something I could just add to my next order, but when I saw the physical dimensions of the part that matched my values I was certain it had to be a different animal. Will that 1/3 size little blue cap really replace the larger black bombs that are on the boards now?

Is there a good, better, best range I should be aware of? They're not too expensive, so I might as well opt for the "better".

How is wholesale resoldering accomplished? Or do you mean to say, just go start to finish and do it all at once instead of checking for progress each time? When you mentioned it, it occured to me that one might (carefully) hot-air gun the soldered side of the pcb and reflow everything at once. I've got a competent Milwaukee hot air gun.
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Old 20th February 2006, 07:26 PM   #4
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I'd use a soldering iron instead of a hot air gun, and just move from one joint to the next, one PCB at a time. Hot air guns will tend to toast the board, melt wire insulation, and maybe cook some parts before the solder reflows.

Yes, newer electrolytic caps are quite a bit smaller than those made 30 years ago. As long as the voltage rating is the same or higher, your new caps will be OK. Be careful about polarity.

If you want to spend more money, you can buy 105C rated caps. You might also look for low ESR types used in switching power supplies. They typically cost just a little more, but its hard to say if you'll hear any difference in the end. On the other hand, the cost difference is only a few cents each, so you won't be blowing a big pile of money if there isn't any audible difference.

When you finish the work, put a note inside the cabinet of the unit somewhere indicating the date and work that you did (like replace caps, resolder, etc.). When someone else gets the amp in 10 years they will know that the caps are probably OK.

I_F
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