SPEC 4 Cuts out after warmup

[rudderless]

I have an exceptionally clean SPEC 4 which, after playing for 2 - 4 hours at low to moderate volume sounding great, distorts in a very low pitch, like a white noise, and then both channels cut out simultaneously. Turned off, and then back on after a day or so, it performs again as described. Last year I replaced the protection circuit relay and the capacitor which attaches to it. Otherwise it is untouched. My thinking is that the amp boards themselves are fine as the sound quality is great and the fact that the problem is not isolated to a single channel, but that something must be going on in the supply side. Does that sound right? Any ideas where to look before delving in?

Thanks in advance for any help tendered.

Jim

 

[xlbee]

My gut says you have a cap failing... or one of the thermal bias diode packs has a broken connection so that the bias increases to the point it goes into protection when it heats up (although I would think that would happen a lot quicker than 2-4 hrs). Don't think it's a bad protection circuit transistor as that wouldn't cause the distortion. What electronics skills/experience and tools do you have?

 

[rcs16]

Exactly, when it fails, get in there and start to measure the power supply voltages with a DMM. If they are okay, then move onto the power amp pcb.
Either determine the source or start re-building it starting with the power supply.

 

[rudderless]

Thanks xlbee and rcs16 for your quick replies. Yes, it's true, the intermittent cutting out was occuring in a much shorter time period, but when I put it on the bench I wasn't able to duplicate it running at lower volumes. Only after reinstalling it in my rack and putting it into regular service did the problem reoccur. If I can duplicate the failure, checking the voltages is within my skill level. Absent a failure on the bench, I definitely can test the components individually. I grew up in an electronics household (Dad was a lifelong Ham operator, he built radios and other equipment) and I took vocational electronics almost 40 years ago, as well as harvested parts from various electronics (TVs, radios, etc) for my Dad. That being said, my practical application of what knowledge I do have is lacking. I have a grasp of the theory behind what's going on but have limited experience as I had a break of 30+ years with little electronics work. I guess that's due to working primarily on my own equipment and I have been blessed with reliability. I have a good DMM, a decent scope, signal tracer, signal generator, etc. And my soldering skills are fairly proficient.

I'll get it back on the bench and try pushing it a little harder than last time in an effort to induce failure and proceed as recommended. If that doesn't get me there, I will move on to the power supply assembly and go through it component by component. Does that sound like a reasonable course of action to you?

Thank you again. You are very kind and generous with your advice.

 

[markthefixer]

GENTLY warm various areas with a hair dryer while on the bench.

NOT GENTLE will induce spurious failures and damage elsewhere.

check:
(use red dmm probe with all but the tiniest portion of the tip insulated with tape
and connect the black dmm probe to a bare metal chassis ground)
awm-113 pins 2,3 for +68v DC regulated
awm-113 pins 18,19 for -68v DC regulated

when in protect check the pins 11 12 and 13 voltages on the AWM-113 meter card
recheck:
awm-113 pins 2,3 for +68v DC regulated
awm-113 pins 18,19 for -68v DC regulated
then check:
awm-113 pin 11 should be about 20 volts dc
awm-113 pin 12 should be 0.000v dc (this is the amplifier output for the right channel)
awm-113 pin 13 should be 0.000v dc (this is the amplifier output for the left channel)

The meter board doesn't heat up that much, so I expect one of the amp boards has the fault,
being that they are next to the power output transistor heat sinks.
Of course following Murphy's law, wherever it is, it will be a PITA to get to.