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Spec 4 with Problems

miklemke

miklemke

AK Subscriber
Subscriber
I just bought a Spec 4 that has issues.

The unit has been worked on in the past. Several resistors on the right amp board were replaced with higher wattage versions.

When the unit does power up, after a few moments, the left channel wattage meter jumps violently a few times and then the protection relay kicks out. The right channel pops thru the speaker. All this with nothing connected to the inputs.

I have +73v on pin 1 and -73 v on pin 3 of the AWR 139 power assembly
I have +73v on pin 1 and -73 v on pin 3 of the AWR 140 power assembly

My fuse assembly is AWR 144, not 143 as in the service manual. It is laid out differently and the pin numbers don't coincide. I have + 90v on pin 1 and -90v on pin 18.

I discovered that the wires for pins 21 and 20 on the right channel were reversed and corrected that. I wonder what damage that caused.

Where do I start?

Thanks
 
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I had a Spec-4 do something like that in the past.
It was caused by a defective 2SA798 5-pin dual-transistor, Pos. Q1.
Weirdest thing happened when I ordered a replacement; I received a transistor marked A798 as it should be, but it was a 3-pin single PNP transistor :scratch2: After some arguing I finally got the right one and the SPEC was fine.
I would also do a re-cap on the input amp's, i.e. replace C1/2/3/4 on both AWH-066's.

rgds,

/tri-comp
 
I corrected the wire terminations for pins 20 & 21, cleaned the connection pins on the amp boards. I powered it up and after about a minute the relay kicked in (should have been a few seconds). I set the DC balance and Idle Current on both channels, right dialed-in better. The transient spikes in the left channel were gone, and it worked for about 10 minutes in total. Both channels sounded OK, but then it went into protection mode.

I do feel better knowing the outputs are good and apparently the amp boards are OK.

Any ideas of next steps.

Thanks
 
Sounds like a relay driver issue or a bad relay timer capacitor..Or both
 
avionic said:
Sounds like a relay driver issue or a bad relay timer capacitor..Or both
Click to expand...

Seconded, you might want to check with EchoWars whether or not the zetex 1056a is a good replacement for the Q13 2sc1384 protection relay(s) driver.

C1 and C2 are the caps of primary interest. C3 and C4 could also be replaced just in case.
 
Will do.

The problem with the wattage meter spiking is back. For the most part, it's the meter on the left channel spiking, with no effect on the sound. It goes off the scale at random, with or without an input.
 
Probably a transistor in the front end of the amp, or maybe a leaky cap (not as likely).

Freeze mist may narrow things down a bit.
 
I switched the left and right amp assemblies. The left wattage meter still spikes. I'm now assuming it's something on the meter amp board that's causing the problem???

Any ideas on where to look next???

Thanks
 
miklemke said:
I switched the left and right amp assemblies. The left wattage meter still spikes. I'm now assuming it's something on the meter amp board that's causing the problem???

Any ideas on where to look next???

Thanks
Click to expand...

The meter amp is a sophisticated beast using a ta7136 and FIVE 2sc945 transistors per channel.

I'd use :
512-KSC1845EBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea
for the 2sc945's

So it sounds like one is misbehaving, I'd try freeze mist to find it.
 
I replaced Q13 and all the electrolytic caps on the meter amp board. I discovered a couple of minor differences between the schematic and actual board, mybe due to the multi voltage issue or a revision.

The relays now seem to work properly, first the left channel in about 5 sec followed by the right channel about 1 sec later, but now the left wattage meter immediately pegs on the high side and stays there. I haven't left it on long for fear of burning-out the winding on the wattage meter.

I may just replace the transistors you mentioned. I'm assuming those are mouser part numbers.

Thanks for your help.
 
Q16 was bad. I replaced it with the recommended substitute and the meter issue was solved, no more spiking.

The unit played fine for about 20 min, then the protection relay kicked out for about 5 sec and went back on and the unit played for another few minutes and the relay kicked out again. I turned the unit off and back on an the unit played for a few more minutes and the relay again kicked out. I turned it off for a few hours and tried again. This time it played for about 30min and then there was a soft pop from the speakers and the relay kicked out and it won't reengage now.

I checked Q13 and it's fine. What next????
 
markthefixer said:
Probably a transistor in the front end of the amp, or maybe a leaky cap (not as likely).

Freeze mist may narrow things down a bit.
Click to expand...

Not to sound like the iddiot that I am, but can you explain this to me.

I bought a can of freeze it stuff on sale at MCM electronics for a buck. One whole dollar, and I've been waiting for a reason to use it.

What would "freezing" a part in his case do. Atvantagously anyways.

I wants to know.

Thanks!
 
Transistors change characteristics based on thermal events, so shocking one with a cold shot can sometimes narrow things down when you're got an oddball intermittent problem.
 
miklemke said:
Q16 was bad. I replaced it with the recommended substitute and the meter issue was solved, no more spiking.

The unit played fine for about 20 min, then the protection relay kicked out for about 5 sec and went back on and the unit played for another few minutes and the relay kicked out again. I turned the unit off and back on an the unit played for a few more minutes and the relay again kicked out. I turned it off for a few hours and tried again. This time it played for about 30min and then there was a soft pop from the speakers and the relay kicked out and it won't reengage now.

I checked Q13 and it's fine. What next????
Click to expand...

Basically start from scratch, check the power supplies for correct levels and stability, then check the protection's inputs from the amplifier outputs for a too-high dc offset. Sounds like something else went wrong, probably with an amp - judging by the pop.
 
Power tested OK at 72.7v on all four legs.

DC balance was definitely a problem. Although I had previously set both channels to near 0, the left channel was at 900mv. I readjusted and again measured after the unit was on for about 10min. The voltage drifted between +20mv and -16mv. That's the best I could get it. The right channel was better, but still wouldn't hold 0.

I do have some Bourne trim pots that I'll install at VR1. Any other suggestions as to settling the amps down?

Thanks.
 
After adjusting the DC balance down as mentioned in the previous post, the unit worked for about 2 hrs. I checked the DC balance at various points and it was erratic, drifted up to 400mv. At a later point the protection relay tripped and has not reengaged even after being off overnite.

This is the right channel, the left seems OK.
 
Check the DC offset of the right channel right at the output pin on the board. It's possible the front end transistor is bad, also could be a driver transistor that has stopped working like a transistor. It may test ok with a meter, but have no current gain. I would also recommend replacing the speaker relay drive transistor. The gain of the transistor is critical and if it falls off due to age the transistor will start to heat up and then fail. I used 2SC13840S to replace it, available at Digikey.
 
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