Spec 2 Troubles

laatsch55

laatsch55

Super Member
My bought brand new in 1975 Spec 2 has gone into protection mode and will not come out, even cycling on/off switch.
I've searched a lot of threads the last couple days to make sure I wasn't wasting anyone's time.
Protection light on, right channel reads 10 watts on meter, DC offset a week ago was 82mv rt channel, 8mv left. I understand its a changeout of the 2SA798 Trans, got some coming. Also read Echowar's "replacing the relay" checked the coil on said relay and it is indeed open.
Is there anything I'm missing here, Also while Im in there what else should
be changed ? Thank You Gentlemen for any and all advice.
 
Welcome to AK laatsch55. I am sure you will get a reply to your problem soon. Good luck.
 
Yes, Echowars I did find that, among others. Having owned this amp since buying it new I believe I'll recap all boards while its down, P.S. , Meter Amp and poweramp. The TO-220's onthis board test ok but do show some signs of heat stress.The TO-220 drivers will got swapped out too. The pots do not respond to cleaning and did read markthefixers thread on swapping with Bourns 12 turns #3266 (sealed) Anything Im overlooking here ?
I heeded the warning about loosening up the STV-4H hold down screws and after removing, took outputs and sockets off heatsink to keep stress off that 32year old wiring.If you think of anything else I'd appreciate you letting me know.
I may have a few questions when its time to fire it (poor choice of word) up! Thanks Gentlemen
 
Single-turn devices (Bourns 3386H-Series) are preferred for bias and current limiting adjustment on the Spec 2, mainly because the bias adjust range is limited anyway, and you can get more than enough resolution with a single-turn pot. 12 or 25-turn pots are overkill, and simply make adjustment a chore rather than the simple affair that it should be. 500-ohm bias pot # would be 3386H-501LF-ND, and the 100-ohm current limit adjust would be 3386H-101LF-ND (Digikey numbers). The bias on the Spec 2 is quite dependant on the heating of the pre-driver transistors, so expect to monitor the bias for an hour while waiting for it to stabilize.

It's nice to replace the current limit pots too, but if you do so the amp will have to be re-adjusted. This requires a 4-ohm dummy load capable of a minimum of 200W, a scope, and a signal generator. And someone who can follow the SM instructions.
 
Thanks Echowars, had not ordered the pots yet. I was going to replace all semiconductors on the board. 32 years is a pretty good service life and i have most of the stuff on hand.
I do have a dual trace scope, audio frequency generator and the service manual.
Can the dummy load be some 270 series Ohmite 100 watt wirewounds in series, or is that to resistive?
 
Yes they are two ohms, got a couple dozen of the darn things laying around from a large map printer I junked out.
When its time to set the current limiters should the gain on the amp be set to max? Can't seem to find that in the SM, nor the driving frequency but did find that in another thread 1Khz correct?
 
laatsch55 said:
When its time to set the current limiters should the gain on the amp be set to max?
Click to expand...
The gain will be whatever it needs to be to do what the manual requires. You'll likely start with the gain all the way down, and increase it till you see the circuit beginning to clip the signal.
Can't seem to find that in the SM, nor the driving frequency but did find that in another thread 1Khz correct?
Click to expand...
1KHz is fine.

Neglected to mention that you need a true-RMS meter to read the output of the amp, in addition to the scope, so that you can determine the point where the current limiter is beginning to take effect with a reasonable degree of accuracy.

Might also mention that the amp is being pushed pretty hard when you are making this adjustment, and will get blazing hot in no time at all. Don't dawdle...get it done and get out, then let it cool for a few minutes and do the other channel.
 
True RMS as in a Simpson 260 or Fluke 77?
Appreciate the warning on the heat, I do have a tendency to "dwadle" especially when its extremely interesting !

RMS--- Root-Mean-Square.

Thanks Echowars!
 
Simpson? Analog? Not a chance...

Find a proper DMM. I think the 77's are averaging, not RMS. I think you have to go to the 80-series or 170-series to get true RMS. I just bought a new Fluke 179, and it is a great meter, but too spendy for most AK members, who prefer to roll the dice with a $6.99 Harbor Freight POS. :rolleyes:

If funds are an issue, consider that you'll be measuring a sine wave, so even an averaging meter would probably do the job. But try not to cheap-out here if you can. The work you do is only as good as the tools you use.
 
Geez, I don't think any of my meters are true RMS.:tears:
I think (smack if I'm wrong EchoWars) that you'll be OK with the Fluke 77.
That's my favorite everyday meter, if I need to monitor two channels at once I break out the Fluke 83, and for dead nuts accuracy I have a Sinclair DM450.
 
Money is not an issue when it comes to this stuff. I do a lot of oilfield electrical work, No problem upgrading, once again Thanks.
 
I've thought for quite awhileThe '77 was true RMS. Just dug out the manual, averaging it is.
Hey Echowars, when you're wrong is it a small thing that is hardly noticed or is it FRONT page material?
 
Happens all the time...I just keep it quiet.:smoke:

But yeah, when it hits the news here, seems to be Page 1 material.:whip:
 
EW... Mistake ?

We just can't find anyone who is old enough to remember when that was :D

John
 
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Had a go at the relay direct swapout. It was close to begin with but had to spend 20 minutes reconfiguring one end of the relay to get the pins where they needed to go.
Yes the relay cover says 115v AC. This was the sacrificial lamb and just used to see if it was possible.
 
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