pioneer sx-1280 acp-63 (vr1) adjustment

karel.b

Active Member
Hi all,
I've tried to adjust dc balance and idle current on my sx-1280 because one channel was little louder then the other. To adjust the dc balance with the existing vr1 (both channels) is almost impossible. Idle current (vr2), not so bad. It is working OK now, BUT:
I'd like to replace them with Bourns trimmer resistors from Mouser, but I do not know the right P/N. (vr1, acp-063=150k; vr2, acp-032=100 ohms)
If someone has the right Mouser P/N for the two of them, I'd be very grateful for sharing the info with me.
Thanks
 
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sx-1280 awh-074L vr1 150k 25 turns offset adjust 652-3296P-1-204LF
sx-1280 awh-074L vr2 100 ohms 1 turn idle current 652-3386H-1-101LF
 
I've tried to adjust dc balance and idle current on my sx-1280 because one channel was little louder the(a)n the the other.
Hard to believe that this issue is due to what is probably a minor "DC" offset. The gain should not change due to "DC" offset, "AC" gain is fixed by feedback action. What were the #'s. I suspect it is something else.
The only way to track down or verify a issue like this, is to first identify, at what part of the gain stages it is coming from. Other than that, you are taking a wild guess.
You need to do some testing with instruments, a audio signal generator connected ("Y") to both L/R aux inputs (use 50mV/1KHz). Then using a good AC voltmeter, measure the output of the pre-amp first. Pull the pre-power jumpers (powered off of course) and measure each L/R with volume pot set to max. Gain is supposed to be 1/0.15 = 6.667(16.5dB), so your o/p should be around 50mV*6.667 = 0.333Vrms. At this point you can also engage the tone control ckt to see if is altering the gain and perform many other tests such as THD, frequency response(s), volume control tracking, etc.
If the pre-amp section is fine, load the power amp with 8-16 ohms loads(100-200W), put pre-power jumpers back in and measure the gain of the power amp next. It should have a gain of 40x, so your ACVoltmeter should measure 0.333*40=13.31Vrms.
Not to say that changing those old pots and re-adjusting them, is not a bad thing to do, but I do not think it is the reason that you have as the cause for the issue at hand.

Good Luck
Rick
 
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I DIDN'T MISS the first line....

Ac voltmeters that do this (0-1v accurately) are not as common as I'd like. So I was not going to discuss the situation until after he had the amp tuned up.

THEN I was going to ask him about the cleaning and re-capping status.

THEN I was going to advise:
1. a speaker channel swap at the back panel
2. a pre-out/amp in channel swap to see if the hot channel moved (eliminates or proves problem in power amp )
3. flipping the stereo / mono switch to mono to exonerate or prove a preamp problem

and so forth moving around in the system.
 
Hi Mark,
Yes, I had a problem with the tone switch, sometimes it would cut the right channel off and other time it would be less audible, when in off position. After taking the switch right out and cleaning it, it works fine. My mistake not to mention it in the first place !!!!!!!!! then the ajustments. My apology.
 
Ac voltmeters that do this (0-1v accurately) are not as common as I'd like.
Sure they are and have been for a long while now. I use a old HP3478A, it is very accurate at everything it does. 300mV(100-20KHz) is spec'd as 0.29% plus 163 counts on a 5-1/2 digit meter. It measures up to 100KHz in the 300mV range and 300KHz in the 30V range. Not too bad for something designed in 1981.
IIRC, I bought mine for around $50 which is a steal considering what they cost new in 1980's.
 
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Hi Mark,
I ordered the VR1 and VR2 for my SX-1280 from Mouser, but the VR1 (652-3296P-1-204LF) spec. shows 200k, not 150k. Is it OK?
Thanks
 
I'm not MTF (I wish was yesterday afternoon!) but the 200k will work just fine! Even better! The offset VR works in a small range to balance the zero volts at the center. A little more isn't going to hurt.
 
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100K works fine as long as the input pairs are reasonably matched, and makes adjustment a bit easier than it would be as 200K.

As far as cranking any volume knob to max, careful wit dat chit. Small interruption and you space it off and next thing you know your amp or speaker or both are a smoking ruin. Kind of a waste of time anyway, since on pretty much any old Pioneer it's a switch issue (and occasionally the amp feedback resistors are out of spec, if the issue is a mild one).
 
I've already ordered the 652-3296P-1-204LF (200k) as Mark recommended. If it is only a matter of easier adjustment, then it should be OK.
Thanks
 
Finally I found some time to replace the VR1 and VR2 on the amp board (AWH-074) in my sx-1280. It is so much easier to adjust the new ones, especially the VR1.
I told my daughter to bring hers to me, so I can do the same operation on that one; (since she was coming to my place anyway). It is pretty easy to replace the VR1 and VR2, just a few screws and the board is accesible. But to my surprise, the VR1 in this receiver is different value!!!!! In mine is 150k, in hers is 47k (measuring 52k). It seems to be an original part, not a replacement. The VR2 is the same, 100 Ohms.
Would anyone know what why the differece? The schematic diagram shows 150k.
I'm little confused and I'd like to know if 150k would be OK to install in the circuit.
Thanks in advance for any info.
 
I have 200K trimpots, as per MTF recommendation.
I should have made myself clear as what the question was. Woud Pioneer use a different values of the trimpots?(150K in one, 47K in another). And is it OK to use 200K, when the original is 47K?
Sorry for the confusion and thanks in advance for any input.
 
It is a variable resistor, which can be turned down to the appropriate resistance needed.

It is rather hard to turn a resistor UP past it's maximum value.

Yes, it is OK to use it.
 
OK, I installed the 200K's, they work perfectly!!! The receiver works perfectly!!
Thank you Mark, again.
 
47k vs. 150K

I have seen this difference also. I just rebuilt 2 1280s and one had 150K v1 and the other had 47 ish K.
I stayed with the 47K variable resistor, and found that one of the Darlington pair was causing the inability to adjust the DC offset to zero volts.
I have asked the question before about the different values used in the V1 position, but did not get an answer.
Might have been easier to replace V1 with 200K as it may have given enough adjustment.
Fun stuff this is huh? I love it!
Glad to hear you got your 1280 up and running. They are a fine machine.:thmbsp:
Regards,
J.T.
 
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