SX-980 DC Offset Problem

TechGuru87

TechGuru87

New Member
Hi All, I am restoring an sx-980, recapped all boards, replaced transistors, diodes, resistors etc. from parts lists found on AK. When the unit came in the power supply was toast and offsets and idle current was way out of spec. After the restore I couldn't for the life of me get the dc offset in both channels to settle down, I adjusted the new 100K multi-turn trim pots all the way counter clockwise but still offset was in the negative, so I cut the jumpers at R5 on the amp boards just to dial in to as close to zero as possible. Got left channel to around 0mV but then would constantly jump +/- 40mV on left channel & +/- 100mV as low as -140mV on right channel. I found a mod suggested by Echowars so I went ahead and cut the trace between R21 & R22 and bridged the cut trace with a 330uF 10v cap. That calmed the dc offset way down but now sets at -0.5mV on left channel and -4.0mV on the right channel. The idle current is spot on.
No matter what I do I cannot get the right channel the same as left channel. Not sure why I can't get out of the negative offset. I know these offsets are within spec and the receiver sounds great but is the mismatch going to be a problem down the road? Any suggestions greatly appreciated. Thanks...

Here are some voltage readings

Power Supply: AWR-156
pin 16: +13.4
pin 18: +7.4
pin 20: +34.6
pin 22: +51.6
pin 23: +22.7
pin 25: -22.7
pin 27: -49.8
pin 28: -13.5

Power Amp Boards AWH-073
pin 1: -49.8
pin 2: +13.1
pin 4: +22.67
pin 5: -22.67
pin 8: +51.4
pin 12: +53.6
pin 22: -53.6


AWH-073 Power Amp Board x 2
*C1: CEANL 2.2uf/50v, 647-UFG2A2R2MDM (2.2u/100V/5mm) (Audio Grade)
*C2: CEANL 2.2uf/50v, 647-UFG2A2R2MDM (2.2u/100V/5mm) (Audio Grade)
*C11: CSSA .22uf/25v, 505-MKS2C032201B00JA WIMA (.22u/63v/5mm)
*D1,D2,D6,D7: 1S2471 = 512-1N4148
*D3,D4: 10E2 = 512-1N4004
*Q1 2SA979 dual transistor pnp bcecb 100v 50ma 0.4w 250hfe 150mhz
two gain matched 512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe
*Q2 2SC1775 = 512-KSC2383YTA (to-92 ecb 160v 1a .9w 50mhz 160-320hfe)
*Q3, Q4 2SC1885 = 512-KSC2690AYS to-126 ecb 120/a160 1.2a 1.2w/20w 175mhz 35-320hfe512
*Q5 2SA912 = KSA1220AYS to-126 ecb 120/a160 1.2a 1.2w/20w 175mhz 35-320hfe
*Q6 2SD381A - PREDIRVER Heatsinked = MJE15032G
*Q7 2SB536A - PREDRIVER Heatsinked = MJE15033G
*Q8: 2SC869 = 512-KSC2383YTA (to-92 ecb 160v 1a .9w 50mhz 160-320hfe)
*R25,R26 (Metal Film/1W,RN1H 4R7K): 594-5083NW4R700J 4.7 ohms 2 watts
*R14,R15 150 OHM 1/2 WATT 594-5083NW150R0J 150 OHM 2 WATTS
*VR1: 100ohm, 652-3296P-1-101LF 100ohm 25 turn
*VR2: 100Kohm, 652-3296P-1-104LF 100Kohm 25 turn
*two 330uF 10v caps for Echowars mod. one for each amp board
 
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Did you thermally couple the KSA992's that made up the 2SA979's? Does the offset adjustment behave when the idle is sitting at 0mv? Also, I'm not real fond of the 2383 being used as Q2. Something on the positive side is not pulling it's weight.
 
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Where does the trimmer rest after you adjusted the offset to the near zero volts? Are they centered up or slammed to the max (either direction)?
In the current condition, it's looks fine to me. +-40mV's, either side of zero volts, on the center rail is okay.
Does the relay engage?
 
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kramden said:
Did you thermally couple the KSA992's that made up the 2SA979's? Does the offset adjustment behave when the idle is sitting at 0mv? Also, I'm not real fond of the 2383 being used as Q2. Something on the positive side is not pulling it's weight.
Click to expand...
Hi Kramden, Yes, they are sandwiched together back-to-back with some thermal compound and heat shrink wrapped. Actually I am unable to get idle current to zero without unplugging the main output transistors but lowered offset very little. I couldn't find any reference to use anything other than the 2383 for Q2, what do you recommend I use? What component on the positive side could be the cause? Thanks...
 
zebulon1 said:
Where does the trimmer rest after you adjusted the offset to the near zero volts? Are they centered up or slammed to the max (either direction)?
In the current condition, it's looks fine to me. +-40mV's, either side of zero volts, on the center rail is okay.
Does the relay engage?
Click to expand...
Hi Zebulon1, the offset trim pots are slammed all the way counterclockwise and idle current trim pots almost all the way clockwise and can't drop idle to 0mV as Kramden asked me to test. This can't be right as there is no way to get it to the positive side, it acts as if the trim pots have little affect. The relay clicks on after power on. I ran some test speakers on it and cranked it to nearly 80 watts on the meters for both channels without engaging the protect circuit. The audio appears to be distorted under heavy bass notes so I don't trust this to go back to the customer yet because something if off somewhere just not sure where?
 
Connect the outputs to load resistors, input a sine wave, monitor the output with a scope and run it up till it just starts to clip. Do both channels clip at the same output? Does it make rated output? Does the positive or negative side clip first?
 
A guy in a 780 thread had the same issue, he said he changed the bias bjt (2sc1775 ->ksc1845f ) and new ecaps.
 
rcs16 said:
CEANL = Nichicon UKL or film, kemet F612
Click to expand...
So what you are saying I should replace Q2 (2sc1775) with ksc1845f, instead of the 2383, and replace C1&C2 (2.2 uf 50v ) with 1uf 100v film cap, I checked I only have 1uf 63v Wima's on hand. will that work? thanks...
 
The 63v will work much better, mainly because it will fit.
Line level signal, a few mV's is all, at that point in the circuit.
Try the 1uf if its not too much trouble. I don't think it matters that much. Only a coupling cap.
I never did it but wouldn't hesitate to try it. See if it settles things down. You can decide later if you want to keep it in circuit and add some 2.2's to your next order.
 
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zebulon1 said:
The 63v will work much better, mainly because it will fit.
Line level signal, a few mV's is all, at that point in the circuit.
Try the 1uf if its not too much trouble. I don't think it matters that much. Only a coupling cap.
I never did it but wouldn't hesitate to try it. See if it settles things down. You can decide later if you want to keep it in circuit and add some 2.2's to your next order.
Click to expand...
Thought I had 1uf wima but they are .1uf, I do have 1uf 100v UKL's though...
 
zebulon1 said:
Try'em, see what happens.
Click to expand...
I only swapped out the 2383 for 1845 and left the 2.2uf 100v UFG's because I didn't have 4 of any other caps mentioned. The good news is the 1845's fixed it, I now can dial in the dc offset to +/- .01 mV both channels and idle current still spot on and I have full use of the trim pots. So, should I still order the 1uf or 2.2uf wima's or UKL's to replace C1&C2? I guess the 2383 which I found somewhere on AK as a better sub for the 1775 but not in this case. Thanks for everyone's help...
 
Good news.
Order what you took out, probably the 2.2uf's.(?)
Pre stage components need to be fast. The voltages and current are not a big concern.
The 2383's are good when installed in power supplies and protection circuits, where the 1775's are used.
 
Good to hear c1845f fixed the issue.
C2383 & its complement A1013 are used for low power push-pulls, current sources, regulators etc.
Up to you what to want to use, for an i/p coupling cap, I recommend kemet F612 (which is similar to the old panasonic ecq-v) , if you can get one to fit, will never have to change again.
 
rcs16 said:
Good to hear c1845f fixed the issue.
C2383 & its complement A1013 are used for low power push-pulls, current sources, regulators etc.
Up to you what to want to use, for an i/p coupling cap, I recommend kemet F612 (which is similar to the old panasonic ecq-v) , if you can get one to fit, will never have to change again.
Click to expand...
I have found on AK that others have replaced the two 2.2uf 50v caps with a single 1uf 100v film cap on the AWH-073. Looks like the Kemet F612 will just fit. I'll add them to my order. All I have left to do is replace the function lamps with LED's while waiting for the caps and should be good to go. Thanks for everyone's help...
 
The only use I get out of the 2383’s and his complement are in the power supply of HRS series Sunfire subwoofers. Just seems like in this application, it is overkill.
 
Where did you come up with your transistor substitutions?

Right at post 1 when I dropped in, I said - q2's gonna give him future problems in addition to whatever else is upsetting it.

with the current unavailability of ksa1381, these are forgivable:
Q3, Q4 2sc1885
512-KSC3503dstu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.46 ea

Q5 2sa912
512-KSA1381estu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.53 ea
 
markthefixer said:
Where did you come up with your transistor substitutions?

Right at post 1 when I dropped in, I said - q2's gonna give him future problems in addition to whatever else is upsetting it.

with the current unavailability of ksa1381, these are forgivable:
Q3, Q4 2sc1885
512-KSC3503dstu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.46 ea

Q5 2sa912
512-KSA1381estu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.53 ea
Click to expand...
Hi Mark, Sorry I don't see any other post from you except this one, but Kramden said he was not fond of using 2383 for Q2, but no one said don't use the 2383 until rcs16 posted that someone else had a similar problem with a 780, even though all the voltages checked out, audio output was good I knew something was still wrong and decided to change out 2383 with 1845 for Q2 and hope for the best. That fixed the dc offset imbalance. As for where I came up with my list it was somewhere on AK but not sure exactly? Since some parts were not stocked or unavailable I used subs that were recommended but may not have been for this exact model or board number instead just the part number. I was not aware of how sensitive this circuit is and the 2383 would be a bad choice. I've done quite a few sx_50 series but not many sx_80 series which are not as forgivable for error. Is there an updated tried and true parts list including subs for out of stock parts for the 980 for everyone to reference so this problem doesn't happen to someone else? Thanks for everyone's help.
 
Actually, that was pretty good first shot for anybody.

I just had a cap list in my notes (don't know if I published it) I Wish it was more extensive (semiconductors for example) , but that's as far as I could stretch, then or now. Maybe in the future.
 
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