Cant set dc balance on gwh-109

supereagle

Active Member
Ok unit started a low hum sx-1980,rebuilt power supply completely also used echowars mods and moved heat causing trans to giant heatsinks .Rebuilt gwh-109 and gwh-110 ,gwh-109 has the strange black and yellow small round thermisters .Hooked to dbt went dim set gwh-110 ,gwh-109 left channel 3.5v dc offset idle current is 1.0mv ,cannot adjust dc offset below 3.4v .Any ideas where to start or where to check? Checked pins on power supply all normal .
 
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pin 17=76.5v pin 9=33.4v pin 11= -34.1v pin 18= -77.5v pin 13=-13.35v pin 16= 7.94v pin 7=12.6v pin 13=13.3v pin 14=13.3v pin15=4.8v pin 16 =7.9v​
 
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[QUOTE=" .Any ideas where to start .[/QUOTE]
model number of unit concerned and some history please .
 
Sx-1980 sorry i forgot to put that in .Upgraded power supply with jfet mods and problem transistors moved to big heatsinks ,amp boards upgraded and main caps replaced .also sorry i had pin numbers in a straight line and when i hit post it moved them.
 
did you change any of q1, q3, q5, q7, q9, q11?

If you used dual 992's you may have shifted the operating point of q1 and thus need R9 (39k) R103(120k) and VR1 (150k) changes.
Q1 collector sets at 1/2 (R5 (39k) and R7 (39k) of B2 (80v) which is 40 volts.

Might have to cut jumper b to bring in R103 if the trimpot is maxed out - but the Q1 ksc1845 needs to be gain matched with the KSA992 pair that make Q3.
If the trimpot is at ZERO ohms, check if R103 is in circuit and if so put the B jumper back in - BUT ONLY AFTER CRANKING VR1 BACK TO MAXIMUM RESISTANCE!!
 
Yes I did change q1 through q11. I didn't want to leave anything to chance , I will check r103 and report back .
 
ok checked R9=39k R103=2k VR1 repaced with update R5=39K R7=39K .I cut blue jumper wire b now my dc offset on gwh-109 is 1.302v and idle current is 1.0mv . I did test R49 R51 both are supped to be 470 ohms they are 220ohms those are the 2 resistors that have the black and yellow thermisters by them.
 
Correction: R109 (120,000 ohms or 120k), not R103 (2000 ohms or 2k) - the print I had to use was smudgy.

ok checked R9=39k R103=2k VR1 repaced with update R5=39K R7=39K .I cut blue jumper wire b now my dc offset on gwh-109 is 1.302v and idle current is 1.0mv .

R109 is supposed to be 120,000 ohms
what is the part number you used for VR1 replacement? 652-3296P-1-204LF ? That is a 200,000 ohm resistor and should be adequate in adjustability.
Cutting the blue jumper wire got you closer (1.302 vs 3.5v), you make NO mention of the value of VR1 and NO mention of adjusting it after cutting the jumper.
We NEED to know the DC offset voltages at each end of VR1's adjustment range.



I did test R49 R51 both are supped to be 470 ohms they are 220ohms those are the 2 resistors that have the black and yellow thermisters by them.

That is part of the power limiter circuit and will NOT affect dc offset or idle current until the amp is pumping out 200 watts or so.
 
Double checked R109 120K yes VR1 is correct dc offset range is 3.6v=1.302v it would not come below 3.5v until blue jumper was cut .
 
Double checked R109 120K yes VR1 is correct dc offset range is 3.6v=1.302v it would not come below 3.5v until blue jumper was cut .
We NEED to know the DC offset voltages at each end of VR1's adjustment range.

VR1 = 0 ohms - HOW MANY VOLTS?
VR1 = 200,000 ohms - HOW MANY VOLTS?
 
Vr1 is in fact 652-3296p-1-204lf 200k also voltage is -77.5v at VR1 also took readings off all pins on power supply awr-154.R97=1.348v R103=1.348V. VR1 range is 1.323v =2.598v
 
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Vr1 is in fact 652-3296p-1-204lf 200k also voltage is -77.5v at VR1 also took readings off all pins on power supply awr-154.R97=1.348v R103=1.348V. VR1 range is 1.323v =2.598v
OK, I didn't express myself well enough.

what resistance is VR1 at when the dc offset is reading 1.323v ?
This is with the blue wire cut which adds R109's 120,000 ohms to the circuit?
what resistance is VR1 at when the dc offset is reading 2.598v ?
 
I appoligize i missed the ohms on VR1 . VR1 1.320v =0.9 ohms VR1 2.598v=0.00ohms does this seem normal
 
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I appoligize i missed the ohms on VR1 . VR1 1.320v =0.9 ohms VR1 2.598v=0.00ohms does this seem normal

No, VR1 is 0 ohms to 200,000 ohms. Power is OFF reading ohms?
PLUS I have no confirmation of the status of jumper b to b. sigh...

From the emitter of q1 to the -80v regulated and filtered ( R107, C1 100uf & C5 0.047uf) rail:
we have either

39,000 ohms (R9 39,000 ohms + VR1 0 ohms) to 239,000 ohms (R9 39,000 ohms + VR1 200,000 ohms)

or if jumper B is cut:
159,000 ohms (R9 39,000 ohms + R109 120,000 ohms + VR1 0 ohms) to 359,000 ohms (R9 39,000 ohms + R109 120,000 ohms + VR1 200,000 ohms)
 
From Q1 emitter to VR1 159k ohms and that was at 2.670v. now at 1.350v it was still 159k .I also checked from Q1 emitter to r107 and it was 159k ohms.Blue jumper wire is still cut.
 
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You had better start double checking that whole q1 emitter to -80v circuit leg. Check for shorts to the traces. While doing the checking (POWER OFF!!!) remove the Q1 emitter lead from the solder pad, thus isolating this leg of resistors. I didn't do ALL that typing just for the FUN of it, I had a PURPOSE in mind:
From the emitter of q1 to the -80v regulated and filtered ( R107, C1 100uf & C5 0.047uf) rail:
we have either

39,000 ohms (R9 39,000 ohms + VR1 0 ohms) to 239,000 ohms (R9 39,000 ohms + VR1 200,000 ohms)

or if jumper B is cut:
159,000 ohms (R9 39,000 ohms + R109 120,000 ohms + VR1 0 ohms) to 359,000 ohms (R9 39,000 ohms + R109 120,000 ohms + VR1 200,000 ohms)

Could it be that your pot installation is goofed up? You should get 200,000 ohms of resistance variation, just like I posted a few posts back.

It does not fill me with confidence in the readings when I ask: "Power is OFF reading ohms?"
and I get back:
I'm sorry I had it hooked to dbt I will check it when I get home .
 
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I'm sorry the first readings were botched up , the readings this time were taken after i ran VR1 to top of adjustment then removed gwh-109 for ohm readings from top pin of VR1 to emitter of Q1 . I then reinstalled ran VR1 to lowest adjustment then removed and rechecked from top pin of VR1 to emitter of Q1 .If pot instilation was goofed up how could i adjust voltage ? Am i not taking readings at right places ?I very much appreciate all your help more than you know ,and i want to follow all your instructions to the T .
 
Removed emitter from Q1 measured 40kohms , replaced VR1 same number pot , measured 0.600mv at lowest setting and measures 158k ohms from emitter of Q1 to VR1
 
Like you said i better double check , i have been over this board at least ten times and everything from trans to resistors to zeners checks out,i also dont see any shorts on board or traces of shorts .
 
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