DC Offset wont drop below .25v

Excuse me, I looked at the post with the voltage readings table.

I see a power supply problem.
One that explains your initial problem with the lack of DC offset voltage adjustment range.

AWR-101 pin 14 should be -51.5 volts DC and is -13.7 volts DC.

It feeds the AWH-050 power amp pins 1,2, 16 & 17 for the input portion of the amp, AS WELL AS EACH CHANNEL'S INDIVIDUAL offset voltage adjustment source network, which EXPECTS to have +36.5v DC AND -51.5v DC. This network has a pair of ground referenced voltage dividers, one (62K, 1K) for the +36.5v DC that makes +0.52v DC , and one (82K, DIODE, 6.8K) for the -51.5v DC that makes -4v DC. Those two resultant voltages go to a divider (100k, 10k pot, 9.1k) that feeds R19 (or R20 for the other channel) into the power amp's input section. THE VR1 and VR2 pots should have +0.180v DC at one end and -0.220v DC at the other end.
With just -13.7v instead of -51.5 these voltages will be messed up - probably not going negative enough across the pot.

AWR-101 pin 15 voltage also has a problem, it should be -19 volts DC and is -4.28 v DC,
HOWEVER this pin is dependent upon pin 14's -51.5 (-13.7) , so it should be ok when pin 14's voltage is corrected.
This pin 14 voltage only feeds the phono preamp awf-011 pin 2 so it's not affecting your offset.

Troubleshooting that regulator:
AWR-101 pin 13 shows the correct -13v from the negative regulator's reference voltage OF -13v dc, meaning a lot is working.
I suspect Q2 2sb507p may be damaged. Voltage readings to ground of Q2 and Q4 are needed.
 
Excuse me, I looked at the post with the voltage readings table.

I see a power supply problem.
One that explains your initial problem with the lack of DC offset voltage adjustment range.

AWR-101 pin 14 should be -51.5 volts DC and is -13.7 volts DC.

Heck yea! ;)
Could of turned into a 15 page thread :whip:
 
Thank you Sir!
Q2 KSA940
E -13.7v
B -14.5v
C -.065v

Q4 KSA1220AYS
E -12.8v
B -13.4v
C -14.8v

First I will assume that "C -.065v" really means -65.0 v DC

Second the series pass element q2, a to-220 ksa940 transistor is hard to put in backwards... <grin>, and thus with the base at -14.5v DC and the emitter at -13.7v DC, the 0.8v while a slight bit high, is correctly biased.

That leaves Q4, the feedback element.
This backwards (easy to do with a to-126) would be an e - c reversal, thus the transistor would never be biased on.
Hopefully the ksa1220a is undamaged.
 
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Yes, My apologies. The actual reading on my meter, (Off the DBT) for Q2 is
Q2 KSA940
E -13.7v
B -14.5v
C -065.8v
For some reason, when taking those readings the E & B readings went out 2 decimal places to the right, (which I omitted in the post) but on the collector, it only went out 1 and added the a zero in front which threw me off. Again, Sorry for my error. I will slow down

20180909_073719.jpg


This morning I checked pin orientation on Q4. My routine when installing transistors is to pull up the datasheet on the PC on my bench and verify for orientation. I know from my short time in this hobby that the to-126 casing has 3 dots molded into the face. I then check which pins are ECB..
Screenshot 2018-09-09 09.54.38.png

I then orientate the transistor to the symbol on the board and match the legs to the symbol.
20180909_101044.jpg
Is this correct?
I pulled Q2 and Q4 this morning after reading Marks last post and they tested good on my Micronta Transistor Checker and by doing the 6-way diode test on my meter.
 
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Well done Mark,
ksa1220a-y orientation looks good to me. Always good to verify the devices before insertion/soldering
Nice to see you are using lead grabbers.
Could recommend to put some 100nF/100V film caps (Panasonic ECQ-E) across the big ecap terminals(red bleed resistors) to do a better job of filtering higher freq supply noise.
 
Just wanted to clarify, Q4 was installed correctly (the first time) and I still have the issue with offset not dropping below 0.29v
 
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please re-post the current voltage readings for q2 e,c,b and q4 e,c b, and let us please refer for now to the problem as not having -51.5v at awr-101 pin 14 .

please check the resistors (R12, R13, R14, R15, R16) one leg lifted and verify the actual circuit connections - i.e. bad / cracked traces or solder joints.
something doesn't add up.

R12 (1.5K), C20 (100uF, 63V) is an input power filter to provide "wake up" current through R13 (1.5k) to the Q4 feedback element. The regulator is sensitive to ANY hum from this point - thus the huge capacitor at C20.

R14 (5.6k) and R15 (15k) are the voltage divider that monitors the output voltage and feeds the base of Q4, the regulator circuit "wants" to maintain the base of Q4, 0.6v lower(pnp!) than the Q4 emitter voltage which is set by ZD2 and R16 (2.7k).
That math is:
voltage in x (R14 / (R14 + R15)) = voltage out
51.5 x (5600 / (5600 + 15000)) = 14v


The only other way I can see an operating circuit putting out -13.7v dc is that R15 (15k brown green orange) is somehow shorted resulting in Q2 base being at the same voltage as Q4 emitter. Then it would be regulating at -13.7v dc
Thing is I can SEE R15 in the picture, and it has the correct color codes.

We start out with the zener voltage, and add about 0.6v absolute as the "turn on" voltage for q4 which is found at q4 base
then we lose 0.6v from the base of q2 to the emitter of q2. for the final voltage fed to the voltage divider bridge.

Hope that this, (AND I) makes sense here.. headaches make it hard to concentrate. Thus I post my "logic" and train of thought, as a check...
 
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Ok.. Well.. I spent a few hours desoldering and checking resistors as requested.. Fair enough.
I took the voltage readings all PS pins, along with Q2 & Q4 and recorded them. Nothing changed since the last post so I went straight down the list of resistors Mark wanted checked.

R12 checked good. So since I had couple new C20's caps on hand I replaced it.
Powered up immediately after that change and nothing new.
Continued with all the remaining resistors and they checked good. I even pulled legs on a few more..
Screenshot 2018-09-09 17.57.10.png

At this point I was pretty disappointed there was nothing to replace.
Went thru and checked continuity on a bunch of trace contacts. Nothing. Touched up a few solder joints too.. Nothing stood out tho.
Then I thought I may as well check other items that are in the pin 14 trace. Checked another resistor with no issues, then I put my lead grabbers on C22 while it was still in circuit.

I now realize the meter was set to resistance, but I pulled it anyway and changed it.
I just happen to have a few 1000pf 100v ceramics on hand so I tossed one in.
Powered up and checked Pin 14 at 51v
From that point I knew I'd be able to adjust offsets, and I am!

I'm wondering if this was the actual problem or it was something simple as touching up a few solder joints?

At any rate, I need to thank all you guys for putting up with my mistakes and bad typing errors.
 
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tin whiskers or old flux maybe . :idea: ceramics are the last ones to check in circuits like these .. it does happen but wasnt in this case unless the heat cleared the 50k resistance ..
 
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