Phase Linear 400 - DC offset problem
- Thread starter audiogymp
- Start date
I've made some more measurements, which I've consolidated with the ones from my earlier post. They include the ones requested and a couple more that might assist.
R17: 64.0 -> 62.1V
R18: 62.1 -> 61.9V
D6: 61.7 -> 61.4V
D7: 61.7 -> 61.2V
D8: 61.1 -> 60.8
R19: 61.2 -> 61.0
R20: (61.0 assumed) -> 60.7V
Q6 red/c: 62.3V
Q6 brown/e: 61.2; 60.8V
Q6 black/b: 61.0; 60.9V
C7: 61.1 -> 40.6V
R15: 61.2 -> 40.7
When I tester D6-8 in circuit I got several hundred mV (anode to anode) and about 1.5V in reverse direction. None stuck out as different.
Using diode tester on Q6 in circuit gave the following results. I used the colour assignments that are hand-noted on Diagram III-5, which appeared to be correct from my inspection of the circuit.
b+e-: 650
b+c-: 648
e+b-: 1080
e+c-: 655
c+b-: 1300
c+e-: 1090
My component tester was not able to identify Q6 in circuit.
R17: 64.0 -> 62.1V
R18: 62.1 -> 61.9V
D6: 61.7 -> 61.4V
D7: 61.7 -> 61.2V
D8: 61.1 -> 60.8
R19: 61.2 -> 61.0
R20: (61.0 assumed) -> 60.7V
Q6 red/c: 62.3V
Q6 brown/e: 61.2; 60.8V
Q6 black/b: 61.0; 60.9V
C7: 61.1 -> 40.6V
R15: 61.2 -> 40.7
When I tester D6-8 in circuit I got several hundred mV (anode to anode) and about 1.5V in reverse direction. None stuck out as different.
Using diode tester on Q6 in circuit gave the following results. I used the colour assignments that are hand-noted on Diagram III-5, which appeared to be correct from my inspection of the circuit.
b+e-: 650
b+c-: 648
e+b-: 1080
e+c-: 655
c+b-: 1300
c+e-: 1090
My component tester was not able to identify Q6 in circuit.
@#$%! I missed Q5, D4, D5 as possible candidates.
Q9, D10 are very outside chance.
Problem is that very little current flowing through R17,18 so not getting correct voltages.
This is typically caused by something failed open circuit or a transistor not being turned on.
C8,9 makes incircuit test of Q6 unreliable.
Voltages at Q6 b and e show that it's not being turned on due to insufficent current/colt drop across R19 and R20
Measure Q5 voltages with respect to ground. The collector will be about +60V, expect -0.7V approx.
Diode check Q5, D4, D5 incircuit may be ok, c5 may cause some issue with this.
Q9, D10 are very outside chance.
Problem is that very little current flowing through R17,18 so not getting correct voltages.
This is typically caused by something failed open circuit or a transistor not being turned on.
C8,9 makes incircuit test of Q6 unreliable.
Voltages at Q6 b and e show that it's not being turned on due to insufficent current/colt drop across R19 and R20
Measure Q5 voltages with respect to ground. The collector will be about +60V, expect -0.7V approx.
Diode check Q5, D4, D5 incircuit may be ok, c5 may cause some issue with this.
Maybe the penny has dropped?
Initial fault causes some excessive dc on speaker line, for
example +5Vdc
Path D13, R35, Q7e-b raises voltage at Q5c so current
through R17, R18 slows so voltage at Q10b, e and speaker line
increases. Q5c voltage raises a little more, slows current
further,,, speaker line voltage increases some more. This
cycle is repeated until a steady state, +60Vdc on speaker
line.
R41, R34 Q7e, b is another path to alter Q5c voltage.
So the problem is to find the cause of the initial dc offset
(+5Vdc in this example) rather than the cause of the +60Vdc
which is just the circuits reponse.
I need to digest this and try to work out a strategy.
Others welcome to chime in.
Initial fault causes some excessive dc on speaker line, for
example +5Vdc
Path D13, R35, Q7e-b raises voltage at Q5c so current
through R17, R18 slows so voltage at Q10b, e and speaker line
increases. Q5c voltage raises a little more, slows current
further,,, speaker line voltage increases some more. This
cycle is repeated until a steady state, +60Vdc on speaker
line.
R41, R34 Q7e, b is another path to alter Q5c voltage.
So the problem is to find the cause of the initial dc offset
(+5Vdc in this example) rather than the cause of the +60Vdc
which is just the circuits reponse.
I need to digest this and try to work out a strategy.
Others welcome to chime in.
I'll measure some voltages to see if they cast some light on the part of the circuit we need to focus on. I'll go for points we have figures for in Diagram III-5.
I know it's late in the piece but it might be worth mentioning a couple of points from the SM - items 4 and 11 of the section VI. Troubleshooting Guide. There's also section V. Test and Alignment, Step #7 at (b). Q1 and Q2 seem to be a possible cause of DC offset, as well as Q3 and Q4. I expect you've considered this but I thought I'd mention it since the net is expanding a little.
I know it's late in the piece but it might be worth mentioning a couple of points from the SM - items 4 and 11 of the section VI. Troubleshooting Guide. There's also section V. Test and Alignment, Step #7 at (b). Q1 and Q2 seem to be a possible cause of DC offset, as well as Q3 and Q4. I expect you've considered this but I thought I'd mention it since the net is expanding a little.
Here are some voltages I measured from around Q1-7, for comparison to the figures in Diagram III-5 (remember that the rail voltage here with DBT is +/- ~65V).
Q1: c +24.1; e +23.5
Q2: c +23.5; e +23.5
Q3: c +24.6; b=Q1c
Q4: c +25.1; b=Q2c
Q5: c +60.7; e -64.5; b -64.5
Q7: c +60.5; e -64.5; b +60.7
A lot of this is affected by the DC offset. However, the Q1-2e voltage is quite a bit higher than expected and it's not clear to me why this might be altered by the offset.
Please let me know if you think I need to re-check any measurements.
Q1: c +24.1; e +23.5
Q2: c +23.5; e +23.5
Q3: c +24.6; b=Q1c
Q4: c +25.1; b=Q2c
Q5: c +60.7; e -64.5; b -64.5
Q7: c +60.5; e -64.5; b +60.7
A lot of this is affected by the DC offset. However, the Q1-2e voltage is quite a bit higher than expected and it's not clear to me why this might be altered by the offset.
Please let me know if you think I need to re-check any measurements.
Certainly, Q1, Q2 may cause dc offset.
I'm reluctant to request any further measurements until I think through what might be happening. The scenario
outlined above may be happening. also might not. The original thought was the problem was the +60Vdc on the
speaker line. Need time to think.
I will recheck the troubleshooting guide.
Last measurements are helpful. The standouts are Q5 b=e, normally means a short. Diode test this tr.
Also Q7e looks wrong, expect +61.5V, not neg 64..V, recheck
I'm reluctant to request any further measurements until I think through what might be happening. The scenario
outlined above may be happening. also might not. The original thought was the problem was the +60Vdc on the
speaker line. Need time to think.
I will recheck the troubleshooting guide.
Last measurements are helpful. The standouts are Q5 b=e, normally means a short. Diode test this tr.
Also Q7e looks wrong, expect +61.5V, not neg 64..V, recheck
Suggest diode check Q5, in circuit is fine since you are looking for a b-e short.
Been trawling through the forums. High dc does come up once in a while but no good suggestions on how to tackle.
General path taken is replace the outputs(done) then replace Q1,2,3,4 if problem persists.
I assume you don't have a variac. MIGHT be able to slowly crank up the voltage to the point of dc on the line before
it max's out +60V.
I will unpack my PL400 and practice on that rather than your gear, I'm betting it's got the same problem.
Need to make some space on the bench.
recheck this voltage.
Been trawling through the forums. High dc does come up once in a while but no good suggestions on how to tackle.
General path taken is replace the outputs(done) then replace Q1,2,3,4 if problem persists.
I assume you don't have a variac. MIGHT be able to slowly crank up the voltage to the point of dc on the line before
it max's out +60V.
I will unpack my PL400 and practice on that rather than your gear, I'm betting it's got the same problem.
Need to make some space on the bench.
Sorry for the delay - out of town yesterday. Will do the checks/re-checks suggested shortly and edit this post.
I don't have a variac but I don't think it would help. I have read posts in which it has been reported that the circuit is unstable at reduced input voltage and that variac cannot be used because of this. DBT appears to be the recommended way to go. With rail voltages getting to +/- 65, I think the circuit should be behaving somewhat normally but it has been at the back of my mind that there may be some effect from DBT. I can go for a higher wattage to get higher rails if you think I should - current doesn't seem to be running away at the moment as long as speaker terminals remain OC. (I got a little ahead of myself after replacing the outputs and had test speakers hooked up - DBT stayed bright and probably saved my new outputs.)
I rechecked Q7e and got -63.7, so I don't think any real change. Thew amp was cold. If I made a mistake before, it might have been measuring from the wrong side of R37. III-5 indicates that Q7e should be close to rail voltage. For b and c, I'm not sure if I'm seeing cause or effect.
Diode test for Q5 in circuit are:
b+e-: 588
b+c-: 567
e+b-: 1412
e+c-: 729
c+b-: OC
c+e-: OC
Cheers, Jon
I don't have a variac but I don't think it would help. I have read posts in which it has been reported that the circuit is unstable at reduced input voltage and that variac cannot be used because of this. DBT appears to be the recommended way to go. With rail voltages getting to +/- 65, I think the circuit should be behaving somewhat normally but it has been at the back of my mind that there may be some effect from DBT. I can go for a higher wattage to get higher rails if you think I should - current doesn't seem to be running away at the moment as long as speaker terminals remain OC. (I got a little ahead of myself after replacing the outputs and had test speakers hooked up - DBT stayed bright and probably saved my new outputs.)
I rechecked Q7e and got -63.7, so I don't think any real change. Thew amp was cold. If I made a mistake before, it might have been measuring from the wrong side of R37. III-5 indicates that Q7e should be close to rail voltage. For b and c, I'm not sure if I'm seeing cause or effect.
Diode test for Q5 in circuit are:
b+e-: 588
b+c-: 567
e+b-: 1412
e+c-: 729
c+b-: OC
c+e-: OC
Cheers, Jon
Last edited:
Hi mbx
I just realised I've been making a fairly stupid mistake, which effects my c and e designation on some transistors. I had been treating the arrow in the symbols as pointing in the c-e direction instead of the arrow always being on the emitter terminal. For NPNs what I have reported is correct. However, for PNPs I have had the e and c designations backwards.
This affects what I reported for Q3, Q4 and Q7 (ie c and e are backwards) but shouldn't affect other measurements. This will explain the confusion about Q7e but I hope it hasn't otherwise lead you too far astray. Sorry - still on the steep part of the learning curve.
Jon
I just realised I've been making a fairly stupid mistake, which effects my c and e designation on some transistors. I had been treating the arrow in the symbols as pointing in the c-e direction instead of the arrow always being on the emitter terminal. For NPNs what I have reported is correct. However, for PNPs I have had the e and c designations backwards.
This affects what I reported for Q3, Q4 and Q7 (ie c and e are backwards) but shouldn't affect other measurements. This will explain the confusion about Q7e but I hope it hasn't otherwise lead you too far astray. Sorry - still on the steep part of the learning curve.
Jon
Just a recap for those wanting to chime in.
Approx +60Vdc on left speaker line. Outputs already replaced.
Amp on DBT, rails at about 65V.
Uses PL14A main board, refer schematic.
Measurements to hunt down this voltage shows unexpectedly low current
through path R17, R18 down to Q5 collecter which is at about +60Vdc.
So high voltage path via Q10be, Q11be and R38.
Q10 replaced, red herring.
Incircuit diode test kinda suggests Q5 is OK (see below) however Q5be
voltages are not OK, Vb=Ve.
Suggest measure Q5Vbe with one probe on Q5b/R14, the other on Q5e/R16, mV.
Also diode check D4 then D5, in circuit is OK. Maybe the DBT is affecting
the Q5Vbe, try to increase the rails voltage/wattage a little.
Now for the spanner in the works.
If some other transistor (Q1,,,4) caused a small yet significant dc offset
say 5-10V, that would raise the voltage at Q5c (via D13, R35, Q7 or R41,
R5, Q7) causing reduced currect through R17,18, raising the voltage at Q10b
and Q11b and the speaker line. This cycle would continue until steady state
maybe unti 60V at Q5c?? Under this scenario the problem is the 5-10V offset
not the 60V... Maybe a variac would help, maybe best to replace Q1,,,4
I don't think pulling the outputs/fuses would help. Q8,9 look like current
sensing, no big deal. Radical plan would be to pull fuses, pull Q7,8,9,Q10
and tie Q5c to GND then variac p-on. DO NOT DO THIS, consider replacing Q1,,,4.
A few side notes.
Q5
Interpretation of incircuit testing is a bit of a lottery.
I suspect you have the e+b- and e+c- results the wrong way
around. ie, expect
e+b-: 729
e+c-:1412 = e+b-(729) and b+c-(567) =(1296 or abouts)
However point is I don't see a Q5 b-e short
I don't see any significant value in the White Oak Capacitor upgrade
verses simply sourcing caps from Mouser etc... The WO main board does
look nice and has some minor improvements. The asking is not a lot of
$$$, also mandatory if the original board/tracks are damaged.
The troubleshooting section of the service manuals uses a variac to
gradually ramp up the voltage, however others claim a variac should
not be used citing instability.
My PL400 (blown outputs?) is now on the bench so I can practice on that.
Approx +60Vdc on left speaker line. Outputs already replaced.
Amp on DBT, rails at about 65V.
Uses PL14A main board, refer schematic.
Measurements to hunt down this voltage shows unexpectedly low current
through path R17, R18 down to Q5 collecter which is at about +60Vdc.
So high voltage path via Q10be, Q11be and R38.
Q10 replaced, red herring.
Incircuit diode test kinda suggests Q5 is OK (see below) however Q5be
voltages are not OK, Vb=Ve.
Suggest measure Q5Vbe with one probe on Q5b/R14, the other on Q5e/R16, mV.
Also diode check D4 then D5, in circuit is OK. Maybe the DBT is affecting
the Q5Vbe, try to increase the rails voltage/wattage a little.
Now for the spanner in the works.
If some other transistor (Q1,,,4) caused a small yet significant dc offset
say 5-10V, that would raise the voltage at Q5c (via D13, R35, Q7 or R41,
R5, Q7) causing reduced currect through R17,18, raising the voltage at Q10b
and Q11b and the speaker line. This cycle would continue until steady state
maybe unti 60V at Q5c?? Under this scenario the problem is the 5-10V offset
not the 60V... Maybe a variac would help, maybe best to replace Q1,,,4
I don't think pulling the outputs/fuses would help. Q8,9 look like current
sensing, no big deal. Radical plan would be to pull fuses, pull Q7,8,9,Q10
and tie Q5c to GND then variac p-on. DO NOT DO THIS, consider replacing Q1,,,4.
A few side notes.
Q5
Interpretation of incircuit testing is a bit of a lottery.
I suspect you have the e+b- and e+c- results the wrong way
around. ie, expect
e+b-: 729
e+c-:1412 = e+b-(729) and b+c-(567) =(1296 or abouts)
However point is I don't see a Q5 b-e short
I don't see any significant value in the White Oak Capacitor upgrade
verses simply sourcing caps from Mouser etc... The WO main board does
look nice and has some minor improvements. The asking is not a lot of
$$$, also mandatory if the original board/tracks are damaged.
The troubleshooting section of the service manuals uses a variac to
gradually ramp up the voltage, however others claim a variac should
not be used citing instability.
My PL400 (blown outputs?) is now on the bench so I can practice on that.
If these are volts, then something is wrong with input transistors or grounding. What voltage is on base of Q1? Check ground wires from input connector to board and check R2. With power off, check resistance between input ground and speaker ground. Should be either 2.7 or 56.
problem (to me, anyway) looks like input floating or Q1 open.
Further reading shows burnt tracks are an issue, check under magnifying lamp.
Many comments that amp needs near full voltage to bias up ok, still undecided on that.
Some links,
http://www.diyaudio.com/forums/solid-state/182404-recapping-phase-linear-400-a-2.html
http://synthetizer-sche.chez-alice.fr/power ampli/phase linear/repair.htm
Many comments that amp needs near full voltage to bias up ok, still undecided on that.
Some links,
http://www.diyaudio.com/forums/solid-state/182404-recapping-phase-linear-400-a-2.html
http://synthetizer-sche.chez-alice.fr/power ampli/phase linear/repair.htm
Thanks, Steve. I did your tests first and I'll report before proceeding with mbz's because I think there may be something interesting here.
First I confirmed the measurements from Q1 and Q2
Q1c (measured at D2 cathode) was +24.0V
Q1e (measured at R6 leg with trace running to Q1&2) was +23.5
Q2c (measured from D1 cathode) was +23.5
Q2e (see Q1e) was + 23.5
I then measured Q1b (at R1 and R3 legs with trace running to Q1) and I got +14.8V both times. From the circuit, this looks surprisingly high. Could this be the cause?
I checked resistance from input ground to speaker ground and it was as bout 2.5 Ohms (close to what we expect since the problem is with the left channel). It seemed to jump around a bit but it's a bit of a stretch for my meter going this low. Not sure if it means anything.
First I confirmed the measurements from Q1 and Q2
Q1c (measured at D2 cathode) was +24.0V
Q1e (measured at R6 leg with trace running to Q1&2) was +23.5
Q2c (measured from D1 cathode) was +23.5
Q2e (see Q1e) was + 23.5
I then measured Q1b (at R1 and R3 legs with trace running to Q1) and I got +14.8V both times. From the circuit, this looks surprisingly high. Could this be the cause?
I checked resistance from input ground to speaker ground and it was as bout 2.5 Ohms (close to what we expect since the problem is with the left channel). It seemed to jump around a bit but it's a bit of a stretch for my meter going this low. Not sure if it means anything.
Check voltages on the other sides of those resistors, but this looks like a major part of the problem. Looks like Q1 and q2 should be changed. If subbing, make sure B,C, and E are correct.
Steve/mbx
I checked voltages on R1 and R3. R1 had 14.8V on both sides. R3 measured 0V away from Q1b. So am I right in thinking that the 14.8V is coming from Q1?
I diode checked Q1 and Q2 in circuit.
Q1
b+e-: 697
b+c-: 688
e+b-: OC
e+c-: OC
c+b-: OC
c+e-: 207
Q2
b+e-: OC
b+c-: OC
e+b-: OC
e+c-: OC
c+b-: OC
c+e-: 760
It looks to me like Q1 might have failed OC c+e- and Q2 has failed CC (Q2c+e- reading through D2 and Q1). Q1c+e- doesn't look right. Thoughts?
mbx gave me a couple of links at post #56. The diyaudio thread has some X-sistor sub suggestions at post #25. It says:
The recommendation seems general but it was made in the context of a PL14B board and mine is PL14A.
Q1 and Q2 are Motorola MPS5172 on my board, (not TIS97, which the schematic shows for the PL14B board). I tried to buy MPS5172 with the last Mouser order but they had none in stock and I couldn't find any suggested subs at the time. I can get 10 of ZTX694B at RS without too much skin.
Is this a workable sub?
Thanks, Jon
I checked voltages on R1 and R3. R1 had 14.8V on both sides. R3 measured 0V away from Q1b. So am I right in thinking that the 14.8V is coming from Q1?
I diode checked Q1 and Q2 in circuit.
Q1
b+e-: 697
b+c-: 688
e+b-: OC
e+c-: OC
c+b-: OC
c+e-: 207
Q2
b+e-: OC
b+c-: OC
e+b-: OC
e+c-: OC
c+b-: OC
c+e-: 760
It looks to me like Q1 might have failed OC c+e- and Q2 has failed CC (Q2c+e- reading through D2 and Q1). Q1c+e- doesn't look right. Thoughts?
mbx gave me a couple of links at post #56. The diyaudio thread has some X-sistor sub suggestions at post #25. It says:
Here are some suggestions on transistor types for the driver board that seem to work well in Phase Linear amps (no warranty expressed or implied):
Input diff amp NPN's - ZTX694B, buy 10 or more & match Hfe's - this will give lower output offset voltages, assuming there is not another issue.
Input diff amp NPN's - ZTX694B, buy 10 or more & match Hfe's - this will give lower output offset voltages, assuming there is not another issue.
The recommendation seems general but it was made in the context of a PL14B board and mine is PL14A.
Q1 and Q2 are Motorola MPS5172 on my board, (not TIS97, which the schematic shows for the PL14B board). I tried to buy MPS5172 with the last Mouser order but they had none in stock and I couldn't find any suggested subs at the time. I can get 10 of ZTX694B at RS without too much skin.
Is this a workable sub?
Thanks, Jon
