Phase Linear 400 - DC offset problem
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Now that irks me just a little.
For me, Q5 b-e voltage of a few mV is very significant, Q5 is not turned on ... Causes can be D4, D5 and ???
Diode test D4 then D5 in circuit. You may get some climbing results due to C5.
Suggest measure voltage at negative rail (R16/R14 junction) then at D4, D5 junction, black probe connected to ground.
D4, D5 should be providing a stable 0.6(Ge) 1.3(Si) volt drop acorss Q5b-e and R16.
The Q5b-e voltage I measured yesterday was less than a few mV, it was 0.3mV - so close to zero it's near limit of detection.
When I measured D4 and D5 in circuit I got (lower figures Anode to Anode):
D4: 597; 1756
D5: 619; 1750
There didn't seem to be any drift because of C5.
With power I got the following voltages relative to ground (black probe).
R16/R14 junction: -63.7
D4 (away from D5): -63.7
D4/D5 junction: -63.7
D5 (away from D4, ie -ve rail): -63.7
There was some drift in these voltages but they appeared to settle pretty close to the same voltage - so no apparent voltage drop across D4 and D5. Does this suggest that R14 or C5 might be conducting too much?
FYI - R11 and C5 are shown in reversed position in the schematic.
When I measured D4 and D5 in circuit I got (lower figures Anode to Anode):
D4: 597; 1756
D5: 619; 1750
There didn't seem to be any drift because of C5.
With power I got the following voltages relative to ground (black probe).
R16/R14 junction: -63.7
D4 (away from D5): -63.7
D4/D5 junction: -63.7
D5 (away from D4, ie -ve rail): -63.7
There was some drift in these voltages but they appeared to settle pretty close to the same voltage - so no apparent voltage drop across D4 and D5. Does this suggest that R14 or C5 might be conducting too much?
FYI - R11 and C5 are shown in reversed position in the schematic.
Remember that we are still working on the dc/biasing of the amp, no audio input. So like all good caps C5 should block any dc, so there
will be no dc current flow through C5 and R11.
D4, D5 and R14 act as an approx constant current source, ie 1.2V/3300 ohms = 0.36mA
How can D4, D5 anode/cathodes be +63.7V. They diode test ok, maybe failing short circuit under load?
Alternatively there is no current from, Q4 suspect.
Measure Q4 b,c,e voltages with respect to chasis. Previous measurements had a b-e voltage of
Q4: e +25.1; b=Q2c = +23.5 b-e = 1.6V
Also measure the voltage drop across R12, repeat on R9
Suggest you replace Q3 and Q4 if you have spares available
Check the trace from Q5b to Q4c, maybe just do a resistance measurement with amp powered off.
I'm not happy with the results from D4 and D5, suggests a short under load however may be due to no/minimal
current flow. If no improvement after Q3 and Q4, I'd replace D4 and D5, after than fni...
I don't have Q4 spares yet - ordered but Mouser was out of stock. They will come but I'm not sure when. After I do the suggested measurements perhaps we could return to whether Q4 replacement is critical at this stage.
EDIT: It looks a bit like replacement of Q4 will be unavoidable, if I'm understanding things correctly.
I measured continuity from Q5b-Q4c and it was CC.
Powered on I read the following voltages:
Q4b: +23.3; Q4e: +24.6; and Q4c: -64.5
R12 V drop: 0.0mV
R9 V drop 410mV
So it looks like no current through Q4. Am I right in thinking that with a PNP, if b is -ve relative to e, there should be current?
EDIT: It looks a bit like replacement of Q4 will be unavoidable, if I'm understanding things correctly.
I measured continuity from Q5b-Q4c and it was CC.
Powered on I read the following voltages:
Q4b: +23.3; Q4e: +24.6; and Q4c: -64.5
R12 V drop: 0.0mV
R9 V drop 410mV
So it looks like no current through Q4. Am I right in thinking that with a PNP, if b is -ve relative to e, there should be current?
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Keeping in mind I'm not a tech, so only basic explainations available.
You are correct regarding PNP transistors. The arrow indicates the direction of the voltage drop, assuming normal forward bias operation.
Typically, the voltage drop is about 0.6-0.7V. Your measurements show 1.3V. As you increase the Vbe, the transistor works harder/higher
current. Maybe the higher Vbe caused Q4 to "burn out"??? Doubtful, Q4e voltage is higher than expected because of no current flow and
no associated Vdrop across R12, which will probably be 400mV like R9 giving a reasonable(?) Q4 Vbe.
I can't reconcille the no current through R12 other than Q4 suspect faulty. With current flow through Q4 it will hopefully correct Q5 Vbe
turning it on so there is current flow down R17,18,19,,, branch.
Mouser has MPSA13 on back order, expected delivery about Sept 13. If you need something sooner I could check my spares, worst case
remove a Q4 from one of my units. I'm away from home maybe 10 days so no real advantage I think, maybe best to wait, install Q4 and
problem remains.... Maybe someone more knowledgable will chime in and confirm we are on the right/wrong track.
R12 Vdrop confirms no current flow through Q4.
You are correct regarding PNP transistors. The arrow indicates the direction of the voltage drop, assuming normal forward bias operation.
Typically, the voltage drop is about 0.6-0.7V. Your measurements show 1.3V. As you increase the Vbe, the transistor works harder/higher
current. Maybe the higher Vbe caused Q4 to "burn out"??? Doubtful, Q4e voltage is higher than expected because of no current flow and
no associated Vdrop across R12, which will probably be 400mV like R9 giving a reasonable(?) Q4 Vbe.
I can't reconcille the no current through R12 other than Q4 suspect faulty. With current flow through Q4 it will hopefully correct Q5 Vbe
turning it on so there is current flow down R17,18,19,,, branch.
Mouser has MPSA13 on back order, expected delivery about Sept 13. If you need something sooner I could check my spares, worst case
remove a Q4 from one of my units. I'm away from home maybe 10 days so no real advantage I think, maybe best to wait, install Q4 and
problem remains.... Maybe someone more knowledgable will chime in and confirm we are on the right/wrong track.
mbz - is "MPASA13" a typo or were you suggesting it as an alternative for MPSA93?
In Post 62 I said:
I bought 3 each of Q3 and Q4, which are MPSA55 and MPSA93, respectively in the channel we are concerned with (and surprisingly reversed in the other channel, which seems to be against the recommendation in the service bulletin). These x-sistors were a little pricey, which is why I only got a few. Mouser didn't have the A93 in stock, so they will be coming in a later shipment.
If the other channel has an MPSA55 in place of the MPSA93 for Q4, does this suggest that MPSA55 might be a viable substitute for MPSA93? If so, I could use an MPSA55 as a replacement for Q4.
In Post 62 I said:
I bought 3 each of Q3 and Q4, which are MPSA55 and MPSA93, respectively in the channel we are concerned with (and surprisingly reversed in the other channel, which seems to be against the recommendation in the service bulletin). These x-sistors were a little pricey, which is why I only got a few. Mouser didn't have the A93 in stock, so they will be coming in a later shipment.
If the other channel has an MPSA55 in place of the MPSA93 for Q4, does this suggest that MPSA55 might be a viable substitute for MPSA93? If so, I could use an MPSA55 as a replacement for Q4.
Apology, typo, I meant MPSA93, I can only go off the service bullitens.
laatsch55 in post 65 said MPSA92 and 93 were a suitable replacements for Q3, Q4
with MPSA92 being the preferred. Do you have a spare MPSA92 for Q4?
I'm having second thoughts about Q4. It's certainly not turned on, so Q5 Vbe is not
correct/turned on so voltage at Q10b is way too high. But Q4 is essentially controlled
by it's base voltage, if this is high then Q5 will off, so no current flow through R17
R18, R19 and a high Q10Vb. So a high Q4Vb can(?) cause high dc on speaker line. The
cause would be low current flow through Q2 (already replaced!)
Heads spinning. If you have a spare MPSA92 then consider replacing Q4. First check
service bullitens. Otherwise wait for MPSA93, it will give me more time to think
about Q4b being too high
laatsch55 in post 65 said MPSA92 and 93 were a suitable replacements for Q3, Q4
with MPSA92 being the preferred. Do you have a spare MPSA92 for Q4?
I'm having second thoughts about Q4. It's certainly not turned on, so Q5 Vbe is not
correct/turned on so voltage at Q10b is way too high. But Q4 is essentially controlled
by it's base voltage, if this is high then Q5 will off, so no current flow through R17
R18, R19 and a high Q10Vb. So a high Q4Vb can(?) cause high dc on speaker line. The
cause would be low current flow through Q2 (already replaced!)
Heads spinning. If you have a spare MPSA92 then consider replacing Q4. First check
service bullitens. Otherwise wait for MPSA93, it will give me more time to think
about Q4b being too high
With no current/volt drop across R12 and a Q4 Vbe of over 1 volt, Q4 is suspect faulty.
Suggest diode check this tr, did not see it in previous posts. Replace Q4 when spares arrive.
If Q4 does not turn on then Q5 is not turned on so near rails voltage on speakers.
Looking a bit further, Q4 is controlled by Q2c voltage. If Q2 is not conducting then Q4b will be off
but Q2 already replaced.
Suggest diode check this tr, did not see it in previous posts. Replace Q4 when spares arrive.
If Q4 does not turn on then Q5 is not turned on so near rails voltage on speakers.
Looking a bit further, Q4 is controlled by Q2c voltage. If Q2 is not conducting then Q4b will be off
but Q2 already replaced.
laatsch55
Super Member
Every transistor on the board can be checked in circuit on the basic transistor check except Q8,Q9. To separate the control board from the backplanes desolder the basedrives for the outputs, both positive and negative. Problems on the backplaners can manifest on the control board. Bad output insulators, solder splash, it doesn't take much....
Found this interesting thread on the phoenix site that kinda matches you situation.
http://forums.phxaudiotape.com/show...se-Linear-Amplifiers?highlight=latch+tendency
I need to read it another 4-5 times to get my head around it, but it talks about a
race condition at startup where Q3 and Q4 don't start up in time so Q5 remains off
and results in near rails voltage on the speaker lines.
The fix is easy to implement however it should be stressed that this only solves
the race condition at startup. There are many possible failures that result in high
dc on speakers, so a separate analysis is required in each case.
Suggest diode check Q3 and Q4. Q1 and 2 have already been replaced.
A side note, I've ordered the Don Imlay speaker protection board, solution appears
to integrate nicely in the PL400. The original PL400 design strikes me as being
somewhat fragile so the WO control and back plane boards are next on my list
http://forums.phxaudiotape.com/show...se-Linear-Amplifiers?highlight=latch+tendency
I need to read it another 4-5 times to get my head around it, but it talks about a
race condition at startup where Q3 and Q4 don't start up in time so Q5 remains off
and results in near rails voltage on the speaker lines.
The fix is easy to implement however it should be stressed that this only solves
the race condition at startup. There are many possible failures that result in high
dc on speakers, so a separate analysis is required in each case.
Suggest diode check Q3 and Q4. Q1 and 2 have already been replaced.
A side note, I've ordered the Don Imlay speaker protection board, solution appears
to integrate nicely in the PL400. The original PL400 design strikes me as being
somewhat fragile so the WO control and back plane boards are next on my list
I'm glad to be giving something back, even if it's just to show others not to do what I done.
I'll test Q3,4.
Laatsch55 - can I clarify? should I be disconnecting the wires running to the base of each of the two drivers for the channel?
EDIT - In circuit tests of Q3,4 follow.
Q3
b+e-: OC
b+c-: OC
e+b-: 658
e+c-: 1710-1760 (continued climbing)
c+b-: 653
c+e-: OC
Q4
b+e-: OC
b+c-: OC
e+b-: 1708
e+c-: OC
c+b-: 1949
c+e-: OC
The b- values for Q4 look too high and may be consistent with failure to switch on.
I'll test Q3,4.
Laatsch55 - can I clarify? should I be disconnecting the wires running to the base of each of the two drivers for the channel?
EDIT - In circuit tests of Q3,4 follow.
Q3
b+e-: OC
b+c-: OC
e+b-: 658
e+c-: 1710-1760 (continued climbing)
c+b-: 653
c+e-: OC
Q4
b+e-: OC
b+c-: OC
e+b-: 1708
e+c-: OC
c+b-: 1949
c+e-: OC
The b- values for Q4 look too high and may be consistent with failure to switch on.
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Q3 looks ok, the e+c- test is probably just charging up C3, expect climbing.
Regarding Q4, yes the b- measurements are high. In circuit measurement can be
tricky. For e+b-: 1708 . Maybe eb has failed open circuit and the path taken is
Q4e-R12-R9-Q3be-D1-Q4b. So you have 2 pn junctions = 1.3V and 2 1k resistors.
c+b-: 1949 Samiliar scenario, alternate path being R14-D15-D14-R7-Q4b
You could break this possible alternate path by removing F2 or F3.
Not really the outright failure I was looking for, you can try different things and
measurements but at the end of it you are never 100% sure until you replace the
part and then find out there is no change.
Voltages from III-5 and the Pheonix post show Q5 is not switched ON, Q4 controlls
Q5. Maybe Q4 faulty? Q4 controlled by Q1 (already replaced)
Regarding Q4, yes the b- measurements are high. In circuit measurement can be
tricky. For e+b-: 1708 . Maybe eb has failed open circuit and the path taken is
Q4e-R12-R9-Q3be-D1-Q4b. So you have 2 pn junctions = 1.3V and 2 1k resistors.
c+b-: 1949 Samiliar scenario, alternate path being R14-D15-D14-R7-Q4b
You could break this possible alternate path by removing F2 or F3.
Not really the outright failure I was looking for, you can try different things and
measurements but at the end of it you are never 100% sure until you replace the
part and then find out there is no change.
Voltages from III-5 and the Pheonix post show Q5 is not switched ON, Q4 controlls
Q5. Maybe Q4 faulty? Q4 controlled by Q1 (already replaced)
Apology, I was looking at the wrong page when I composed the post, looking at page III-5, F2, F3 referring to the fuses.
The post was meant to highlight how tricky in circuit measurements can be, in each case Q4 e+b- and Q4c+b-
there is an alternate path that MAY explain the higher than expected readings especially if Q4 has failed open.
Removing either fuses for the power transistors will break the path for the Q4 c+b- case. If you are bored you could
try it, maybe we both learn however regardless Q4 should be replaced, the problem will remain then ...
MPSA93s arrived - positive signs for fix.
Before replacement, I removed F2,3 and re-measured Q4 b- with diode tester: b-c+ gave OC, b-e+ not much change. This supports theory that Q4 was not opening.
Replaced Q4 above the board (cut pins and soldered new component to old pins). I would always recommend this for Q1-Q4 because the PCB traces are fine and too fragile. The heavier traces on a lot of the other parts of the board are not so bad (can be worked with care).
After replacement of Q4 DC offset seemed to be corrected (F2,3 restored) - limited time but will re-check this (results not ambiguous but affected by disbelief). Other results:
V drop across R12 went up to 245mV
Q5 (pins relative to ground): b-63.2; e-63.3; c-0.8 (fairly consistent with III-5)
Q4e and Q3e: 12 V (cf 22V in III-5)
Q1e and Q2e: -0.5 (consistent with -0.6 in III-5)
Q1c and Q2c: 11.2/11.1V (cf 21 in III-5)
This is looking promising to me. Will need to re-solder D9,10 for further checks.
Any suggestions about things I should check?
Cheers, Jon
Before replacement, I removed F2,3 and re-measured Q4 b- with diode tester: b-c+ gave OC, b-e+ not much change. This supports theory that Q4 was not opening.
Replaced Q4 above the board (cut pins and soldered new component to old pins). I would always recommend this for Q1-Q4 because the PCB traces are fine and too fragile. The heavier traces on a lot of the other parts of the board are not so bad (can be worked with care).
After replacement of Q4 DC offset seemed to be corrected (F2,3 restored) - limited time but will re-check this (results not ambiguous but affected by disbelief). Other results:
V drop across R12 went up to 245mV
Q5 (pins relative to ground): b-63.2; e-63.3; c-0.8 (fairly consistent with III-5)
Q4e and Q3e: 12 V (cf 22V in III-5)
Q1e and Q2e: -0.5 (consistent with -0.6 in III-5)
Q1c and Q2c: 11.2/11.1V (cf 21 in III-5)
This is looking promising to me. Will need to re-solder D9,10 for further checks.
Any suggestions about things I should check?
Cheers, Jon
MPSA93s arrived - positive signs for fix.
Before replacement, I removed F2,3 and re-measured Q4 b- with diode tester: b-c+ gave OC, b-e+ not much change. This supports theory that Q4 was not opening.
Replaced Q4 above the board (cut pins and soldered new component to old pins). I would always recommend this for Q1-Q4 because the PCB traces are fine and too fragile. The heavier traces on a lot of the other parts of the board are not so bad (can be worked with care).
After replacement of Q4 DC offset seemed to be corrected (F2,3 restored) - limited time but will re-check this (results not ambiguous but affected by disbelief). Other results:
V drop across R12 went up to 245mV
Q5 (pins relative to ground): b-63.2; e-63.3; c-0.8 (fairly consistent with III-5)
Q4e and Q3e: 12 V (cf 22V in III-5)
Q1e and Q2e: -0.5 (consistent with -0.6 in III-5)
Q1c and Q2c: 11.2/11.1V (cf 21 in III-5)
This is looking promising to me. Will need to re-solder D9,10 for further checks.
Any suggestions about things I should check?
Cheers, Jon
Before replacement, I removed F2,3 and re-measured Q4 b- with diode tester: b-c+ gave OC, b-e+ not much change. This supports theory that Q4 was not opening.
Replaced Q4 above the board (cut pins and soldered new component to old pins). I would always recommend this for Q1-Q4 because the PCB traces are fine and too fragile. The heavier traces on a lot of the other parts of the board are not so bad (can be worked with care).
After replacement of Q4 DC offset seemed to be corrected (F2,3 restored) - limited time but will re-check this (results not ambiguous but affected by disbelief). Other results:
V drop across R12 went up to 245mV
Q5 (pins relative to ground): b-63.2; e-63.3; c-0.8 (fairly consistent with III-5)
Q4e and Q3e: 12 V (cf 22V in III-5)
Q1e and Q2e: -0.5 (consistent with -0.6 in III-5)
Q1c and Q2c: 11.2/11.1V (cf 21 in III-5)
This is looking promising to me. Will need to re-solder D9,10 for further checks.
Any suggestions about things I should check?
Cheers, Jon
Q5 is still not turning on fully, need 0.3-0.65V drop across b-e.
Maybe between measurements there was some mains bounce
Q4e, Q3e at 12V is probably partly(?) attributed to power up with DBT
V drop across R12 went up to 245mV gives 0.245mA. At full power 80V rails
the current will be about 0.57mA, so 0.245mA is not doing any damage.
Likewise for Q1c and Q2c
Certainly keep an eye on the bias and dc offset.
Tempted to leave D9, D10 permanently out. This isolates the over current
protection circuit.
Also tempted to apply the fix to C6 as mentioned in the Pheonix post/link.
Connect 2 diodes across C6, one anode to +ve side of C6, cathode to ground.
Other diode cathode to +ve side of C6, anode to ground. Just about any diode
with a 0.65V drop will do, 1N4148...
Maybe between measurements there was some mains bounce
Q4e, Q3e at 12V is probably partly(?) attributed to power up with DBT
V drop across R12 went up to 245mV gives 0.245mA. At full power 80V rails
the current will be about 0.57mA, so 0.245mA is not doing any damage.
Likewise for Q1c and Q2c
Certainly keep an eye on the bias and dc offset.
Tempted to leave D9, D10 permanently out. This isolates the over current
protection circuit.
Also tempted to apply the fix to C6 as mentioned in the Pheonix post/link.
Connect 2 diodes across C6, one anode to +ve side of C6, cathode to ground.
Other diode cathode to +ve side of C6, anode to ground. Just about any diode
with a 0.65V drop will do, 1N4148...
