Phase Linear 400 - DC offset problem
- Thread starter audiogymp
- Start date
Nicely spotted, mbz. If I'd seen that before my last order, I'd already have a bunch of them.
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.
I should stick with what is recommended in the service bulletin and I'll need to put together another shopping list. I'll include a few more MPSA93s. It might make sense to include some replacements for Q8 and Q9 but these are 2N1304 and 2N1305, which aren't stocked and I couldn't find any recommended subs. Any recommendations would be appreciated.
There might be another hiatus while I fill out the shopping list. I went a little overboard last time, so I'll have to work out some other things I need.
Edit - just saw Steve's post. will put in an order for ZTX694B with RS today.
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.
I should stick with what is recommended in the service bulletin and I'll need to put together another shopping list. I'll include a few more MPSA93s. It might make sense to include some replacements for Q8 and Q9 but these are 2N1304 and 2N1305, which aren't stocked and I couldn't find any recommended subs. Any recommendations would be appreciated.
There might be another hiatus while I fill out the shopping list. I went a little overboard last time, so I'll have to work out some other things I need.
Edit - just saw Steve's post. will put in an order for ZTX694B with RS today.
The 2N1304/5 are the germanium V/I limiters and may be a little tough to find. The amp will work fine without them at least for troubleshooting purposes. If you look at the 400 Series II you will notice the same type of VI limiting circuit but with silicon transistors and a couple of biasing resistors, that could be a solution if your germaniums are bad.
Craig
Craig
Ed has all the germaniums you want, contact him on Phoenix, user name oldphaser
EDIT: I have been watching this thread and because your amp is STILL not running, I posted the link to the guys that know these amps in and out. Hopefully you will get concrete results and your amp is up and running soon
EDIT: I have been watching this thread and because your amp is STILL not running, I posted the link to the guys that know these amps in and out. Hopefully you will get concrete results and your amp is up and running soon
Last edited:
laatsch55
Super Member
Q6 is the bias transistor on the backwall. Do an out of circuit test on it. D9, D10 isolate the protection circuit. To remove any potential problems involving the protection circuit, desolder either leg of D9, D10. 2N3439 will sub for for the 40327. Install a heatsink though. Full rail voltage on a 400 is 80 volts.Check D13, has to be out of circuit as it will show .078mv/dc in each direction. Q7, Q10 are the predrivers, no they don't have to be matched., A good sub for the 2N3403 is a 2N5088, You will have to remove the heatsink and install on the 5088. Q1,Q2, can be a TIS 97, GES 97, 5172 and really a host of others, MPSA18's even. Q3,Q4 MPSA93's or 92's...with MPSA92 being the preferred part.
Check the diodes on the backwall, not often but they can go out, once again, out of circuit. Dead shorts will show on an in circuit test if it is indeed shorted.
Check the diodes on the backwall, not often but they can go out, once again, out of circuit. Dead shorts will show on an in circuit test if it is indeed shorted.
laatsch55
Super Member
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
$$$, also mandatory if the original board/tracks are damaged.
Those improvements are not "minor"....
verses simply sourcing caps from Mouser etc... The WO main board does
look nice and has some minor improvements. The asking is not a lot of
$$$, also mandatory if the original board/tracks are damaged.
Those improvements are not "minor"....
Good input from laatsch55. Apologies on the "minor" improvements, difficult to judge from a pix. Noted the opamp? film caps, film resistors.
Didn't like single turn VR's. Each to their own. Will order main board and probably backplane. Will diy main caps.
Contribution from Northwinds has been "immeasureable".
D13 being damaged along the way rather than root cause?
Didn't like single turn VR's. Each to their own. Will order main board and probably backplane. Will diy main caps.
Contribution from Northwinds has been "immeasureable".
D13 being damaged along the way rather than root cause?
From sregor...Q1,2 tested faulty, must be replaced.
From laatsch55
Out of circuit test of Q6
Out of circuit test of D13, D14, D15
2N3439 will sub for for the 40327
good sub for the 2N3403 is a 2N5088
Q1,Q2, can be a TIS 97, GES 97, 5172
Q3,Q4 MPSA93's or 92's...with MPSA92 being the preferred part.
Suggest running the tests, all are passive/unit powered down.
Then only 1 more order for mouser...
From laatsch55
Out of circuit test of Q6
Out of circuit test of D13, D14, D15
2N3439 will sub for for the 40327
good sub for the 2N3403 is a 2N5088
Q1,Q2, can be a TIS 97, GES 97, 5172
Q3,Q4 MPSA93's or 92's...with MPSA92 being the preferred part.
Suggest running the tests, all are passive/unit powered down.
Then only 1 more order for mouser...
She will be running soon!
I've made a start but will need to continue tomorrow. The amp may have some surprises left.
The ZTX694Bs arrived today (faster than expected), so I was going to start with Q1 and Q2 replacements to see whether this resolved things. When I removed Q1 (and hit my first bit of lifting trace - grrr) I tested it out of circuit and it looks OK - component tester recognised it and diode tester gave readings of 691 and 697 with b+ but otherwise OC.
In light of this I went to the other tests before continuing with the Q1,2 replacements.
Q6 tested OK out of circuit - diode tester gave readings of 654 and 653 with b+ but otherwise OC.
TBC
The ZTX694Bs arrived today (faster than expected), so I was going to start with Q1 and Q2 replacements to see whether this resolved things. When I removed Q1 (and hit my first bit of lifting trace - grrr) I tested it out of circuit and it looks OK - component tester recognised it and diode tester gave readings of 691 and 697 with b+ but otherwise OC.
In light of this I went to the other tests before continuing with the Q1,2 replacements.
Q6 tested OK out of circuit - diode tester gave readings of 654 and 653 with b+ but otherwise OC.
TBC
62vauxhall
Addicted Member
FWIW, I embarked on a PL 700B repair project a couple of years ago which led me to the Phoenix site. It was not a new to me amp but one owned since 1990. Initially, the objective was restore to original but I soon learned, as you have experienced, the traces lift easily.
A replacement stock board was made available but once installed, bias could not be adjusted. Consensus was there was a short somewhere but I could not find it. After approximately 125 responded to posts on Phoenix, member Lee suggested sending it for professional help. It got the White Oak conversion making it a stupefyingly better amplifier.
From beginning to end the attempted repair process took many weeks. Diagnosis from afar is time consuming for those offering assistance and for me, frustrating with the re & re component testing and attempted fixes being as taxing to the PC board as they were.
A few months ago, I acquired a near mint PL 400 for what I considered a good price. Although fully working, the opinions from Phoenix were of disasters waiting to happen - just a matter of time.
Since I knew no history of the 400 and did not want to experience again the delicate nature of another original PL board, I cut to the quick and replaced it with one from White Oak. For me, it was a case of once bitten twice shy with the benefit of an improved amplifier.
A replacement stock board was made available but once installed, bias could not be adjusted. Consensus was there was a short somewhere but I could not find it. After approximately 125 responded to posts on Phoenix, member Lee suggested sending it for professional help. It got the White Oak conversion making it a stupefyingly better amplifier.
From beginning to end the attempted repair process took many weeks. Diagnosis from afar is time consuming for those offering assistance and for me, frustrating with the re & re component testing and attempted fixes being as taxing to the PC board as they were.
A few months ago, I acquired a near mint PL 400 for what I considered a good price. Although fully working, the opinions from Phoenix were of disasters waiting to happen - just a matter of time.
Since I knew no history of the 400 and did not want to experience again the delicate nature of another original PL board, I cut to the quick and replaced it with one from White Oak. For me, it was a case of once bitten twice shy with the benefit of an improved amplifier.
Thanks mbz for summarising the outstanding tests.
As stated in post 73: Q6 tested OK out of circuit - diode tester gave readings of 654 and 653 with b+ but otherwise OC.
Since then I did the following tests out of circuit:
There was a suggestion from laatsch55 about desoldering a leg of D9 and D10 to remove problems involving the protection circuit. Is this something I should try?
As stated in post 73: Q6 tested OK out of circuit - diode tester gave readings of 654 and 653 with b+ but otherwise OC.
Since then I did the following tests out of circuit:
D13 tested OK: 594, OC
D14 tested OK: 542, OC
D15 tested OK: 542, OC
I replaced Q1 and Q2 with a matched pair of ZTX694Bs (B=514 with component tester). The Bs for the removed pair were quite different (one almost double the other). However, new Q1 and Q2 made no difference to DC offset.D14 tested OK: 542, OC
D15 tested OK: 542, OC
There was a suggestion from laatsch55 about desoldering a leg of D9 and D10 to remove problems involving the protection circuit. Is this something I should try?
Q8,9 are part of the protection circuit/over current sensing. Probably not really needed. For your situation
this is the least of your concerns so yes, happy that you lift/remove D9, 10. Doubtful that it will make a difference
however it will remove a possible source and simplifies the circuit.
After Q1,2 replacement what is the voltage on Q1b?
Did you have replacements for Q3,4,5?
this is the least of your concerns so yes, happy that you lift/remove D9, 10. Doubtful that it will make a difference
however it will remove a possible source and simplifies the circuit.
After Q1,2 replacement what is the voltage on Q1b?
Did you have replacements for Q3,4,5?
Q1b voltage after replacement of Q1,2 is +15.2V (was +14.8 before replacement). Interesting - I should have checked this straight off. It's hard to imagine what might be causing this if it isn't a faulty Q1. I don't see how Q3,4 could be responsible.
I will open D9,10 if you think it is worthwhile. But I'm a little gun-shy about dealing with traces after Q1,2. When I tested D13, I cut and re-soldered above the board and I'm inclined to do this wherever possible.
I bought 3 each of the parts used on the board for 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). Mouser didn't have the A93 in stock, so they will be coming in a later shipment. It looks like A93 is the critical Q3,4 part because it can be used for either and must be used for Q4. But I have MPSA55, so could replace Q3, with a like part at least.
Q5 is 40327, for which you recommended 2N3439 as a sub. I bought 5 when I replaced Q10, so still have some left.
I will open D9,10 if you think it is worthwhile. But I'm a little gun-shy about dealing with traces after Q1,2. When I tested D13, I cut and re-soldered above the board and I'm inclined to do this wherever possible.
I bought 3 each of the parts used on the board for 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). Mouser didn't have the A93 in stock, so they will be coming in a later shipment. It looks like A93 is the critical Q3,4 part because it can be used for either and must be used for Q4. But I have MPSA55, so could replace Q3, with a like part at least.
Q5 is 40327, for which you recommended 2N3439 as a sub. I bought 5 when I replaced Q10, so still have some left.
I'm not a tech, so maybe you should be listening to others.
For me there are two possible fault scenarios. One where Q5
has failed so there is no current/volt drop down branch
R17,18,19. The other fault scenario is where a small amount
of dc leaks onto the line (Q1,2???) Q5c voltage rises
choking off current down branch R17,18,19.
Suggest measuring Q5b-e voltage, ie, one probe on R14, other on R16.
Coarse measurements show Vb roughly equal Ve, not the expected
0.5-0.6V. Maybe D4 and D5 have failed so we don't get the right b-e
voltage. Other possibility is because of DBT, the rails voltage is
not high enough for Q5 to bias ok (this is going on other peoples
comments, not sure I agree, rails at about 65V...)
With 60V on the speaker line, and 40.7 at R15/C7, 23V at Q1,2 emitters is
not surprising. I can't reconcile the 15V at Q1b apart from leakage.
Transistors correctly orientated?
I would lift D9 and D10 to simplify things.
For me there are two possible fault scenarios. One where Q5
has failed so there is no current/volt drop down branch
R17,18,19. The other fault scenario is where a small amount
of dc leaks onto the line (Q1,2???) Q5c voltage rises
choking off current down branch R17,18,19.
Suggest measuring Q5b-e voltage, ie, one probe on R14, other on R16.
Coarse measurements show Vb roughly equal Ve, not the expected
0.5-0.6V. Maybe D4 and D5 have failed so we don't get the right b-e
voltage. Other possibility is because of DBT, the rails voltage is
not high enough for Q5 to bias ok (this is going on other peoples
comments, not sure I agree, rails at about 65V...)
With 60V on the speaker line, and 40.7 at R15/C7, 23V at Q1,2 emitters is
not surprising. I can't reconcile the 15V at Q1b apart from leakage.
Transistors correctly orientated?
I would lift D9 and D10 to simplify things.
I checked terminals of the ZTX694Bs with component tester and they were the same pin-out arrangement as the MPS5172s they replaced. I checked the spec sheet now to confirm and it is consistent with installation orientation. So I'm pretty sure Q1,2 are correctly oriented.
I measured Q5b-e voltage and it came in at 0.3mV (yes, millivolts). This looks like it might be a possible problem.
I haven't lifted D9 or D10 yet.
Can I say that I appreciate your tenacity with this fix and the help I've received from others. While I'm not ready to throw in the towel, I can understand if this has taken up more of your time than you bargained for. I'm doing this to learn as much as to save money but in the end there is a plan B on this one and I can shell out the two and a bit hundred for a WO board if the fix fails. I appreciate your generosity with help but I wouldn't want to be taking up inordinate amounts of your time if you are starting to feel over it.
I measured Q5b-e voltage and it came in at 0.3mV (yes, millivolts). This looks like it might be a possible problem.
I haven't lifted D9 or D10 yet.
Can I say that I appreciate your tenacity with this fix and the help I've received from others. While I'm not ready to throw in the towel, I can understand if this has taken up more of your time than you bargained for. I'm doing this to learn as much as to save money but in the end there is a plan B on this one and I can shell out the two and a bit hundred for a WO board if the fix fails. I appreciate your generosity with help but I wouldn't want to be taking up inordinate amounts of your time if you are starting to feel over it.