Hello, Citation 16A with black face plate so it's a 16A. Works on both channel, lots power could acheive bias on both channel for 50mV across 0,47ohms output transistors emitters. Issue with one of the channel with 0,56volts DC offset.
Other channel is fine down to 10mV.
New caps on both boards, original tested resistors and transistors and zeners.
New 15T Bourns tripots on both channels. Solder reflowed.
Here's the voltages readings I get from trouble channel;
Q7 collector; 0,22v
Q12 collector; 64v
Q12 emitter; -0,6v
CR5 anode ; -4v
Q8 emitter; 0,7v
Q15 emitter; 64v
Q15 collector; 1v
Q18 collector; -0,7v
Q18 emitter; -2,2v
all voltages are a bit more negative value than supposed as per schematics and the good channel.
Is Q1 needs more investigation?
All small ceramic caps are there and tested OK, very high resistance.
R26 and R10 are new 3,9Kohms 3 watts type.
+ and - 10v are OK.
Does a leaky transistor could do that? Or leaky ceramic??
Thanks.
Other channel is fine down to 10mV.
New caps on both boards, original tested resistors and transistors and zeners.
New 15T Bourns tripots on both channels. Solder reflowed.
Here's the voltages readings I get from trouble channel;
Q7 collector; 0,22v
Q12 collector; 64v
Q12 emitter; -0,6v
CR5 anode ; -4v
Q8 emitter; 0,7v
Q15 emitter; 64v
Q15 collector; 1v
Q18 collector; -0,7v
Q18 emitter; -2,2v
all voltages are a bit more negative value than supposed as per schematics and the good channel.
Is Q1 needs more investigation?
All small ceramic caps are there and tested OK, very high resistance.
R26 and R10 are new 3,9Kohms 3 watts type.
+ and - 10v are OK.
Does a leaky transistor could do that? Or leaky ceramic??
Thanks.
The offset pot VR1 should be able to take care of this.
What range of offset does VR1 give when adjusting it?
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Yes, VR1 should drop it down, but I cannot reach a lower value with pot adjustment?? This is a brand new tested 1 Mohms 15T Bourns trimpot.
Of course I did compare all voltage on the good channel, but I don't have a bunch of new transistors and other parts in hand and I like to rely on more experimented members with the Citation 16A here....
Of course I did compare all voltage on the good channel, but I don't have a bunch of new transistors and other parts in hand and I like to rely on more experimented members with the Citation 16A here....
So the half volt of offset is the minimum possible, with VR1 set all the way to one end?
Something is wrong, then. The input stage is a dual matched transistor in a single package,
perhaps it has a problem, if all the capacitors were replaced.
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Bias issue persist???
Hello, just got new or NOS 2N2916 dual NPN tranasistors for replacing Q1.
Still with a new device , issue persist, Lower DC offset I can set is 0,56volts.
Here's Q1 voltages readings, both Emitters, Base and Collectors are the same.
Q1
E= -0,9v
B= -0,23v
C= +7,6v
Case is 0V...
All caps are replaced now, none of the original ceramics are on the board.
CR5 and CR6 are new also.
Both CR3 and CR4 are ok, at +/- 9,4 v
C1 and C5 are new replaced.
again all resistors measures fine to specs. no components overheats.
Plan on replacing CR1 and CR2....
will be back
Thanks.
Hello, just got new or NOS 2N2916 dual NPN tranasistors for replacing Q1.
Still with a new device , issue persist, Lower DC offset I can set is 0,56volts.
Here's Q1 voltages readings, both Emitters, Base and Collectors are the same.
Q1
E= -0,9v
B= -0,23v
C= +7,6v
Case is 0V...
All caps are replaced now, none of the original ceramics are on the board.
CR5 and CR6 are new also.
Both CR3 and CR4 are ok, at +/- 9,4 v
C1 and C5 are new replaced.
again all resistors measures fine to specs. no components overheats.
Plan on replacing CR1 and CR2....
will be back
Thanks.
DC offset issue persist
Hello, just got new or NOS 2N2916 dual NPN tranasistors for replacing Q1.
Still with a new device , issue persist, Lower DC offset I can set is 0,56volts.
Here's Q1 voltages readings, both Emitters, Base and Collectors are the same.
Q1
E= -0,9v
B= -0,23v
C= +7,6v
Case is 0V...
All caps are replaced now, none of the original ceramics are on the board.
CR5 and CR6 are new also.
Both CR3 and CR4 are ok, at +/- 9,4 v
C1 and C5 are new replaced.
again all resistors measures fine to specs. no components overheats.
Plan on replacing CR1 and CR2....
will be back
Thanks.
DC offset
Hi, after waiting for some parts, did all of the zener diodes replacements 10v and 5,1 v
Did also replaced 1N914 diodes.
Did tested R4 for 22kohms OK.
Tooked all pc board transistors for hfe tests
Q2-3-4-5-6-7-9 tested ok with hfe equal or better than 70 with Ib : 1mA
Q8-10-11 tested OK with hfe equal or better than 170 with Ib: 1mA
Q12-15-17 tested OK with hfe 48 and better and close.
?? Q13 have a lower hfe at 36 still with Ib : 1mA a MPSU60 PNP transistor ??
Q19-20 did measured Ok with hfe :190 with Ib: 1mA
Q21-22 are are hfe of 66 and 70 with Ib of 10mA
So basically Q13 have a lower hfe , may this pull down the DC offset to a constant 0,55v with VR1 all the way to -9,85 volts?
Considering his complementary transistor Q12 at a hfe of 50
Again all solder was reflowed and all resistances tests and measure OK.
All caps are new.
Lower voltage is +9,9v and - 9,85v
+65v and -65v for Vcc.
I did interchanged the good pc board driver on the other channel output transistor and issue remains on pc board either output transistors I can have 10mV DC offset, but not with this pc board .
thanks.
Hi, after waiting for some parts, did all of the zener diodes replacements 10v and 5,1 v
Did also replaced 1N914 diodes.
Did tested R4 for 22kohms OK.
Tooked all pc board transistors for hfe tests
Q2-3-4-5-6-7-9 tested ok with hfe equal or better than 70 with Ib : 1mA
Q8-10-11 tested OK with hfe equal or better than 170 with Ib: 1mA
Q12-15-17 tested OK with hfe 48 and better and close.
?? Q13 have a lower hfe at 36 still with Ib : 1mA a MPSU60 PNP transistor ??
Q19-20 did measured Ok with hfe :190 with Ib: 1mA
Q21-22 are are hfe of 66 and 70 with Ib of 10mA
So basically Q13 have a lower hfe , may this pull down the DC offset to a constant 0,55v with VR1 all the way to -9,85 volts?
Considering his complementary transistor Q12 at a hfe of 50
Again all solder was reflowed and all resistances tests and measure OK.
All caps are new.
Lower voltage is +9,9v and - 9,85v
+65v and -65v for Vcc.
I did interchanged the good pc board driver on the other channel output transistor and issue remains on pc board either output transistors I can have 10mV DC offset, but not with this pc board .
thanks.
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DC offset
HI thanks for answering Mooly, yes there is a 0,55volts dc at the speakers output with inputs to ground. Other channel is below 10mV DC. Both channel bias are fine and solid at 50mV DC.
It is a strait baseline on the scope no hf noise other than very low power supply normal residuals . and even if I did not yet replaced the big ps caps.
It did try both inputs and probes and have a 100MHz scope here, so I can go quite high HF but only yo see a clean normal DC value....
All zeners are replaced, but it seems that CR5 is a bit low at 4,8 v ?
Will go to try replacing Q13, as it is a long weekend here and won't be able to have the MSPU10 parts ....
HI thanks for answering Mooly, yes there is a 0,55volts dc at the speakers output with inputs to ground. Other channel is below 10mV DC. Both channel bias are fine and solid at 50mV DC.
It is a strait baseline on the scope no hf noise other than very low power supply normal residuals . and even if I did not yet replaced the big ps caps.
It did try both inputs and probes and have a 100MHz scope here, so I can go quite high HF but only yo see a clean normal DC value....
All zeners are replaced, but it seems that CR5 is a bit low at 4,8 v ?
Will go to try replacing Q13, as it is a long weekend here and won't be able to have the MSPU10 parts ....
Here is what I would do...
First question. Has the amp got an unknown history ? I ask that because you mustn't assume anything when it comes to faultfinding.
Does the voltage on the wiper of the preset (one end of R19) swing correctly from minus 10 to plus 10 volts ?
Has any unauthorised work been done with regard to bridging and bridge mode ? What voltage do you see on each side of R13. I would assume the point marked 'E7' is at zero volts but check.
CR5 you mention isn't an absolute voltage. There should be 5.1 volts or thereabouts across the Zener, and you should see -10 on the anode of the diode which is derived from CR4.
First question. Has the amp got an unknown history ? I ask that because you mustn't assume anything when it comes to faultfinding.
Does the voltage on the wiper of the preset (one end of R19) swing correctly from minus 10 to plus 10 volts ?
Has any unauthorised work been done with regard to bridging and bridge mode ? What voltage do you see on each side of R13. I would assume the point marked 'E7' is at zero volts but check.
CR5 you mention isn't an absolute voltage. There should be 5.1 volts or thereabouts across the Zener, and you should see -10 on the anode of the diode which is derived from CR4.
Amp is being used in hifi set-up, normal dust, no overheated pc traces,
E7 reads 0volt.
R19 measures 1 Megohms, and swing +9,9v and -9,85v.
Bridge wiring is not used, only E4 wire on both channel goes to ground.ie: power supply center tap.
Voltage on R13 across on bad pcb; 0,19v and from ground to each side of
R13: 0,58v and 0,54v
Voltage on R13 good pcb: 0,009v and 0,046v.
Again, using the bad pcb to the other channel output stage does still have this behaviour with a 0,55v dc at speakers posts.
Thanks.
E7 reads 0volt.
R19 measures 1 Megohms, and swing +9,9v and -9,85v.
Bridge wiring is not used, only E4 wire on both channel goes to ground.ie: power supply center tap.
Voltage on R13 across on bad pcb; 0,19v and from ground to each side of
R13: 0,58v and 0,54v
Voltage on R13 good pcb: 0,009v and 0,046v.
Again, using the bad pcb to the other channel output stage does still have this behaviour with a 0,55v dc at speakers posts.
Thanks.
I'll give it some thought tomorrow all being well.
A typo crept into my reply above, I meant R3, not R13 but you guessed that I think.
Quick things to consider:
1/ Check the DC voltage at the junction R1 and R2. Lets be sure that no stray DC voltage is appearing at the input causing a 'valid' offset condition. Compare the voltage with the good channel.
2/ The caps around the feedback network, C4 and C5 could imbalance the DC conditions if one were short or leaky. The AC performance would still be OK. That's definitely one to check.
3/ C3 is needed for stability but I'm not 100% sure what the effect would be if leaky.
4/ Check the voltage across R5 (560 ohm) is around 650 millivolts give or take. Compare with the good channel.
5/ Compare the voltages across the current mirror resistors, that's R6 and R7, R8 and R9, R24 and R25. Each pair should be similar. Compare to the good channel.
I'll look closer tomorrow, but have a look at the above first.
A typo crept into my reply above, I meant R3, not R13 but you guessed that I think.
Quick things to consider:
1/ Check the DC voltage at the junction R1 and R2. Lets be sure that no stray DC voltage is appearing at the input causing a 'valid' offset condition. Compare the voltage with the good channel.
2/ The caps around the feedback network, C4 and C5 could imbalance the DC conditions if one were short or leaky. The AC performance would still be OK. That's definitely one to check.
3/ C3 is needed for stability but I'm not 100% sure what the effect would be if leaky.
4/ Check the voltage across R5 (560 ohm) is around 650 millivolts give or take. Compare with the good channel.
5/ Compare the voltages across the current mirror resistors, that's R6 and R7, R8 and R9, R24 and R25. Each pair should be similar. Compare to the good channel.
I'll look closer tomorrow, but have a look at the above first.
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