Hi guys,
I'm currently repairing a C-Audio SR606, have made good progress, got it out of "protect" mode and passing audio on both channels. This required replacement of the +18v Zener diode and the TL071 at the input, and the R1 resistor in the protection circuit of channel B (all failed open.)
However, I can't set the bias right on channel B. I am reading 396mV between the P and N drive terminals, where I have been informed there should be 350mV. Adjusting VR1 (bias pot) does not affect this reading, in either direction. I have replaced VR1 with a known good pot and replaced the BF422 transistor in the bias circuit with a known good MPSA42. The other two resistors (1k5, 2k7) also measure good. The driver transistors 2SA958/2SC2168 measure good on my transistor tester. There's no DC at the amp output so presumably no shorted output devices.
I should add that I'm still powering it via a safety lightbulb, to mitigate any accidents caused by my ham fists and butter fingers, so the rails are a little lower than they should be - but the bias reading on Channel A is the correct 350mV value, so I assume Channel B's bias ought to work too!
I don't want to put it back into service without the bias working correctly. Can anyone suggest the next part to check?
Thanks, Matt
I'm currently repairing a C-Audio SR606, have made good progress, got it out of "protect" mode and passing audio on both channels. This required replacement of the +18v Zener diode and the TL071 at the input, and the R1 resistor in the protection circuit of channel B (all failed open.)
However, I can't set the bias right on channel B. I am reading 396mV between the P and N drive terminals, where I have been informed there should be 350mV. Adjusting VR1 (bias pot) does not affect this reading, in either direction. I have replaced VR1 with a known good pot and replaced the BF422 transistor in the bias circuit with a known good MPSA42. The other two resistors (1k5, 2k7) also measure good. The driver transistors 2SA958/2SC2168 measure good on my transistor tester. There's no DC at the amp output so presumably no shorted output devices.
I should add that I'm still powering it via a safety lightbulb, to mitigate any accidents caused by my ham fists and butter fingers, so the rails are a little lower than they should be - but the bias reading on Channel A is the correct 350mV value, so I assume Channel B's bias ought to work too!
I don't want to put it back into service without the bias working correctly. Can anyone suggest the next part to check?
Thanks, Matt
Hi
I have a number of C Audio amps and I have serviced and reset the bias on all of them. I use a higher bias of around 450mv.
Adjusting VR1 should alter the bias.
If it does not and you are sure the transistors are all ok and the resistors are ok and the wiring and connections around TR6 are ok I think you might find that R21 or R22 are not the correct value. I had to change a number of these on my amps to make things work out. I have found that 2.7K for R22 works and 1.5K for R21 works.
If you are changing out any burned out resistors use higher wattage as C audio scrimped on the wattage on a few resistors that have a high voltage through them.
Don
I have a number of C Audio amps and I have serviced and reset the bias on all of them. I use a higher bias of around 450mv.
Adjusting VR1 should alter the bias.
If it does not and you are sure the transistors are all ok and the resistors are ok and the wiring and connections around TR6 are ok I think you might find that R21 or R22 are not the correct value. I had to change a number of these on my amps to make things work out. I have found that 2.7K for R22 works and 1.5K for R21 works.
If you are changing out any burned out resistors use higher wattage as C audio scrimped on the wattage on a few resistors that have a high voltage through them.
Don
Hi Don
That is very interesting i was also struggling like member "studio45"
With my issue i could not turn the vr1 down far enough to reach 350mv as another expert suggested on another forum.
Do you suggest 450mv for an sr707?
thanks for your help.
Hi
I was not suggesting any value for the bias.
I have a lot of the amps 404,606 and 707. When I buy an amp I test it before using it. I found the bias on the amps varied considerably. The average across all the amps I bought was about 550mv.
The Hitachi output lateral mosfets on the amp are so fast that I am not sure a high bias does much for the sound. I just use about 60 or 70mv per output mosfet.
When working on my amps I change R24, R34 and R50 for 27k 1w resistors.
You also mention using a light bulb. I use a light bulb on a flying lead across the fuse holder in the soft start system instead of the fuse. I soldered the ends of the flying lead to a dead fuse. A normal light bulb set up does not work as it often causes the soft start to oscillate.
don
I was not suggesting any value for the bias.
I have a lot of the amps 404,606 and 707. When I buy an amp I test it before using it. I found the bias on the amps varied considerably. The average across all the amps I bought was about 550mv.
The Hitachi output lateral mosfets on the amp are so fast that I am not sure a high bias does much for the sound. I just use about 60 or 70mv per output mosfet.
When working on my amps I change R24, R34 and R50 for 27k 1w resistors.
You also mention using a light bulb. I use a light bulb on a flying lead across the fuse holder in the soft start system instead of the fuse. I soldered the ends of the flying lead to a dead fuse. A normal light bulb set up does not work as it often causes the soft start to oscillate.
don
Hi AMV8
I hope i am not to side step too far away from the original question.
I just wonder, would switching out the old tl071's in the C-audio's for perhaps opa134's which have a better spec be an option? i read they are a drop in replacement option.
My R24 R34 and R50 are already 27ohm 1w. i think it does get hot in those areas.
thanks neops : )
I hope i am not to side step too far away from the original question.
I just wonder, would switching out the old tl071's in the C-audio's for perhaps opa134's which have a better spec be an option? i read they are a drop in replacement option.
My R24 R34 and R50 are already 27ohm 1w. i think it does get hot in those areas.
thanks neops : )
Hi qwerp, great minds think alike eh 
I'm still working through the problem - I think I have some more broken parts to find. On the faulty board I am measuring about -5v DC at the bottom of the bias resistor chain, on the working board I have about +5v there, that can't be right! So I can turn *up* the bias now, but it still won't go down beyond 396mV. I theorise that this is because the bias adjust circuit isn't connected to both supply rails properly. This may be a problem further back in the circuit.
Still finding occasional conflicts between what's on my board and the schematics too. I definitely have a later/earlier revision. But it's close enough to make some sense.
I think I could probably put the amp back into service without anything blowing up. But I would not sleep easy at night. Those MOSFETs are nearly unobtanium now and £lots....don't want to risk wrecking the whole lot of them.
Thanks for your help so far
I'm still working through the problem - I think I have some more broken parts to find. On the faulty board I am measuring about -5v DC at the bottom of the bias resistor chain, on the working board I have about +5v there, that can't be right! So I can turn *up* the bias now, but it still won't go down beyond 396mV. I theorise that this is because the bias adjust circuit isn't connected to both supply rails properly. This may be a problem further back in the circuit.
Still finding occasional conflicts between what's on my board and the schematics too. I definitely have a later/earlier revision. But it's close enough to make some sense.
I think I could probably put the amp back into service without anything blowing up. But I would not sleep easy at night. Those MOSFETs are nearly unobtanium now and £lots....don't want to risk wrecking the whole lot of them.
Thanks for your help so far
(darn, i lost my post again , I must remember to right click save before posting....shall try again)
hi studio45
the only way i found my issue was to replace a link in the circuit from the other channel board
I hope this does not sound too obvious or a waste of time...but here goes...
if you have two chains A and B
A is working
B not working
by removing links from B and replacing with links from A...even if it means breaking traces or simply removing a few single components and soldering wires to reach over to the working board. it is possible to find the fault, sometimes it is as fast this way to pin point the section of the circuit at fault than to replace all the components and triple checking all the pads and traces.
when the failed sction of the circuit is pin pointed you can focus all your energy on repairing that part (pads traces replace components)
i have no idea if this is at all helpful. but worth a try.
btw the fans are expensive too. not easy to get one at a reasonable cost with the same spec.
kind regards
hi studio45
the only way i found my issue was to replace a link in the circuit from the other channel board
I hope this does not sound too obvious or a waste of time...but here goes...
if you have two chains A and B
A is working
B not working
by removing links from B and replacing with links from A...even if it means breaking traces or simply removing a few single components and soldering wires to reach over to the working board. it is possible to find the fault, sometimes it is as fast this way to pin point the section of the circuit at fault than to replace all the components and triple checking all the pads and traces.
when the failed sction of the circuit is pin pointed you can focus all your energy on repairing that part (pads traces replace components)
i have no idea if this is at all helpful. but worth a try.
btw the fans are expensive too. not easy to get one at a reasonable cost with the same spec.
kind regards
Hi
I have not found any advantage in changing the TL071. I do not think it is a weak spot.
If you want to make improvements to the amp I would change out the electrolytic capacitors on the pcb for higher quality ones. I have used Elna Silmic.
After that it is a bit more difficult. I have modified the wiring. Keeping low voltage away from high voltage and AC away from DC. It means opening the wiring looms and requires some rewiring but it does help clean up the signal. However care must be taken due to the voltages involved.
I also meant to add earlier that R24 R34 and R50 should be raised slightly off the pcb to help air flow.
Don
I have not found any advantage in changing the TL071. I do not think it is a weak spot.
If you want to make improvements to the amp I would change out the electrolytic capacitors on the pcb for higher quality ones. I have used Elna Silmic.
After that it is a bit more difficult. I have modified the wiring. Keeping low voltage away from high voltage and AC away from DC. It means opening the wiring looms and requires some rewiring but it does help clean up the signal. However care must be taken due to the voltages involved.
I also meant to add earlier that R24 R34 and R50 should be raised slightly off the pcb to help air flow.
Don
Now then, thanks for all your input so far - a week has passed, and the damn thing's still on the bench.
There's something really fishy going on with channel B. The main symptom that I can identify, is that the current source for the driver stage is sourcing a lot more current than its brother on channel A.
On ch. A, there is little to no voltage drop measurable across the large 4k7 power resistors in the driver stage. On ch B, they are dropping nearly 30v, indicating quite a lot of current is flowing there.
I have replaced all the transistors in the ch B driver stage with known good units, and still the problem persists (and nothing else has changed either).
Has anyone got a good idea why this might be happening?
TIA,
Matt
PS Still on the lookout for "SR DRIVER 3 ISS 4" schematics, if any has seen them...my boards are some later version than is depicted in the commonly available PDF (which is marked "REVISION 2")
There's something really fishy going on with channel B. The main symptom that I can identify, is that the current source for the driver stage is sourcing a lot more current than its brother on channel A.
On ch. A, there is little to no voltage drop measurable across the large 4k7 power resistors in the driver stage. On ch B, they are dropping nearly 30v, indicating quite a lot of current is flowing there.
I have replaced all the transistors in the ch B driver stage with known good units, and still the problem persists (and nothing else has changed either).
Has anyone got a good idea why this might be happening?
TIA,
Matt
PS Still on the lookout for "SR DRIVER 3 ISS 4" schematics, if any has seen them...my boards are some later version than is depicted in the commonly available PDF (which is marked "REVISION 2")
Hi
The test procedure for the amp is
1 Test the power supply and make sure the correct voltages are being delivered to each channel. If unsure what is happening replace the fuse in the soft start with a 100w 240v light bulb. This drops the voltage to about 60 % of the normal.
2Test the input pair, check current to each transistor and ensure no dc out to the next stage.
3 Test the next stage; again check the driver pair and the current mirror also the standing currents and ensure no dc out of the stage
4 Check the next driver stage and the current mirrors
5 check the output transistors. The easy first check is to see if any source resistors have any dc voltage across them. They should not have.
6 Check step by step frm the power supply omwards.
don
The test procedure for the amp is
1 Test the power supply and make sure the correct voltages are being delivered to each channel. If unsure what is happening replace the fuse in the soft start with a 100w 240v light bulb. This drops the voltage to about 60 % of the normal.
2Test the input pair, check current to each transistor and ensure no dc out to the next stage.
3 Test the next stage; again check the driver pair and the current mirror also the standing currents and ensure no dc out of the stage
4 Check the next driver stage and the current mirrors
5 check the output transistors. The easy first check is to see if any source resistors have any dc voltage across them. They should not have.
6 Check step by step frm the power supply omwards.
don
Hi neops yes replaced those fans. Turns out maybe the old ones weren't so bad anyway, but probably a good idea after 25 years of service!
After checking every single MOSFET on ch B on a transistor tester, I found 3 PMOS (J50) reading gate capacitance of >100uF, where all others read under 100pF. I guess they must be bad?
However, incorrect voltage readings still persist even with them removed, so they weren't the cause of the fault.
Thanks for the test procedure AMV8, I'll get on that today....
After checking every single MOSFET on ch B on a transistor tester, I found 3 PMOS (J50) reading gate capacitance of >100uF, where all others read under 100pF. I guess they must be bad?
However, incorrect voltage readings still persist even with them removed, so they weren't the cause of the fault.
Thanks for the test procedure AMV8, I'll get on that today....
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