Not very experienced here and running in circles so hoping for an helicopter view from more experienced users or an hindsight from experiences..
I am able basically to set only three out of four biases in their sweet spots (300-320mV).
The NPN left channel high-side bias is the problematic one (measured at R302 and set by R326 trimpot) as turning the pot makes no sensible change; the bias at startup is rather low at 50mV (while all other biases rise very promptly to sweet spots) and sloooowly rises up to 200-250mV at maximum over some 10 minutes or so.
I checked the output board, NPN side, comparing the channels and while components all seem to be working (diodes are ok, transistors in circuit appears to be ok too - except bias voltages on 0,2ohn 3W emitter resistors which are rather different on the 6 output transistors), I thought everything was ok.. then read about the Odep system (the front led is always lit as if the amp is fully working) and thought it might be the one to draw current from the output transistors somehow..
I checked voltages on the odep board, and it had not been possible to properly set odep + of the channel in question. I happened to find a resistor which sensibly drifted out of spec (R134). Substituted it and then hoped.. but still the amp bias is not working and odep + is still not
I then decided to take out the six output transistors but they all seem (seem, meaning that my diode tester procedure might not be fool proof, but I would assume they are all ok) to be ok and put them back in.
Of course I'll try to post a schematic as soon as possible, but any hint how to go on with what possibly to check will be appreciated, thanks!
I am able basically to set only three out of four biases in their sweet spots (300-320mV).
The NPN left channel high-side bias is the problematic one (measured at R302 and set by R326 trimpot) as turning the pot makes no sensible change; the bias at startup is rather low at 50mV (while all other biases rise very promptly to sweet spots) and sloooowly rises up to 200-250mV at maximum over some 10 minutes or so.
I checked the output board, NPN side, comparing the channels and while components all seem to be working (diodes are ok, transistors in circuit appears to be ok too - except bias voltages on 0,2ohn 3W emitter resistors which are rather different on the 6 output transistors), I thought everything was ok.. then read about the Odep system (the front led is always lit as if the amp is fully working) and thought it might be the one to draw current from the output transistors somehow..
I checked voltages on the odep board, and it had not been possible to properly set odep + of the channel in question. I happened to find a resistor which sensibly drifted out of spec (R134). Substituted it and then hoped.. but still the amp bias is not working and odep + is still not
I then decided to take out the six output transistors but they all seem (seem, meaning that my diode tester procedure might not be fool proof, but I would assume they are all ok) to be ok and put them back in.
Of course I'll try to post a schematic as soon as possible, but any hint how to go on with what possibly to check will be appreciated, thanks!
Assuming the microtech1000 schematic I have is similar but fewer output transistor pairs:
check the diode stack d301-304 or D401-404 to see if one is shorted across.
You need to also check the Vbe transistor q318 or q418 to see if the voltage across is too low. If it or any of the components say c306 c326 C406 c426 is shorted across, the bias voltage will not be high enough.
Q107 or q108 and Q207 Q208 the current limiters should not be crimping the drive to the predrivers Q307 Q318 Q417 Q419 too much. Affected somehow by the odep which I can't spot on the schematic.
Note diode check on DVM is done at 2 v so transistors or diodes can pass that check and break down at voltages they actually operate at. Checking current C to E through microamp scale of meter series a 47k resistor from a 12 v or 24 v supply is a better check. Or on diodes backwards current.
Same with capacitors, resistance check on DVM is even lower than 2 v these days. I don't know what idiot redesigned all the meters to check resistance at a few millivolts but makes DVM past 2010? fairly useless on ohms scale. Everything is "open".
check the diode stack d301-304 or D401-404 to see if one is shorted across.
You need to also check the Vbe transistor q318 or q418 to see if the voltage across is too low. If it or any of the components say c306 c326 C406 c426 is shorted across, the bias voltage will not be high enough.
Q107 or q108 and Q207 Q208 the current limiters should not be crimping the drive to the predrivers Q307 Q318 Q417 Q419 too much. Affected somehow by the odep which I can't spot on the schematic.
Note diode check on DVM is done at 2 v so transistors or diodes can pass that check and break down at voltages they actually operate at. Checking current C to E through microamp scale of meter series a 47k resistor from a 12 v or 24 v supply is a better check. Or on diodes backwards current.
Same with capacitors, resistance check on DVM is even lower than 2 v these days. I don't know what idiot redesigned all the meters to check resistance at a few millivolts but makes DVM past 2010? fairly useless on ohms scale. Everything is "open".
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Thanks a lot @indianajo I will check that as time allows, please let them coming shall you consider something worth to be checked
A quick update, with amp on I measured the left channel d301-2-3-4 diode stack and indeed the voltage at its ends is lower than optimar.
The voltage of the good channel (d401-4) is 2050mV at startup rising to about steady 2080 mV after some minutes, while the bad left stack voltage is 1300mV at startup slowly rising to 1800mV or so after some minutes which is no good.
I pulled a leg up and multimeter tells me all four diodes are ok.. I am tempted to substitute all four to avoid the hassle of properly measure them and be sure thery are ok, not sure I can do it right the way as need to check if I have them around.. will update..
The voltage of the good channel (d401-4) is 2050mV at startup rising to about steady 2080 mV after some minutes, while the bad left stack voltage is 1300mV at startup slowly rising to 1800mV or so after some minutes which is no good.
I pulled a leg up and multimeter tells me all four diodes are ok.. I am tempted to substitute all four to avoid the hassle of properly measure them and be sure thery are ok, not sure I can do it right the way as need to check if I have them around.. will update..
Hi geoturbo,
Is the attached the correct schematic? If not, would you link to a better source?
Thanks,
Steve
Is the attached the correct schematic? If not, would you link to a better source?
Thanks,
Steve
Steve, the Macrotech MA2400 schematic should be close enough, I am however attaching the Crown Microtech 2400 schematic as a reference (I am not sure where I sourced it from - googling for sure, however it is not available on the official site for some reason).
I see more differences from the MT1000 mentioned by @indianajo but they're all close relatives anyway me thinks.
Need to try swapping out the 1n4004 diodes next.. thanks all
I see more differences from the MT1000 mentioned by @indianajo but they're all close relatives anyway me thinks.
Need to try swapping out the 1n4004 diodes next.. thanks all
If I understand correctly, voltage across R302 doesn't respond to R326 pot. What is collector-emitter voltage across Q318? Does it respond to R316? How does it compare to the other channel? (Both in voltage and behavior re R316 counterpart?)
If base of q317 to base of q319 is not 3.6 v or higher, the diode stack d301 to d304 can't develop 2.4 v to 2.8 v. This seems to be a belt + suspenders approach of separating the bases of drivers q301 to q305. Either pair of points low in voltage will destroy any idle bias current. I don't see any dotted lines indicating q318 or d301 to d304 are installed on the heat sink. There are seven parts to possibly be shorted around Q318.
BTW, J500-9 to J600-9 could be shorted, too, taking the whole Vbe (q318) stack down in voltage.
BTW, J500-9 to J600-9 could be shorted, too, taking the whole Vbe (q318) stack down in voltage.
Thanks a lot guys, your shoulder support is much appreciated.
As for q318 or d301 to d304 correct, they are not installed on the heat sink.
Around q18 I see no shorted component, r329 and c309 are not present btw (I might have a different revision board).
Voltage b/w J500-9 to J600-9 on the good channel is about 3.4v whilst this is 2.5v on the bad one.
………………………………...…………………damn I just shorted something around q01 or q12 on the good channel, work for later or maybe should try to fix this one first now as I lost the references to good voltage measurement, deng I guess who does not make anything does not make mistakes………………………………………………
When I earlier measured - now I need to go by heart as I did not record them - q318 voltages they seemed in line with what were those measured on the formerly good channel (something around 20V between collector and emitter and 65mV - 0.065V - between base and emitter).
Resistance over r326 trim pot appeared linear and with no shorts/interruptions (still good pot me thinks), measured voltage with amplifier turned on was kind of weird as in a concave (contrary of u-shaped) way, being the bias current maximum (90-100mA whilst the bias current should be set to 300-320mA) at one single spot, while the behaviour of the good channel was linear.
The bias current was diminishing turning the pot either way.. fiddling with the pot up and down and putting the pot in the sweet spot again made somehow reach a higher bias current in the 200/250mA territory (behaviour of voltage remained a concave behaviour), hoping my description makes sense and you have further suggestions..
Thanks a lot so far
base of q17 to base of q19 is about 3.4 v in the good channel whilst I am measuring only 2.4v in the bad one.
As for q318 or d301 to d304 correct, they are not installed on the heat sink.
Around q18 I see no shorted component, r329 and c309 are not present btw (I might have a different revision board).
Voltage b/w J500-9 to J600-9 on the good channel is about 3.4v whilst this is 2.5v on the bad one.
Correct, I am not able to correctly set bias (as specified in the service manual this should be set at 300/320mV measuring voltage across r302) via r326 pot measuring voltage across r302.
………………………………...…………………damn I just shorted something around q01 or q12 on the good channel, work for later or maybe should try to fix this one first now as I lost the references to good voltage measurement, deng I guess who does not make anything does not make mistakes………………………………………………
When I earlier measured - now I need to go by heart as I did not record them - q318 voltages they seemed in line with what were those measured on the formerly good channel (something around 20V between collector and emitter and 65mV - 0.065V - between base and emitter).
Resistance over r326 trim pot appeared linear and with no shorts/interruptions (still good pot me thinks), measured voltage with amplifier turned on was kind of weird as in a concave (contrary of u-shaped) way, being the bias current maximum (90-100mA whilst the bias current should be set to 300-320mA) at one single spot, while the behaviour of the good channel was linear.
The bias current was diminishing turning the pot either way.. fiddling with the pot up and down and putting the pot in the sweet spot again made somehow reach a higher bias current in the 200/250mA territory (behaviour of voltage remained a concave behaviour), hoping my description makes sense and you have further suggestions..
Thanks a lot so far
Sorry about your misfortune on the working channel. As a precaution, I place shrink tube around meter probes so that there's minimal probe length exposed; same thing re the grounding ring on scope probes. I think I advocate fixing the new problem before returning to the original. Good luck.
I suspect Q318 is not being supplied with the correct operating current (d301-2-3-4 diode stack probably innocent), but I need to dig deeper to understand how the ODEP might be intervening .
I suspect Q318 is not being supplied with the correct operating current (d301-2-3-4 diode stack probably innocent), but I need to dig deeper to understand how the ODEP might be intervening .
You can pull any leg of d1 to d4 to get it out of the circuit while you test base of q317 to base of q319. Turn on briefly to take measurement. You can use two pamona grabbers instead of probes, to do the measuring whlle you turn power switch on and off. If it is below 3.2 v that stack or the parts outside P600-9 P500-9 have a problem.
Don't see how Q318 could be 20 v c to e. diode stack would clamp it through q317 & q319 to ~3.4 v.
Don't see how Q318 could be 20 v c to e. diode stack would clamp it through q317 & q319 to ~3.4 v.
Pulling a leg out of d1-4 the measured value between q17-q19 bases is about 2.4v, shall we assume then that the fault is still on the output board?
I will need to measure again voltages on q18 I don't have correct values now.
I will need to measure again voltages on q18 I don't have correct values now.
Could be in the components around Q18 on the output board. Could also be in that large number of components behind p500-9 and p600-9. Suspect those things are the odep. Check R341 R342 R343 R309, sometimes emitter sampler resistors get blown when the output transistors go. If that is not it isolating in the area around Q100 Q103 Q101 Q102 is going to be difficult.
Thank you Indianajo, this is a tricky one indeed, it is putting me back running in circles.
I mentioned Odep in the object because besides not being able to correctly set the high bias on the output board, I also found that I could not properly set odep+ by its trimmer. By chance I found r314 resistor on the Odep circuit (see low left hand corner of schematic) which was out of specification, substituting it voltages at U103 came back to normal (as those in the good channel) but trimmer r132 was not properly working anyway.
Maybe some active components around U103 were impacted by extra current demand which probably heated r314 in the first place?
There appears to be an ocean of components between U103 and Q318..
I mentioned Odep in the object because besides not being able to correctly set the high bias on the output board, I also found that I could not properly set odep+ by its trimmer. By chance I found r314 resistor on the Odep circuit (see low left hand corner of schematic) which was out of specification, substituting it voltages at U103 came back to normal (as those in the good channel) but trimmer r132 was not properly working anyway.
Maybe some active components around U103 were impacted by extra current demand which probably heated r314 in the first place?
There appears to be an ocean of components between U103 and Q318..
I remember reading btw that the odep is pulling down the Voltage Translator Stage, assuming from Q103 base (your "isolating in the area around Q100 Q103 Q101 Q102 is going to be difficult" phrase lighted half a bulb..)
Tracing and understanding the odep circuit is head-spinning. Maybe we can determine if some of the current sources are in shut-down.
If the amp is active, the Q105 and Q110 current sources have to be active. Would you measure the voltage across R109? Should be about 0.6V if the circuit is active. In like manner, what is the voltage across R140?
Thanks!
If the amp is active, the Q105 and Q110 current sources have to be active. Would you measure the voltage across R109? Should be about 0.6V if the circuit is active. In like manner, what is the voltage across R140?
Thanks!
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