I had completely overlooked this statement...
That, of course, put an end to any further attempts for now.
Installing a new transistor only really makes sense if you get a variable transformer ("variac"), which allows you to slowly increase the operating voltage while monitoring the current consumption.
As soon as anything unusual happens (DC at the output, etc.), immediately remove the voltage!
That's always my basic procedure when I receive an unknown device for testing.
But the question still remains: Why did the transistor burn out? Have you checked all the solder joints carefully? Is there a small solder short somewhere?
It is an absolute mystery! As I said initially, all I did was remove and re-cap the Main A amp board. I have scrutinised the board multiple times and simply cannot see any issues. So frustrating. Why did I not just leave it alone 🙁
Don't be frustrated. You have many fellow sufferers. 🤢
And the old saying holds true: You learn from your mistakes...
What you're experiencing isn't uncommon when working on power amplifiers.
For one thing, the transistors are sensitive. And the energy stored in the large electrolytic capacitors in the power supply is often forgotten. Even though the amplifier has been turned off for a long time, there's still a residual charge. A moment's carelessness when unplugging a connector or soldering on an installed circuit board can have dire consequences.
Always check the large capacitors for complete discharge. If they aren't, discharge them through a sufficiently high-resistance resistor (>1 kOhm).
And the old saying holds true: You learn from your mistakes...
What you're experiencing isn't uncommon when working on power amplifiers.
For one thing, the transistors are sensitive. And the energy stored in the large electrolytic capacitors in the power supply is often forgotten. Even though the amplifier has been turned off for a long time, there's still a residual charge. A moment's carelessness when unplugging a connector or soldering on an installed circuit board can have dire consequences.
Always check the large capacitors for complete discharge. If they aren't, discharge them through a sufficiently high-resistance resistor (>1 kOhm).
2SJ200 being a power FET device, can not easily be damaged the way you described (dead short throughout..??). This could potentially happen (only?) if you caused a short before the protection circuit, e.g. where the FET is soldered on the PCB. Also, its gate had to sit at the potential required to fully open that FET (I think even 1V could have caused the damage; 4V would have fully opened that FET and let full available current from the power supply to run through)... These two fault instances can not easily just happen, so yes it is a mystery to me as well how you managed to cause such damage.
Also, I see that you were powering up the amp with boards missing (boards unplugged). I do not believe this is okay... and it could be one of the reasons why you are where you are now. Does the service manual state anywhere that you can power up the amp with some (a particular one, a combination of... a few) boards missing..??
See if anyone (a TV repair service, a HiFi audio shop that also provides repairs) will attempt to fix the amp... just let them know what happened, what you did, the damage so far (know to / caused by you...) and who knows... you might get lucky.
Also, I see that you were powering up the amp with boards missing (boards unplugged). I do not believe this is okay... and it could be one of the reasons why you are where you are now. Does the service manual state anywhere that you can power up the amp with some (a particular one, a combination of... a few) boards missing..??
See if anyone (a TV repair service, a HiFi audio shop that also provides repairs) will attempt to fix the amp... just let them know what happened, what you did, the damage so far (know to / caused by you...) and who knows... you might get lucky.
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Assuming all four output transistors are removed, I would suggest a variation on @maudio's suggestion above. Tack a 22k resistor between the negative terminal of C408 and the emitter of Q415. This should restore DC feedback and allow the frontend to bias. With luck, you should see ~0V at Q415 emitter and about +3.5V at Q414 emitter. The Q414 emitter should respond to adjustment of the RT402 bias pot. Set the pot to minimum spreader voltage.
Take advantage of the absence of the output devices to confirm bias voltages at the gate resistors R430-R433 and viable source resistors R434-R437.
If all looks good, remove the 22k resistor and install a single pair of N and P outputs, no load attached. With more luck, the protection circuits will be satisfied and the relay will close. Install the remaining pair and adjust offset and bias.
Good luck!
Ah I have put all 8 (working) output transistors back in the unit now and put it all back together. Still getting high current draw and protection mode.
Some more observations that I took when the output transistors were removed:
With the amp powered off, Q466 measured about 40V (residual cap voltage) at pin 2, -200mV ish at pin 1, and 225mV ish at pin 3.
With it powered on, I get 57V at pin 2, -15V ish at pin 1, and 15V ish at pin 3.
Driver transistor measurements when powered on:
Q464
B - -5v
C - 60v
E - -6v
Q465
B - -11.5V
C - -61V
E - -10V
Q414
B - -53v
C - 61v
E - -54v
Q415
B - -60V
C - -61V
E - -59V
All of these transistors tested fine when out of circuit.
Some more observations that I took when the output transistors were removed:
With the amp powered off, Q466 measured about 40V (residual cap voltage) at pin 2, -200mV ish at pin 1, and 225mV ish at pin 3.
With it powered on, I get 57V at pin 2, -15V ish at pin 1, and 15V ish at pin 3.
Driver transistor measurements when powered on:
Q464
B - -5v
C - 60v
E - -6v
Q465
B - -11.5V
C - -61V
E - -10V
Q414
B - -53v
C - 61v
E - -54v
Q415
B - -60V
C - -61V
E - -59V
All of these transistors tested fine when out of circuit.
All voltages negative and collectors of both drivers at -60-ish volt, looks like you are missing the positive supply rail. Always start with verifying the power supply rails. Blown rectifier might cause this.
Correct, the front end of the power amp has a seperate supply. But it looks like you're missing the main power supply to the final stage, the B+ coming from C702 in the schematic. Could be a missing connection, or D705 is blown.
The amount of fusible resistors in this amp is insane btw. Are you sure they are all still in decent shape?
The amount of fusible resistors in this amp is insane btw. Are you sure they are all still in decent shape?
Gotcha. This is why I should probably stick to IT 😂
I have checked all the fusibles and they measure fine. But that was a few days ago. I will do the following and post the results:
Please let me know if I should check anything else while I'm there.
I want to build a dim bulb tester but the lack of availability of incandescent bulbs is not helpful
I have checked all the fusibles and they measure fine. But that was a few days ago. I will do the following and post the results:
- Check the power supply voltages
- Re-check the fusibles
- Check D705
- Check the voltages to/from C702 are correct
Please let me know if I should check anything else while I'm there.
I want to build a dim bulb tester but the lack of availability of incandescent bulbs is not helpful
Not sure about the UK but filament bulbs are not hard to get here, it's still legal to sell old stocks and a lot of shops do so. Otherwise try second hand stores.
I would not install the outputs yet, follow the suggestion from BSST to close the loop and verify everything is healthy first.
I would not install the outputs yet, follow the suggestion from BSST to close the loop and verify everything is healthy first.
Yeah they're properly outlawed here so it's gonna be an eBay job I think, and even at a quick glance I can only see packs of 10!
FYI - you can usually get heat lamps from 150W upwards for your dim bulb tester. You know the lamps used for keeping food hot, or maybe used for a sick animal or lizard etc.
Again the fusibles - every amp I have ever repaired with those they have gone open or way out of whack.
Again the fusibles - every amp I have ever repaired with those they have gone open or way out of whack.
I've had a bit of a breakthrough
I completely removed the Main A board that I recapped and replaced it with the spare one I have from a donor amp
It still draws about 300W and the right channel's MOSFETs get very warm... But it comes out of protection and stays out of protection!!!
I guess I need to look at the offset and bias settings as they're likely to be way out of whack. Is there anything else that might cause this? The amp always ran warm even when idling and apparently this is normal.
I completely removed the Main A board that I recapped and replaced it with the spare one I have from a donor amp
It still draws about 300W and the right channel's MOSFETs get very warm... But it comes out of protection and stays out of protection!!!
I guess I need to look at the offset and bias settings as they're likely to be way out of whack. Is there anything else that might cause this? The amp always ran warm even when idling and apparently this is normal.
Sigh
I got the bias for the right channel down to 35mv where it should be and the power draw shot right down
Then I went for the left channel bias adjuster and it was also showing high, and I stupidly tweaked it anticlockwise rather than clockwise to reduce, at which point one of the main fuses popped and now the J200s and K1529s on that channel are all shorted.
Great.
Anyone got any spares they can sell me, or is there a modern equivalent I can use?!
Two steps forward, one step back...
I got the bias for the right channel down to 35mv where it should be and the power draw shot right down
Then I went for the left channel bias adjuster and it was also showing high, and I stupidly tweaked it anticlockwise rather than clockwise to reduce, at which point one of the main fuses popped and now the J200s and K1529s on that channel are all shorted.
Great.
Anyone got any spares they can sell me, or is there a modern equivalent I can use?!
Two steps forward, one step back...
They might not all be shorted if they are in parallel - take out one of each and measure again? If two of them are in parallel for example, and one is shorted, you will see a short on your DMM (I haven't looked at the schematic but its a likely scenario)
The amp has two K1529s and two J200s per channel.
One J200 survived, both K1529s died. Which sucks as I have spare J200s but only one spare 1529 😢
One J200 survived, both K1529s died. Which sucks as I have spare J200s but only one spare 1529 😢
It looks like I can use modern close equivalents IRFP340/9240 - as per this thread: https://www.diyaudio.com/community/threads/sony-ta-f770es-output-replacement.364196/ I just need to modify the temperature compensation circuit slightly.
Would anyone be able to tell me exactly what I need to change? Then I can get all the output transistors replaced with these new ones, the temp compensation adjusted, and the whole situation put to bed!
Would anyone be able to tell me exactly what I need to change? Then I can get all the output transistors replaced with these new ones, the temp compensation adjusted, and the whole situation put to bed!