I think just replacing those and hoping for the best probably is wishful thinking tbh.
If just one output transistor has failed short circuit it will also show as a short across all the others because they are in parallel. That's a small point because each set should really be from the same batch so that they share current equally. In other words if you have to replace one then all three should be changed.
When they are removed you may well find the 0.2 ohms are OK, or at least all but one.
I would be surprised if Q315/316 had failed. Same for the TL431's. Unless they are reading obviously short circuit or at least very low ohms then they are impossible to check on a meter.
If the 100 ohm reads high or open circuit with one leg removed then yes, that has failed. That would also put Q314 under suspicion.
The 2k2, again check out of circuit.
The 2200uF cap will be fine I'm sure.
The meter beeping once etc can be caused by caps charging and also it is polarity dependent (which way round the meter leads are) as the meter test voltage/current will turn on or off semiconductor junctions. This is why parts needs checking when out of circuit rather than soldered in place.
If just one output transistor has failed short circuit it will also show as a short across all the others because they are in parallel. That's a small point because each set should really be from the same batch so that they share current equally. In other words if you have to replace one then all three should be changed.
When they are removed you may well find the 0.2 ohms are OK, or at least all but one.
I would be surprised if Q315/316 had failed. Same for the TL431's. Unless they are reading obviously short circuit or at least very low ohms then they are impossible to check on a meter.
If the 100 ohm reads high or open circuit with one leg removed then yes, that has failed. That would also put Q314 under suspicion.
The 2k2, again check out of circuit.
The 2200uF cap will be fine I'm sure.
The meter beeping once etc can be caused by caps charging and also it is polarity dependent (which way round the meter leads are) as the meter test voltage/current will turn on or off semiconductor junctions. This is why parts needs checking when out of circuit rather than soldered in place.
Thanks Mooly - I will retest all of these again today with the output transistors removed and double check any suspect components not in circuit this time.
No, all are definitely frazzled.
R370 : OL
R348 : 2.5kOhm (should be 75Ohm)
R354 : OL (Shows visible damage)
R352: OL
Correct about Q315 - removed it and it is OK.
Q316 is not OK - there is a short between base and emitter and was the reason for the false positive for Q315.
R337 (100ohm) : OL
Q314 : checks out fine, removed and tested alongside the the one from the working amp and they are very close in values with no shorts.
R341 should be 330Ohms but measures 280Ohms.
Correct! - out of circuit it was OK and reads 2.2 in circuit it reads 350ohms!
On the working amp it reads 2.2k in circuit which was what lead me to think it was not good.
Correct again!
Here are the revised failures..
Thank you!
I have a working left board and the faulty one side by side here. My first attempt was to go through each and every component in circuit looking for opens/shorts and comparing all the resistor values with those on the working board. That turned up a bunch of suspects but luckily Mooly has been guiding me with his wisdom and it turned out, after removing the components and testing them, that quite a few that looked bad were false positives caused by another failed component. So your advice is correct and testing each part in isolation is the way.
Mooly obviously has some experience and was able to evaluate the circuit and where I am likely going wrong - he may well advise me to do exactly as you suggest and remove all the parts and test them. I will do whatever is necessary and am very grateful for all the help. I'm learning something along the way too
Hmmm... so all those burned parts are a classic zapped output stage scenario... don't under estimate how difficult and time consuming this may become. The big big problem diagnosing at a distance is trying to piece together all the evidence and that is much more difficult than actually being able to work on it for real.
Also I don't want you to feel you have invested time and money and then after a lot of work still not end up having a working amp.
If you still want to carry on then there are a few tricks we can use to hopefully simplify it a little.
Another very real issue is getting replacement parts. Don't buy off eBay as there are so many fake and remarked parts around and so that then means we might be down to using alternative devices.
Also I don't want you to feel you have invested time and money and then after a lot of work still not end up having a working amp.
If you still want to carry on then there are a few tricks we can use to hopefully simplify it a little.
Another very real issue is getting replacement parts. Don't buy off eBay as there are so many fake and remarked parts around and so that then means we might be down to using alternative devices.
I understand and I would like to carry on. It's worth a try and I have some time to spend. I won't be upset if it ends up not being successful.
Here are the parts so far and some possible replacements all found on digikey.com....
Transistors:
Q321/327/323 (2SA1943 PNP) => [2SA1943-OQ-ND]
Q317 (2SD669A NPN) => [Fairchild's KSC2690YSFS-ND]
Q318 (2SB649A PNP) => [Fairchild's KSA1220AYSFS-ND]
Q316 (2SA1015) => [KSA1015YTACT-ND]
Resistors:
R370/354/352 0.2/1W 5% Fusible => [0.2ACCT-ND]
R348 75/0.5W 5% Metal Oxide Film => [A142677CT-ND]
R337 100/1W 5% Fusible => [A121516CT-ND]
R341 330/0.5W 5% Metal Oxide Film => [A142692CT-ND]
I will do the same checks now on the right channel and try to see what problems lie there...
OK...
Those look reasonable replacements. The UK Digikey site doesn't bring up the KSC2690 but I assume that is the complementary match to the KSA1220 (which does come up).
What I would suggest... how we go about this for faultfinding... is for you to initially fit just one single pair of output transistors, an upper NPN and the lower PNP device and leave the other two pairs out of circuit.
Also we might override the bias generator by linking out Q316 from C to E. That forces a zero bias condition.
And use of the bulb tester is a must.
Those look reasonable replacements. The UK Digikey site doesn't bring up the KSC2690 but I assume that is the complementary match to the KSA1220 (which does come up).
What I would suggest... how we go about this for faultfinding... is for you to initially fit just one single pair of output transistors, an upper NPN and the lower PNP device and leave the other two pairs out of circuit.
Also we might override the bias generator by linking out Q316 from C to E. That forces a zero bias condition.
And use of the bulb tester is a must.
Yep - it is the complementary match.
So I should desolder 2 of the NPNs and only fit one of the new PNPs?
Connect C to E on Q316?
Yes - Bulbs should arrive any day now and I will assemble the device
Interestingly - I went through the right side board and there doesn't appear to be anything obviously wrong there! No shorts/opens and all the resistors measure the correct values.
This was the side that got very hot very quickly last time I powered up the amp. Could that kind of behaviour be the result of the left side being blown or does it indicate possible problems i'm not seeing on the right board or perhaps on the main board?
Thinking about it - there have to be problems elsewhere, it can't just be the left board because the original damage caused by the swapped cables did not hurt the left board. I can make that assumption because after the probe slip mishap on the new amp swapping in the left board from the old amp fixed it - ergo the left board survived the original mistake.
Yes, just have one pair fitted, and it doesn't matter which one of the three in either row it is.
C to E on Q316 to be shorted or you can link the cap C315 that is across the same transistor. Whichever is easiest to work with.
The other board has to be treated with the same logic. If you can find no obvious parts failed then it has to be powered up and voltage measurements and observations made to determine the problem/s.
C to E on Q316 to be shorted or you can link the cap C315 that is across the same transistor. Whichever is easiest to work with.
The other board has to be treated with the same logic. If you can find no obvious parts failed then it has to be powered up and voltage measurements and observations made to determine the problem/s.
Excellent, thanks!
I will make the dim bulb tester tomorrow and then it's a case of waiting for the new parts to turn up - could be a few weeks with things as they are.
Is there any point in checking out any parts of the main board while waiting or is it best to start with the two left and right boards before moving on?
Is it worth removing and testing every single component on the right board, caps, diodes etc. if there is nothing wrong with any of the output transistors or resistors?
I wouldn't remove any more parts beyond anything that obviously seems faulty. The possibilities for introducing more problems are endless doing that. Careful measurement is used to reveal problem areas at each stage in testing. If you look at those threads I linked to earlier you will see the methodology. We never replace parts in hope, we take measurements every time instead.
As a general rule any high value resistors will be fine, anything over about 8k can not see more 0.5 watt dissipation even if you put it across one of the rails.
As a general rule any high value resistors will be fine, anything over about 8k can not see more 0.5 watt dissipation even if you put it across one of the rails.
Gotcha
Today I assembled the DBT.
It seems to work I think - I tried a plugging in another working lamp and that worked - the tester bulb did not light but the regular lamp did. I tried with a toaster and that lit up the tester - is that expected?
I ordered three different bulbs so I have..
The digikey parts should turn up early/mid next week
2x 150W
2x 200W
2x 300W
Which would be best for the amp?
The parts from DigiKey should turn up early/mid next week
The 150 W is best. Even that is a bit high tbh but it will have to do.
A toaster has a low resistance and so would appear as a low resistance in series with the bulb, the bulb getting the majority of the power available... so it lights brightly. The bulb sets the maximum current that can be drawn.
The same would happen if the amp drew to much current. The lamp would light as it sees more voltage across it and the voltage powering the amp would fall away.
------------------------------------------
A 150 watt bulb across 230 volt mains would draw around 0.65 amps (I=W/V)
The resistance is about 354 ohms (R=V/I)
So one to think about
If you measure the actual resistance of the bulb on your meter it will read only a few ohms... so how does that work then A few ohms across 230 volts would/should draw loads and loads of current... but the bulb doesn't
A toaster has a low resistance and so would appear as a low resistance in series with the bulb, the bulb getting the majority of the power available... so it lights brightly. The bulb sets the maximum current that can be drawn.
The same would happen if the amp drew to much current. The lamp would light as it sees more voltage across it and the voltage powering the amp would fall away.
------------------------------------------
A 150 watt bulb across 230 volt mains would draw around 0.65 amps (I=W/V)
The resistance is about 354 ohms (R=V/I)
So one to think about
If you measure the actual resistance of the bulb on your meter it will read only a few ohms... so how does that work then
A few ohms across 230 volts would/should draw loads and loads of current... but the bulb doesn't So one to think about
If you measure the actual resistance of the bulb on your meter it will read only a few ohms... so how does that work then
Hmm, good question - I had to google around to find that out. So the resistance of the filament quickly rises as it gets hot which is a result of the high voltage. More volts => more heat => more resistance => less current.
Is that right?
Shame I went for the 150W and not the 100W. I think I was reading somewhere that the bulbs wattage should be double that of the test device. Will it be OK with the 150W bulb?
That's absolutely right
A 150 watt bulb should still protect the output stage under a fault condition, it is after all still putting a hard limit on what can be drawn.
Hi Mooly
Hope you're keeping well!The DigiKey parts showed up this afternoon - so I made the replacements.
As it stands now....
Left board:
Q328 Output transistor is the only NPN in play.
Q321 Output transistor is the only PNP in play.
Q316 is shorted between E and C.
The driver transistor pair Q317 (NPN) and Q318 (PNP) have been replaced.
All frazzled resistors have been replaced.
Right board:
No obvious shorts or opens found.
So what's the next step? Fire her up with the DBT and take some readings?
At this stage should both boards be connected or is it better to do one side at a time as seemed to be the case in the thread you posted earlier?
Should there be any load attached?
Which measurements should I take? The bias offset between TP1/GND and between TP2/TP3 on the left board and TP5/GND and TP6/TP7 on the right?
Definitely work on only one channel at a time, leave the other disconnected.
No load is to be connected at any point at this stage.
The bulb must be in place at all times.
Make sure the output transistors have been fitted correctly and that any insulation washers are back in place.
-------------------------------------------
The first step is just to get the amp to power up without the bulb lighting (although it may flash brightly for a second at power on). If you get to that point then we check the DC output offset which should be close to zero.
You may need to measure the offset before any speaker relay which may or may not be functional at this point. Measure between ground and the point marked VOP L on the diagram which located at the junction of the 0.05 ohm resistors.
Good luck
No load is to be connected at any point at this stage.
The bulb must be in place at all times.
Make sure the output transistors have been fitted correctly and that any insulation washers are back in place.
-------------------------------------------
The first step is just to get the amp to power up without the bulb lighting (although it may flash brightly for a second at power on). If you get to that point then we check the DC output offset which should be close to zero.
You may need to measure the offset before any speaker relay which may or may not be functional at this point. Measure between ground and the point marked VOP L on the diagram which located at the junction of the 0.05 ohm resistors.
Good luck
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