5. Rectifier diodes shorted!?
6. What about the BD135/BD136?
7. If transformer had a defective secondary winding and the output transistors were shorted current has been very high. Are the PCB tracks all still conducting end to end?
8. What about the mains fuse? Also blown?
9. If the mains fuse was still OK what value does it have?
10. “some blown resistors and transistors in the phono power supply.” Unusual! Did it die from overvoltage/ligtning strike?
No something is incorrect. My hypothesis based on extremely high education says that it otherwise would work OK 😀
6. What about the BD135/BD136?
7. If transformer had a defective secondary winding and the output transistors were shorted current has been very high. Are the PCB tracks all still conducting end to end?
8. What about the mains fuse? Also blown?
9. If the mains fuse was still OK what value does it have?
10. “some blown resistors and transistors in the phono power supply.” Unusual! Did it die from overvoltage/ligtning strike?
No something is incorrect. My hypothesis based on extremely high education says that it otherwise would work OK 😀
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To @fencki
Here are some key voltages. Can you measure them and see if you have obvious differences. Can you see the progression working back from the rails, the magic number of around 0.6 to 0.7 volts across each junction.
Have a measure and tell us what you actually have.
Here are some key voltages. Can you measure them and see if you have obvious differences. Can you see the progression working back from the rails, the magic number of around 0.6 to 0.7 volts across each junction.
Have a measure and tell us what you actually have.
Hi.
There is one update.
I connecrted the amp and tried to hear some music.
Nothing changed from previous checks.
now i have a very low and disorted music coming from the outputs.
One channel no DC-offset - other one still 380mV.
now i am even more confused.
how long can I power the amp without destroying the output transistors (without heatsink)
🤔
There is one update.
I connecrted the amp and tried to hear some music.
Nothing changed from previous checks.
now i have a very low and disorted music coming from the outputs.
One channel no DC-offset - other one still 380mV.
now i am even more confused.
how long can I power the amp without destroying the output transistors (without heatsink)
🤔
problem solved with low volume and disorted: tape monitor was pressed
🙄
stupid mistake!
i have music playing from both channels.
now i have only one output with high DC-offset (380mV) and disotion with higher levels...
second one is almost zero (6-8mV) and no disortion
i am very confused why it works now... i have just reconnected it to the case...
🤔
now i have to find the problem with DC-offset.
🙄
stupid mistake!
i have music playing from both channels.
now i have only one output with high DC-offset (380mV) and disotion with higher levels...
second one is almost zero (6-8mV) and no disortion
i am very confused why it works now... i have just reconnected it to the case...
🤔
now i have to find the problem with DC-offset.
Last edited:
It 100% depends on the current through them. For a working amp I would the time at less than a couple of seconds. If the amp is faulty and no current flows then they will be OK indefinitely. SImple answer is never ever run without the heatsink.
Measure the current in the faulty channel and compare to the good one. Do this with no load attached and by measuring across the 0.22 ohm (or 0.47 in your case) resistors and calculate the current.
I measured about 300mV so it should be about 640mA on the good side.
On the bad side, i have only 40mV on one of the 0R47 reistors. the second 0R47 resistor has 0V.
On the bad side, i have only 40mV on one of the 0R47 reistors. the second 0R47 resistor has 0V.
Was the bad channel tested with or without a load? Always test with no load for faultfinding.
Did you measure the key voltages I mentioned in post#43?
Did you measure the key voltages I mentioned in post#43?
yes, i did measure without input and without load.
Ua ist about 24V now, when transistors work.
R28: 23,8V
R26: -23,6V
R20: 23,2V
R19: -23V
TR3 (Collector): 23,95V
TR1 (Collector): -23,7V
Looks Ok for me...
Ua ist about 24V now, when transistors work.
R28: 23,8V
R26: -23,6V
R20: 23,2V
R19: -23V
TR3 (Collector): 23,95V
TR1 (Collector): -23,7V
Looks Ok for me...
Those are low but we need to know why 🙂
You are going to have to do a lot more measuring to build up a picture of what is happening.. Look at the image I posted at post #43 and the eight measurements. The actual measured supply voltage is important because all the other voltages relate to that.
There is no easy way, you have to do all the measurements. Its the only thing that can give us all a clue what is happening.
You are going to have to do a lot more measuring to build up a picture of what is happening.. Look at the image I posted at post #43 and the eight measurements. The actual measured supply voltage is important because all the other voltages relate to that.
There is no easy way, you have to do all the measurements. Its the only thing that can give us all a clue what is happening.
I have done the 8 measurements. it is here 2 posts before.
all 8 measurements look OK to me.
or did I miss something else?
all 8 measurements look OK to me.
or did I miss something else?
There is nothing obvious there.
TR1 and TR3 collectors and the R19 and R20 readings show TR5 and TR7 are being turned on fairly hard and that is reducing the current the output stage will pass.
If either R19 or R20 (the 2k2's) had gone seriously high in value it could do this kind of fault and they do have around 20 volts or more across them in normal conditions. That's enough to cause failure on low wattage parts. They would either need checking out of circuit or if that is difficult than just dab another resistor across each in turn. Anything in the 10k to 100k region should work to prove it or not.
TR1 and TR3 collectors and the R19 and R20 readings show TR5 and TR7 are being turned on fairly hard and that is reducing the current the output stage will pass.
If either R19 or R20 (the 2k2's) had gone seriously high in value it could do this kind of fault and they do have around 20 volts or more across them in normal conditions. That's enough to cause failure on low wattage parts. They would either need checking out of circuit or if that is difficult than just dab another resistor across each in turn. Anything in the 10k to 100k region should work to prove it or not.
R19 and R20 are 3W and new from mouser.
and i only tried it a few seconds out, so the are not burned i think...
maybe the outputs defect?
and i only tried it a few seconds out, so the are not burned i think...
maybe the outputs defect?
It is certainly very strange. The outputs seem to show a reasonable base/emitter volt drop and so if they were faulty it would suggest that they are open in the collector channel but I think that would show as a (very) high offset... unless both were faulty in the same way. That would be a bit of a coincidence though.
There are things you can do to work on this.
If you disable the good channel it would allow you to use a DBT. You could then remove TR5 and TR7 and see if the output stage draws high current and lights the bulb. Assuming it didn't you could then look at the output stage in detail.
The good channel looks like it could be disabled simply by lifting one end of both 4.7 ohm base resistors to the output transistors. You have to do that to stop the good channel lighting the bulb.
I'll look in again tomorrow 🙂 that's me done for today.
There are things you can do to work on this.
If you disable the good channel it would allow you to use a DBT. You could then remove TR5 and TR7 and see if the output stage draws high current and lights the bulb. Assuming it didn't you could then look at the output stage in detail.
The good channel looks like it could be disabled simply by lifting one end of both 4.7 ohm base resistors to the output transistors. You have to do that to stop the good channel lighting the bulb.
I'll look in again tomorrow 🙂 that's me done for today.