Hi All
I fancied attempting a repair for fun and picked this up for £9 on ebay. Dim bulb tester stays off. Relay clicks but no sound through headphones other than the faint "leakage" sound you get on input selectors.
DC at both speaker terminals is around zero. Right channel DC at bias adjust points is 17mv which is normal but around 2v on the left channel. Can I assusme that the power transistors and possibly drivers are blown on the left. There is also a hum when the AUX input is selected.
If so need replacements for 2SA1633 / 2SC4278 and possibly a 2SD1762 driver. Can you suggest good equivalents which are easily available.
Any advice would be appreciated.
Best Wishes
I fancied attempting a repair for fun and picked this up for £9 on ebay. Dim bulb tester stays off. Relay clicks but no sound through headphones other than the faint "leakage" sound you get on input selectors.
DC at both speaker terminals is around zero. Right channel DC at bias adjust points is 17mv which is normal but around 2v on the left channel. Can I assusme that the power transistors and possibly drivers are blown on the left. There is also a hum when the AUX input is selected.
If so need replacements for 2SA1633 / 2SC4278 and possibly a 2SD1762 driver. Can you suggest good equivalents which are easily available.
Any advice would be appreciated.
Best Wishes
Help!
I have done some further investigation. If you go through "Source Direct" the amp appears to work normally with audio through both channels via headphones. It reads around 0.5 v across the biasing resistors for the left channel (too high ), right channel normal. With the tone circuit engaged (Source Direct off) the left channel becomes just a hum and it reads around 22v (very high!) across the left channel biasing resistors, right channel normal. The relay enagages as normal.
What faulty componenet am I looking for here or where should I look?? Can I assume the transistors are OK as it works with source direct OR could it be the biasing resistors? I don't want be desoldering everything without a theory. All I have is a DVM and a transistor checking device.
Any help would be appreciated.
I have done some further investigation. If you go through "Source Direct" the amp appears to work normally with audio through both channels via headphones. It reads around 0.5 v across the biasing resistors for the left channel (too high ), right channel normal. With the tone circuit engaged (Source Direct off) the left channel becomes just a hum and it reads around 22v (very high!) across the left channel biasing resistors, right channel normal. The relay enagages as normal.
What faulty componenet am I looking for here or where should I look?? Can I assume the transistors are OK as it works with source direct OR could it be the biasing resistors? I don't want be desoldering everything without a theory. All I have is a DVM and a transistor checking device.
Any help would be appreciated.
Assuming the 17mv bias is the voltage across the 0.22 ohms then 17mv sounds OK and would equate to about 77ma (or half that if across both in series).
The 2 volts bias (and 22 volts and 0.5 volt you later mention) all point to (again assuming you are reading across the 0.22 ohms) the resistors being open circuit which in turn suggests the output stage semiconductors will have failed short circuit.
The 2 volts bias (and 22 volts and 0.5 volt you later mention) all point to (again assuming you are reading across the 0.22 ohms) the resistors being open circuit which in turn suggests the output stage semiconductors will have failed short circuit.
Thanks for replying Mooly. You have helped me many times before.
Just out of interest why does the relay not prevent the amp from operating and why does it appear to work OK when used via source direct?
Any suggestions for suitable transistors? I did find some potentials for the power transistors but the details are on another PC. I am not sure about the drivers though.
Just out of interest why does the relay not prevent the amp from operating and why does it appear to work OK when used via source direct?
Any suggestions for suitable transistors? I did find some potentials for the power transistors but the details are on another PC. I am not sure about the drivers though.
The honest answer to that one is that is that there could be several reasons why it works with headphones and so it is easier to work with something definite like the voltage you mention.
Just think for a moment... you say you have 22 volts across the channel biasing resistors, and as I mentioned earlier I'm assuming this is across the 0.22 ohm emitter resistors as shown here.
So 22 volts across 0.44 ohm (the two 0.22's in series) means a current of 50 amps is flowing... which isn't really possible. So the most likely scenario is that the resistor/s are open.
Although a relay is shown on the circuit diagram it isn't immediately obvious what it does. The speaker feed as far as I can see is direct (apart from switches).
So that is the starting point, to confirm what is or has happened. Are those resistors open and are the transistors shorted.
It would also be worth measuring the DC voltage at the input to the power stages as the source direct switch is operated. It should be zero volts across R201 and R202.
Attachments
Hi all
Got home early today due to a poer cut at work! Did some dismantling of the unit to get a better look. It appears that the 4 power transistors have been replaced along with one of the voltage regulator transistors. Will remove these and test when I have more time along with resistors. The transistors had no pads, just heat sink compound so they could have shorted that way?
Got home early today due to a poer cut at work! Did some dismantling of the unit to get a better look. It appears that the 4 power transistors have been replaced along with one of the voltage regulator transistors. Will remove these and test when I have more time along with resistors. The transistors had no pads, just heat sink compound so they could have shorted that way?
Its a case of being methodical and doing all the usual checks and tests. No insulating pads could be a big issue unless the transistors are all plastic types.
The power transformer gets warm but testing the yellow and white wires / red and black wires there are only mv's not volts. Is it a faulty power supply? I did get a small shock when I was handling it though. Also tested the red and black wires on the board and only mv's.
Fuse is good and also tested it with a replacement.
At a loss at the moment.
Fuse is good and also tested it with a replacement.
At a loss at the moment.
You should measure AC voltage going into the bridge.
If no AC is present then disconnect the transformer secondary and see if the voltage comes up. If not then either the transformer or primary or primary wiring is suspect.
Bridge rectifiers usually fail with at least one of the four diodes going short but that would cause serious overload and should be blowing fuses.
If no AC is present then disconnect the transformer secondary and see if the voltage comes up. If not then either the transformer or primary or primary wiring is suspect.
Bridge rectifiers usually fail with at least one of the four diodes going short but that would cause serious overload and should be blowing fuses.
Hi
I have been distracted by other little projects. OK problem is the front end on the left channel only. Left hand biasing resistor reads 2volts with source direct on and around 24v with source direct off. So I assume problem is in tone amp but not sure where to start without pulling everything out.
Any pointers would be appreciated.
I have been distracted by other little projects. OK problem is the front end on the left channel only. Left hand biasing resistor reads 2volts with source direct on and around 24v with source direct off. So I assume problem is in tone amp but not sure where to start without pulling everything out.
Any pointers would be appreciated.
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