Just a suggestion, if TR11 is really open circuit, then you would see negative supply voltage on the speaker terminals of that channel (if you have no speaker/load connected), with no lamp tester -37V, with a lamp tester a lower voltage (but not that low) if the rest of the circuit is working correctly?
You would probably get audio out of the working channel (left channel?) even if the right channel is blown, as long as you don't have a speaker connected to the blown/faulty channel.
Also looking back did you get capacitors off ebay for this? Worst place in the world, they can be fake parts, with smaller capacitors stuffed inside larger capacitor cans.... Would give you all sorts of problems if you replaced the main PSU caps with them.
You would probably get audio out of the working channel (left channel?) even if the right channel is blown, as long as you don't have a speaker connected to the blown/faulty channel.
Also looking back did you get capacitors off ebay for this? Worst place in the world, they can be fake parts, with smaller capacitors stuffed inside larger capacitor cans.... Would give you all sorts of problems if you replaced the main PSU caps with them.
Thanks for the input, ohdsp.
Did a quick check at the speaker terminals, tested for AC, couldn't find any voltage present. Switched over to DC and it recorded -.24dc right channel and -.17dc left channel.
Give this a quick try, can confirm that with the lamp limiter I can get audio right ch all be it with a lot of buzzing. The Left channel is just buzzing.
The Left channel is TR12...
I got some from eBay but not the larger PSU caps, I don't think.
Going to spend a bit more time later soldering the output transistors so I can test them all out of circuit. Will test the diode John Paul suggested also.
Cheers.
Speaker outputs with no load connected should be sub 100mV ideally. If the supply voltage is collapsing with the bulb tester that can throw things off a bit.
Lots of buzzing may suggest a supply diode (bridge rectifier)/supply capacitor issue, looks like there are some capacitors in parallel with the bridge rectifier(C29-C32) maybe one of those is failed as a short? Maybe there is a grounding issue, have you checked the 0V/ground connection between the PSU and the amplifier PCB?
Just looking at inside pics, can you disconnect the PSU PCB from the Amplifier PCB? If you can, and if you connect the mains with a bulb tester and the bulb is bright, and the supply voltages lower than +/-37V, then the problem is more likely with the PSU.
Lots of buzzing may suggest a supply diode (bridge rectifier)/supply capacitor issue, looks like there are some capacitors in parallel with the bridge rectifier(C29-C32) maybe one of those is failed as a short? Maybe there is a grounding issue, have you checked the 0V/ground connection between the PSU and the amplifier PCB?
Just looking at inside pics, can you disconnect the PSU PCB from the Amplifier PCB? If you can, and if you connect the mains with a bulb tester and the bulb is bright, and the supply voltages lower than +/-37V, then the problem is more likely with the PSU.
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So you have removed just the two outputs from the faulty channel and they read OK. The other channel is intact.
I think the next quick test is to lift R18 and also R50 and R52 to ensure the driver stages are isolated from the supply and for you then to power the amp up and confirm the good channel is now 100% OK with no buzzing etc.
If it is all OK then that indicates a problem around the driver/output stage and so we can look at testing the driver stage (taking for now that the outputs are indeed OK) and try and figure out what is going on.
The above will prove for sure whether the PSU or the suspect channel of the amp is at fault.
I think the next quick test is to lift R18 and also R50 and R52 to ensure the driver stages are isolated from the supply and for you then to power the amp up and confirm the good channel is now 100% OK with no buzzing etc.
If it is all OK then that indicates a problem around the driver/output stage and so we can look at testing the driver stage (taking for now that the outputs are indeed OK) and try and figure out what is going on.
The above will prove for sure whether the PSU or the suspect channel of the amp is at fault.
I disconnected the power supply board from the main board and powered on, lamp limiter didn't light up and I get a read of 26.6 VAC on each live output.
I lifted all 4 OT's from the board for testing, I've soldered them back in but can easily lift them again if need be?
Lets concentrate on this one channel then, the right one. Lets leave the left channel disconnected for now. I'm wondering if you have problems with both channels given what has happened.
With no speaker attached we need to know:
1/ Is the bulb lit brightly?
2/ What is the DC offset at the speaker terminals?
3/ What voltage is across that 18 volt Zener diode.
With no speaker attached we need to know:
1/ Is the bulb lit brightly?
2/ What is the DC offset at the speaker terminals?
3/ What voltage is across that 18 volt Zener diode.
Bulb lit means excess current. The low Zener voltage is probably because the rails are to low at this point.
The offset is wrong but not in the way that would suggest a short in the output stage.
Did you check the bridge as JP mentioned earlier?
If that seems OK then we have to isolate the high current draw possibilities in this channel we are working on to initially reach a state where the rails are correct and the bulb is out.
I'm trying to think of ways that are the least work for you and that get the quickest results
Lifting R21 and R23 isolate the output transistors.
You can risk switching on in that state. If there are no problems with the drivers it will be OK, if there are problems then R49/51 and R19 may suffer. You can lift R49, R51 and R17 to totally isolate it all.
In that the bulb should be out. If not then the PSU has a fault such as a leaky or shorted diode in the bridge.
You said the preamp was disconnected... a floating input would send a working channel crazy and could cause all sorts of weird symptoms. Make sure its all coupled up correctly before going deeper.
The offset is wrong but not in the way that would suggest a short in the output stage.
Did you check the bridge as JP mentioned earlier?
If that seems OK then we have to isolate the high current draw possibilities in this channel we are working on to initially reach a state where the rails are correct and the bulb is out.
I'm trying to think of ways that are the least work for you and that get the quickest results
Lifting R21 and R23 isolate the output transistors.
You can risk switching on in that state. If there are no problems with the drivers it will be OK, if there are problems then R49/51 and R19 may suffer. You can lift R49, R51 and R17 to totally isolate it all.
In that the bulb should be out. If not then the PSU has a fault such as a leaky or shorted diode in the bridge.
You said the preamp was disconnected... a floating input would send a working channel crazy and could cause all sorts of weird symptoms. Make sure its all coupled up correctly before going deeper.
Sorry, Mooly but I'm not entirely sure how to test the bridge rectifier.
There are 4 pins, I see the + and - pins on the schematic. They appear to be showing open circuit on both Ohms and diode setting. between live and the pin to its right the readings jump about all over the place and its the same between the bottom 2. Between the bottom left and top Right getting 01.0 Ohms and 000 on the diode setting...
There are 4 pins, I see the + and - pins on the schematic. They appear to be showing open circuit on both Ohms and diode setting. between live and the pin to its right the readings jump about all over the place and its the same between the bottom 2. Between the bottom left and top Right getting 01.0 Ohms and 000 on the diode setting...
You need to test the bridge rectifier with the power off, and ideally the power supply capacitors discharged.
Earlier in the thread there is the PCB image which shows the diode direction between the pins.
If you measure across each diode, with the diode test function on your multi meter, you should see 0.6V to 0.7V shown on the meter with the diode in forward conduction and open circuit in the reverse direction. Meter might display it differently (like in mV or something, and all zeros for open). If you get any other reading something is wrong.
Earlier in the thread there is the PCB image which shows the diode direction between the pins.
If you measure across each diode, with the diode test function on your multi meter, you should see 0.6V to 0.7V shown on the meter with the diode in forward conduction and open circuit in the reverse direction. Meter might display it differently (like in mV or something, and all zeros for open). If you get any other reading something is wrong.
Bulb lit means excess current. The low Zener voltage is probably because the rails are to low at this point.
The offset is wrong but not in the way that would suggest a short in the output stage.
Did you check the bridge as JP mentioned earlier?
If that seems OK then we have to isolate the high current draw possibilities in this channel we are working on to initially reach a state where the rails are correct and the bulb is out.
I'm trying to think of ways that are the least work for you and that get the quickest results
Lifting R21 and R23 isolate the output transistors.
You can risk switching on in that state. If there are no problems with the drivers it will be OK, if there are problems then R49/51 and R19 may suffer. You can lift R49, R51 and R17 to totally isolate it all.
In that the bulb should be out. If not then the PSU has a fault such as a leaky or shorted diode in the bridge.
You said the preamp was disconnected... a floating input would send a working channel crazy and could cause all sorts of weird symptoms. Make sure its all coupled up correctly before going deeper.
Ok, Mooly. I lifted R49, R51 and R17 as you said. The lamp limiter lights up for a split second on initial power up then goes out completely and stays out.
This shows the power supply is basically OK, no shorts or anything drawing current.
Just to be sure... those resistors I have now lifted as mentioned above, they where from the right channel. This channel still has its OT's soldered down. Its the other side which has the OT's lifted.
Cheers
Lets stick to just one channel for now, the one with the resistors lifted. Also DO NOT connect any speakers to either channel.
It's very difficult to try and think of easy ways for you to test this and I'm sure it must seem like we are going around in circles sometimes... if I had this in front of me I would approach it very differently
Lets try this. If you now refit R49 and R51 but leave R17 isolated then the amplifier should still power up without the bulb lighting however the DC offset voltage may now be a very high negative voltage.
No current can flow because there is now no way for any bias voltage to develop between the driver transistor base's and thus turn on these transistors. It doesn't matter what voltage the base's are at, the output stage will draw no current although the output voltage will follow what the base's are doing.
Let's see if it gets this far and still has the bulb out.
It's very difficult to try and think of easy ways for you to test this and I'm sure it must seem like we are going around in circles sometimes... if I had this in front of me I would approach it very differently
I just appreciate the help and your knowledge. As long as your happy to help I don't mind how we go about it!
Just firing up the soldering iron now will report back.
Cheers.
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