I bought a pair of these mono blocks with one not working and I had a quick look at it this evening.
The main transformer is never powered, even if some relays are engaged at power on.
It has some 'automatic' selection between 110/230V mains, with a 'power PCB' with a bunch of relays and logic circuitry, and I guess this is causing the power never to reach the main transformer, or maybe some protection circuitry that kicks in..
It seems to work so far that it indicates 230 mains is selected.
Previous owner claims it was blowing fuses, but I can't even get it far enough to power on properly. I disconnected the trafo secondaries to avoid smoke. Fuses are checked (found two). Seems the stuff working is powered by a small 'standby' SMPS (logic circuitry, relays and power button LED).
A schematic would be of great help to get further, but I have not been able to find anything online. Hoping somebody here would be able to help out?
EDIT: Maybe an admin could change the name of this thread to 'fault tracing' or similar instead of 'schematic'?
The main transformer is never powered, even if some relays are engaged at power on.
It has some 'automatic' selection between 110/230V mains, with a 'power PCB' with a bunch of relays and logic circuitry, and I guess this is causing the power never to reach the main transformer, or maybe some protection circuitry that kicks in..
It seems to work so far that it indicates 230 mains is selected.
Previous owner claims it was blowing fuses, but I can't even get it far enough to power on properly. I disconnected the trafo secondaries to avoid smoke. Fuses are checked (found two). Seems the stuff working is powered by a small 'standby' SMPS (logic circuitry, relays and power button LED).
A schematic would be of great help to get further, but I have not been able to find anything online. Hoping somebody here would be able to help out?
EDIT: Maybe an admin could change the name of this thread to 'fault tracing' or similar instead of 'schematic'?
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All I could find --so far is that it "senses " the mains voltage automatically and that its class "D" with a "high bias " switch for analogue operation class "A" up to (about ) 45 watts .
No sorry still cant find a schematic .
No sorry still cant find a schematic .
It seems to be a 3pair EF output stage with the option of higher bias, but I have not looked closely on the amp board yet. Can not see any caps for a lower supply rail voltage (class H type). Since it has high power with high rail voltages (around 65V I think), the Iq can't be very high before the power dissipation goes through the roof. My guess would be 0.3-0.4A in the high bias mode. This will only give a few watts of 'class A' operation, which also depends on the load. 4 ohms will get very little class A operation. However, I have seen benefits on distortion spectrum in other amps when turning up the Iq, so it could be a good thing even with the amp operating out of class A.
I hope somebody here has access to the schematic and is prepared to share it, chances are not good for me getting it up and running without a schematic.
I want to do some basic tests of the power supply today. Plan to feed the transformer directly via a light bulb with no loads connected to see that the trafo/rectifier/caps are operational.
I hope somebody here has access to the schematic and is prepared to share it, chances are not good for me getting it up and running without a schematic.
I want to do some basic tests of the power supply today. Plan to feed the transformer directly via a light bulb with no loads connected to see that the trafo/rectifier/caps are operational.
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PSU seems fine, but I found blown outputs on the negative side of the amp. I guess it was protection preventing it from firing up the main power. There are no output relays, but there is a UPC1237 on it, so I guess that is used to cut the power instead.
I borrowed the amp board out of the 'working' amp, and it then powers up briefly, but cuts out likely because of dc offset. I'm measuring 3-4V offset at the output with the functional amp module, that stayed on in the other amp (never measured offset though). Maybe I should look at front end supply, seems to be separate.
PCB's are colored red, so it's almost impossible to follow traces on them..
I borrowed the amp board out of the 'working' amp, and it then powers up briefly, but cuts out likely because of dc offset. I'm measuring 3-4V offset at the output with the functional amp module, that stayed on in the other amp (never measured offset though). Maybe I should look at front end supply, seems to be separate.
PCB's are colored red, so it's almost impossible to follow traces on them..
Some actual progress! 🙂
It turned out that the transistors in the cap multiplier for the front end were blown. One had a short and the other open circuit, so one rail missing to the front end. Transistors were 2sa1023 & complementary, and they seem rated Vcb 70V, and the actual voltage is 80V.. Replaced them with MJE 340/350 instead, they can take more current and more voltage. I think I will 'upgrade' these in the other amp too, just to avoid the same fault.
It now powers up as it should with the working amp PCB, and has 1mV offset on the output. The faulty amp PCB is still to be investigated, and parts to be ordered.
It turned out that the transistors in the cap multiplier for the front end were blown. One had a short and the other open circuit, so one rail missing to the front end. Transistors were 2sa1023 & complementary, and they seem rated Vcb 70V, and the actual voltage is 80V.. Replaced them with MJE 340/350 instead, they can take more current and more voltage. I think I will 'upgrade' these in the other amp too, just to avoid the same fault.
It now powers up as it should with the working amp PCB, and has 1mV offset on the output. The faulty amp PCB is still to be investigated, and parts to be ordered.
I found faulty MJE 340/350's (pre drivers..?) 3171/1930 drivers (hard to find), and all three NJW0302's (PNP's) blown with shorts between some pins. NPN's seem fine. Protection diodes from output to rails were also shorted. Have ordered new parts, and hope it will come back to life with those replaced.
My reasoning is that the cap multiplier transistors for the front end went first, and caused the rest(?) I have no idea about the topology of the amp though, since no schematic has been found. I can only see it's EF with three output pairs.
I hope I prevented it from happening again, with some sturdier transistors in the cap multiplier. I did replace those in the other amp too.
My reasoning is that the cap multiplier transistors for the front end went first, and caused the rest(?) I have no idea about the topology of the amp though, since no schematic has been found. I can only see it's EF with three output pairs.
I hope I prevented it from happening again, with some sturdier transistors in the cap multiplier. I did replace those in the other amp too.
I update a bit here again, maybe it will help somebody else working on one of these in the future.
Parts are ordered, and now I'm waiting for parts again (same as with my KX amp build) 🙁
I tried to follow some traces on the amp PCB today, and made a sketch of the output stage. I added the op-amp just to get it to run and be able to simulate bias voltage.
It is a bit odd that it has no decoupling caps on the rails, no output zobel, and no resistor in parallel to the output inductor. No 'power ground' on the amp board, except for the speaker ground going via the protection circuitry. Emitter resistors are fairly high resistance too. I think it's the first time I've seen the B-E resistors too.
No base resistors on the outputs either. I think I will have a look at stability when/if I get it up and running.
I was not able to follow the feedback network further than the first resistor.
It seems the front end is a symmetrical design.
Parts are ordered, and now I'm waiting for parts again (same as with my KX amp build) 🙁
I tried to follow some traces on the amp PCB today, and made a sketch of the output stage. I added the op-amp just to get it to run and be able to simulate bias voltage.
It is a bit odd that it has no decoupling caps on the rails, no output zobel, and no resistor in parallel to the output inductor. No 'power ground' on the amp board, except for the speaker ground going via the protection circuitry. Emitter resistors are fairly high resistance too. I think it's the first time I've seen the B-E resistors too.
No base resistors on the outputs either. I think I will have a look at stability when/if I get it up and running.
I was not able to follow the feedback network further than the first resistor.
It seems the front end is a symmetrical design.
Attachments
I have replaced a MJE350E, which seemed to be the only blown transistor in the front end.
I then thought I would like to check the functionality of the front end first, so I put two 1,7k resistors between the base pads for the drivers, and connected the midpoint of these resistors to the speaker terminal to connect to the feedback network, and get stability.
Should work, right?
I only get abt 1,5V DC, and seems there is no ac-amplification (finger on input, nothing on the output).
I'm feeding the front end with +-50V, even if it should be 80V. I'm thinking this should be enough, but not sure. I do see some 35V zeners on the PCB.
I have done in circuit measurements on all resistors comparing them with the working amp, and they all check out. Transistors are measured in circuit too with diode tester, and I get diode voltages between legs on all of them, and no shorts.
Same goes for the zeners, I measured the forward voltage in circuit with diode tester, and it seems fine.
I guess next step will be to try the full 80V, but I doubt it will help.. Suggestions are welcome. Feels like I'm nearing the end of the road for this amp resurrection project.. I think I would need a schematic to be able to check voltages and currents in the circuit.
Too bad I spent I nice sum on ordered parts for the output stage.
Thinking about it now, I could actually try to measure the zener voltages when it's powered.
I then thought I would like to check the functionality of the front end first, so I put two 1,7k resistors between the base pads for the drivers, and connected the midpoint of these resistors to the speaker terminal to connect to the feedback network, and get stability.
Should work, right?
I only get abt 1,5V DC, and seems there is no ac-amplification (finger on input, nothing on the output).
I'm feeding the front end with +-50V, even if it should be 80V. I'm thinking this should be enough, but not sure. I do see some 35V zeners on the PCB.
I have done in circuit measurements on all resistors comparing them with the working amp, and they all check out. Transistors are measured in circuit too with diode tester, and I get diode voltages between legs on all of them, and no shorts.
Same goes for the zeners, I measured the forward voltage in circuit with diode tester, and it seems fine.
I guess next step will be to try the full 80V, but I doubt it will help.. Suggestions are welcome. Feels like I'm nearing the end of the road for this amp resurrection project.. I think I would need a schematic to be able to check voltages and currents in the circuit.
Too bad I spent I nice sum on ordered parts for the output stage.
Thinking about it now, I could actually try to measure the zener voltages when it's powered.
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I had to try the above front end test on the working amp, and that had almost 0 offset, and did respond to finger on the input too. So, there is still something wrong with the front end of the problem amp. Maybe next step will be to wire them both up and compare voltage levels at random points..
Suggestions from experienced 'amp troubleshooters' are welcome 🙂
Suggestions from experienced 'amp troubleshooters' are welcome 🙂
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After a lot of hours spent, I think I'm beginning to see the light in the tunnel.
I have found more blown transistors, also in the protection part of the amp, but yesterday I did a quick test with a pair of output transistors from my parts bin. It seems to work as it should, offset and idle current (a little bit higher with the other outputs).
I spent abt two days with a lamp shining though the PCB to follow the traces, and draw a schematic of the amp-section in ltspice. I'm not sure if it's a good idea to post it here, so I post a schematic that is already out on the internet.
This amp is very similar in construction with some exceptions:
-Extra transistor to cut the current sources for the IPS (mute I guess)
-Only BJT's, no FET's
-EF triple OPS
-Three pairs of outputs
It uses MJE 340/350 for VAS and pre-drivers, and that makes it 'slow'. The miller compensation pole is at abt 1kHz, and if I remove the miller caps in the sim, it goes to 2k.. I think it would be interesting to use good VAS transistors and maybe another type of compensation. OLG is not very high in the amp, so I think it could be a candidate for high OLBW. The rest of the transistors seem to be up for the job, it's the MJE's that limits it.
Anyway, from the preliminary measurements I did on the working amp, distortion is very low, even if 'normal bias' is selected. It does however take quite some time to come up to the 'working bias' in low bias setting (abt 70mA/device). Selecting high bias (abt 200mA/device), it it basically stable from start up.
I have the correct outputs on order (error in first order, I ordered 0281, not 0302), so I'm expecting it will be to just solder them in and re-tune the bias.
There is also a trimmer on the current source for the upper half of the IPS.
I'm not sure how to tune this. I was thinking I would look at distortion spectrum, and adjust it. Or should I instead measure voltages or current between the halves?
Suggestions are welcome!
I have found more blown transistors, also in the protection part of the amp, but yesterday I did a quick test with a pair of output transistors from my parts bin. It seems to work as it should, offset and idle current (a little bit higher with the other outputs).
I spent abt two days with a lamp shining though the PCB to follow the traces, and draw a schematic of the amp-section in ltspice. I'm not sure if it's a good idea to post it here, so I post a schematic that is already out on the internet.
This amp is very similar in construction with some exceptions:
-Extra transistor to cut the current sources for the IPS (mute I guess)
-Only BJT's, no FET's
-EF triple OPS
-Three pairs of outputs
It uses MJE 340/350 for VAS and pre-drivers, and that makes it 'slow'. The miller compensation pole is at abt 1kHz, and if I remove the miller caps in the sim, it goes to 2k.. I think it would be interesting to use good VAS transistors and maybe another type of compensation. OLG is not very high in the amp, so I think it could be a candidate for high OLBW. The rest of the transistors seem to be up for the job, it's the MJE's that limits it.
Anyway, from the preliminary measurements I did on the working amp, distortion is very low, even if 'normal bias' is selected. It does however take quite some time to come up to the 'working bias' in low bias setting (abt 70mA/device). Selecting high bias (abt 200mA/device), it it basically stable from start up.
I have the correct outputs on order (error in first order, I ordered 0281, not 0302), so I'm expecting it will be to just solder them in and re-tune the bias.
There is also a trimmer on the current source for the upper half of the IPS.
I'm not sure how to tune this. I was thinking I would look at distortion spectrum, and adjust it. Or should I instead measure voltages or current between the halves?
Suggestions are welcome!
Seems the referenced schematic did not upload, so I try again.
I found the trimmer in the IPS current source is for offset adjust.
I found the trimmer in the IPS current source is for offset adjust.
Just to finish off the thread, I'm now listening to two working amps, and they sound pretty good, but I have no way of exploring the power reserve in my small apartment..
I ended up replacing a load of transistors and zeners before the blown amp was operational. It turned out the other 'working' amp had a problem with the mute function too, and that turned out to be a current source transistor in the front end that measured ok with diode check, and had the correct current when the amp was on, but did not cut the current properly when mute was activated. Some kind of internal leakage in the transistor I guess.
I ended up replacing a load of transistors and zeners before the blown amp was operational. It turned out the other 'working' amp had a problem with the mute function too, and that turned out to be a current source transistor in the front end that measured ok with diode check, and had the correct current when the amp was on, but did not cut the current properly when mute was activated. Some kind of internal leakage in the transistor I guess.