The schematic I downloaded from the internet differs substantially from the amplifier I have on my bench. It shows MPSA42 devices and a different number of diodes, as well as different capacitors. The unit I'm working on has MPSL01 devices, so I suspect it's an older model than depicted on schematic I have.
Problem in brief>
Right channel clipping positive peaks at 80 watts output (observed on bench). Customer complains it's sometimes making loud static/noise.
This far, I have checked voltage drop across all .33-ohm emitter resistors. Also did an ohmmeter check of same parts. It looks like all the output devices are functioning, based on seeing about 300mV across all resistors. This draws my attention to a possible problem on the driver PCB piggyback. Unfortunately, my schematic is the wrong version.
If someone (Nelson?) has the right one for s/n 770811, I would appreciate a copy, Thank you!
Problem in brief>
Right channel clipping positive peaks at 80 watts output (observed on bench). Customer complains it's sometimes making loud static/noise.
This far, I have checked voltage drop across all .33-ohm emitter resistors. Also did an ohmmeter check of same parts. It looks like all the output devices are functioning, based on seeing about 300mV across all resistors. This draws my attention to a possible problem on the driver PCB piggyback. Unfortunately, my schematic is the wrong version.
If someone (Nelson?) has the right one for s/n 770811, I would appreciate a copy, Thank you!
I pulled the heat sink/spreader apart to have a look at the power devices.
Customer tells me this amplifier was serviced recently by another shop, but apparently unsuccessfully, as the "noise" problem returned a week after repair.
It looks like these output devices have been replaced. Whether they are the right parts or not is something I don't know at this time, but they could be contributing to the early clipping problem. The do seem to be conducting, as I read a few hundred mV across all emitter resistors.
Thus far, I have replaced the heat sinked parts on the positive rail side of the driver board as well as the small signal MPSL01 devices (with MPSA42). No change in the early clipping. I've also replaced the 470uF cap. Having done that, I'm starting to become suspicious of the output devices again.
Customer tells me this amplifier was serviced recently by another shop, but apparently unsuccessfully, as the "noise" problem returned a week after repair.
It looks like these output devices have been replaced. Whether they are the right parts or not is something I don't know at this time, but they could be contributing to the early clipping problem. The do seem to be conducting, as I read a few hundred mV across all emitter resistors.
Thus far, I have replaced the heat sinked parts on the positive rail side of the driver board as well as the small signal MPSL01 devices (with MPSA42). No change in the early clipping. I've also replaced the 470uF cap. Having done that, I'm starting to become suspicious of the output devices again.
check out #3+4 under
https://www.diyaudio.com/community/...nd-schematic-collection-of-all-models.175138/
The image in post #3 shows different versions of 0R33 resistors - in the upper part a MOX version and in the lower part a carbon version.
I hardly believe that this condition is still the original.
P.S.: The serial number on back plate of the power amp device is not always the right way to find out the existing version - the PCB version itself is decisive, because with some repairs complete boards are exchanged.
this thread provide additional information
https://www.diyaudio.com/community/threads/discrepancies-threshold-400a-schematics.55849/
https://www.diyaudio.com/community/threads/discrepancies-threshold-400a-schematics.55849/
As it turns out, that schematic is identical to the one I have. So the idea that this amp is modified explains the deviation from schematics. I have no idea what the modification is. The caps are a different value and there are three 10uF/100V axial caps on this board. The 470uF is the only one that jives with the schematic.
Not only has the amp been modified, it's been unsuccessfully serviced by another shop recently for this same problem (intermittent noise). The noise is the waveform collapsing sometimes as the amp heats up under full signal conditions. It's a random condition and I suspect it stopped long enough for the other shop to call it "fixed" and they returned it to customer only to have the noise reappear days later.
My observation is that it is incapable of sinking normal current in the positive swing as the other channel. This channel starts clipping positive peaks around 80 watts while the other channel clips symmetrically at 136 watts. I have not yet determined if that's the fault of the 1990s era output devices that were installed by someone, or a problem with the driver board.
I've replaced the heat sinked devices on the positive side of the driver board as well as all of the small signal transistors on that half of the circuit, so far, with no improvement. I'm reluctant to start replacing output devices due to the number and cost, but having already replaced the positive half of the driver PCB, there's not much left to suspect.
in post #12 under
https://www.diyaudio.com/community/threads/nad-3130-difficult-repair.398739/#post-7336825
I have mentioned a troubleshooting approach of power amp section from the integrated amplifier NAD 3130.
The same steps must actually be possible even on this model despite the fact, that Vbe multiplier and output power buffer uses a different topology.
Basically checking DC conditions of the front-end alone as first step and as second step additional with only the driver stage of output buffer.
https://www.diyaudio.com/community/threads/nad-3130-difficult-repair.398739/#post-7336825
I have mentioned a troubleshooting approach of power amp section from the integrated amplifier NAD 3130.
The same steps must actually be possible even on this model despite the fact, that Vbe multiplier and output power buffer uses a different topology.
Basically checking DC conditions of the front-end alone as first step and as second step additional with only the driver stage of output buffer.
Well when I have an early clipping problem the FIRST thing I check is the DC rails. But may be I need to check them under load. But then, this is a shared power supply, so the other channel would be performing poorly if the power supply were at fault.
Having replaced all the silicon in the positive side of the driver PCB, leaves me with the unhappy scenario of replacing all of the power transistors next. They are not the original parts and perhaps are not the correct types for the application.
My next thought it is to break feedback loop and trace to what stage I have clip-free signal. Some transistor amps oscillate when the feedback loop is interrupted though.
The biggest challenge is that this amp is not in agreement with the published schematic. According to Mr. Pass, this amplifier has modifications done to it so it deviates from published documentation.
Having replaced all the silicon in the positive side of the driver PCB, leaves me with the unhappy scenario of replacing all of the power transistors next. They are not the original parts and perhaps are not the correct types for the application.
My next thought it is to break feedback loop and trace to what stage I have clip-free signal. Some transistor amps oscillate when the feedback loop is interrupted though.
The biggest challenge is that this amp is not in agreement with the published schematic. According to Mr. Pass, this amplifier has modifications done to it so it deviates from published documentation.
I see. Then one must create a new schematic of the present power amp boards to find out in detail, what modifications were made.
Furthermore, it is very important that the supplier of the new transistors knows exactly that they were produced by On-Semi.
Keep in mind, that there are a lot of counterfeits in the marked
https://sound-au.com/counterfeit.htm
https://www.diyaudio.com/community/threads/counterfeit-transistors.115281/
https://www.diyaudio.com/community/threads/my-transistors-original-or-copy.82638/
Furthermore, it is very important that the supplier of the new transistors knows exactly that they were produced by On-Semi.
Keep in mind, that there are a lot of counterfeits in the marked
https://sound-au.com/counterfeit.htm
https://www.diyaudio.com/community/threads/counterfeit-transistors.115281/
https://www.diyaudio.com/community/threads/my-transistors-original-or-copy.82638/
I'm pretty certain there was not such a version shipped. The diodes D7 and D8 are where we normally had
resistors.
I think what I'm going to try next is swap the driver PCBs just to narrow down the search to either driver or power components.
When I repared a couple of 4000's I took out all the outputtrannies and measured them in a test jag, 15v and 0-3 amps.
Amps was one '78 and a '79 and driverstages was totally different.
Those transistors that did not work ok was replaced and matched to those working in the other amp.
All cascodes was swapped out for MJ15015 and MJ15016.
All emitter resistors were swapped out as the base voltage resistors in the cascode.
"Someone" burned out the speakers and the took some parts with them, but now they work great. Had to swap a powerswitch and it was not easy to find, but after a long search I found a replacement.
They measured really nice, about 0.02 % up to 250w in 4 ohms, if I remember correct.
/Figge
Amps was one '78 and a '79 and driverstages was totally different.
Those transistors that did not work ok was replaced and matched to those working in the other amp.
All cascodes was swapped out for MJ15015 and MJ15016.
All emitter resistors were swapped out as the base voltage resistors in the cascode.
"Someone" burned out the speakers and the took some parts with them, but now they work great. Had to swap a powerswitch and it was not easy to find, but after a long search I found a replacement.
They measured really nice, about 0.02 % up to 250w in 4 ohms, if I remember correct.
/Figge
If there's no objection to swapping the driver boards (assuming L-R identical), I want to try that to narrow down the fault. If it IS the power transistors, I can only imagine that hfe is poorly matched.
Yes you can swap driverboards.
I presume you have checked voltage and currents on the cascode drivers, and output drivers?
Does the currents and voltages look ok in the earlier stages?
The circuit for "soft class A", or what it was called, I think can not be the reason for this.
Figge
I presume you have checked voltage and currents on the cascode drivers, and output drivers?
Does the currents and voltages look ok in the earlier stages?
The circuit for "soft class A", or what it was called, I think can not be the reason for this.
Figge
I swapped the driver boards and the asymmetrical clipping on the right channel persists. I've pretty much vindicated the driver PCB. So now to pull the output transistors on the heat sink and look at each on the curve tracer.
Notice how the negative side (right) has all the same device part number, while the positive (left) side has mismatched devices? I suspect the cascode circuit is sensitive to hfe matching.
