If removing Q3 doesn't work, C1 is my semi educated guess. If its value is too low you will lose bass response, and you may be overdriving the input trying to hear something out of an amp with no response below 1KHz or so. Assuming low to no DC output on your pre you can temporarily short C1 as a test. If that make the amp work, replace C1 with another of at least 1 uf.
Edit: Could you have swapped C1 and C5?
Edit: Could you have swapped C1 and C5?
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Posted the values in the earlier post. Exactly same for both channels.
Remove Q3.. and replace with another one? At the moment I do not have a replacement ZTX450. I need to remove from a working F5 if needed.
No, I have not swapped C1 and C5. I can check if C1 is faulty. I can short it and check. There is no DC output on my preamp. It is the same preamp which drives my F5.
Thanks.
Have done that earlier. I will double check once again.
Is it the case with Q4 as well? I will remove and check.
Thanks everyone for chipping with help. I really need.
Good point...one strand of wire bridging signal to ground can wreak havoc...
Checked R1-R4 with DMM and all the values are spot-on.
Removed Q3 as suggested. No change in the behavior of the circuit.
Measured preamp output DC offset at 1.2mv. With that confidence shorted C1. Gain jumped and bass came back. Did not listen to enough music. It was just for a few seconds, since amp offset jumped to 140mv when C1 is shorted. I will replace with a different cap and check. There seems to be some hope.
Will post the developments.
Thanks.
Removed Q3 as suggested. No change in the behavior of the circuit.
Measured preamp output DC offset at 1.2mv. With that confidence shorted C1. Gain jumped and bass came back. Did not listen to enough music. It was just for a few seconds, since amp offset jumped to 140mv when C1 is shorted. I will replace with a different cap and check. There seems to be some hope.
Will post the developments.
Thanks.
Tragedy...
I bought new 1uF input capacitor, unmounted the PCB from the heatsinks, replaced the old 1uF with the new one. Resoldered the removed Q3 previously and reseated the JFETs, which had gone through some bit of manhandling earlier. Checked for shorts, cleaned up and mounted back on the heatsinks.
Connected this amp board to preamp and speaker. Started playing music for the first time from this board. No problem. Gain was there, frequency response was there. Extremely delighted. Quickly changed the old test speaker to my regular Fostex fullrange, eager to hear the music.
Turned on, nothing happens. Noticed one of the wires of the primary of the transformer has come loose from the connector. Reconnected and switched on. Nothing. Checked fuse. Blown. Replaced fuse. Blown again.
Disconnected my speaker. Connected the light bulb back in series with the power and checked after replacing the fuse. Bulb glows full bright indicating short. Disconnected the amp from the power supply. Bulb comes up and turns off, indicating the problem is with the amp board.
With the amp connected back, the bulb continues to glow. Amp output measures around 220mv. Checked heatsinks for any short with the MOSFETs. No problem.
Need advice, as to how to debug the faulty board. Are the outputs blown? or is the other transistors? I am completely disappointed. I am not back to square one, but back to -10.
TIA.
I bought new 1uF input capacitor, unmounted the PCB from the heatsinks, replaced the old 1uF with the new one. Resoldered the removed Q3 previously and reseated the JFETs, which had gone through some bit of manhandling earlier. Checked for shorts, cleaned up and mounted back on the heatsinks.
Connected this amp board to preamp and speaker. Started playing music for the first time from this board. No problem. Gain was there, frequency response was there. Extremely delighted. Quickly changed the old test speaker to my regular Fostex fullrange, eager to hear the music.
Turned on, nothing happens. Noticed one of the wires of the primary of the transformer has come loose from the connector. Reconnected and switched on. Nothing. Checked fuse. Blown. Replaced fuse. Blown again.
Disconnected my speaker. Connected the light bulb back in series with the power and checked after replacing the fuse. Bulb glows full bright indicating short. Disconnected the amp from the power supply. Bulb comes up and turns off, indicating the problem is with the amp board.
With the amp connected back, the bulb continues to glow. Amp output measures around 220mv. Checked heatsinks for any short with the MOSFETs. No problem.
Need advice, as to how to debug the faulty board. Are the outputs blown? or is the other transistors? I am completely disappointed. I am not back to square one, but back to -10.
TIA.
Look carefully for any loose wire that could be causing a short - or mounting hardware that shifted
Pull and measure DCR from Source to Drain on your output devices. If 0, they are dead and need replacing.
Offset jumping to 140 mV without C1 indicates you are getting some DC on your pre-out. Take a look at that after you get the amp running again.
Pull and measure DCR from Source to Drain on your output devices. If 0, they are dead and need replacing.
Offset jumping to 140 mV without C1 indicates you are getting some DC on your pre-out. Take a look at that after you get the amp running again.
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OK. Will check the output devices. Can I check them without pulling them. I can turn on the circuit and measure with the light bulb on. I measured across R16, 17, R18 and R19. They are showing around 0.5V
I measured the DC on my preamp. It shows 1.2mv.
Thanks.
Turned on, but drawing 1A each. The amp should draw ~100W if at full mains voltage. What is your bulb's power rating? If only 100W it could look full bright, even though it is only at 50W.
I noticed Q4, too. No idea how the current source is working on the one good channel with that bias point.
I noticed Q4, too. No idea how the current source is working on the one good channel with that bias point.
I will measure Vbe (means Voltage across Base and Emitter isn't it?) of Q4 again. What I measured before on the good and the bad channel is .32v and .32v This is from my previous noting. The good channel is still working perfectly.
I will also change Q4 on the faulty channel and check again.
You may be right. The bulb I am using is a 100w one. I may think it is bright and not necessarily full bright. What should be the bias point on Q4? According to Papa's schematic, voltage across Emitter and Collector of Q4 should measure 4.3v and in my case it is measuring 5.3V on the working channel.
Thanks.
Do you have fuses in the rails? If not, can you rig up something in the 3A range temporarily? I think it is time to try without the bulb, or maybe a 300W bulb.
What do you have for inrush limiting and what is your line fuse rating? What is your mains voltage?
I would expect Q4 Vbe (yes base to emitter) to be a lot closer to 0.6V. Odd that the one channel works like that. The high Q4 Vce reading means that your FETs need higher than average voltage to turn on.
What do you have for inrush limiting and what is your line fuse rating? What is your mains voltage?
I would expect Q4 Vbe (yes base to emitter) to be a lot closer to 0.6V. Odd that the one channel works like that. The high Q4 Vce reading means that your FETs need higher than average voltage to turn on.
Bob, I am running on 220v power line. I do have a 10ohms, 10A surge thermistor in series with the power line to protect startup surge. Also I have a line fuse to the transformer primary of 2.5amps. My bulb is a 100w bulb abs have access to a 200w one as well.
I have replaced all the ztx and FETs in the system and there is n change to the state of the problem, the bulb continues to glow. I suspect the mosfets might have gone bad. I am rebuilding the amp from scratch on a new board. I have a new set of matched mosfets in anticipation of a aleph JX. I have discarded the problem board. I will visit it after I get the new one running.
Cheers.
It's not likely that the FETs died while you were changing speakers. Especially since they seem to be drawing only an amp now.
I have a feeling that inrush had popped the fuse, even with your thermistor. You're getting 22A surge, I think that you just got lucky earlier not blowing them. The difference between the amps could be just the build up of tolerances. I'd try a 2.5a slow blow fuse or a pair of thermistors in series. Hope you haven't disassembled it yet.
I have a feeling that inrush had popped the fuse, even with your thermistor. You're getting 22A surge, I think that you just got lucky earlier not blowing them. The difference between the amps could be just the build up of tolerances. I'd try a 2.5a slow blow fuse or a pair of thermistors in series. Hope you haven't disassembled it yet.