Overall those voltages and the DC offset look good .
I have picked up on a 3 anomalies ..
1/ Voltage at Q2 collector +75.7V looks correct but it should be the same as the voltage at Q6 base (+74.7V ?).
2/ Voltage at Q1 collector -78.0V looks correct but it should be the same as the voltage at Q5 base ( -76.4V ?).
Maybe connecting directly to the bases of Q6 and Q5 causes oscillation and upsets the readings (quite possible).
3/ There is 450uA flowing through R14 ( according to all the other connected voltage drops ) which should give a voltage drop of 0.99V across R4 and not 1.6V (77.2V - 75.6V).
The current through R1 is 8.79mA and thro R4 8.95mA giving 0.479W and 0.497W dissipation so change them to 1W handling as " PRR " advised ( if you already have not done so ) .
I have picked up on a 3 anomalies ..
1/ Voltage at Q2 collector +75.7V looks correct but it should be the same as the voltage at Q6 base (+74.7V ?).
2/ Voltage at Q1 collector -78.0V looks correct but it should be the same as the voltage at Q5 base ( -76.4V ?).
Maybe connecting directly to the bases of Q6 and Q5 causes oscillation and upsets the readings (quite possible).
3/ There is 450uA flowing through R14 ( according to all the other connected voltage drops ) which should give a voltage drop of 0.99V across R4 and not 1.6V (77.2V - 75.6V).
The current through R1 is 8.79mA and thro R4 8.95mA giving 0.479W and 0.497W dissipation so change them to 1W handling as " PRR " advised ( if you already have not done so ) .
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I made the measurements using a digital oscilloscope and the accuracy of the DC values changed based on what scale I had selected and how long the Rms averaging was. I think they are all +- a tiny bit, so the slight anomalies you found are things I noticed too. I'm not too worried about the tiny mismatches, if that makes sense.
I replaced the main output transistors today, re-did the thermal heatsink pads with new grease, and turned it on. It didn't blow a fuse or catch on fire, so I'm stoked!
I slowly adjusted the bias potentiometer and measured the voltage across the output resistors using a regular DC volt meter this time. I can get a DC voltage drop between 0.0mV and 10mV at the most. I set it at 9.0mV.
The amp seems to pass a 100hz sine wave like a champ all the way up to amplifier rail voltages and down to about 20mV of AC noise with no input signal. I tried watching the output on the oscilloscope while adjusting the bias but couldn't notice any difference between min and max adjustment on the potentiometer. Should I be able to see something? I was hoping to see some crossing point distortion but maybe that's hard to see at lower frequencies?
I'm really happy with this and I thank you all for your help. I don't need more bass in the home theater since I already have one of these, but now I can have twice as much!
Thank you all so much!
I slowly adjusted the bias potentiometer and measured the voltage across the output resistors using a regular DC volt meter this time. I can get a DC voltage drop between 0.0mV and 10mV at the most. I set it at 9.0mV.
The amp seems to pass a 100hz sine wave like a champ all the way up to amplifier rail voltages and down to about 20mV of AC noise with no input signal. I tried watching the output on the oscilloscope while adjusting the bias but couldn't notice any difference between min and max adjustment on the potentiometer. Should I be able to see something? I was hoping to see some crossing point distortion but maybe that's hard to see at lower frequencies?
I'm really happy with this and I thank you all for your help. I don't need more bass in the home theater since I already have one of these, but now I can have twice as much!
Thank you all so much!
Attachments
Great news on the working amp 🙂
On the issue of viewing crossover distortion whilst adjusting the bias , this is something I have not tried but suggest the amp needs to be driving a dummy load with approx. 10W . I know "nigelwright 7557" views the crossover distortion whilst adjusting the bias but I don't know the detail .
😎 photo .
On the issue of viewing crossover distortion whilst adjusting the bias , this is something I have not tried but suggest the amp needs to be driving a dummy load with approx. 10W . I know "nigelwright 7557" views the crossover distortion whilst adjusting the bias but I don't know the detail .
😎 photo .
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Another test - Check out the Vf of PN junctions for all transistor ahead of the drivers. This can usually be done in circuit.
Here's an educational link that covers it:
Test a transistor with a multimeter.
The most common failure mechanism for bipolar junction transistors is to go short circuit from one leg to another. This can be catastrophic in a typical DC coupled design blowing the output transistors.
Here's an educational link that covers it:
Test a transistor with a multimeter.
The most common failure mechanism for bipolar junction transistors is to go short circuit from one leg to another. This can be catastrophic in a typical DC coupled design blowing the output transistors.
Update:
I've had a 120hz hum for a while and finally found the source. I detailed it in another thread that can be found here:
the dreaded Boston acoustic VR2000 sub woofer hum
Thank you again everyone for the help! I'm glad I had this thread to reference again since I forgot a few things!
I've had a 120hz hum for a while and finally found the source. I detailed it in another thread that can be found here:
the dreaded Boston acoustic VR2000 sub woofer hum
Thank you again everyone for the help! I'm glad I had this thread to reference again since I forgot a few things!
Great thread! Thank you for the schematic JZ. It has been very helpful in my attempt to solve the same problem with a VR2000 that I've inherited. It too was blowing a fuse each time I flipped the switch from off to on. I've replaced the large caps, 4 output transistors, and a few defective resistors. No luck, the fuse still blows. The one thing that I am not sure about is the 120V transformer and whether it is bad. I disconnected the transformer from the amp board to check it without a load. The primary has 0.8 ohms of resistance, the secondary has 2 ohms and 0.8 ohms on its two outer taps, white and orange respectively. On the secondary, the white pair of outer taps reads 32V (16V from outer to center tap) and the other orange pair of outer taps reads 110V (54V from outer to center tap). Are these the correct secondary voltages I should be getting from this transfomer?
Hey I'm glad this was helpful for you!
The voltages your are measuring from the transformer sound right, but it's been a while since I worked on this.
When I was troubleshooting, I removed the four largest output transistors from the board and then powered it on. This was the only way it would not blow the fuse, so I knew it was related to a short in one of them.
Are you certain your output transistors are all healthy and working as intended? Do any of them get really hot when the fuse pops? Do you have a thermal camera you can use to look for the heat from a short circuit?
The voltages your are measuring from the transformer sound right, but it's been a while since I worked on this.
When I was troubleshooting, I removed the four largest output transistors from the board and then powered it on. This was the only way it would not blow the fuse, so I knew it was related to a short in one of them.
Are you certain your output transistors are all healthy and working as intended? Do any of them get really hot when the fuse pops? Do you have a thermal camera you can use to look for the heat from a short circuit?
Awesome! Thanks for the quick response JZ. I'm pretty sure it is not the large output transistors. Two of the original ones were bad. So I replaced all 4 of them with new ones of same model stated in the initial post on this thread. I'll do another test and remove them as you suggest, turn it on, and monitor for hot spots best I can. I don't have a thermal camera and so my temperature gun will have to do. During this test, perhaps I should use a dim bulb tester so I can bypass the fuse from blowing.