Does anyone here know these amps?
It came to me fried. A bunch of fuses were missing and some were jumped with thin wires. I replaced all the output transistors and driver transistors. And I checked the rest of the transistors on the driver boards on diode check. One trace was fried on each driver boards, but those have been fixed. Nothing else looks burned. I've checked the diodes for shorts in circuit as well. Output resistors measure good. I reflowed the solder on all the molex connectors, it was failing.
When I power it up with the dim bulb tester, I get a bright bulb and DC on the output. With 60W, 100W, and 200W bulbs. With any of the output modules. On either channel, with either driver board.
I found some instructions from the designer on the canuckaudiomart forum. He said it can be tested with one output module. He said to put it on the variac and that the DC should go down to near zero at around 30-40VAC. When I try it the DC goes down to around 3-5V by 60%. But the transistors are starting to get hot to the touch.
I'm confused because this seems to be a simple amp and I don't know how I could have missed any damage. Both channels are behaving exactly the same way.
I haven't had the bavery or stupidity to plug it straight into the wall and see if it functions normally at full line voltage.
The fuses for this amp are odd sizes and were oddly expensive. The short 6A fuses were $4 each from digikey!
Here's the schematic.
It came to me fried. A bunch of fuses were missing and some were jumped with thin wires. I replaced all the output transistors and driver transistors. And I checked the rest of the transistors on the driver boards on diode check. One trace was fried on each driver boards, but those have been fixed. Nothing else looks burned. I've checked the diodes for shorts in circuit as well. Output resistors measure good. I reflowed the solder on all the molex connectors, it was failing.
When I power it up with the dim bulb tester, I get a bright bulb and DC on the output. With 60W, 100W, and 200W bulbs. With any of the output modules. On either channel, with either driver board.
I found some instructions from the designer on the canuckaudiomart forum. He said it can be tested with one output module. He said to put it on the variac and that the DC should go down to near zero at around 30-40VAC. When I try it the DC goes down to around 3-5V by 60%. But the transistors are starting to get hot to the touch.
I'm confused because this seems to be a simple amp and I don't know how I could have missed any damage. Both channels are behaving exactly the same way.
I haven't had the bavery or stupidity to plug it straight into the wall and see if it functions normally at full line voltage.
The fuses for this amp are odd sizes and were oddly expensive. The short 6A fuses were $4 each from digikey!
Here's the schematic.
Attachments
Stuck to taking guesses.
Hot transistors, one would likely assume.
Bias pot is bad/dirty/open or amp oscillating.
Or since you have DC offset.
Could be issues with current source transistors
or diodes used in current source circuits.
Otherwise when handed shotgun repair attempts
in the past. Sometimes the repair attempt caused
more issue and had to repair those first.
Then try to find original problem.
So observe closely solder joints / damaged traces
DC offset generalized could be bad transistor or
open/shorted connections or any current source/mirror
or differential circuit not balanced by faulty or
open transistors/diodes
Hot transistors, one would likely assume.
Bias pot is bad/dirty/open or amp oscillating.
Or since you have DC offset.
Could be issues with current source transistors
or diodes used in current source circuits.
Otherwise when handed shotgun repair attempts
in the past. Sometimes the repair attempt caused
more issue and had to repair those first.
Then try to find original problem.
So observe closely solder joints / damaged traces
DC offset generalized could be bad transistor or
open/shorted connections or any current source/mirror
or differential circuit not balanced by faulty or
open transistors/diodes
Hi,
I have the Audio Design 10A. Similar in many respects. Do you have Molex connectors to the different modules? These are most likely badly tarnished and need cleaning or even better.. replaced by removal and direct soldering. Does the 20A have a bridging module? This can become defective as well.
Cheers
I have the Audio Design 10A. Similar in many respects. Do you have Molex connectors to the different modules? These are most likely badly tarnished and need cleaning or even better.. replaced by removal and direct soldering. Does the 20A have a bridging module? This can become defective as well.
Cheers
Tried what Audio1Man suggested. No outputs getting hot, but still around 30V DC. Decided to plug it directly into the wall and risk blowing something. It didn't blow anything, but the amp is still DC.
To sumarize, I've replaced all the outputs and drivers with original part numbers. Pulled and checked all the other semiconductors on all junctions. Resoldered the molex connectors. And the issue is with both channels. I have verified the power supply rails are okay. And I've checked all NPN/PNP transistors are in the right positions according to the schematic.
Can anything on the motherboard be at fault? I've checked for shorts here too. I pulled U1 to see if that changed anything and it didn't, I think that part of the circuit is just for the LEDs?
I have zero ideas right now.
Here's some pictures.
To sumarize, I've replaced all the outputs and drivers with original part numbers. Pulled and checked all the other semiconductors on all junctions. Resoldered the molex connectors. And the issue is with both channels. I have verified the power supply rails are okay. And I've checked all NPN/PNP transistors are in the right positions according to the schematic.
Can anything on the motherboard be at fault? I've checked for shorts here too. I pulled U1 to see if that changed anything and it didn't, I think that part of the circuit is just for the LEDs?
I have zero ideas right now.
Here's some pictures.
Attachments
Assuming you have Audio1Man's test still in place (good move!), next step is to discover why the amp output doesn't bias to about 0V. Report amp output voltage, and base voltages at Q2 and Q3. Report voltages across R115 and R120, and voltage at Q10 base and Q10 emitter re ground. I hope this will point us in the right direction.
I've replaced the small signal transistors now too. With original part numbers. And checked multiple times to make sure all transistors are in the right spot.
I have Audio1Man's jumper still in place.
With one output module in, I'm getting about 15V at the output with the board I've replaced the small transistors on (on DBT). And 20V with the other board with the original small signal transistors still.
All of the following measurements are with the board I changed the small signal transistors on:
Output voltage settled to 11-12V Output with 100W bulb. About 23V on the power supply rails with this bulb.
If I probe the base of Q2 and 3, the voltage starts at about what the output voltage is, and then both that voltage and the output voltage climbs up a few volts over a few seconds then stabilizes. When I take the probe off, the output voltage goes back down. Weird?
0.6V across R115 and R120 (47ohms)
At Q10, Base voltage is 11.5V. Emitter voltage is 200mV higher than the base voltage. (With output voltage at 11.18 a this moment)
I have Audio1Man's jumper still in place.
With one output module in, I'm getting about 15V at the output with the board I've replaced the small transistors on (on DBT). And 20V with the other board with the original small signal transistors still.
All of the following measurements are with the board I changed the small signal transistors on:
Output voltage settled to 11-12V Output with 100W bulb. About 23V on the power supply rails with this bulb.
If I probe the base of Q2 and 3, the voltage starts at about what the output voltage is, and then both that voltage and the output voltage climbs up a few volts over a few seconds then stabilizes. When I take the probe off, the output voltage goes back down. Weird?
0.6V across R115 and R120 (47ohms)
At Q10, Base voltage is 11.5V. Emitter voltage is 200mV higher than the base voltage. (With output voltage at 11.18 a this moment)
