Hi Guys,
I had a blown channel on Mark IVCM amplifier (1970's), replaced almost everything.
However, I am still getting DC on speaker outputs (around 7.6VDC) with load and full power.
The funny thing is I am getting around 8VDC (or a little worse) on the original channel which was NOT rebuilt?
This is one of those amplifiers that does not have bias setting and where "everything has to be right" for it to function correctly.
I do not think this is a coincidence, but don't understand what could be "common" to both channels. Wiring, ground, power supply, chasis?
Thanks!
Marko
I had a blown channel on Mark IVCM amplifier (1970's), replaced almost everything.
However, I am still getting DC on speaker outputs (around 7.6VDC) with load and full power.
The funny thing is I am getting around 8VDC (or a little worse) on the original channel which was NOT rebuilt?
This is one of those amplifiers that does not have bias setting and where "everything has to be right" for it to function correctly.
I do not think this is a coincidence, but don't understand what could be "common" to both channels. Wiring, ground, power supply, chasis?
Thanks!
Marko
Measure the voltage between the speaker return connector, and the ground buss on the circuit board. If it is 7.6 v, there you are. BGW750 soldered the speaker return to the circuit board. Nearly every other model used something flimsier, less reliable, and easier to disassemble.
For example punch down blocks (AMP KKR) in an amp I own, that oxidize after 10+ years. Remove wire & replace in the fork. Or circles of copper on the circuit board meant to have a ring terminal screwed to it. Again, inadequate contact force can allow the copper to oxidize. Those, you can put a stainless steel star washer between the ring terminal and the circuit board. https://www.mcmaster.com/star-washers/internal-tooth-lock-washers/
For example punch down blocks (AMP KKR) in an amp I own, that oxidize after 10+ years. Remove wire & replace in the fork. Or circles of copper on the circuit board meant to have a ring terminal screwed to it. Again, inadequate contact force can allow the copper to oxidize. Those, you can put a stainless steel star washer between the ring terminal and the circuit board. https://www.mcmaster.com/star-washers/internal-tooth-lock-washers/
SAE MKIVCM? If so check the 12 Ohm resistors on the center tap of each transformer, chassis ground to circuit ground.
Craig
Craig
Indianajo: BGW750?
But if the connection is broken, say due to oxidation or cold solder, how would that put DC on the outputs? DC has nowhere to drain? There should be no DC on the circuit ground?
Craig: Thanks, I will check them out.
But if the connection is broken, say due to oxidation or cold solder, how would that put DC on the outputs? DC has nowhere to drain? There should be no DC on the circuit ground?
Craig: Thanks, I will check them out.
I did not replace SE4010 and 2N3638 nor filter capacitors. The good (original) channel had about 150mV without load, unfortunately I did not measure it with load.
For relatively minor DC offset you'll have to monkey with the 2N4036 differential transistors, they're in sockets for that purpose. Did you replace the 1/4" input jacks, were they grounded to the chassis or insulated?
Craig
Craig
Actually, it seems to me that the offset was BETTER when the gain (hfe) of 2N4036 was different. Originally they came like 56 and 76 on one side and 73 and 54 on the other? Maybe this was done on purpose?
I did not replace the input jacks, I believe they were not connected to the chasis.
In any case I don't have the amp now. I gave it to the shop. Spent a way too much time trying to figure out what the problem was.
I did not replace the input jacks, I believe they were not connected to the chasis.
In any case I don't have the amp now. I gave it to the shop. Spent a way too much time trying to figure out what the problem was.
As I said, 99.9999999999% of solid state power amplifiers don't have the speaker connectors soldered to the output transistor PCB. Hard to make a bad solder joint when there isn't one. Other methods can oxidize at the interface of two metal parts.
Please don't ask for help for a product you have shipped off to the repairman. It is difficult to take a voltage measurement from a distance of kilometers