NAD 2100 in protection, -45V rail too high

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Electoneman

Electoneman

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Joined 2016
mandu said:
The 4 ohm 8 ohm selector switch is defective.
Only one side functioning.
Pull all 4 secondary fuses and measure ac voltages on the secondary side to ground.
Change the switch position and measure again.
If defective, replace the switch.
Regards.
Click to expand...

When I flip the switch to 4 ohm, it does drop the voltages everywhere. But, I'll give it a shot. I am measuring with the switch in the 8 ohm position.
 
M

mandu

Member
Joined 2010
With fuses F503, F505, F506, F504 removed,
Switch in 8 ohms position what are the voltages between
Ground to F503, Ground to F505, Ground to F506, Ground to F504
and with switch in 4 ohms position what are the voltages between
Ground to F503, Ground to F505, Ground to F506, Ground to F504 ?
Make sure the primary is set to 120 volts instead of 100 volts.
Regards,
 
Electoneman

Electoneman

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Joined 2016
I have two 2100s and the high voltage power envelope rail uses 10000/80 and the regular rail 10000/50. Same with the 3100 integrated amp and 7100 receiver. Proper DC voltage on the lower power rail after rectification should be +/- 45vDC, so the 50v caps should be fine. The 2100 service manual does not list voltages. The 3100 service manual does.
 
M

mandu

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Joined 2010
If you have no load and power on in 8 ohms position, then (power off or) turn to 4 ohms position, the power supply capacitors will not discharge fast enough. It will show more than your calculated value.
The rails are not discharging equally, meaning you may be seeing a higher negative voltage. This will not be a problem if the output transistors are drawing some current.
Regards.
 
Mooly

Mooly

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Electoneman said:
That pic is a little foggy. Which specific transistors and diodes?
Click to expand...

You need to check them all. If in doubt then remove them and see if the low voltage rail falls back to normal.

D436 (top one) is the commutation diode. If you have -71 volts on the anode (the end that goes to Q438 collector) then you need to check that diode for being short/leaky. Just pull it from the board and see if the voltage falls.

Also, if you do have -71 volts on the anode then there is also a problem around the rail switching transistors (those lower three in the diagram).

There is a low (ish) impedance path via R462 (the 100 ohm) to enable the voltage on your main reservoir cap to rise if there were a fault around Q450. That is why you need to check ALL those semiconductors.

(the original service manual isn't exactly clear but it displays OK for me, all component numbers are discernable)
 
Electoneman

Electoneman

Member
Joined 2016
Mooly said:
You need to check them all. If in doubt then remove them and see if the low voltage rail falls back to normal.

D436 (top one) is the commutation diode. If you have -71 volts on the anode (the end that goes to Q438 collector) then you need to check that diode for being short/leaky. Just pull it from the board and see if the voltage falls.

Also, if you do have -71 volts on the anode then there is also a problem around the rail switching transistors (those lower three in the diagram).

There is a low (ish) impedance path via R462 (the 100 ohm) to enable the voltage on your main reservoir cap to rise if there were a fault around Q450. That is why you need to check ALL those semiconductors.

(the original service manual isn't exactly clear but it displays OK for me, all component numbers are discernable)
Click to expand...

Thank-you. I have a couple different scans of the 2100 service manual, but I also use the 3100 integrated amp service manual as it has the voltage specs printed.
 
Electoneman

Electoneman

Member
Joined 2016
A little update. I've pulled all the commutation diodes and measured them out of circuit. All measure ok. I've also pulled every transistor in the amplifier stages (numbered 400) and all test ok with hfe in the proper range. I used my dmm to check the resistors and they are within spec. I pulled the four voltage rail fuses and measured centre voltage again. You could watch the filter caps discharging on the dmm and once it got below 500mv, click - the relays engaged. Put the fuses back in and centre voltage goes back up to around 6vDC and stays there in protection.

My next step is to order all replacement parts and re-populate the board.
 
Electoneman

Electoneman

Member
Joined 2016
I'm having difficulty finding an original pair of 2SD1238. I don't want to try any Chinese or ebay clones for obvious reasons. Two cross-reference replacements are the 2SC2581 and 2SC2837. Specs are:

2SD1238 / 2SC2581 / 2SC2837

PC: 80w / 100w / 100w
Vcb: 90v / 200v / 150v
Vce: 80v / 140v / 150v
Veb: 6v / 6v / 5v
IC max: 12A / 10A / 10A
Freq: 20mhz / 20mhz / 60mhz
hFE: 120 min / 50 / 50-180

Would either be a safe replacement? Is one better than the other?

Thank-you.
 
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