Hi,
I have good basic electrical engineering knowledge but little experience. I now have a problem is out out my league, so if any of you can give me a a head-start I would be very grateful.
I got a NAD-amplifier from the dump, that I try to repair. Except from starting up, it did not "do" anything and the first fix seemed simple enough. BR101 looked bad, so I replaced it. That made the LED on the board work again, but the amp still did not work. Then I noticed how R167, R154 and R156 got smoking hot (I saw smoke) within a minute. I then switch off the amp.
So the left channel and the right channel have resistors that carry way too much current. Notice that the resistor R167 has a different function than R156 and R154.
Then some measurements: R154 and R156 had a voltage of 15V (and rapidly rising) R167 measured 90V
In simpler amplifier diagrams I can calculate the expected current running through stages. Here my lack of experience is showing. In this schematic I did not see how current sources set the circuit.
Then I registered on this forum hoping that posting my question here teaches me how to go about with this one.
The service manual is attached to this posting
I have good basic electrical engineering knowledge but little experience. I now have a problem is out out my league, so if any of you can give me a a head-start I would be very grateful.
I got a NAD-amplifier from the dump, that I try to repair. Except from starting up, it did not "do" anything and the first fix seemed simple enough. BR101 looked bad, so I replaced it. That made the LED on the board work again, but the amp still did not work. Then I noticed how R167, R154 and R156 got smoking hot (I saw smoke) within a minute. I then switch off the amp.
So the left channel and the right channel have resistors that carry way too much current. Notice that the resistor R167 has a different function than R156 and R154.
Then some measurements: R154 and R156 had a voltage of 15V (and rapidly rising) R167 measured 90V
In simpler amplifier diagrams I can calculate the expected current running through stages. Here my lack of experience is showing. In this schematic I did not see how current sources set the circuit.
Then I registered on this forum hoping that posting my question here teaches me how to go about with this one.
The service manual is attached to this posting
One ordinarily puts an amplifier under test on an AC current limiter to slow down the emmision of smoke to allow one to analyze parts. Usually called a "light bulb box", because the old 60 W incandescent bulb had characteristics that were perfect for this job. If it lit up with no signal going in, the amp had a DC problem.
2nd step is usually to measure DC out on the speaker outputs, or since this amp has protection relays, the input to the relays. Say relay 101 pin 4 & 6, or more conveniently the output sides of L105 & L106. DC greater than 100 mv shows a problem.
Basic information for new repair hobbiests: Electricity from one hand to the other over 25 v can stop the heart. Use an alligator clip lead to connect the negative probe of a DVM to the speaker ground, an only use one hand to probe. Furthermore 1 v at high currents can burn the flesh to charcoal. Wear no rings, bracelets or other metal jewelry on hands or neck while probing the amp.
I do advise taking DC data with a DVM before replacing anything.
Happy hunting.
2nd step is usually to measure DC out on the speaker outputs, or since this amp has protection relays, the input to the relays. Say relay 101 pin 4 & 6, or more conveniently the output sides of L105 & L106. DC greater than 100 mv shows a problem.
Basic information for new repair hobbiests: Electricity from one hand to the other over 25 v can stop the heart. Use an alligator clip lead to connect the negative probe of a DVM to the speaker ground, an only use one hand to probe. Furthermore 1 v at high currents can burn the flesh to charcoal. Wear no rings, bracelets or other metal jewelry on hands or neck while probing the amp.
I do advise taking DC data with a DVM before replacing anything.
Happy hunting.
Thanks for the tip. I removed the capacitors. They do not short and there measured capacitance is around the printed 220uF. Powering up the amp without these did not fix the the problem with the high voltage over R167.
I measured again, it's 90V.
BTW: wouldn't a short in those capatitors yield a voltage of 50V over the 82Ohm resistors? I measured 15V.
Capacitance at 2 v (test voltage of meters) does not predict leakage current at 50 v. Put a milliammeter in series and put 50 V on them if you want a real test. Current should decay down to single digit microamps. Electrolytic capacitors are known time fuses, they cause the owner to buy another product in 1 year (cheap ones) or 10 years (middle price) or 30 years(best available). Select long service life ones as replacement, 8000-10000 hours, if you do buy new ones.
It is alternately possible that D101 D 107 are shorted backwards. Easily checked with diode scale of dvm. 1n4148 are a bit stressed at 50 v, 1n4003 would be a more long term part choice but cost $.02 more.
As far as R167, there is 100 v between the rails. It is possible Q129 or 131 is shorted CE, or Q121, 123, 125, or 127. Check that the voltage across these CE is at least 2 v. I'd use MPSA42/92 as replacement. When checking voltages on TO92 transistors a pamona grabber as a test probe for one point keeps one from using two hands on a 90 v source.
It is alternately possible that D101 D 107 are shorted backwards. Easily checked with diode scale of dvm. 1n4148 are a bit stressed at 50 v, 1n4003 would be a more long term part choice but cost $.02 more.
As far as R167, there is 100 v between the rails. It is possible Q129 or 131 is shorted CE, or Q121, 123, 125, or 127. Check that the voltage across these CE is at least 2 v. I'd use MPSA42/92 as replacement. When checking voltages on TO92 transistors a pamona grabber as a test probe for one point keeps one from using two hands on a 90 v source.
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Thanks for you safety concern. Would not be the first time I zapped my selves, although that was with 230V AC.
A minimum of 2V for Uce is good advice.
All diodes seem ok.
As for the transsistors:
Q129: 97V
Q131: 3.6V
Q121: 2.3V
Q125: 79V
Q127: 0.8V Thought I had a winner here, but that's to be expected, the voltage of one of the diodes the Ube of Q125 for the other diode
The MPSA42 has a different pinout so I'll have to search through my inventory to see if I have a suitable replacement or order some new ones.
A minimum of 2V for Uce is good advice.
All diodes seem ok.
As for the transsistors:
Q129: 97V
Q131: 3.6V
Q121: 2.3V
Q125: 79V
Q127: 0.8V Thought I had a winner here, but that's to be expected, the voltage of one of the diodes the Ube of Q125 for the other diode
The MPSA42 has a different pinout so I'll have to search through my inventory to see if I have a suitable replacement or order some new ones.
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Bingo!
Sorry, I had to expand schematic up to 400% to see transistors were ecb.
I got some BC639/640 from newark/farnell 2 years ago that were BCE, which you can turn around backwards with the flat away from the OEM position. they were only 80 vceo though. The regulated voltage on C111 is below 16, it might be alright.
Worst I zapped myself was 400 v from electrolytic capacitor in a 6 v car radio. Was thumb to finger, didn't go through my heart. No warnings in the Ford car manual 1959 of course. Was a 1956 radio in the 1959 car, had a vibrator to to transformer to boost the voltage that the 59 radio didn't have. In 59 they used 12 v anode tubes.
Sorry, I had to expand schematic up to 400% to see transistors were ecb.
I got some BC639/640 from newark/farnell 2 years ago that were BCE, which you can turn around backwards with the flat away from the OEM position. they were only 80 vceo though. The regulated voltage on C111 is below 16, it might be alright.
Worst I zapped myself was 400 v from electrolytic capacitor in a 6 v car radio. Was thumb to finger, didn't go through my heart. No warnings in the Ford car manual 1959 of course. Was a 1956 radio in the 1959 car, had a vibrator to to transformer to boost the voltage that the 59 radio didn't have. In 59 they used 12 v anode tubes.
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