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NAD 214 Troubleshooting

kennycrippen

kennycrippen

Member
I’ve been restoring a NAD 214 power amp with my 9 yo son for a fun learning project. He’s been doing great with soldering and staying interested. Initial symptoms were: unit powers on, soft clipping light stays on, no relay click. Front lights stayed on even after unplugging the unit suggesting caps holding charge.

When I opened it up it was full of grime and dog hair. Main filter caps and the next 4 biggest caps (C210, C211, C212, C213) on the power supply were all leaking. Pulled them all, cleaned the board up meticulously and replaced. Also replaced all of the other caps on all 3 boards. Finally, I replaced the IC201 opamp.

I’ve been testing along the way. Symptoms listed above persist with and without driver boards plugged in. DC offset on both channels is darn near 0. I currently have the bias trimmer pots turned all the way down.

I’m pretty decent at hands on soldering but still learning a lot about troubleshooting. I hesitate to throw any more parts in here with hopes of resolving the issues. Any advice would be greatly appreciated. Any known transistors that are prone to failure? Possible bad relay? Tips on how to and where to focus testing? I have a DMM and have used it for testing caps, transistors etc. Hoping to high heaven it’s not the transformer. Thank you!
 
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Cannot for the life of me find IC202. I’ve replaced IC201 with a new component and I did see IC202 listed in the service manual but have never yet found it… on power supply board I assume?

EDIT: Found it. Long skinny one. The “I” is obscured by the screen printed circle overlapping and I had been reading as C202 this whole time.
 
dr*audio said:
How are you testing the DC offset? You can't test it at the speaker terminals if the speaker relay is off. You have to test it at the emitter resistors.
Click to expand...
Indeed, I was testing at the terminals. Another thing to test, thank you. Will test IC202 first and report back, might be a few days.
 
kennycrippen said:
Indeed, I was testing at the terminals. Another thing to test, thank you. Will test IC202 first and report back, might be a few days.
Click to expand...
Better to test the dc offset first, you more likely have a failed output/driver/PE transistor.
Test between the output emitter resistors as per the doctor
 
Ok, a few days turned into just a few hours. ;) DC offset is still good, very near 0 measuring at emmiters. Here are the pin readings of IC202:

1: 0.13V
2: 0.3V
3: 0
4: 0.25V
5: 0.2V
6: 0.9V
7: 0.5V
8: 0.3V
9: 0.475V

From what brief research I've done pin 9 is way too low.
 
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A few maybe's, Yeah pin 9 looks low but it comes from unregulated supply (off main caps) so it should be ok unless R230(18k) has drifted high.

Pin 6 is the relay drive, it sets a low voltage between R228 and the relay for current flow, 0.9V looks ok.

Check fusible resistor R229, when stressed fusibles drift high/open, measure incircuit, 680ohms. Generally all fusibles should be checked.

Pin 2 is dc offset 300mV is high also pin 4 is GND, would have liked to see 0V, but since Pin 6 is ok assume no issue with pin 2.

Check R229, R230.
 
Check TH101 and TH102. If they are open the relay will not activate. Also, you said the caps leaked. It may only have been glue but if it was really leaked electrolyte, it can eat the traces on the board, so if any caps on the board leaked, examine the traces nearby, carefully. If they look brown instead of green, scrape off the solder mask and examine the copper to make sure it isn't eaten. Repair as necessary bridging any gaps with wire and solder, not just solder.
 
Thanks guys. TH101 and TH102 look good, both measure around .4 ohms (MM fluctuates a bit but stays under 1).

R229 right around 660 ohms in circuit. R230 I probably need to pull to measure, it's jumpy on the MM.

From what I initially found on the bottom of the board the traces seemed to be in tact but I can check again once I get the board back off. Attached a pic of the cap leakage for fun, it was pretty ugly. Main filters were pretty bad too.

Curious if there's a better way to get to the bottom of the power supply? Thus far I've been unscrewing it from the top and propping it up after desoldering the 4 large wires that are soldered into one driver. It sure would be handy if the bottom cover was easily removable. I don't really see a way to do that unless the large heat sinks are removed in which case I would need to remove the driver boards.
 

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Got a read on R230 in circuit. Stable around 770k o_O So it seems I should replace that one, haha.

Tempted to put a BoM together of a number of other resistors/diodes so I'm not spending $5 shipping for a $.33 part. @mbz How did you figure R230 is 18k? The schematic is too blurry to read. I plugged the color bands into an online calculator before I saw you had posted the value and it gave me 16k. I'm probably doing something wrong, I input brown/blue/orange/gold. I will likely need to calculate values for a lot of the resistors using the color bands.

Edit: I see what I did now. My tired eyes saw blue instead of gray. Gray in sequence returns 18k.
 
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kennycrippen said:
Thanks guys. TH101 and TH102 look good, both measure around .4 ohms (MM fluctuates a bit but stays under 1).

R229 right around 660 ohms in circuit. R230 I probably need to pull to measure, it's jumpy on the MM.

From what I initially found on the bottom of the board the traces seemed to be in tact but I can check again once I get the board back off. Attached a pic of the cap leakage for fun, it was pretty ugly. Main filters were pretty bad too.

Curious if there's a better way to get to the bottom of the power supply? Thus far I've been unscrewing it from the top and propping it up after desoldering the 4 large wires that are soldered into one driver. It sure would be handy if the bottom cover was easily removable. I don't really see a way to do that unless the large heat sinks are removed in which case I would need to remove the driver boards.
Click to expand...
That looks like glue. From your readings on the IC, it should be turning the relay on, so you may have a bad relay, an open trace, open R229, or maybe the 52V supply is bad. Pin 9 on the IC is low and should be turning the relay on.
 
Thanks @dr*audio for the pointers. I had assumed that was cap leakage because the caps were all bulging like crazy. Makes more sense that it's glue since there isn't PCB damage.

Might take awhile to pull together a parts list/order. Once I have a replacement in hand for R230 I'll swap it in and report back.
 
Might be more to this one other than R230 going high/open.

With pin 6 at about 1V means there is about 50V across R229 and the relay so the relay should(?) operate.
Suggest continuity check the relay coil, ie connect probes to D210 take reading then reverse polarity/connections and repeat measurement, same result?

Schematic has R229 as 1.5k but you measured about 600ohms, check R229 for markings.
Probably just a late change in value for relay dropout issues?
 
Schematic has R229 as 1.5k but you measured about 600ohms, check R229 for markings.
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I think you're looking at the 216 specs, which I was also doing earlier. R229 on the 214 page is a blob of black smudge, ha. :oops: I'll check the markings, but you did mention earlier that it should be 680 ohms, so I'm assuming that's what the markings will tell me too.

I'll test D210 later this evening.
 
D210 coming in at .6v one way and 2.8v or so the other, I think that’s good? Measured D211 and get the same result.

Confirmed R229 value and it tests in spec.
 
Tested all resistors on the power supply board and put an order together. Should get to swapping in R230 and any other components that need it within a week. Tracked down most of the transistors just in case, a few of those are difficult to find modern replacements for. Grabbed all the diodes too just in case. They're cheap.

Thanks for getting me jump started in the right direction @mbz and @dr*audio
 
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