Help-Troubleshooting & schematic for older NEC A520E amp - diyAudio
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Old 6th April 2009, 05:10 AM   #1
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Default Help-Troubleshooting & schematic for older NEC A520E amp

Hi-
I know nothing about troubleshooting solid state equipment (my electronics 'formative years' were definitely in the tube era).
However, my latest thrift store find is just too nice to be consigned to the parts bin without making an effort at repair, so.....
Please help!
I need a schematic if possible- I've looked but haven't had any luck.

It's a NEC A520E Integrated Amplifier.
Obvious symptom: It shuts down after a couple of minutes of louder playing, though will play for hours at (very) low volume.

I had a look at the power amp board, and there is a NEC UPC1237H protection IC which controls the relay (to the speakers), and I assume that is what is opening the relay. I can see the relay contacts opening. (The IC also controls power-on delay, power-off disconnect, etc).
uPC1237H Datasheet

The UPC1237H looks for DC offset at the outputs (among other things) and disconnects the speakers if the offset is too high.
Checking the DC at the speaker terminals (this is why I have cheap speakers for testing!)- one channel has 'volts' of offset- rising with volume. Other channel has no offset.
On the power amp board, there are test points (TP1,2,3,4) which seem to indicate the same voltage appearing on the adjacent power resistors next to the output transistors. On the (assumed) faulty side, the TPs (both) are showing several volts, increasing with volume level. On the other side the TPs show 0 volts (Voltages measured relative to ground.)

I'd appreciate any help you can give, and ideas of 'where to look next'.

Thanks
John

I'll see if I can get a couple of pics to show here....Click the image to open in full size.
Click the image to open in full size.
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Old 6th April 2009, 03:14 PM   #2
jaycee is offline jaycee  United Kingdom
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A lot of the principles are exactly the same as tube circuits - just the voltages are a hell of a lot lower!!

OK - the protection circuit is doing its job, so you can assume that circuit is ok. The good thing is, you have a working channel - this means you can take readings at particular points and compare them with the non working side.

Try and find the signal level inputs to the power amp section. Check whats being fed into it - it might well be that the power amp is fine, and the preamp part has the fault. If severing the connection between the two is easy (eg a cable or connector) you can even try injecting an attenuated signal (portable CD player, iPod, anything with a volume control) directly to the power amp and check it out that way.
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Old 6th April 2009, 03:18 PM   #3
jaycee is offline jaycee  United Kingdom
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Hm, i'm wondering.. it looks like the centre piece contains the power supply and output transistors - but the rest of the power amp might be on another board connected by wires. My old Teac A919 was like that. It had compensation capacitors all over the place!

It might help us if you can get some more pictures, particularly an over view of the whole circuit.
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Old 6th April 2009, 03:35 PM   #4
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Thanks.
I'll get some more pics later today- you are correct in that the PS and the power amp are on the board I showed in the pic.
Looks like the phono preamp board is tucked against the side of the amp, and the rest of the preamp is up near the volume/tone controls at the front.
It looks like the signal comes to the power amp board via a couple (one/channel) of cables attached to wirewrap pins. So part of the 'next step' is to isolate the power amp as you suggest, by unwrapping those connections.

Whether it is the cause of the problem or a symptom, I've found a transistor (on small heatsink) on the power amp board which seems to have been very hot at some point in the past- board is discoloured and the solder tabs holding the sink to the board are gone. It is stone cold now, though. It is certainly a candidate for possible replacement if I can figure out a part#- it's marked C2298 (I'll recheck#) and the only ref I can find to that part# is a modern SMD by Siemens.

I did have a check around the power amp board (power off) checking for shorts across the transistor legs but found nothing there- mostly high resistances. (I'd read that they often show dead short after failing?).

More to come

John
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Old 6th April 2009, 04:50 PM   #5
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Default more pics

A couple of pics showing the overall layout of the amp:
PSU and PA board is between the heatsinks.
Power goes to to the other boards from here (Red/Gry/Brn/Purps) to the other boards. Click the image to open in full size.
Another shot showing the Phono/Pre on the left behind the shield and heatsink, input selector switches are at top left, then signal goes to volume and tone boards at the (right) front of amp. Then signal to PA board via two Wh/Wh-Blu/Red ribbon-style cables.
Click the image to open in full size.

Transistor on sink-somewhere I read that this is part of the bias circuit? It's not clear to me yet that even one channel is working perfectly- this transistor seems to be shared by both channels? Left channel signal cable (?) behind transistor heatsink.
Click the image to open in full size.

Cheers
John
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Old 6th April 2009, 06:11 PM   #6
jaycee is offline jaycee  United Kingdom
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That lone transistor on the heatsink will be some kind of regulator judging by the big power resistor and the small (zener most likely) diode next to it.

Bias thermal feedback is pretty much always mounted on the main heatsink, per channel. It's either a transistor, a string of diodes or in some cases a thermistor.

I can see 2 driver transistors per channel and some small blue presets which probably adjust the bias. So my guess is the front-end circuitry is elsewhere.

I've got a feeling it's on the left, as shown in the second picture. That's exactly how my Teac was.


edit: hmm, i just noticed the TO-92 sized transistor behind one of the driver transistors on the third picture. I bet that's the thermal compensation. If so, that's a bit nasty!
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Old 6th April 2009, 06:21 PM   #7
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Quote:
Originally posted by jaycee

Bias thermal feedback is pretty much always mounted on the main heatsink, per channel. It's either a transistor, a string of diodes or in some cases a thermistor.
There's a component bolted on to one of the driver transistors on each side- marked C14/C15- with two black leads back to wirewrap pins. The pins have a diode marking on the PCB. I assumed this was some sort of thermal sensor??

John
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Old 6th April 2009, 06:34 PM   #8
jaycee is offline jaycee  United Kingdom
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Ah yes, now i see what you mean... yeah, that will be the bias thermal sensor
The TO-92 i mentioned then, is probably the "VBE multiplier", basically it's a constant voltage source that controls the idle current through the output stage.

Anyway.. that isn't your problem!

At a guess you are going to need to get that board out on the left (i suppose its on the right if you have the recievers front towards you) and look at it. My guess is it will have the phono sockets on it, but there will be a clear division and then there will be the power amp's front end circuit. There's probably a load of signal wires running from those phono sockets over to the selector switches.
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Old 6th April 2009, 06:47 PM   #9
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Jaycee-
Thanks for all the help!

I'm headed down to the shop in a few minutes, and will try disconnecting the front end from the PA.

I'll report back.

Cheers
John
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Old 6th April 2009, 07:20 PM   #10
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OK-
Problem is in the preamp/driver stages somewhere.

With the feed to the PA disconnected- DC offset problem goes away. When I connect the (bad) right channel feed to the left PA channel, offset appears on the left channel.

I was hoping the problem would be on that nice, big , accessible PA board!

I'll work back and see if I can swap channels to figure out which board contains the problem.

Later

John
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