Technics SU-V series AN7062N input amplifier

Hi!


I brought a second SU-V85A back to life that had blown output transistors, drivers and some of the other components in the current amp circuit.
The amp works but when I turn up the volume the top of the sine wave clips pretty quickly on the channel that had issues. I traced it back to the input amplifier on the board behind the faceplate (AN7062N). All voltages check out except for the one on the collector of Q402 which is too high and increases with volume.
I replaced the 2sa1123 with a KSA1220 just to be sure but the issue stays the same. There is no clipping on the input of the chip so I think it might be the chip itself, do you know of any replacement or reputable seller of these ?

Thanks



Technics su-v85a.jpg
 
I have the spec sheet but it just shows a block diagram and max ratings so that's not very helpful. I checked both D402, Q402 and C412 and they seem all fine. I don't usually just go around changing parts but I can't make sense of this circuit. Pin 11 and 8 are supposed to be the outputs but I read nothing on them. I only get signal on the collector of the 2 transistors and on the one for the right channel there is a big dc offset.
 
I have the spec sheet but it just shows a block diagram and max ratings so that's not very helpful.

Post it here... please. Or at least give us the part number from the chip itself.

OK, I did some digging and found both the schematics and the chip data sheet.

For starts you should compare the behaviour of the two channels against one another ... pin 11 against pin 8, and their associated parts. I would check C408, C410, C412 and D402 first.
 
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I did, it's the AN7062N :) but you can find it here.

Yep, found it the hard way :D

This is from my previous post...
For starts you should compare the behaviour of the two channels against one another ... pin 11 against pin 8, and their associated parts. I would check C408, C410, C412 and D402 first.

It would also be helpful to check them against their mirrors on the other channel.
 
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Didn't see your update on you previous post.

The chip handles both channels actually, (except for power rails and ground connections each side of the chip is one channel. I compared both channels and like I said before, the only difference is visible on the collector of Q402. D402, c412 and c410 test fine, R408 reads 819ohms. I'll check c408 tomorrow.
 
Didn't see your update on you previous post.

The chip handles both channels actually, (except for power rails and ground connections each side of the chip is one channel. I compared both channels and like I said before, the only difference is visible on the collector of Q402. D402, c412 and c410 test fine, R408 reads 819ohms. I'll check c408 tomorrow.

The voltages are indicated on the schematic, so you can verify where the strangeness begins.

I've never worked on this particular model so...
I'm adding the spec sheet so the others can chime in.
The service manual is HERE
 

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  • AN7062N_PanasonicSemiconductor.pdf
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collector of Q402 as I said :eek:. That really is the only place I measure a difference between both channels. Voltages are on the low side because I'm plugged in to a dim bulb tester but other then that they're ok

Ok so all the other parts in that chain read correctly?
Don't forget that signal also travels off board on pin 7 of j10 and would be affected by parts around Q462 as well...
 
Sure looks like it yes.

I assumed the previous stage was feeding the high voltage into q462 which is the first stage of the voltage control amp but I didn't really check voltages around that transistor. I will give that one a close look tomorrow and report back.
 
Well I didn't dig far enough in the current drive amp. D506, D508 and IC502 were toast as well. I had some 8.2V zener diodes so I replaced D506 and 508 but for the IC I'll have to order. RC4558 seems very close in spec (those M5219P look like perfect candidates for fakes on ebay)
 
Yes I did, the cable is fine (I don't think I would have sound if it wasn't). I tried disconnecting that pin and the result was that all pins on Q402 were at 57V (supply rail) and the output of the power amp was sitting at -33V, I forgot to compare to the result of removing the corresponding pin on the other channel, I'll do that tomorrow.
 
Yes I did, the cable is fine (I don't think I would have sound if it wasn't). I tried disconnecting that pin and the result was that all pins on Q402 were at 57V (supply rail) and the output of the power amp was sitting at -33V, I forgot to compare to the result of removing the corresponding pin on the other channel, I'll do that tomorrow.

About the only thing I can think of, if all parts confirm, is bad solder, broken trace, etc. It wouldn't be the first time I've fixed something by retouching the solder...

Maybe some of the others have ideas...
 
I redid all the solder joints around these parts, it's always possible I missed something but I doubt it. I will disconnect the output of the preamp IC from the voltage control on the working channel tomorrow and see what voltage I get, this way I will probably be able to narrow it down to either the AN7062N I suspected in the first place, or something further down the road. Thanks for your help so far.
 
Success :) !

I disconnected pin 10 on J10 (equivalent of pin 7 but on the working channel) and the dim bulb lit up bright and kept the amp from fully powering up so no way to compare voltages on the main board. I did however compare voltages around the AN7062N pre amp ic and those were perfectly symmetrical on both channels so I assumed this part was ok and refocussed my attention on the main board. I started checking resistor after resistor which took me some time but one before last, just when I was ready to give up, I found that R460, 1 ohm, was measuring 1.2Mohm. Although on the outside it didn't look burnt at all it opened up probably when the outputs fried and actually protected the voltage control amp from damage. I replaced it with a 4.7ohm resistor because I don't have 1ohm and all voltages now check out, no more clipping on the positive half and bias is adjustable :D. Now I just need to order some 1ohm resistors and we'll be done :)