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
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
Get the spec sheet for that VCA chip, find out what it expects... then you've got a basis to know what is and isn't right. Randomly changing parts is not likely to fix the issue.
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.
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.
Last edited:
I did, it's the AN7062Nbut you can find it here.
Yep, found it the hard way
This is from my previous post...
It would also be helpful to check them against their mirrors on the other channel.
Last edited:
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 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
Last edited:
collector of Q402 as I said
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...
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. Now I just need to order some 1ohm resistors and we'll be done
!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
. Now I just need to order some 1ohm resistors and we'll be done - Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.