You could look at these I suppose and check the voltages. All four chips will have a similar arrangement on these pins.
When a pin is 'low' as driven by the BA612 chip the appropriate LED also lights. The 'High' logic level looks to be around +18 volts give or take. You could check all control pins apart from the selected input are 'High'.
I wouldn't overthink at this point though. The chips have suffered an overvolts condition and that has most likely zapped them I'm afraid.
When a pin is 'low' as driven by the BA612 chip the appropriate LED also lights. The 'High' logic level looks to be around +18 volts give or take. You could check all control pins apart from the selected input are 'High'.
I wouldn't overthink at this point though. The chips have suffered an overvolts condition and that has most likely zapped them I'm afraid.
Now i have replaced the IC 756 with a new LC7818 and there is no difference, same crosstalk as before.
Next i will replace the IC 757 with a new LC7818.
Is it plausible that IC 756 that i removed is OK for use if i have to replace IC 755 or IC 758?
Next i will replace the IC 757 with a new LC7818.
Is it plausible that IC 756 that i removed is OK for use if i have to replace IC 755 or IC 758?
I'm very surprised at that tbh.
Yes, there is a good chance it could be OK if the symptoms are identical with the replacement fitted.
I find the block diagram easier to follow for looking at the signal path. All the inputs and outputs are confusingly drawn as left and right together. So tuner in left goes to IC756 and tuner in right goes to IC 755 and so on.
The output from those two IC's looks to be the line coming out of the bottom of them. Its difficult matching everything up with the scanned copy of the manual being in so many parts.
Do any of the tape monitor switches change the fault?
I think you are just going to have to work through those IC's and see if replacement of one changes things... and I don't like saying that as way to fault find 🙂 but the manual is just so hard to follow in scanned form.
Yes, there is a good chance it could be OK if the symptoms are identical with the replacement fitted.
I find the block diagram easier to follow for looking at the signal path. All the inputs and outputs are confusingly drawn as left and right together. So tuner in left goes to IC756 and tuner in right goes to IC 755 and so on.
The output from those two IC's looks to be the line coming out of the bottom of them. Its difficult matching everything up with the scanned copy of the manual being in so many parts.
Do any of the tape monitor switches change the fault?
I think you are just going to have to work through those IC's and see if replacement of one changes things... and I don't like saying that as way to fault find 🙂 but the manual is just so hard to follow in scanned form.
Ok, i will replace the IC755 with the previously removed IC756 and see what happens.
Like I say, I'm really struggling with the actual circuit to match everything up. At this point I would say swap the remaining two, one at a time with the removed and assumed OK parts.
What about this?
Also is there any possibility there could be a solder splash or blob shorting any of the data lines out (from the work done on the power supplies).
Give the board a super intense visual look over.
What about this?
Also is there any possibility there could be a solder splash or blob shorting any of the data lines out (from the work done on the power supplies).
Give the board a super intense visual look over.
Both L and R from tuner goes to IC756 and so does the rest of the sources to IC756.
No crosstalk detected with the Vcr/Tape monitor (IC757). IC756 and IC757 is in the "Audio Section).
IC755 and IC758 is in the "Audio Record signal Section".
I will give it good look👍(again).
That's an interesting result. So it looks like we are on the right lines with suspecting the chips but I never like it when replacing something doesn't quite fix things...
Unfortunately there is no way to prove all this easily without just replacing them and the trouble is any combination of them could have been damaged. The overvolts applies to all of them.
It is a promising result though because I couldn;t really see anything else beside the chips that could do this. Still give the board a really good look over though.
Unfortunately there is no way to prove all this easily without just replacing them and the trouble is any combination of them could have been damaged. The overvolts applies to all of them.
It is a promising result though because I couldn;t really see anything else beside the chips that could do this. Still give the board a really good look over though.
I replaced the IC758 with the "old" IC757 and the crosstalk is same as when i replaced the IC755 with the "old" IC756, almost gone.
Now i´m tempted to replace the "old" IC756 with the "old" IC758, but i think i may be better off waiting for the new LC7818´s to arrive?
After all, new is new, and the old ones may have different degrees of breakdown.
Now i´m tempted to replace the "old" IC756 with the "old" IC758, but i think i may be better off waiting for the new LC7818´s to arrive?
After all, new is new, and the old ones may have different degrees of breakdown.
I have now installed the remaining 2 new LC7818 at IC755 and IC758 and i think the crosstalk is worse than with the old IC756 and IC757.
It is worst playing CD and can now be heard on the Tape side buttons too.
With 4 new LC7818 one would think it should work as intended?
Maybe the IC´s i got are no good, at least the two latest, or maybe all the new ones are bad despite the LC7818 marking on these?
Should i replace the two latest with the old IC756 and IC757 or place an order of unmarked LC7818´s from another source?
Decisions, decisions...
It is worst playing CD and can now be heard on the Tape side buttons too.
With 4 new LC7818 one would think it should work as intended?
Maybe the IC´s i got are no good, at least the two latest, or maybe all the new ones are bad despite the LC7818 marking on these?
Should i replace the two latest with the old IC756 and IC757 or place an order of unmarked LC7818´s from another source?
Decisions, decisions...
I would say having reached this point and with no change to the fault despite all new chips that this is really getting into the realm of signal tracing with a scope and signal source.
Chip replacement was the obvious step and particularly given the history of over volting the rails. I really don't know what to suggest to you next.
This looks to be where the Source and Tape inputs are selected. Check the voltages on the control pins and make sure only 1 line is active at once. They look to be 'active low' meaning zero volts enables the input. Check only one line is active at any one time.
Chip replacement was the obvious step and particularly given the history of over volting the rails. I really don't know what to suggest to you next.
This looks to be where the Source and Tape inputs are selected. Check the voltages on the control pins and make sure only 1 line is active at once. They look to be 'active low' meaning zero volts enables the input. Check only one line is active at any one time.
Thank you, i will check the pins next time i have this apart.
As for now it works fine listening to the CD source and the tuner or other sources turned off or the opposite.
Problem can be detected playing CD with the tuner on and when using source AV1 or AV2 with volume knob turned halfway to full both can be heard, but it seems that the cd input disturbs the most.
This never happens with the older units, like my HK930, with a mechanical source selector.
Thanks again for your time.
As for now it works fine listening to the CD source and the tuner or other sources turned off or the opposite.
Problem can be detected playing CD with the tuner on and when using source AV1 or AV2 with volume knob turned halfway to full both can be heard, but it seems that the cd input disturbs the most.
This never happens with the older units, like my HK930, with a mechanical source selector.
Thanks again for your time.
This needs quantifying... I thought you were hearing the audio breaking through at almost normal volume. If it is very very faint (and I would say it shouldn't be audible at normal volume settings) then you have to ask is this a 'real' fault.
CMOS bi lateral switches like these chips will use have a very high off resistance but also junction capacitance and that can leak audio, more noticeably higher frequencies. Also remember the CD input will see the highest signal levels and that will make the issue appear worse.
This was the issue before i replaced the IC755 with the old IC756.
With the IC755 replaced it was clearly better, but no significant difference after i installed the new IC´s for IC755 and IC758, so i guess the old IC755 was the problem.
Well now all the LC7818 are new, and the amp probably works like it should despite the slight crosstalk from CD at loud volume settings.
This is no problem as i wont play CD while listening to other sources, so for now it´s good.