replacing CMOS IC's in a PRophet 5 rev. 3.3

[radiance]

I'm about to order new CMOS chips for my Prophet 5 rev. 3.3 and I'm wondering...do I REALLY have to replace them all?
Or are there just a few that are most prone to fail?

 

[radiance]

Ok, maybe better to buy a cheap logic probe to find out which are the actual dead chips instead of replacing them all ??

Or is there more to it, finding a dead logic chip?

 

[wtmnmf]

It would be helpful to know what problem you are having with it 

Get the schematics.  Understand the schematics in terms of functional blocks.  Try to locate a suspect block based on the type of failure you are experiencing.  Are all of the chips seeing the correct supply voltages?  Any getting hotter than the others?  If there are socketed chips and you can swap some around, try that and see if the problem moves with the chip or changes in some way. Get an EPROM burner/IC tester and test all of the socketed stuff.  If you don't have an oscilloscope, now is the time to get one.

I wouldn't remove soldered chips unless you have a good desoldering tool and are confident of doing the the work without doing more harm than good.  If you have found the defective chip and have only basic tools, then go ahead and remove it and repair any damage done.  If you know it's the bad chip and can sacrifice it, cut the legs off first; it'll make desoldering easier and lower the chances of lifting a pad or trace. Use a nice machined pin socket.

If I'm looking at something without a schematic or much of a clue, here's what I do:

Look for a mechanical fault (90% of all "electronics" failures). Bad ribbon connectors, dirty contacts, broken pots/switches, etc. Check for cold solder joints. You can learn alot about a circuit while running your eyes over it looking at the solder joints.

80's stuff with a battery? Check it! It may be pulling down the logic supply if it's dead. It may have leaked and taken out nearby components or a trace. Sometimes factory settings need to be reloaded after a new battery is installed before the unit will wake up.

Swap boards around.  Swap components around.

Check IC pins.  Power?  Look for floating inputs. Look for outputs that are not at logic levels.  Check the bypass caps, they tend to fail from power cycling, and they can fail by shorting.

Check that the microprocessor is in a good state (initial conditions met) and has a good clock.

There are many things that you can learn about a circuit without much of a clue or any documentation, but since the service manual is available you don't need to bang your head on the wall too hard 

Good luck, I hope get that nice synth running!

 

[radiance]

Thanks for that!

I've the docs and I "kinda" know what going on and were on the boards.
So far I've recapped the PSU and replaced ALL tantalum caps on the PSU, main and voice board.
The synth IS running but it has some problems that seem CMOS related. All LEDs are flickering and you hear this as well in the audio.
It seems that the pots (or ratter the CMOS IC's on the potentiometer board) are throwing out random values.
When turning all knobs this can be solved but it's always a different pot and after a while the problem comes back without me having toughed any pot.

After searching for this problem I found that the original 4000 series CMOS chips are prone to fail and that it's wise to replace them all and be done with it.
This however, seems overkill and I'll much ratter have a logic probe and search for a bad chip every time something flaky happens.

 

[Andy Peters]

Thanks for that!

I've the docs and I "kinda" know what going on and were on the boards.
So far I've recapped the PSU and replaced ALL tantalum caps on the PSU, main and voice board.
The synth IS running but it has some problems that seem CMOS related. All LEDs are flickering and you hear this as well in the audio.
It seems that the pots (or ratter the CMOS IC's on the potentiometer board) are throwing out random values.
When turning all knobs this can be solved but it's always a different pot and after a while the problem comes back without me having toughed any pot.

After searching for this problem I found that the original 4000 series CMOS chips are prone to fail and that it's wise to replace them all and be done with it.
This however, seems overkill and I'll much ratter have a logic probe and search for a bad chip every time something flaky happens.

You're wasting your time with a logic probe. You need an oscilloscope.

-a

 

[radiance]

I have a scope...how do I track logic IC's with a scope?


..BUT...

replacing all tants and cleaning all pots seemed a fix to all flickering weirdness.
All good now.

Thanks all!