Dipthonizer repair question

[theehman]

Working on a vintage Colorsound Dipthonizer for a customer.  He got it from a seller and it wasn't auto-wahing.  You could hear the filters change as the rotary switch was turned, just no wah action.  He was poking around in it to see if he could find the issue and blew out the zener diodes.  I've replaced the diodes and it's back to the filter state he received it in.  I've checked the voltages and replaced all ICs and it's still not working properly.  Looking for suggestions of anything I might have overlooked.  Is it possible that the CA3080 I installed are counterfeit or is this not a thing just yet?

[antonis]

Can't find a schematic for the time being but, as far as I know, it should consist of two parallel filters (1/2 CA3080 followed by 1/4 RC4558..)
Check around 4558 respective caps.. There should be 2 pairs (of equal value each pair) and of 1/3 - 1/4 ratio..

[ElectricDruid]

Is it possible that the CA3080 I installed are counterfeit or is this not a thing just yet?

It's possible. Where did you get it? The Alfa Rpar AS3080E clones are very good (at least as good as the selected 3080 chips and better than the originals in several key parameters) if you decide you need a reliable replacement. There are still good CA3080s around though if you want to keep it more original.

[R.G.]

Yep, it's possible. Pretty much any IC can and will be counterfeited if there's money in it. I read that counterfeit chips are a massive problem to > Chinese < manufacturers, too, so take some comfort there if you can.

I suggest falling back on your voltmeter. Instead of easter-egging parts, read and write down all the pin voltages. Also look for whether the voltage to pin 5 is right, and varies with triggering/input level.

As a hint, the voltage on pin 5 of the 3080 can never get over one silicon-diode-drop without killing the chip. If pin 5 ever got more than about 1ma, it kills the chip if it was real.

The LM13700 is a good replacement. If I did a board for a Dipthongizer, I'd use the LM13700.

[theehman]

Good info, guys!  I checked my emails and it looks like these CA3080 may have come from an ebay seller.  I'll get voltages tomorrow and post them.

[Rob Strand]

When in doubt you can measure the characteristics of the device yourself.  Set up a test circuit and drive the IABC pin.   If the device is real and working the device behaviour can be verified against the datasheet, not only that it works but also Gm.

A less fundamental approach is to simply inject a test signal into the unit, then instead of driving the OTA's from the circuit's Envelope detector you drive them from a DC voltage derived off a pot.  If you can adjust the pot manually and get a good filter sweep then you have some confidence the OTAs are working and also a good load of the audio signal processing circuit is working.   If you measure the voltage on the pot you have a good idea what range of voltages to expect.

If you get that far then perhaps the fault is in the Envelope detector.   You can check by injecting a test signal then following the signal through  the Envelope detector.   If that checks out OK perhaps the ripple filter caps aren't faulty to there is a DC offset making the output of the Envelope detector drive the filters outside of the normal frequency range.

The idea is to break the problem down and check locale areas of the circuit.

[bean]

Not sure if you have it but here's the schematic theehman along with my redraw (different component numbering and there may be some small other differences). I have voltages if you need them, although they are from a later version where I used LM13700 instead of CA3080.

[bean]

Actually, I'll just go ahead an pot them. You might get differences in the Bias Input pins on the 3080 from my 13700. IIRC, comparing both in a different circuit one was about half the other but I can't say if that's typical.




Two more things that might help (not mine but I had them in my folder).