API 554 problem, replacement FET?

Greetings,

I have an API 554 on my bench that is behaving oddly.  The mid band works properly, but the high and low seem to stay engaged whether the EQ in/out switch is engaged or not.  Also, they only seem to cut - even when turned to the boost positions. 

I think the switching FETs are to blame, but the part number (TIS-75) has been obsoleted.  The only equivalent shown on alltransistors.com is also obsolete.  I sent an email to API, but no reply.  Can anyone recommend a suitable replacement?

Schematics for the EQ are in the Tech Docs section here:
https://groupdiy.com/index.php?topic=44738.msg561000#msg561000

Thanks,
Mike

Based on those particular set of circumstances, it could be that V+ from the EQ in switch is not making it to the HF and LF boards. Could be a bad wire maybe. I would open it up and check voltages. Check to see V+ is making it to the common term of each switch.

Actually the switching in general looks a little dodgy. There are 10M resistors on the gates of the output JFETS! Maybe just cleaning around those solder joints with 99% iso would help.

Otherwise, you can't check voltages on a lot of the gates because of the high Z. You will have to measure at the caps if you have very high Z probes or maybe even back out to before the 1M.

Possible replacement JFET would be J175 maybe if maximum voltage limits are within range. Switching source voltages are -16 for on and +16 for off.

Thanks for the reply squarewave, I will check the switch first.  If that isn't the issue I will clean with isopropyl and try to check those voltages with my VTVM.    J175 does look like it should work.

Them more I think about it, the 10M resistors on those gates is crazy. A little dirt could conduct enough to pull the gate down below Vgs and cause signal to leak through regardless of the switch controls. I would do a sweep at max level with EQ in / out to make sure EQ is actually switching properly. Actually what would do is replace the control circuitry if it's not too difficult.

JFETs used as on /off switches are not very critical as long as they meet max voltage parameters.

JFETS used a limiter/compressors can be more difficult with Vgs sensitivity.

JR

JohnRoberts said:

JFETs used as on /off switches are not very critical as long as they meet max voltage parameters.

JFETS used a limiter/compressors can be more difficult with Vgs sensitivity.

Click to expand...

Ok, but that's not my concern. If you look at the circuit there are 10M resistors in series with the gates of the JFETS in series with the outputs of the EQ sections used for switching. So even the slightest particle of dirt or excess flux could pull the gate in one way or another. If it happens to be anywhere around 0V, the JFET will remain on regardless of the control voltage. Or worse, it might only pass signal that exceeds a certain voltage.

i had this problem a while ago.... https://groupdiy.com/index.php?topic=44395.0

had several i was working on at the same time, i don't recall the exact fix for this one,
or if that one made it out of the shop...

but at least one of them was merely a bad 741  (like they aren't  ALL bad)

Almost every manufacturer in Pro Audio that uses FETs for audio switching uses that same circuit, with a small ceramic cap, so it ramps quickly on and off. I don't remember a case where an SSL was affected by accumulated dirt or flux. Your condenser mikes have resistors from 60 meg to 1000 meg connected to the diaphragm your breath and mouth moisture are constantly coating. It's possible to corrupt those examples but it takes awhile. The gate of a FET is gig ohms. 10 meg is a small value by comparison. The 741 is there in this case to just turn it on and off. Audio does not go through it. However a quieter IC might benefit a noise free transition as they are running open loop when controlling a FET. I use MC33071's. Any that are used in boost or cut operations, should be premiums to reduce noise and distortion while boosting or cutting. The only flavor should be the API2520 amplifiers, circuitry design and capacitor choice.

J111 and J112 should work and are N channel, as is the TIS75. The pin out is a mirror of the TIS75 so just turn it around. Pin 2 is the same, pin 1 and 3 reversed. If the J175 is used it would also work but in reverse. Pushing the in button would be pushing in to bypass. These are very fast analog switches and need the slow down installed in the 554 (R=10M, C=>.1Uf). Used also in early sample and hold circuits.