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Author Topic: OD2 is always maxed out no matter the setting [SOLVED]  (Read 1660 times)

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Cpt Adama

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When probing test point F on OD2 at 10 I get about 23Vp-p. When the setting is changed to zero the voltage stays the same.
The front panel works fine and I can see the digits changing value from 10 to 0.
I swapped U14 & 13 to see if the TC9176 was defective, but this did not have an effect.
The chip works and I can get about 14.8V at pin 15 for the output to OD2 circuit. Changing the setting from 10-0V also changes the voltage accordingly on pin 15.
I'm a bit stumped on this problem.  :facepalm:
Any Ideas?
« Last Edit: Time Format by Cpt Adama »
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MarshallJMP

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Re: OD2 is always maxed out no matter the setting
« Reply #1 on: Time Format »

Most common thing that happends here is a bad "H" wire or a bad connection. If you have no connection it will be always full on.

It could also be the vactrol but they rarely fail. To check if current is going through the led measure the voltage over R50 (1k).

Next is the driver circuit around U2 and the feedback from the vactrol.
« Last Edit: Time Format by MarshallJMP »
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Cpt Adama

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Re: OD2 is always maxed out no matter the setting
« Reply #2 on: Time Format »

Most common thing that happends here is a bad "H" wire or a bad connection. If you have no connection it will be always full on.

It could also be the vactrol but they rarely fail. To check if current is going through the led measure the voltage over R50 (1k).

Next is the driver circuit around U2 and the feedback from the vactrol.

H wire checks out OK for continuity and voltage is read off the PCB and on the Amp Board on both sides of the wire. The voltage at the H wire does not change when I run the OD2 gain from 0-10.

R50 has zero voltage over the resister. In addition U2 (TL072CP) pin 1 is suppling 14.69V at the Vactrol (U3) pin(1). If I measure the voltage drop from Vactrol pin 1 to R50 I get the same 14.69V.

U2 input pin 2 is getting variable voltage from OD2 gain control, I can run from 0-10 and see the voltage change, but the output on U2 pin 1 remains constant at 14.69V. Shouldn't that voltage vary as the pin 2 input varies? Could the TL072 be bad?

Also Pin 4 on U2 is receiving 15V power supply to the chip
« Last Edit: Time Format by Cpt Adama »
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Cpt Adama

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Re: OD2 is always maxed out no matter the setting
« Reply #3 on: Time Format »

I was able to reference another working MP-1 and Pin 1 on U2 should definitly change with the input of pin 2. I'm going to order one the TL072CP chips and replace it and see if that fixes the problem. I'll post an update with the results. Thanks to MarshallJMP for leading me in the right direction.  :thumb-up:
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rnolan

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Re: OD2 is always maxed out no matter the setting
« Reply #4 on: Time Format »

Hope that gets it fixed  :thumb-up: fingers crossed  :wave:
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MarshallJMP

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Re: OD2 is always maxed out no matter the setting
« Reply #5 on: Time Format »

I don't think the problem is the LF353 (is there a TL072 in it?) but the vactrol since there is no current going thru R50. Could be that the led inside the vactrol is busted. Since part of the vactrol is used for feedback to the LF353 and it doesn't respond the LF353 is trying to push current through it and that's why you have 14V constant on pin 1 of U2. So IMHO the LF353 is not the problem.
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Cpt Adama

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Re: OD2 is always maxed out no matter the setting
« Reply #6 on: Time Format »

Now that I'm looking at the schematic again I see they list an LF353, but yes there is a TL072CP there. I used my multimeter and tested the diode and it reads about 2V. I'm away for a couple of days but I'll take look and see if the other chips are LF353 or if the were replaced with the TL072CP. I'm hoping its the chip because I could not find any Vactrol anywhere.
« Last Edit: Time Format by Cpt Adama »
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MarshallJMP

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Re: OD2 is always maxed out no matter the setting
« Reply #7 on: Time Format »

Strange, so on diode test you get 2V over the led of the vactrol? Is R50 ok? Maybe R50 is disconnected from the vactrol? But it seems there's no current flow since you measure the same voltage on pin 1 and 2 of the vactrol. So if you measure with an ohm meter from pin 2 to ground you should have 1k, can you test that when you get back?
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Cpt Adama

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Re: OD2 is always maxed out no matter the setting
« Reply #8 on: Time Format »

So for the Op-Amp (U2) difference from the schematic, ADA must have made a change at some point. I can tell these are OEM chips as the solder joints are wave soldered. I was also able to check out 2 other version 2 MP-1's and they as well had the TL072CP chips. So ADA must deem this as a viable substitute.
« Last Edit: Time Format by Cpt Adama »
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Cpt Adama

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Re: OD2 is always maxed out no matter the setting
« Reply #9 on: Time Format »

The LED portion of the Vactrol (U3) on my multimeter on diode test (power off on MP-1) positive lead on pin 1 (anode) and negative lead on pin 2 (cathode) shows 1.6V. From the VTL5C4/2 data sheet: LED Forward Voltage Drop @ 20ma: 2V (1.65V Typ.).

With the positive and negative leads reversed shows 2.7V. From the VTL5C4/2 data sheet: LED Reverse Breakdown Voltage: 3V

R50 measures 995 ohms. (1K from schematic, so 0.5% low and in tolerance)

This is the part where I am definitly confused. When I measure directly across R50 leg to leg I get no voltage drop. When I measure from pin 1 of the Vactrol (anode) to the ground side of R50 I get the 14.7V that is being applied to pin 1 of the Vactrol from the output of U2. When I measure from pin 1 to pin2 on the Vactrol I get the 14.7V. Why would I not get a voltage drop from pin 2 that goes to R50 and the other side of R50 that goes to ground? :crazy:

So if U2 (TL072CP) is not the problem could it be that the sensor side of the Vactrol is inop? How would I test that?
« Last Edit: Time Format by Cpt Adama »
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MarshallJMP

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Re: OD2 is always maxed out no matter the setting
« Reply #10 on: Time Format »

Are you sure the ground side of R50 is actually grounded? Verify that with an ohm meter measuring from ground side of R50 to the starground.

Edit:If you get 14.7V between pin 1 and 2 that should mean that you have an open circuit? Do you have a small power supply? What I would try is get the vactrol out of circuit, start at 1V and keep an eye on the current into the led and make 100% sure it works. This way you can also check if the resistors inside work. If you vary the current you should also see a variation in the resistance.
« Last Edit: Time Format by MarshallJMP »
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Cpt Adama

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Re: OD2 is always maxed out no matter the setting
« Reply #11 on: Time Format »

R50 is grounded to the star-ground.

On the Vactrol, I removed it from the CB and started a 1v and about 10ma and the resister side is open, jumped it up to 12V and 12ma which is right in the center of it's rated values and should be 150-200 ohms and it shows open.  >:(

The LED shows current and voltage when power is run thru it, but now that I took it out of the circuit the diode test shows open. >:(

These seem to be unobtainium anyone with a spare VTL5C4/2 you would like to sell me? O:-)
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MarshallJMP

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Re: OD2 is always maxed out no matter the setting
« Reply #12 on: Time Format »

Small bear in the US sells them. Parkin elmer stopped producing these years ago but a company called xvive makes replacements now. Never used them but I'm sure they will work.

https://smallbear-electronics.mybigcommerce.com/photocoupler-dual-xvive-vtl5c4-2/

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Cpt Adama

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Re: OD2 is always maxed out no matter the setting
« Reply #13 on: Time Format »

As MarshallJMP predicted, it was in fact the Vactrol VTL5C4/2 that was inoperative. It's been replaced and now OD2 controls the Vp-p as it should.

MarshallJMP your a real superstar at these forums and I really do appreciate your expertise and help. I sent you a little something in PayPal, have a couple of pints on me 🍺🍺.

Keeping 80's rack units out of the landfill one component at a time.  :metal:
« Last Edit: Time Format by Cpt Adama »
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MarshallJMP

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Ok great  :thumb-up: BTW no need to send me anything, I rather have that you donate something to the depot.
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