Funny meter issue during EZ760 calibration

[Potato Cakes]

Hello, friends!

So after a long, long time, I've finally finished my EZ760, or at least building it. When I started the calibration process, I encountered an issued with the meter that I haven't even heard of. I started with Meter Zero trimmer (R192) all the way down as the instructions state then I started to turn it up to get the meter to zero. When I started to turn it up to find the zero point, it would go past where I wanted, with a noticeable lag between when I turned the trimmer and when it would actually move. Then when I tried to turn it down, same thing in reverse. I hooked a DMM up to it and saw the voltage was doing a seesaw effect between + and - voltages. When it was increasing voltage, after it crossed 0VDC it would ramp up quickly to it's peak, then back down. When crossing 0 again, it would keep going down but at a much slower pace. So I hooked the meter back up to it and sure enough I could watch the needle do exactly what I was reading the voltages do. There is a BC212L that seems to be involved in the metering circuit. This is all taking place with no signal present at the input.

Has anyone seen this before? Could a bad transistor be doing this?

Thanks!

Paul

 

[Potato Cakes]

So I discovered the problem with that guy. I am using a JLM Power Plant and I had one rail is marked for +48 I had trimmed for +24 thinking that it would be fine, but it is not. I put it in the other side and now that part seems to be solved.

A new thing that needs to be solved is that I can't adjust the R192 enough to get the meter to "0".  I metered one side of the connection and it is +14VDC, and it is unaffected by the meter trim. I measured the other side that is affected by R192 and the most I can get is +8VDC. Chasing it backwards, the resistor before the trimmer has +24V on one side then has the 8V on the other side. The value  of both the trimmer and the resistor appear to be correct. The other board exhibits the same problem, so it seems unlikely that I have made the exact same mistake, but I am pretty good and messing up stuff.

If anyone has a working one of these or has the voltages for the test points on this unit I would greatly appreciated. I'm kinda in the dark where to chase it down if all the values and continuity tests seem to be good so far.

Thanks!

Paul

 

[Potato Cakes]

Update.

I just pulled the Compressor and Expander cards thinking that the issue is somewhere in that area. I checked the component values and orientation, cleaned up some suspect solder joints and put it back in. Now I have 20VDC on the - meter connection and the same range (.658-8.50VDC) on the + pin. So I'm thinking that the + pin that is affected by the Meter Zero is correct, and there is something fishy regarding the - pin. My test point 1-5 voltages are 15.5, 22.7, 21.1, and 1.1, respectively. Is the voltage for T.P. 5 about what I should be getting?  I'd love to wrap this up this weekend and focus more on house projects. DIY is very distracting!

Thanks!

Paul

 

[Potato Cakes]

It seems that when the card is out of the case, I get the higher voltage for the Meter Signal -V connection. I put it back in the case and we are back where we were. T.P.'s 1-4 now measure 15, 23.9, 13.5, .77, respectively. I still think the voltage for T.P. 4 is approximately what I should be getting for Meter -V. Could someone confirm this?

Thanks!

Paul

Edit: I meant to say Meter Signal +. It is fed directly from R116 as shown in the schematic below.

 

[Potato Cakes]

Looking at the schematic and confirming with a DMM I see that one side of the Meter Range (R116) is connected to nothing, as are several other trimmers. I do not understand how a trimmer can be doing it's job without some sort of reference or at least with difference voltage values between CCW and CW. For an experiment, I attached the end with no connection to the chassis and I then had about 6V at the Meter Signal + connection and was able to zero the meter. Without it, I get the 13V on all three connection of R116 (CW, CCW, wiper) and adjusting the trimmer gives me no change to the voltage.

If the trimmers marked without a ground reference and are opened ended are correct, could someone give me quick explanation? Or should I go ahead and connect the free end of the trimmer to ground? Connecting them to ground seems to solve my problem and hopefully allows me to move on with the calibration, but if that is not correct, then I absolutely have no clue what the problem is.

Thanks!

Paul

 

[Potato Cakes]

So I found some voltages on the manual for the Q2 version of the Compex ADR. When I get home from this tour leg I'll dig in and let you guys know how it goes.

http://www.floatingpointaudio.com/adr-Compex-by-q2-audio-user-manual-v2.pdf


Thanks!

Paul

 

[Harpo]

... There is a BC212L that seems to be involved in the metering circuit....

No. There is a BC212B that seems to be ... 
The transistors with L index come with a different pinout.

 

[Potato Cakes]

I think that is what I meant to type, but now that you mention it, I'll have to double check. That would be hilarious if that would be the case.

Thanks!

Paul

 

[Potato Cakes]

Just got home, it is indeed a 212B and not L like I had typed earlier. I was really hoping that I had put the wrong transistor in Q44's place. That would have solved some issues for me.

So back to head scratching...

Thanks!

Paul