Total Recall - low to zero voltage on IC4-B pins

[dlblumberg]

Hi there,

First, should the 'Total Recall' be put through biasing procedure before expecting IC's to show approximately correct voltages?

I haven't done the biasing yet, but most of my voltages are good - all except pins 5, 6 and 7 on IC4.  I've got 0 on 5 and between .2 and .3 on 6 and 7.  Even without biasing I would have expected them to be higher than that, no?  And, for the life of me, I can't figure out the mistake that's led to this condition.  Any suggestions?

Thanks,

David

[jimilee]

Have you checked your components for correct values and polarities? Have you reflowed all of your solder joints?

[dlblumberg]

Hi there, Jimilee,

Yes, I have indeed done what you asked.  Any thoughts?

David

[jimilee]

Nope, try biasing it. Send us voltages and pics of bot sides clear up close in focus.

[thomasha]

Check for shorts and the polarity of c18, if its open to ground it would make sence that pn 6 and 7 are almost at 0

[dlblumberg]

Thanks, thomasha,

I will check that out!  It is true that even prior to biasing, the voltages on those IC4 B pins should be closer to correct, right?

Thanks,

David

[dlblumberg]

Hi there, thomasha,

I checked it out - no luck there.

I should add that as I said before, pin 5 is zero voltage.  But both pin 6 and pin 7, when I initially probe them are both around -.3 volts.  But in both cases, when I leave the probe there, the voltages start counting up, as in -.3, -.29, -.28, -.27, etc.  As long as I leave the probe there, they continue counting up.

Any other idea's? 

Thanks,

David

[thomasha]

Hi,
I just realize I was looking at IC3B! My mistake.

Both extremes of bias2 trimmer. It should be 0 and VD. If you don't have VD the problem is prior to the trimmer. Set bias2 at half and the voltage should be VD/2 at pin 5.

This also answer your question about the biasing. Both bias trimmers must be at least near half of the turn, otherwise the voltages will be either to negative or 0v.

[dlblumberg]

Hi Thomasha,

Okay, I just posted pics of the board.

I do indeed, have both bias trimmers at the halfway mark.  Re: the bias 2 trimmer, no matter where I the wiper, it is at 0 volts.  That is, all throughout the wiper's rotation, it still reads zero volts.  So, you're saying the problem is somewhere prior to the trimmer.  Where might a good place be to look?

David

[dlblumberg]

Here is the other pic.

[dlblumberg]

Hi Thomasha,

If the VD isn't getting there, it almost sounds like maybe a VD track could be messed up.  I looked at R45, and there doesn't seem to be any problem with it.  But I can't tell from the schematic exactly how VD gets to R45.  Maybe looking more closely at the tracks on the actual board would help.  I guess I'll try that.

[dlblumberg]

Correction, looking at the layout I can see how the VD gets to R45.  I'll have to look closely at that area.  Thanks.

[jimilee]

Correction, looking at the layout I can see how the VD gets to R45.  I'll have to look closely at that area.  Thanks.

Do you have the BP cap in the right spot?


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[dlblumberg]

I just found the problem.  Somehow the track between R45 & IC7 pin 8 had been severed.  No continuity there.  Thanks so much, Thomasha, for turning me in the right direction to spot the fault!  Thanks for your help too, Jimilee.

David

[jimilee]

I just found the problem.  Somehow the track between R45 & IC7 pin 8 had been severed.  No continuity there.  Thanks so much, Thomasha, for turning me in the right direction to spot the fault!  Thanks for your help too, Jimilee.

David

Nice! Enjoy it, its a great delay.


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