freestompboxes.org

Great pedal - over designed CMOS switching

I just picked up a "dead" Univibe UV-1 with pedal for less than a 10 box of strings - couldn't resist.
Pedal is the "last" revision with the single board - no chip on the switch board, etc.
Board has a 2-12-03 date but I think this is the "1997" version as it says REV A.

No power or LED's - nothing jumps out as a problem - but beyond confirming that power is getting to the regulator, not sure where to go next.
There is 18 vdc on pin 1 of the UA7815 regulator and 15 vdc on pin 3. (Looking at the schematic - https://www.freestompboxes.org/viewtopic ... 00&start=0) I can't determine how power is turned on - trial and error seem to indicate that a plug had to be in the output jack.) Can't find what controls U4's switching.
One thing I see on my board revision are more diodes in the power section - see attachment, one in parallel with the 100nf dc input cap, and the second is parallel to the 100 ohm dropping resistor for A+
Initial check showed a bad switching contact in the Expression Jack (Middle Contact - Connects U10 (8/9) to On/Off switch (?????) but physically closing that contact didn't help.)

Is there a diagram of how the switching works, i.e. this is low when the On/Off is On which turns this Chip on, which turns on he Red LED?
I suspect that U10 was once changed and may still be the issue.
but would like to be able to work through the switching to see if the logic works further down - i.e., is there a way to see if the Chorus/Vibrato logic works or to see if the unit works and it is just the logic of the Effect/Bypass switch that is not.
I'm sure it is a bad solder joint, or U10 didn't get replace well, but there is some information missing from the schematic and this item seems to have dropped off the radar so I'm hoping someon here can save me some frustration ah show me the way
Thanks
Mike

The resistor to the left of the leftmost diode you have circled looks well fried, is it open circuit?

Well one thing leads further down the rabbit hole
So I replaced the 100 ohm dropping resistor that "creates" the A+ voltage that looked fried
Just because I used a 1/2 watt instead of the 1/4 watt it replaced.
"cleaned up" the solder connections for U10
did a close inspection of the board.

Fired it up and from how quickly the 100 ohm resistor heated up it is evident that something is drawing way too much current down the A+ line. I've not be able to find a schematic that shows the diodes and where they were added but I did pull the Vintage switch as it seemed it wasn't working and might have been a place where the A+ might be shorted.

A visual doesn't show anything so I assume that it is most likely one of the logic chips has gone bad
I'm going to start with U10 since none of the lights are working

What can be helpful in this case, since you've isolated the A+ net to be the culprit is to, one by one, cut the VCC pin of the IC's connected to the A+ line. This way you'll easily isolate which IC causes the short. After that replace the faulty one and fix the others by resoldering the cut pin with a "blob" of solder.

Turned out to be the classic "$100 dead system for a $.10 part" issue
After working my way through the A+ supply chain, it turned out that the 100nf capacitor (located near U5) that was between A+ and ground was shorted
Removed that and the voltage worked
Now I need to go back and reconnect a few items - and replace the "Vintage" switch.

I did learn a lot about the circuit.
Still don't know why there is A+ going to the control board in the ribbon cable
All the control circuits appear to be either Vbias or B+

Thanks for the help
Michael