Help troubleshooting my Theremax

Hi Guys.
My name is Carlos. I'm hoping you can help me troubleshooting a Theremax clone I built from scratch

I used an older revision of the pcb (version 9505B) that I modified for current specs (version C)

The thing is that I don't have the voltages that are supposed to be present in some parts of the circuit.

I followed the troubleshooting info at PAIA with no results.

I only "modded" the control panel, using two separated pcb for the jacks and the pots respectively. 
I triple checked the board and the daughterboards and they are ok.

Also checked all the resistor values before installing, and after visually (in some cases I pulled up some resistors that I doubt and remeasured them)

Maybe there is a crack or a bad solder joint I'm not seeing

Thanks in advance for helping.

Here are the voltages I'm geting:

LM339n

1:0,03v            14:0,15v
2:0,03v/7,55v   13:1,68v/2,10v
3:8,06v            12:0v
4:4,01v            11:4,00v
5:3,96v            10:0,15v
6:4,01v            9:0v
7:3,97v            8:0,49v
(Pin 2 goes high when I short circuit it with pin 3 and stays like that until I short it with pin 1)

Q1

C:7,08
B:2,11
E:1,55

________
Q2

C:7,02
B:2,20
E:1,80
________
Q3

C:7,03
B:2,20
E:1,72
________
Q4

C:7,05
B:2,20
E:1,63
________
Q5

C:8,05
B:0,02
E:0
________
Q6

C:8,05
B:0,02
E:0,06
________
Q7

C:8,05
B:0,03
E:0,01
________

Q8

C:4,07
B:0,59
E:0,01
________
Q9

C:4,90
B:0,59
E:0,01



Q10

C:7,60
B:3,82
E:3,50
________
Q11

C:7,58
B:3,79
E:4,16
________
Q12

C:4,05
B:0,31
E:0,01

lm748
1 1,34
2 5,83
3 5,80
4 0
5 1,26
6 4,03
7 8,06
8 3,44

Carlos, I don't see anything obviously wrong with those voltages, though I'm not a Theremax expert. 

There is a user manual here:  http://www.paia.com/manuals/docs/9505_pages_200.pdf

Generally:

1. I'm surprised they are using a comparator IC to do linear stuff (WTF?).

2. I'm surprised they don't use a standard voltage regulator (typical PAiA corner cutting).

3. I'm surprised they are powering the LM748 below the recommended 10V minimum (a huge no-no).

If you don't have access to a scope, you might check with an audio probe the bases of Q8 and Q9 for audio tone, tuning L1 thru L4 until you hear it.  Make sure you use a plastic tuning tool, not a metal screwdriver.

Dewster
Thanks for replying
I have the manual, but is not very usefull in the troubleshooting. The circuit seems very finicky and there are lotsa thing that can go wrong with it.

I tought the same with the zener, specially if the tunning stability depends on the voltage. I'm using a switching PS but with a 8,2v zener I could not get more than 8,07v in V+

Its the first time I see a LM748. I tought thats because its an old rare cousin of the 741 and I started with electronics in 2001.

I checked for audio at the base of those transistors but I can't hear a thing. I also checked for audio at the junction of the diodes that mix the fixed/variable oscillators and I can't hear a faint whistle. Maybe the oscillators arent working, or they have very small amplitude.

I used standard shielded audio cable for the antennas. As I didnt buy the kit, maybe I had to use coaxil.

Or maybe are the diodes/transistor substitutions (I used oa glass germaniums for the diodes and BC458)

hi carlos,

you're sure about "bc458"?  should be bc548 as substitutes for the 2n4124?

did you follow that how to tune your tmax video? (better audio) i like that one.: 1.06 minutes!

as many here i've built the kit, without the box, for a looney all in one instrument idea i had in mind. but to my shame never finished the project. i finished all the soldering and the circuitboard works fine and has the rca mod and a fancy uv-led for that peculiar gate/velocity..thing. it just gathers dust here and reminds on my better days as a (s)tinkerer. so if you like to pay for the shipping, i would happily send it to the bottom of the world.

Yes! Sorry about the mispelling. There are BC548, only oriented backwards (cbe pinout)

I would love if you could send me your board to Argentina. Don't know how we can do it though.

Maybe with one working I could make the other work

Carlos, do you have access to a scope?  I think it's important to use a scope when doing Theremin building / design, otherwise you're flying blind.

carlos,

sorrysorrysorry, i was a bit fast with my offering and i have to apologise. i did not try the unit before writing and now i tried it. it's deadsilent, curses! shame on me to make false hopes, but i have to troubleshoot that unit firstly. (running away & crying out loud and looking for a zener-diode)

...and back alive. silly panicking-me.

all looks and sounds so far fine and it seems that i've already modificated the circuit a bit by information from this very website. changed some capacitors for better volume response and the previous mentioned "rca" switch allows to choose three different timbres, but with the effect that the pitch-field changes too. all mods are reversible, if you don't like them.

i'll p.m you about the mailing and such.

xtheremin... pm sent

dewster, I have a friend who has a scope in his shop, I can borrow his.
There is a french site with scope pictures for this circuit, I'll go and check for similarities.
Anything I have to check in particular?

"Anything I have to check in particular?"  - Carlos

Look for oscillation!  ;-)