VRG build problem [Resolved]

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Pav
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VRG build problem [Resolved]

Post by Pav »

Third VRG unlucky..

In LFO mode all looks good both EXP & LIN. and UP & Down Controls behaving as expected.
In the Normal and One Shot positions the output is stuck on maximum voltage with solid red light , with AND without an
external gate plug inserted.

So far
I have checked power to chips.
I have continuity checked the on-off-on mode switch and it behaves as expected.
From the Sockboard, gate in is 0v with no plug inserted, and gates can be measured on "Uppr" with plug inserted

So im guessing something is pulling the gate signal high elsewhere on the pcb .

Edit : Hmm insert external gate into slew in and gate is unstuck and external gate works in one shot and normal..
checking wires are not crossed.
Last edited by Pav on Wed Dec 15, 2021 10:43 am, edited 1 time in total.
rgds Pav
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Re: VRG build problem

Post by Pav »

testing continues..
Wires were not crossed. Swapped the 8 way connector with one from a working model - no joy.
tested for shorts under the pins of the connector - all as expected.
Reseated DG403 - no joy.
Reflowed some of the relevant circuit - no joy.
No obvious soldering errors overall.

The voltages on the Mode switch pins which pull DG403 pins low
DG403 pin15 pin10
LFO Mode - 13.6v 0v 0v
Trad AD 0v 0v 0v
One Shot 0v 0v 13.6v

I have traced with scope ext gate pulses as far as R55 where i see the pulse (0 -> -8v??) which should mean DG403 is ok.
Still No Joy
rgds Pav
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Re: VRG build problem

Post by Synthbuilder »

What's the voltage at pin 6 of U8? It should be 5V.

What's the voltage at pin 7 of U8? It should be be -13V with no gate applied. It should, however, go to +13V when the slew output is over +10V or so.

The voltage at either side of R55 should be 0V (no gate) to +10V (gate applied). If it is really -8V then something is probably wrong with the DG403.
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Re: VRG build problem

Post by Synthbuilder »

Pav wrote: Tue Dec 14, 2021 9:14 am The voltages on the Mode switch pins which pull DG403 pins low
DG403 pin15 pin10
LFO Mode - 13.6v 0v 0v
Trad AD 0v 0v 0v
One Shot 0v 0v 13.6v
I'm not sure what you're measuring there. There seems to be three voltages for each mode.

In Trad AD mode both pin 10 and 15 should be +14V.
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Re: VRG build problem

Post by Synthbuilder »

Thinking about this some more. Do you have continuity of the 0V (pin 2 of the power header) from your PSU/Dizzy to the VRG main board?
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Re: VRG build problem

Post by Pav »

Hi Tony, sorry to impose on your pre christmas ,

in reverse order

Yes continuity of 0V on pcb and on dizzy checked as well as the cable itself.

In Trad Mode both pin 10 and 15 are 13.6V ... my numbers were incorrect previously for trad.
I was measuring the voltages on the mode switch not the DG403 hence 3 values

Now with no gate plugged in and Trad Mode selected pin6 of U8 is only + 5V , pin 7 + 14v
Voltage of R55 is +9.4V, (same results with an external Gate inserted in gate socket)
same results with one shot.

In LFO mode (using DMM not scope) U8 pin7 -13V pin 6 still +5v and R55 -6V and LED oscillating

I tried a second DG403 - no joy.
rgds Pav
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Re: VRG build problem

Post by Synthbuilder »

Pav wrote: Tue Dec 14, 2021 6:44 pm Now with no gate plugged in and Trad Mode selected pin6 of U8 is only + 5V, pin 7 + 14v
Pin 6 should indeed be +5V. I've just edited that in my original post to avoid confusion for anyone else using this thread.

Pin 7 should be -14V but since the VRG's output is stuck high that you're getting +14V is no surprise.
Voltage of R55 is +9.4V, (same results with an external Gate inserted in gate socket)
same results with one shot.
All the time in 'AD' and 'One shot' modes? It should go low when the gate is low. If not, then suspect a fault around U11. D6 isn't around the wrong way is it?
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Re: VRG build problem

Post by Pav »

Diodes are all positioned correctly.
D6 has +14.x V and 13.96V respectively at either end and diode check says its ok.
I speculatively replaced U8 but made no difference.

I did some more checking of voltages around U11 and D5 has 0V at both ends .(diode check on dmm says its ok.)
which surprised me ..but maybe a reason .
In fact the row D3,R7,R8 D4 all showing 0V too ..which seems dodgy.

Replaced U11 - no joy. Always in Trad AD and One Shot . LFO mode fine.
WAIT A MINUTE - - think the problem is i have used 4011 when i should be using 4001 OOPs :sstorm: :slapfight:
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Re: VRG build problem

Post by Pav »

SOLVED! Sorry to waste your time on a picking error Tony. Lucky i had one 4001 from Banzai ages ago.
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Re: VRG build problem

Post by Synthbuilder »

Pav wrote: Wed Dec 15, 2021 10:42 amSOLVED!
:tu:
Sorry to waste your time on a picking error Tony.
Don't worry. It's easily done. I've done similar things many times.
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