VCO drift...problem. Help Please.

The Roland Jupiter-4 and ProMars Compuphonic Analogue Synthesizer
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Joined: Sun Aug 08, 2010 8:03 am

VCO drift...problem. Help Please.

Postby sir_dss » Fri Dec 10, 2010 7:05 pm

Hello...I just brought back an J4 from Japan and and a really good step down transformer.

I picked up the service manual to calibrate if it was necessary when I got back home.

I'm finding that one of the VCO(voice board D) will not stay in tune...I can tune it using the procedure from the service manual but it will drift after a few minutes and sometimes act as if there was a random LFO signal being sent to it(there isn't).

When adjusting the the trim pots they cause the VCO to jump around if any downward pressure is applied to them. I'm wonder if that's the culprit or could it be something else?

All of the other VCO's are rock solid and totally stable so it must be on the voice board.

Thanks so much!

David Scott Stone

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Joined: Fri Oct 16, 2009 8:53 pm

Postby covariance » Sat Dec 11, 2010 1:22 pm

Swap the voice boards to check if the drift comes from the outer trims or cv generation circuit.
A loose or oxidated connector can also cause drift.
Also check the problematic voice board for any damage or corrosion.
If all seems fine the first component suspicious is the 726 expo converter IC.

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Joined: Sun Aug 08, 2010 8:03 am

Postby sir_dss » Sun Dec 12, 2010 5:22 am

Excellent Suggestions! Thank you so much!

Posts: 20
Joined: Thu May 08, 2008 5:17 am

Jupiter 4 tuning/calibration

Postby starfield » Sun Jul 07, 2013 8:11 am

hi David, i know its been a while but did you have any luck?

im having a similar tuning problem with one of the voice boards on my jupiter 4

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Joined: Sat Sep 29, 2007 3:41 am

Postby spacecho » Sun Jul 07, 2013 3:57 pm


I had similar issues and found a solution by doing the following:
spacecho wrote:Hi,

I recently got a nice fully functional (thank god) JP 4..

A couple days later i switched it on and noticed that one note was out of tune (more than a semitone!)

At times it would play fine,

It might have been suggested elsewhere - but i managed to track down the cause in 2 mins...

The Voice Allocation board (sits in front of the 4 voice cards) has 2 multipin cables that connect to it (white connectors, rainbow colored leads). I simply tapped the left one and found that it is intermittent (moving it brought the synth back in tune)

I have also noticed that the switches that select the bender moluation (vcf, vca, vco) can change the tuning if the contacts are dirty, as do a few of the rotary dials - basically, there are quite a few parts of the synth where an intermittent pot or switch can change the tuning.

The white multi pin headers, as well as the molex connectors used in roland synths seen to go intermittent (probably due to the fact that they are not gold plated, plating gets corroded and oxidises)
I have also fixed a roland rs505 which would lose voices, by cleaning (isoprophyl and deoxit, and reseat connectors) the voice cards.

Maybe any techs on the site can verify my findings, but i think it is a good start (if you are comfortable doing it) to clean and reseat the ribbon cables on this synth.

I would invest in a bottle of isoprophil alcohol, a toothbrush (firm bristles) and a can of caig de-oxit.

What you want to do is apply isoprophil alcohol to the toothbrush and brush inside the ribbon cable contacts, then do the same with the de-oxit. IMPORTANT - make well sure the sprays have dried before re-connecting and applying power!!

The ribbon cables are a known failure point, as are the tuning pots - if you have had a CV mod (4 individual cv sockets) they will also be a major failure point - definitely clean these contacts.

If this doesn't solve your issues - you might need to clean the contacts on the voice board - but you want to make sure you know what you're doing and feel comfortable doing so - removing the voice boards is something a tech should do..

Let us all know how you go - my JU4 stays in tune really well now.

Posts: 20
Joined: Thu May 08, 2008 5:17 am

Postby starfield » Mon Jul 08, 2013 6:00 pm

thanks for the tip! i will try the toothbrush cleaning

I recently recapped the PSU, motherboard and the voice cards with non-polarized Nichicon 85 degree rated caps, and also reflowed some cold solders

i cleaned the voice board contacts already and re-seated the connections

the control board seems to be solid but a couple of the boards are drifting in tune (around 6-12 cents) the drift is on the lower A1 trimmer

I should mention the top A3 is almost dead on +-1 cent on all 4 voice cards.

im wondering if the problem is the lower tuning trimmers since the A3 test note is stable and dead on with all the boards.

I am also going to replace the 726HC transistors, i hear they can affect the vco stability too. wish me luck! will post my results soon

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