Vintage Synth Repair

Hello all... does anyone have any experience of opening & repairing a 4027-1 oscillator? The pulse wave on VCO1 in my 2600 disappeared and upon testing I found that the sawtooth output from the 4027-1 core was fluctuating around 1.5 to 2v instead of the expected 10v... any advice or suggestions on opening & trouble-shooting the submodule would be appreciated.

Cheers,
J.

Opening the submodule can be tedious.

There's a good photo tutorial here:
http://www.arptech.synth.net/decap.htm

I've had good results using an X-ACTO knife instead of a saw to cut around the plastic enclosure. Although those flush-cut Japanese pull saws from Harbor Freight work well. I also just use my fingers to pick at the silicone rubber around the components, followed by toothpicks to get the little bits around and under leads.

Here's a schematic for the 4027-1 in case you don't have one:
http://www.till.com/arptech/pdf/4027.pdf

Good luck!
-Alex

--- In vintagesynthrepair@yahoogroups.com, "James" <jamesdiplock@...> wrote:

>
> Hello all... does anyone have any experience of opening & repairing a 4027-1 oscillator? The pulse wave on VCO1 in my 2600 disappeared and upon testing I found that the sawtooth output from the 4027-1 core was fluctuating around 1.5 to 2v instead of the expected 10v... any advice or suggestions on opening & trouble-shooting the submodule would be appreciated.
> 
> Cheers,
> J.
>

Thanks for the info Alex. You were right, quite a tedious process removing all the silicone from the circuit board but a relatively simple job. Tracked the fault to the usual suspects - bad tantalum caps - all is now well. Interesting that I found the now un-potted VCO has negligable temperature related drift, no more noticable than the other two potted ones over a 4 hour test period. Out of interest, did you re-pot your repaired modules?

Thanks once more for the pointers.

Cheers,
J

--- In vintagesynthrepair@yahoogroups.com, "Alexis V. Rogers" <alexis.v.rogers@...> wrote:

>
> Opening the submodule can be tedious.
> 
> There's a good photo tutorial here:
> http://www.arptech.synth.net/decap.htm
> 
> I've had good results using an X-ACTO knife instead of a saw to cut around the plastic enclosure. Although those flush-cut Japanese pull saws from Harbor Freight work well. I also just use my fingers to pick at the silicone rubber around the components, followed by toothpicks to get the little bits around and under leads.
> 
> Here's a schematic for the 4027-1 in case you don't have one:
> http://www.till.com/arptech/pdf/4027.pdf
> 
> Good luck!
> -Alex
> 
> --- In vintagesynthrepair@yahoogroups.com, "James" <jamesdiplock@> wrote:
> >
> > Hello all... does anyone have any experience of opening & repairing a 4027-1 oscillator? The pulse wave on VCO1 in my 2600 disappeared and upon testing I found that the sawtooth output from the 4027-1 core was fluctuating around 1.5 to 2v instead of the expected 10v... any advice or suggestions on opening & trouble-shooting the submodule would be appreciated.
> > 
> > Cheers,
> > J.
> >
>