Old 24 pin gear capacitor warning

Started by jim-analog, March 10, 2019, 11:04:01 AM

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jim-analog

 Greetings,

The other week I was attempting some modifications to the Hex distortion output from one of my G505s, run thru a GR300. I was having some very strange issues, that I discovered were due to the gtr controller electronics pulling down the +/-15VDC supplies from the 300. Turns out, there were 3 shorted electrolytic caps. I had previously replaced the main power supply caps in the 300 unit (2 large, 2K2uF/35v and 2 smaller 22uf/25v.....the smaller were blue tantalum, which has a tendency to fail short; I replaced with standard electrolytic types).

So, on any of the 505/202/808/707 type guitar controller boards, there are about 16 small capacitors that should be changed. Most of them are 10uF/16V BiPolar types (likely these are ok), the others are 2 x 47uF and 1x .47uf standard type. Due to the decrease in size of caps over time, it's easy to both increase the voltage rating (as the factory 16V was right on the line operating at 15V) to 25V units. 

The job took about an hour, and there were only 2 slightly problematic areas. One is a potentiometer, RESONANCE  that was mounted in a way that made it very difficult to remove the cap near it (one of the main p/s 47uF). I had to lift the metal standoff the pot is mounted to for solder access to the cap. The other issue was the ribbon cable coming in from the 24 pin jack. Over the years, these are getting VERY brittle and fragile! I'm working on a solution to replace the factory "header" with 2 x 12 pin Molex connectors on the control board, and hard wire individual conductors or ribbon cable from the 24 pin connector to them. FYI, the pin spacing is a standard .1"  I'll provide Mouser part #s when I order them (the largest I have in stock is only 10 cond.) and can also put together a list of caps.

So, the short of the story is the main failure items in older gear tend to be inexpensive and relatively easily replaceable. I doubt the cost was even $10 for the all the caps. As an aside, I have second, spare GR300 that I'm replacing ALL of the electrolytic and tantalum caps in (not just the power supply sections). That is a much larger job that requires  major disassembly, but once done, the unit will be as good as new for another 30+ years.

  If any of your 24 pin gear is acting odd (control voltages not sending complete, lack of gain, ripple on notes, etc.) please look into a cap change. I'm glad to answer any questions and will post pix of the boards, etc.. in a follow up.

Regards, Jim

GuitarBuilder

Yep, standard procedure for 20-30 year old electronics.  I've had to refurbish and recap all my Rockman modules, as well as ADA preamps.  This also can significantly reduce any noise issues you might experience.

I'm very interested in your ribbon connector replacement project.  Looking forward to info and pics!
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973