Page 40 of the manual.

No problem. I am at work and looked it up, the .pdf on IPDB looks a bit fuzzy. I would have to check if I have the printed manual and if it is any better than the online scan.

Just the electrolytic caps, they are the ones that 'dry out' and fail over time. Not saying that a tantalum cap cannot be the problem, but shotgun recapping that is typically just the electrolytic caps.

Could be a bad electrolytic cap though I doubt it. Doesn't hurt to replace them but don't expect it to solve your hum (some hum is normal on Sys11 BTW). Heat kills electrolytic caps, not age.

The other caps marked as "B" in the schematic are bypass, or decoupling capacitors. Those also are not likely the source of your hum.

More likely suspects are wiring, connections and the amps.

viperrwk

While you have the bonnet up it wouldnt hurt to make sure all the mounting screws are in the boards as well and that the earth pin on the power cord is in place. It probably wont help the hum but its good to do it.

One or more of the 'lytics could be bad, but you have to ask the question why doesn't Ed or Todd have a cap kit for Sys11 sound? I'm guessing because there isn't much demand to replace the caps on these boards. So be prepared if after you do so the hum is still there.

You may also want to try disconnecting each channel and see if the hum is more prevalent on one or the other. That may further help you figure out where it's coming from.

viperrwk

its MONO so only 1 channel..all speakers are wired together. Check where your sound board wires are run as well. I have seen wires next to the sound board cause minor hum by being next to speaker wires ribbon cable etc. Try wiggling wires near it with the game on. CAREFULLY lol

You're right of course. I should have said "speaker" since channel infers stereo. And I did not ask whether this was a one speaker Pinbot or two, which if the former, definitely would have made no sense.

viperrwk

Age plus heat kills caps. Even at room temp - they do age and dry out. The hotter the cap, the faster it dries out.

But to answer the other poster's question regarding why GPE doesn't carry System 11 caps... answer = "time".
It's on my list of kits to carry but just never got there yet.

If you have a nice, 60hz hum -- do as the others mentioned and check grounds, connections and cable routing.
Dried out caps in a sound section (the smallest caps seem to go first) often result in diminished volume. Of course, it depends on which specific cap goes bad, though.

I had hum on my f-14 and it turned out to be that the wire feeding the volume pot was too close to one of the coils. For some reason it must've been inducing a hum into the audio circuit through that path.

Speaking of simple things to check, often sound hum problem in pins is the result of a "ground loop" condition. Opposite of making certain the sound board is firmy grounded to the chassis with metal screws, removing and insulating the sound board from the mounting points often reduces hum considerably, even though this would seem a counterproductive thing to do.

And this ^ check your audio cable routings, making sure they are away from high power sources such as power cables, transformers, DMDs and displays, etc..

Ed - you're smarter than me and know a HELL of a lot more about electronics than me. Curious your thoughts about two things:

1 - ageing and reforming of capacitors: http://digital.library.unt.edu/ark:/67531/metadc3104/m2/1/high_res_d/thesis.pdf

2 - if I have a 25-year old electrolytic that tests with appropriate ESR, should I replace it anyway?

viperrwk

In audio equipment, this is almost always the case. In a well designed circuit, externally injected noise shouldn't be much problem- not that it can't be in practice.

and/ or high frequency noise.

I think this depends on several things: size/ cost, difficulty in changing later, time you expect to own item/ purpose for being in there in the first place, and possibly most important, the possible consequences to other parts of the circuit should it fail. IOW, if the transistor that's going to die as a result is NLA, and not easily substituted (think germanium) then definitely.

For the most part, they're virtually free for smaller values. If it's already out to test, why not just replace? It's 25 years old, and old electrolytic WILL fail. A $50 power supply cap is another story.

1 - I have done reforming of older caps before. Never with cheap caps but I have done this with computer grade pricey caps. It does work. Always be careful of using older caps, particularly high capacitance ones. If they aren't reformed, it is possible that they will look like a dead short to the power supply and take something out... including the cap itself.

2 - Depends on specific cap and where used. As mentioned by Pin-groom -- don't worry the small stuff, just replace it and be done. Some older computer grade caps that are lightly used (think Wms ~27000uF caps) -- if they're good, leave them alone.

Does your power cord still have all 3 prongs on it?

Here's a good and not too lengthy article that fairly well describes the relationship between safety (earth) ground and ground loop problems.
http://www.soundonsound.com/sos/aug06/articles/qa0806_4.htm

This is something you don't want to experience unless you do it on purpose, I assure you!

I almost shit my pants.

I was testing an older DC/AC inverter that had been donated to a local Scout troop when out of nowhere...BOOM! There was "confetti" and metal slivers everywhere.

The cap was the only problem. Replaced it and she was up and running.

Lessons learned- always photograph, sketch, or have schematics before applying full power, and wear safety glasses when working on electronics. (You already knew the last one, but do you really do it?)

Been there, done that. Almost went into shock. World got grey and distant for a few minutes.....