500-C Topside IBAM first attempt - stealth mode

rufleruf

Poor Impulse Control
I spent a few hours today pursuing an IBAM install on the topside on a 500-C I am restoring. I've done two underside installs and it really is a must do modification / improvement. While I like the idea of being able to make adjustments without 20 minutes of messing with wires and screws and all that, I hated the idea of a big messy board or box hanging out. Sooo here is my solution.

The test subject. IBAM is installed here topside believe it or not. Pretty stealthy right?
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Lots of scrubbing and detailing went into this guy. It's a very late serial number. Oddly it has mismatched output transformers. No evidence of either having been replaced.

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I must admit it is not hooked up on the underside yet, but that's just soldering at this point.

The 33uf's are really fat. Need to get the supermodel version and stuff should fit a bit better.

Feeling a presidential amount of self satisfaction at the moment.
 
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Thanks - I got lucky that this 500-C had a hole that needed filling where that cap normally is, and I had a spare cardboard cover and a random NOS can cap to gut.

Questions:

Do I remove C86 and R131 along with R132?
Can I use 47uf capacitors instead of the 33uf's?
Is there a source for cardboard capacitor covers new? Sometimes they are hard to remove!
 
Yes, get rid of C86, R131 & R132, 47uFs are OK. I don't know of a source for cardboard cap covers. Try a toilet roll or paper towel, mailing tubes might be good. I've found heating the dickens on those black cardboard covers will evenually soften the glue to allow you to pull them off. But you're right some are really hard to remove.
 
That's pretty neat, but I hate those pots. I used them and am always fumbling around trying to get the screwdriver in the slot. My IBAM is also on top, but I didn't care about neatness. The board just sits between the cans and the tubes. Easier to get to.
 
That really is ingenious Matt!!

I assume the cap can slides back on and the board holds it in place/lined up? Fantastic!!

I wouldn't be surprised if you have requests for the purchase of them.
 
In the last picture in the first post the board is installed, so yes, you just slide the cardboard cover off and access the board. If you look closely I used a 2 wide chassis mount terminal strip that's screwed to the can cap base. I then used tie wire to connect the board to the terminal strip. It's pretty well fastened to the can cap base. I was worried about shorts / crossed signals, but the layout sort of naturally avoids that. I am going to buy a nice soldering iron tonight. I've been using this POS that I have to keep filing the tip down on. It's like an english cucumber with a crayon sticking out of it. Takes a steady hand to get the right amount of solder.

I would be willing to make them, but it was a long build - but I suppose I'd get good at it pretty fast and come up with improvements. Also, it's not trivial getting the old can out and wiring everything in, so I'd probably have to do the install too.

thornev - I hear you. I just like a stock appearance - for me the aesthetics of these old Fishers is a lot of the allure. Different strokes and all that.

I'm not excited about these pots, but they are easy to adjust, small and cheap.
 
Matt;

I like it! I assume that you used axial lead tubular cap(s) underneath the chassis to take the place of the OEM black sleeved can cap position you are using as the housing for the IBAM parts. With the small size of current electrolytics compared to the original units used when these were manufactured, it is quite easy to get parts that will fit in the available space. Let us know what the final 7591 cathode currents settle to.

Joe
 
Joe,

Yes, when I got this 500-C it was missing the covered can cap and there was a new-ish radial capacitor floating by its leads there. I used an adhesive backed zip tie mount to secure a new radial cap to the underside. I am probably going to change to a terminal strip though for longevity.

Is there an ideal cathode current? I read a lot of IBAM threads when I was prepping to do this install, but never saw an ideal mentioned.
 
ideal is btw 70% and 80% of max dissipation of the 7591 (19W). Depending on your plate and screen voltages the ma will vary from, 31ma to 35ma in that range.
 
That's R131, comes off the negative side of those bias filter caps. Maybe they needed more resistance to adjust the bias?
 
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