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Need replacement T1 trimmer pot for 5150

VERAULT

VERAULT

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Joined
Jan 30, 2012
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Connecticut, USA
I have an early 5150 board (16KB to 64KB) and I am repairing it. The Trimmer pot for Composite video T1 is mangled and needs replacing. Anyone know the value or where I can source one?
5160_c1.jpg
 
That looks the same as on one of my 5161 backplanes. I think I need one too.
 
Well hopefully someone cant point us in the right direction. I have desoldered it. There is nothing written on it. I am hoping the unit can work without it as it was not going to function in its currnet state anyway. I think its only for CGA composite video adjustment.
 
Its not a pot. It is a variable Capacitor. Note the designation is C1 on the mainboard. The value is 5 to 50 pF According to the IBM technical reference manual page D2. If it is T1 then the value is 5 to 30Pf according to SAMS. And to me it looks fine.
 
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that photo is from a 5160 suposedly not mine. Found the pic on the internet. mine is t1 and as i mentioned its mangled. no sense in showing nothing at all.
 
Yes, the trimmer is mostly there for older CGA cards that used the 14.3818 MHz OSC signal on the ISA bus and used to adjust the phase of the color burst signal. Later CGA and all other video cards don't use it. I suppose one might use it to slightly tweak the time of day "tick", but I doubt that it would be noticeable, particularly in PCs with their own clock chip.

If you're not using CGA composite output, you can probably get away with a small fixed-value cap, say 10 pF.
 
The use of a motherboard trimmer for composite CGA color burst tuning was silly. Put the crystal with trimmer on the CGA itself and leave it off the motherboard. You have to wonder what the boys in Boca were thinking...
 
VERAULT said:
Sorry I thought I updated this thread but I only mentioned briefly the new replacement I used in my other repair thread I opened for the whole board. See the first post:https://forum.vcfed.org/index.php?threads/breathing-new-life-into-1981-ibm-5150.1239263/

The pots I used were these 9-120Pf Sprague variable caps: https://www.ebay.com/itm/202908102783
Click to expand...
Hi there- I read somewhere about taking precautions soldering direct on to the motherboard - I saw a safer method proposed on minuszero.net which suggests soldering to the old legs.. is that the approach you took for these trimmers? cheers! Andy
 
lordhailsham said:
Hi there- I read somewhere about taking precautions soldering direct on to the motherboard - I saw a safer method proposed on minuszero.net which suggests soldering to the old legs.. is that the approach you took for these trimmers? cheers! Andy
Click to expand...

No. Im not a novice. Desoldering a component and replacing it with another is really simple. I have years of experience and proper soldering and desoldering equipment. Now that was not meant to sound boastful. Replacing this capacitor is an extremely trivial job. Now I dont know your skillset nor do I know what tools or equipment you have. If you have at the minimum solder braid, a jar of soldering flux, and a working soldering iron.. this job shouldnt be too difficult for a beginner with patience. Just make sure the temp on the soldering iron is not to hot or you will burn/lift traces.

There are boards you need to take precaution on.. Like corrosed boards.. from exposure to moisture/water or something caustic like battery leakage. This will ruin the solder and make it really hard to remove but if the item you are desoldering has shiny solder your all set and ready to go.

If you dont have the skills and this approach scares you and you want to just add this one capacitor and never solder again.. sure solder to the old legs.. Its just not something I would do.. I have been soldering since I was a kid.. I guess I got the fear out of my system back then.

Make sense?

Edit: By the way, looking at the photo of your board. You still have all those original 3 leg tantalum capacitors.. Those are what you should be worrying about. Those are going to fail SHORTED on you and cause the machine to not boot. I spend a couple weeks this summer replacing all the tantalums on most of my IBM gear. Some were already dead and shorted and some hadnt gotten there yet,, but would have.. sooner than later. The 5150 board this thread is about was purchased dead and had a shorted tantalum. Replacing them all was all the board needed (and the c1 capacitor)!

Here is the formal thread for that early 5150 rebuild. As of this week its pretty much done. Last bit was the floppy drives and they are now fine. https://forum.vcfed.org/index.php?threads/breathing-new-life-into-1981-ibm-5150.1239263/
 
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lordhailsham said:
I saw a safer method proposed on minuszero.net which suggests soldering to the old legs..
Click to expand...
That was in regard to the filter capacitors on the motherboard, and I see on my website, "If possible, use the technique shown at here, because on a 5150 motherboard, soldering the ground and/or power lines on the PCB is problematic, and you could end up damaging the PCB.'' That sentence is targeted at those with nil or poor soldering skills. A competent solderer would ignore the suggestion, knowing how to deal with ground/power joints that are problematic. And even someone with little soldering experience may decide to go and do some internet research on the subject.

On the 5150/5160 motherboard, the trimmer capacitor's legs are neither connected to ground nor a power rail.
 
Opinion varies greatly on the subject of replacing old-but-still-functional tantalum capacitors, and has been discussed quite a few times on these forums.
My opinion (repeat: opinion) is in post #22 at [here].

In regard to the IBM 5150/5160 motherboards, the data at [here] suggests that if someone has poor soldering skills, then perhaps (to reduce the possibility of PCB damage) they limit any 'global' replacement to just the tantalum capacitors on the +12V and -12V lines.
IBM 5150 motherboard type 16KB-64KB: 6 of the 17 tantalum capacitors are on the +5 volt line.
IBM 5150 motherboard type 64KB-256KB: 10 of the 13 tantalum capacitors are on the +5 volt line.
IBM 5160 motherboard: 13 of the 16 tantalum capacitors are on the +5 volt line.
 
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