Small knob on 5160 motherboard

[kishy]

This may have come up before but I don't know what the heck search terms to use to find it.

I have the 256-640k version of the IBM PC/XT 5160 motherboard. On it, near the ISA slots, there is a small knob (potentiometer?).



Counting from the top it is 2 slots down on the left.

What is this knob for? Is this a primitive method of over/underclocking the bus speed? My thought is perhaps it clocks the bus speed down to allow proper card operation if you want to use a faster CPU (or something like that).

 

[Fallo]

What is this knob for? Is this a primitive method of over/underclocking the bus speed? My thought is perhaps it clocks the bus speed down to allow proper card operation if you want to use a faster CPU (or something like that).

It's a trimmer cap connected to the 14 Mhz clock crystal on the motherboard. If you have CGA, it's used to adjust the color burst phase for the composite output. The 5150 has one also, but near the PSU connector.

 

[kishy]

It's a trimmer cap connected to the 14 Mhz clock crystal on the motherboard. If you have CGA, it's used to adjust the color burst phase for the composite output. The 5150 has one also, but near the PSU connector.

Hmm...thanks. Does it impact the operation of anything except a CGA graphics card? In particular, if I use a VGA card with it, will adjusting that knob impact it? Does it only apply to a specific ISA slot?

 

[offensive_Jerk]

Haha, funny..
I took my 5150 apart today and was also wondering the same thing...

 

[Fallo]

Hmm...thanks. Does it impact the operation of anything except a CGA graphics card? In particular, if I use a VGA card with it, will adjusting that knob impact it? Does it only apply to a specific ISA slot?

No, the trimmer cap is only used for the CGA composite output and nothing else. It has no effect on VGA.

The cap is connected to the OSC line on the ISA bus and works with any slot. Adjusting it is a delicate procedure, since you have to do it with the power on. I did it with my 5150 because I was getting a black-and-white picture on my TV. I booted up Troll's Tale with the cover off and very carefully (since you don't want to drop your screwdriver onto the motherboard and short it out) turned the cap until color appeared.

 

[kishy]

Haha, funny..
I took my 5150 apart today and was also wondering the same thing...

LOL funny, I've been meaning to post this for ages but just felt compelled to today for some reason.

No, the trimmer cap is only used for the CGA composite output and nothing else. It has no effect on VGA.

The cap is connected to the OSC line on the ISA bus and works with any slot. Adjusting it is a delicate procedure, since you have to do it with the power on. I did it with my 5150 because I was getting a black-and-white picture on my TV. I booted up Troll's Tale with the cover off and very carefully (since you don't want to drop your screwdriver onto the motherboard and short it out) turned the cap until color appeared.

That makes sense. I figured it would be a pain to adjust with it off then turn it on to see results (or lack thereof). Neat stuff. I suppose that's a sign of how close the clones had to mimick the IBM designs in order to keep their hardware compatible (though I haven't actually seen a clone board with a knob like this, do most clones have some sort of auto-correction mechanism?)

 

[Chuck(G)]

The trimmer is actually hooked in series with the 14.3818 MHz crystal. On the 5150 schematic, it's labeled "color adjust", but not all color cards use it. It has the side effect of slightly tweaking the system clock frequency, so it can also be used to make small adjustments to your time-of-day calculations, as the 5150 uses a periodic interrupt to keep track of the time of day after you've entered it at system load time.

 

[Fallo]

That makes sense. I figured it would be a pain to adjust with it off then turn it on to see results (or lack thereof). Neat stuff. I suppose that's a sign of how close the clones had to mimick the IBM designs in order to keep their hardware compatible (though I haven't actually seen a clone board with a knob like this, do most clones have some sort of auto-correction mechanism?)

Clone motherboards generally don't have a trimmer cap, since the composite output was pretty rarely used. Also, most CGA clones had a clock crystal on the card itself and didn't need to use the one on the mobo.

The Tandy 1000 is an interesting example of this. Its video isn't connected to the OSC line, so it produces somewhat different composite colors than the IBM CGA card.

Adjusting the cap with the power on is a necessity, since there's no way to tell what position will enable the color burst. I just happened to use Troll's Tale because it runs in CGA 640x200 composite mode and has a nicely colorful title screen. It's particularly tricky because as you're turning the cap, you have to keep one eye on the screen to see when the color appears.

 

[Ec1564]

No, the trimmer cap is only used for the CGA composite output and nothing else. It has no effect on VGA.

The cap is connected to the OSC line on the ISA bus and works with any slot. Adjusting it is a delicate procedure, since you have to do it with the power on. I did it with my 5150 because I was getting a black-and-white picture on my TV. I booted up Troll's Tale with the cover off and very carefully (since you don't want to drop your screwdriver onto the motherboard and short it out) turned the cap until color appeared.

Good to know! I have a 64K to 256K motherboard with that varicap broken off! I never tried it out as I didn't want to mess up anything, Now to get a old case/power supply and get the ole thing running.

 

[per]

Good to know! I have a 64K to 256K motherboard with that varicap broken off! I never tried it out as I didn't want to mess up anything, Now to get a old case/power supply and get the ole thing running.

Some of them are missing just the pastic top of the cap, but I don't know why...

 

[Ec1564]

No, it's busted apart. Top part is missing and the plates are loose. I may over the holiday try it out.