The motherboard of the IBM PC 5150 has labels of letters followed by numbers. What are the letters representing? Examples being U1, P1, C1, T1 and Y1.

I am trying to ascertain which component is the crystal oscillator connected to the Intel 8284, so am particularly interested in the T1 and Y1 labels. There is also a C.ADJ label above T1, does this represent "clock adjust"?

  • 2
    The usual component designators are U for an integrated circuit, P for a connector, C for a capacitor, T for a transformer, and Y for a quartz crystal (or other resonator; it symbolizes a tuning fork).
    – Whit3rd
    Commented Dec 30, 2019 at 5:57
  • 1
    There was a standard, but it is no longer current and was never universally followed. In practice various people use various reference designations. For example using two letter prefixes (eg. IC instead of U) has become more common these days. en.wikipedia.org/wiki/Reference_designator
    – user722
    Commented Dec 30, 2019 at 23:09
  • Had not heard of "reference designator". The "P" is for plug a "most-movable connector of a connector pair". The IBM 5150 uses T1 for trimmer, but T is probably more commonly used for transformers. The trimmer is a variable capacitor, so they could have used VC. There is also a component labelled T01 on the motherboard. Commented Jan 1, 2020 at 16:04
  • "T" in this case probably stands for "tuner" which historically is what variable capacitors were often used for.
    – user722
    Commented Jan 1, 2020 at 20:36

2 Answers 2


"C.ADJ" represents "Color Adjust" - the 14.318MHz signal is used to derive the NTSC clock for the CGA card, so adjusting the trimmer affects the colours that appear if the composite output jack is in use.

  • Do you know if the trimmer is the component labelled T1 on the motherboard? Commented Dec 30, 2019 at 20:46
  • minuszerodegrees.net/5150/misc/… calls it T1.
    – john_e
    Commented Dec 31, 2019 at 9:09
  • Good reference. The Trimmer is T1 - it is an adjustable capacitor whose value is 5-30 pF. Technical detail from the link: fine tunes the motherboard 14.31818 MHz clock signal, which in turn, fine tunes the NTSC 3.579545 MHz colour burst signal that the CGA card sends to the NTSC composite monitor. Commented Jan 1, 2020 at 14:27

AFAIK there is just one 14.318MHz crystal (not an oscillator) on the 5150 motherboard, right next to the 8284. It’s a flat metal component with two leads coming out of it from one side, probably marked with 14.318MHz. There might be a solid wire soldered to its housing, going across it. I’m pretty sure it was labeled as Y1 on the PCB.

  • The component labelled Y1 is as you describe, and it tallies with what Whit3rd commented. Why is the crystal not an oscillator? Commented Dec 30, 2019 at 14:41
  • @Single Malt, The crystal is not an oscillator because it needs some external components (usually a number of capacitors and an inverter or, in this case the 8284) to generate a square wave. There are self-contained crystal oscillators available as a single package that contains all the needed components to produce a square wave directly.
    – StarCat
    Commented Dec 30, 2019 at 17:29
  • Right, that makes sense, I understand now. It is the combination of the 8284 and the crystal that can be used to produce the square wave. Commented Dec 30, 2019 at 20:24

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