The schematics available for Atari SM124 differ a little from the actual boards described below. Some of the capacitors have different symbols and some parts are interconnected in a different way.
C716 bipolar capacitor (C714 on schematics, rated 2.2uF 50V) is a single part most likely to fail. It is part of a horizontal deflection circuit and its failure results in a single vertical line on the screen. It has to be replaced with a part suitable for high impulses, for example a polypropylene MKP capacitor. Replacing it with a regular electrolytic capacitor of the same ratings will result in a blowup of that capacitor.
The difference in this revision is a different design of the right-bottom part, where in revision 1.0 there are some cables and resistors soldered. Also, instead of transistors, an integrated operational amplifier is used.
In this revision resistor R617 is added and a position of R614 was changed.
I just came across this site, as I was looking for information about how to recap my two old CRTs. Thanks a lot for the Post. I got two questions:
1) You wrote that the BiPolar capacitor (C716) on the SM124 must not be replaced with an electrolytic one, but instead using a polypropylene MKP capacitor because of the high impulses. At least on my model, the original capacitor is an electrolytic one and it didn’t blow up. Are there electrolytic ones that can cope with the high frequencies?
2) Is this true in general for the BiPolar caps in CRT Monitors? In other words, when re-caping for example an old Commodore 1084S-P, which does also have a Bi-Polar electrolytic capacitor, is it better/safer to replace this also with a MKP cap?
PS: Sorry, if the questions might sound a bit stupid. But I don’t really have much experience with CRTs; however I do not want to throw them away, but instead want to keep them as long as possible.
Hi Ralf! My opinion is based on a hobbyst experience only. It would be great if someone with professional background could say something. I believe there are electrolytic capacitors supportive of high frequencies manufactured today, however I failed to find them in a reliable way, i.e. to figure out the correct markings indicating the HF. They are generally bigger. I did not want to just buy various types and experiment, since the experiments result in small explosions. For 1) I mean it must not be replaced with just a regular electrolytic capacitor with the same ratings. It can be replaced with a HF one. 2) This is true only for the bipolar capacitors used for high frequency switching in the horizontal deflection circuit. There may be other bipolar capacitors, which do not need to be HF. The HF electrolytic capacitors were used on earlier CRT designs. 1084S-P1 seems to have a HF cap (C2524) and 1084S-D1 too (C473). MKP caps seem to be more reliable and probably do not dry out.
Thanks for the quick answer! My 1084S-P uses a Chton 4.7µF 50V as C473. I guess I will try MKP caps then for both CRTs.
Hey, I’ve ended up on here trying to fix my SM124. Mine has a dodgy power switch that won’t turn on. Does anyone know what component to replace it with?
Do you mean the rightmost turn knob?
Yeah the pot with the switch. Mine’s got various things printed on but nothing that corresponds to a part number.
I have no idea, but there are a lot of “potentiometers with on/off switch” available to purchase. Hopefully it can be found looking at the schematic or measuring it.
Hiya! I was wondering if there’s anywhere to find, or if anybody knows, the dimensions of the outer casing of this crt? It’s got a 12” screen as far as I’m aware, but I was wondering about the plastic case as seen in the assembly pictures. I’ve got my eye on an old one but don’t want to buy it before I find out how big it is. So if anybody knows, fire me a reply!