White ceramic resistor hot to hell in an Atari Disco monitor

baritonomarc

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White ceramic resistor hot to hell in an Atari Disco monitor

Hi all,

what does it means when the big white ceramic resistor close to the power regulator goes hot to hell, even desoldering one of it's sides? 'O_O

this is the story so far: i have an Atari Disco AGM20M med res on my Paperboy. The monitor was working when i got it, but during testing of the non working PCB it when black blowing up a safety resistor and taking the HOT, voltage regulator and main fuse with it.

I then substituted the Flyback (i was lucky enought to find a spare one), the safety resistance, the HOT, the power regulator, the fuse (... and all the caps just as it was on my test bench).

Now, the monitor started to work again, but only for an hour or so, then died again.

This time i noticed that the big white ceramic resitor went really hot, up to the point that the tin fused!!! What could be happened?

It's a little frustrating... but i will not give up :) !
 
I have no experience with this Disco, but when I have seen this in Wells Gardner monitors it has always been caused by a shorted voltage regulator.
 
The voltage regulator is not shorted this time, nor the HOT.

I found the diode going from the emitter to the collector of the HOT (the so called "damper diode") shorted... could it be the culprit or is there another candidate for being the badass component makeing that resistor going hot?

Thanks :)
 
It could be, since I don't have a drawing to go off of I'd have to say it could be a contributor. With the diode shorted voltage is going past it and that noise could be the cause of the overheating.
 
The voltage regulator is not shorted this time, nor the HOT.

I found the diode going from the emitter to the collector of the HOT (the so called "damper diode") shorted... could it be the culprit or is there another candidate for being the badass component makeing that resistor going hot?

Thanks :)

Look at the schematic. When that VR shorts, it puts the power resistor straight hot to ground. That's what happened in my Super Sprint (also med res), and the resistor took skin with it. Many of the components in that section are tied to straight ground, and a short would turn the resistor into a space heater.

Edit: I also lost the fly, which probably started the problem. In total I had to replace VR, HOT, and Fly all together or it kept using the HOT as a fuse...
 
Thank you all

The HOT and the VR have not been taken, so i suppose it's not a Fly fault. This time only the damper diode was blown. Having the DD not been replaced ever, i hope it went bad because of the previous problems with the flyback and age

The question is: could the DD drive the resistor hot if shorted? What other component could cause the DD going bad in the AGM20M (the schematic can be seen on Randy Fromm site, linked below)

http://technical-department.servegame.com/monitor_schematic_diagrams/adi_med_res.jpg
 
Thank you all

The HOT and the VR have not been taken, so i suppose it's not a Fly fault. This time only the damper diode was blown. Having the DD not been replaced ever, i hope it went bad because of the previous problems with the flyback and age

The question is: could the DD drive the resistor hot if shorted? What other component could cause the DD going bad in the AGM20M (the schematic can be seen on Randy Fromm site, linked below)

http://technical-department.servegame.com/monitor_schematic_diagrams/adi_med_res.jpg

I'm not seeing that DD on the HOT, can you post some part numbers so I know we're looking at the same parts.
 
The HOT is a BU208A and the DD is a BY399... if you look closely to the schematic you can see it from the emitter and the collector

Perhaps it's a parts sub that's not in my book or perhaps I just need another cup of coffee, but isn't the HOT in this unit Q902 a 2SD900B (2SD870)? And IIRC, the diode depicted in that assembly is internal.


Also,in case you don't already have one, here is the manual.
http://arcarc.xmission.com/PDF_Monitors/Atari Monitor TM-210 2nd Printing Disco 19in.pdf
 
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Hehe, no you dont need another cup of coffee :)

The monitor currently on my work bench has a BU208A as HOT, with a BY399 damper diode. Maybe it has been changed in the past, but this monitor was working so i am not prone to roll it back to the configuration on the schematic i linked.

The manual you linked is not for Disco AGM20M, but for Disco DMC2090DT... they differ, for example, in the neck board: the AGM has only brightness pot in there.
 
The monitor currently on my work bench has a BU208A as HOT, with a BY399 damper diode. Maybe it has been changed in the past, but this monitor was working so i am not prone to roll it back to the configuration on the schematic i linked.

Makes sense now. That said, the HOT has several low power components between it and R901. I would expect them to smoke (or at least get very hot) long before that power resistor desolders itself.

I'd keep looking in the power secondary between the bridge and TP B1. Check your B+ to see if it's in spec. That's a 19", so without a manual I'm guessing ~123VDC. My gut says VR (still), so probably the first thing I would do is pull Q901 and see if the situation changes. If so, replace it.

Last thought: If you removed the VR and/or HOT, did you remember to put the mica back? I know that's pretty obvious, but stuff happens...
 
The mica is there, sure, also with a foil of... i do not really know what is the material, but looks like a polimer or something similar... the collector is isolated from the chassis. Thanks for the remind, anyhow :)

The VR measures good (i have a spare one that gives me the same resistance measures... i wanna think they are not both bad). I am assuming with "pull the VR" you mean desolder it and check for shorts, right?

I am planning to substitute R901, connect it back and measure B+, just to see if the voltage is ok. Could it be measured directly on the VR emitter? I dont see well market test point or solder islands specific for the pourpouse

(thanks a lot for the help :))
 
Thanks for the remind, anyhow :)

NP. Just don't ask how I know... :)

I am assuming with "pull the VR" you mean desolder it and check for shorts, right?

Is it soldered? The voltage regulator IC should be a socketed TO-3.

I am planning to substitute R901

Lift one leg and check it's resistance. If it's good, leave it.

connect it back and measure B+, just to see if the voltage is ok. Could it be measured directly on the VR emitter?

You can. The emitter of Q901 and test point A1 should be the same.
 
VR (and HOT) is a TO-3, right, but pins and collector are soldered to wires going to the PCB and to test the VR desoldering of the legs is needed

Thanks, i will let you know when brand new BY399 will be on my workbench and the monitor ready for the hot-test! :)
 
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VR (and HOT) is a TO-3, right, but pins and collector are soldered to wires going to the PCB and to test the VR desoldering of the legs is needed

I know it was working, but if it were mine I'd take that apart and put in a proper socket. Not only to make future servicing easier, but testing a component in circuit can be deceptive.
 
To check the voltage regulator i desoldered the 3 wires going to it's pins: it's out of the circuit at this stage, even if i dont remove phisically it from the chassis, isnt it? Then i measured the resistances between pins, also inverting the leads of the DVM: i had the same readings i have on the spare VR... do you think i should use the spare one because the old one is not reliable anymore?

thanks :)
 
This is where it gets tricky. Something is definitely wrong, but components in that section take others with them when they go. I'd hesitate to replace it until you know what's going on, but not replacing it may hide the problem. I generally replace HOT and VR in pairs just to save the hassle.

You measured the resistance, but did you do a diode check? Make a 4x4 matrix and perform a diode check (in both directions) on spare and suspect. Compare and see what you think. It may be just fine.
 
diode check matrix done: the VR and HOT are good basing on this test. I a now curious to see if that damper bastard is the culprit of the problem ;)

thanks, i will keep this updated
 
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