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Wells Gardner D9200 - Horizontal Collapse?
Rocketeer2001:
I used thermal paste for a computer's CPU heatsink. I put that on both sides of the original mica sheet and placed that between the HOT and the heatsink. I don't know if the paste has silicone in it; is that specific ingredient a requirement?
I had checked D417 before and it was ok, but I'll check it again. I didn't check D416 yet, but I will. I'll check D422 too.
I tried disconnecting the input signal to the board and turning it on just in case it was the frequency switching transistor running amuck, and I don't get a constant screeching. More like 1 second intervals of a clicking sound (a relay?) with arcing occurring around the HOT.
lilshawn:
i say "silicone" because lots of thermal pastes for computers contain metals (silver etc.) or carbon and cannot be used in this application due to their electrical conductivity.
you need something like MG chemicals 860 ... basically plain white heatsink grease.
if it's gray, chances are pretty good it has conductive stuff in it.
normally it wouldn't matter... but at voltages and currents like this... it does 100%
Rocketeer2001:
The thermal paste I use is grey, so I put my multimeter in continuity mode and shoved the probes into a glob to see if it conducts. It did not. Then again, it is just measuring using the voltage the multimeter can provide, which is only 1v or so. I'm guessing at the higher voltages the HOT puts out, this stuff could conceivably conduct. I'm uncertain, but I'll get some of the MG 860 stuff just to be safe.
The mica sheet was in there, but upon closer inspection it looks like it's de-laminating? Also now has some char marks. I should probably just get a replacement.
I removed the transistor to test it to see if it's toast, and surprisingly it's still good! (see pic) However, I had a suspicion that the part was arcing/grounding too easily, so using my multi-meter I probed within the mounting hole and the back of the transistor and I get continuity. Not everywhere in the hole, but in a few locations. It must have had a thin layer of insulation or something because it was shorting out to the mounting screw and thus into the heatsink. Some of the part has crumbled away around the hole from where the arcing was occurring (see pic), but there are other parts of the hole that look perfectly fine, yet the meter still shows continuity when probed. I'm thinking maybe I got a faulty part (in conjunction with my possibly incorrect thermal paste and de-laminating mica).
I then proceeded to test D416, D417, and D422. The first two were good, but D422 is suspicious. It shows 0.28v in diode mode when the other two show 0.5V. Could this be a contributor?
Also tested my new capacitor and thankfully it's fine!
grantspain:
you can compare d422 to d423, if they read the same then all good
the insulaton sheet you need is for TO-264 transistor package type
Rocketeer2001:
Grant, Good suggestion on comparing to D423 since it's the same part number. After checking, it's got the same voltage; Looks like my D422 is good!
Thanks for providing the package size of TO-264. I wouldn't have known what to look for otherwise. Even so, I'm having trouble finding that size that won't cost an arm and a leg. If I buy 300 of them, it's a bargain!
How do we feel about using silicone sheets rather than mica with compound? If I use the sheets, I can cut it to whatever size I need and I wouldn't need the compound:
https://www.amazon.ca/100x100mm-Efficient-Conductivity-Resistant-Silicone/dp/B096ZNHY8F/ref=pd_sbs_d_sccl_3_2/144-4490169-7451254?pd_rd_w=heB6B&content-id=amzn1.sym.4b791088-03f0-49af-8af1-ae2f52d6208b&pf_rd_p=4b791088-03f0-49af-8af1-ae2f52d6208b&pf_rd_r=6FAWM4WQCSZ53FYBVCVG&pd_rd_wg=4316Q&pd_rd_r=4a0bdac6-e605-491f-bdf8-ecf523b85029&pd_rd_i=B096ZNHY8F&psc=1
Arcade Parts & Repair sells both kinds, but because I'm in Canada it's over $20 for a $2 part...
https://www.arcadepartsandrepair.com/store/integrated-circuits/transistors/mica-transparent-insulator-to-264-package-5-pieces/
https://www.arcadepartsandrepair.com/store/pinball-kits-parts/pinball-parts/mica-silicon-insulator-to-264-package-5-pieces/#reviews
Or I get the MG compound and just reuse the original mica.