VOGONS


First post, by izools

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I'm in the process of restoring a 5160XT with Hercules graphics and all that lovely stuff...

I had a short on the -12V line so replaced all the tantalum caps.

I had only 320K RAM detected - swapped banks 2 and 6 of the AST SixPakPlus that is installed and now 576K detected so faulty chip is now in bank 6, I'll just replace all 9 chips.

Last fault is this 5151 monitor behaving as attached:

I've disassembled and can't see bulging capacitors, or dry solder joints. Didn't go much further as monitor internals give me the willies... If someone knows what might cause this I'd sure appreciate the advice:

https://youtu.be/jPryi-k_XXY

Reply 1 of 7, by izools

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Yeah I've dismantled and discharged him last night and removed all 15 electrolytic capacitors.

I've ordered nichicon replacements for all of them along with an ESR meter to test out the old ones.

I found the SAMS online too, which is a wealth of information:

https://www.google.com/url?sa=t&source=web&rc … G0S0Zt3gCSmgsJr

There's no dry solder joints, it's a really good job IBM did.

The caps don't look swollen and there's no skidmarks on the PCB but they are Teapo caps and Teapo don't have the best reputation.

So, let's see what happens later in the week when new caps arrive

Reply 3 of 7, by mkarcher

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My guess: The transistor T2 in the power supply is broken or has a bad solder joint. It is meant to take a part of the load off the 7815 voltage regulator chip. If it doesn't do that, the 7815 gets quite hot quite fast, and self-protects by reducing output current/voltage as soon as it got hot. After replacing caps, I recommend you to check the +15V on the power supply output to confirm or reject that hypothesis.

Reply 4 of 7, by maxtherabbit

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mkarcher wrote on 2021-06-08, 06:04:

My guess: The transistor T2 in the power supply is broken or has a bad solder joint. It is meant to take a part of the load off the 7815 voltage regulator chip. If it doesn't do that, the 7815 gets quite hot quite fast, and self-protects by reducing output current/voltage as soon as it got hot. After replacing caps, I recommend you to check the +15V on the power supply output to confirm or reject that hypothesis.

Assuming you mean TR24 - the MJ2955 power transistor. I actually had that same transistor fail on my 5151, but the symptoms were quite different. The raster never collapsed fully like OP's monitor it would just get "shaky" after a bit of run time, and B+ didn't get reduced it actually jumped up a little over 16V.

Anyway it's an easy swap at least, no soldering required just mind the plastic shim to keep the can of the transistor from shorting to the mounting plate.

https://www.vcfed.org/forum/forum/genres/pcs- … g-with-my-5151=

Reply 5 of 7, by mkarcher

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maxtherabbit wrote on 2021-06-08, 15:07:

Assuming you mean TR24 - the MJ2955 power transistor.

I do. I read the label "T2" at the base terminal to be the reference number, but T1, T2, T3 seem to be the pins of a connector where that transistor is mounted. The transistor is called TR24.

maxtherabbit wrote on 2021-06-08, 15:07:

I actually had that same transistor fail on my 5151, but the symptoms were quite different. The raster never collapsed fully like OP's monitor it would just get "shaky" after a bit of run time, and B+ didn't get reduced it actually jumped up a little over 16V.

Your symptoms sounds like that transistor is leaky, and passes more current than intended, especially if it gets warm. Because the transistor passes too much current on its own without being controllable, the B+ voltage rises out of regulation, so the 100/120Hz ripple that should be eliminated by the 7815/MJ2955 combination will (partly?) pass through the regulator. If you live in a 60Hz country, the 120Hz ripple can cause interesting interference with the 50Hz refresh rate, causing 20Hz jitter.

On the other hand, in the case of the OP, I assume the transistor doesn't pass too much, but too little current. It might just be corroded pins in the socket. Because the assumed failure is different, the expected symptoms are different, too.

Reply 6 of 7, by maxtherabbit

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maxtherabbit wrote on 2021-06-07, 16:06:

What are you using to replace the big 18uf non polarized cap in the H drive path?

I'd like to emphasize that C505 MUST be non-polarized and low ESR. You cannot replace it with any off the shelf electrolytic. When I did my recap I replaced it with 2 of these film caps in parallel.

https://www.digikey.com/en/products/detail/pa … E1106JF/2567774

Reply 7 of 7, by TheMobRules

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maxtherabbit wrote on 2021-06-08, 17:35:

I'd like to emphasize that C505 MUST be non-polarized and low ESR. You cannot replace it with any off the shelf electrolytic. When I did my recap I replaced it with 2 of these film caps in parallel.
https://www.digikey.com/en/products/detail/pa … E1106JF/2567774

I have used the Polypropylene variant of those Panasonic film caps with good success to replace those large bipolar low ESR electrolytics (in my case only 1 film cap was needed as the original was just 3.3uF / 50V).