Mitsumi SR98 issue after recap (no boot)

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vertical
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Mitsumi SR98 issue after recap (no boot)

Post by vertical »

After some time, I took my MITSUMI SR98 PSU to a friend to recap it as per instructions here

https://www.exxosforum.co.uk/atari/last/psu/index.htm

Capacitors ordered were from Mouser, Nichicon. Also, we did the ghosting fix and installed TOS 2.06 replacing the old 1.06. In the lab, the ST booted fine, we even used my ultrasatan to play some demos/games.

Came home, booted the STE, everything worked flawlessly again using ultrasatan and goex. I could hear a tiny whistle but I switched off and was going to monitor that. Today, two days after, I tried to boot my STE but seems dead. No lights, nothing. Measured with a multimeter and it seems to give 5.35-36V. Noticed that when I switch off, I can hear a tiny electric noise like appearing and fading down in less than a second.

Needless to say that I start to freak out in case my STE has been damaged. Where do I go from here? I will take my STE to the friend's lab in a few days to be checked.

My config is 520 STE, 4MB RAM, GOEX drive (tried unplugging the power but same result), and I replaced the CPU with a MC68HC000 due to having the oldest DMA chip.

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Steve
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Re: Mitsumi SR98 issue after recap (no boot)

Post by Steve »

You'll need to take some more thorough photographs if you'd like us to try and visually identify anything, top side, different angles and underside. Be very careful as you can get shocked if you've had it on recently, I'd recommend unplugging it from the AC and turning the psu to 'on' to drain any power out, then turn it back off again.

edit: you may as well just wait a few days for your friend to have a look. By the way the rectifier looks twice the thickness of the one Exxos provides in his kit. Did you also replace the resistors & the diode update from the guide?
vertical
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Re: Mitsumi SR98 issue after recap (no boot)

Post by vertical »

Thanks for your reply Steve. I will wait, the best I could do myself is to measure with a multimeter. The rectifier is KBU1005G.

https://www.mouser.com/datasheet/2/395/ ... 248375.pdf

Same part as mentioned by exxos. Well, almost, because I see the G at the end. Seems like it replaced KBU1005 since the later is obsolete. I can take better pictures in the following days.
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Re: Mitsumi SR98 issue after recap (no boot)

Post by rubber_jonnie »

Firstly, and I know it's too late now, but if you ever re-cap a PSU I'd always recommend that you test it on a load to begin with, rather than plug into your ST. There are suitable resistors in exxos store that will allow you to do this.

Using a test load will also allow you to tweak the voltages before hooking up to your ST.

If your friend has a suitable bench PSU I'd suggest you hook that up to the 5v rail to see if you're getting a power light and can boot with a good clean PSU.

12V is unnecessary for booting the machine so can be checked later.

Short term might be worth just using a multi-meter to check the mains in via the switch as I have had switches fail. Also check the fuse hasn't failed, both at the mains lead and also the one on the PSU itself.

Also, there is a white ceramic block on the high voltage side next to the large capacitor, use a multi-meter to check that has continuity as it is fusible and may have popped. If it has, let me know as I have (somewhere!) the part number for a suitable replacement.

As @Steve suggested, be careful with these PSUs, as the aluminium heat sinks are live when it's running and can give you quite a shock.
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vertical
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Re: Mitsumi SR98 issue after recap (no boot)

Post by vertical »

Just came back from my friend's lab. We measured everything on the PSU and looks fine and according to exxos instructions. It was running at 5.25V as per instructions.

We then started to check for a short circuit. My suspicion went directly on the CPU. My STE's CPU is replaced to a MC68HC000 bought from exxos store in late March (25th) of this year. The reason for doing so is the DMA issue (have the "bad" one) and my STE was working fine with the UltraSatan after the swap. When we removed the CPU, 5V came back to life on the STE. Clearly we found the guilty one.

Strange thing is that the computer worked for a 30 minutes after the PSU recap. Not sure why the CPU failed. Is it coincidence? Or something affected the CPU from the recap maybe with higher rated capacitors? I can always put the old CPU back (risking to have issues with US) or order another MC68HC000 if the previous died out of just my bad luck.

As I said, together with the recap, we did the ghosting fix so not sure if this can affect somehow. Just mentioning it.

Not an expert on this so any feedback as to how to proceed is appreciated.
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Re: Mitsumi SR98 issue after recap (no boot)

Post by rubber_jonnie »

vertical wrote: Tue Oct 05, 2021 12:56 pm Just came back from my friend's lab.We measured everything on the PSU and looks fine and according to exxos instructions. It was running at 5.25V as per instructions.
I can always put the old CPU back (risking to have issues with US) or order another MC68HC000 if the previous died out of just my bad luck.
Makes me wonder if your HC CPU was a bad one, maybe re-marked or marginal. Might also be worth checking the socket contacts on the CPU and the socket, I have seen them get twisted and touch other pins on the MMU and GLUE, causing a dead machine so maybe also possible on the CPU.

I have also seen the 'barriers' between the contacts on the socket get damaged and contacts touch.

Double check it all and maybe run with the old CPU until you can get another HC from @exxos he has them in his store for £12.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
vertical
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Re: Mitsumi SR98 issue after recap (no boot)

Post by vertical »

Well, the CPU was working fine until 4 days ago. So roughly 6 months without issues since the installation of the HC. Yes, I did check socket contacts after removing old CPU and installing new. I believe it is something else that made it fail, maybe being a bad one. Can't say.
vertical
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Re: Mitsumi SR98 issue after recap (no boot)

Post by vertical »

I put back the original CPU and everything seems normal. Placed order for HC and it is already shipped. Not using US until then, unless maybe using another card temporarily (and maybe test if the PSU recap enough made any difference).
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Re: Mitsumi SR98 issue after recap (no boot)

Post by Groppo91 »

Hi I am in the middle of a recap and spotted the bit about testing the continuity of the ceramic resistor. Mine doesn't bleep so I guess it is dead. It seems to be marked as RGCW3 8.2ohm k noble branded t19. Would I need to source the same or could anyone recommend an easy to obtain replacement?

Edit - I think I was testing it wrong my cheap dvm seems to get a reading when I put it on 2000ohm setting around 1400-1600 although I haven't isolated it from the circuit which I know you should when testing resistors. Should there still be a continuity bleep when on the continuity setting?

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Re: Mitsumi SR98 issue after recap (no boot)

Post by exxos »

It's a 8.2 ohm fusible resistor. You need to test on something like 200ohm resistance scale on your meter to test it properly.
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