Having completed recapping my Mega STe PSU (and
documenting the process) I'm preparing to recap my STe PSU (SR98). I've bought
Exxos' SR98 recapping kit for that and also his
PSU dummy load resistor kit (which I've already used when testing my Mega STe PSU without attaching it to the computer, thus no risk of blowing it up in case I did something wrong).
I have a few questions before I proceed (in return for information to fill in the gaps I'll consider writing yet another "How to" similar to what I did with the Mega STe PSU), this time for the SR98. I've made a preliminary illustration which should help whoever can answer my questions:
- 20171021-200718__MG_2226-Edit.jpg (667.76 KiB) Viewed 7331 times
1) I noticed that the kit didn't include a replacement for
C206 (a small 1uF/50V capacitor close to the power switch). There's also no mention of it in the replacement listing
in the SR98 section of The Last Upgrade -PSU, although I see discussions of a 1uF capacitor which I assume is C206 (relevant text made
bold by me):
SR98 PEAK CURRENT UPDATE November 3, 2015
I have found the SR98's peak current demand isn't as good as it could be. This is generally seen as a dim on the video when the floppy drive is being accessed. So I started to look at this issue.
At first I changed the opto coupler for a faster one (actually same one as used on the ASP PSU) thinking it would give a more rapid response time, but it changed nothing from what I could see. I later changed it back (after doing all the tests below). The faster one might be slightly better in regulation, but it is so slight its probably not worth changing.
I then started to follow the resistor networks on the PCB and found that there is a capacitor which is pretty much directly across the opto's emitter diode.
What this would mean is there is a small delay when the voltage is applied to the diode as it has to charge up a 1uF capacitor. It would then mean the 1uF would delay in turning off also. It is difficult to exactly work out whats going on with the diagram but I did notice when the 5V was turned up to 5.5 - 5.7V things improved a lot.
For this test I used a STE. I always had some odd ghosting on it and wavy lines on the screen. When the voltage was turned up (way to high in fact for 5V) the video noise problems and ghosting pretty much went away. Though I did notice some odd juddering on the edges of objects afterwards. But the most interesting thing was when the floppy drive was turned on, the video did not appear to dim as it did before.
So I found 2 resistors R201, R202 which seemed to be a voltage divider which also goes via the preset pot.
This divider then is feed to a small transistor which in turn charges the 1uF capacitor to the opto diode. R203 is in the mix aswell, seems to be another resistor in the 0V line to the transistor. In anycase, I added 10K resistor across the top of R201, R202 and now the video problems went away at 5.25V. Also the dimming problem is not there. Any lower voltage then all screwy video problems come back. So clearly there is a biasing problem.
I have also replaced the 1uF with a ceramic to rule out aged capacitor issue, Though I am not totally sure it made a difference.
Overall how it looks is when the PSU gets under load rapidly (when floppy motor turns on) It takes to long to charge the 1uF capacitor and the voltage drops (actually only about 0.02V) but this is visible to a quick eye that the video dims. So the idea is to increase the current though the circuit to give a faster charge rate of the capacitor, which will turn the opto diode on faster, and this does seem to be drastically improving stability.
I have also tried lower resistor values down to 2k2, but this did not appear to change anything.
I upped the 1uF to 2uF. Its hard to say but this might have improved some very slight video issues. This particular issue only seems visible on the "drag bar" on GEM windows where the horizontal lines have vertical noise issues about every 1cm. Though without this new PSU mods, those lines start flickering pretty bad. This STE seems to run happiest at 5.25V. Any lower and the video noise issues start coming back.
Overall the peak power can be improved with just a couple resistors and a new capacitor
So the final round up.
R202 2.4 k red yellow red + 1.8k = 1.029k Ohms
R201 2.7k red purple red + 2.2k = 1.212k Ohms
Replace R202 with 1K
Replace R201 with 1.2K
The trimmer pot may need to be adjusted back to 5.00volts on the 5V rail.
I'm a little confused since it appears changing
C206 from 1uF to 2uF makes a difference (is that a ceramic capacitor instead of an electrolytic? Why not just use an electrolytic?), but it's not mentioned in "the final round up" at the bottom of the quote, neither is it part of the recapping kit. Surely all electrolytic capacitors need replacing after so many years, so what should I do to finalize the PSU servicing?
2) I understand (from the above quote's "final round up") that replacing the following two resistors should make a vast improvement to the video output noise, and considered one of two "ultimate improvements" to the PSU apart from recapping/changing the rectifier):
replace
R201 (original value 2.7K/0.25W) with
1.2K/0.25W (new improved value)
and
replace
R202 (original value 2.4K/0.25W) with
1K/0.25W (new improved value)
then (using the small
VR201 trimpot next to the PC101 opto-isolator I assume?), adjust the output voltage on the +5V line to 5.0V (of course with the dummy load resistors in place and not connected to the computer)? Or should that be adjusted to +5.25V? It's a little confusing.
Have I understood the details correctly?
3) There's mention in this thread's first posting as well as in
in the SR98 section of The Last Upgrade -PSU that the output diode (originally an HRW34) should be replaced with a STPS10H100CFP for much better performance (the second of the two "optimal performance" modifications I think).
I assume this is
D202 (a 3-pin TO-220AB package attached to the large silver heatsink next to the 47uF/400V capacitor (C104)?
I see from the datasheet that there are two variants to choose from: the TO-220AB or the TO-220FPAB insulated package. Which one should I go for? Or will either do?
- STPS10H100FCP.png (113.53 KiB) Viewed 7331 times
Here's the schematic by the way (I think I originally downloaded it from
Exxos' schematics/manuals section)
- sr98 schematic.png (448.84 KiB) Viewed 7331 times