H7842A power supply in a DECmate II

[antiquekid3]

This power supply has come up a few times recently in Rainbows. I have one in a DECmate II. I've had the computer for a while and remember it had power supply issues, mostly because I had removed the power supply.

It blows the internal breaker almost immediately upon powering it up with 6W loads on the +5V and +12V rails. I eventually took the PCB out from the enclosure and started doing some more detailed probing with the Fluke, given a visual inspection turned up nothing.

I got to the main switching transistor, a BUV48A, and found that all three terminals appear quite shorted. Even though the schematic seemed to show that there is nothing of sufficiently low resistance between the terminals to cause my Fluke to read quite low, I pulled it out and confirmed my suspicion.

I guess I'll order another one of these, but I'm nervous about putting a new one in if this was not the cause, but merely another symptom. Any thoughts?

Here's a photo before I dusted it out. Not the worst I've seen, but certainly not the cleanest either.

 

[Bitly]

I've been meaning to post this for a while. MIT OpenCourseWare has their power electronics course up on youtube. Most of it is focused on design, but the 4 videos on DC-DC converters is worth a watch if you need to understand the theory better. It does require some basic circuit theory and I haven't found a good online course for that yet.

Power Electronics

CW

 

[antiquekid3]

I brought up the unit without the BUV48A and confirmed that the 7812 is regulating correctly, and the 300V looks good as well. Hopefully a new BUV48A will return it to operation.

 

[antiquekid3]

I tested the power supply after soldering in the new transistor. The package was slightly different, requiring me to find a longer screw to hold the tab to the heatsink. To my satisfaction, the power supply came right up with all voltages spot on.



After installing the supply in the machine, even the hard drive came to life, though with noisy bearings. Unfortunately, WPS is password protected. Looks like no word processing for me—darn.



The RX50 didn't seem to want to behave with any disks I had, so I swapped in a Gotek my friend brought, and a little fiddling later, we were able to get a number of disk images booting.

 

[ftcnet]

Sounds like you have a Gotek working as a RX50 replacement on a DECmate II? That's impressive. I'm sure I recall recent mention that a Gotek has problems emulating a RX50. Please share which Gotek variant you are using with your DECmate II?

 

[antiquekid3]

It's an Artery AT32F415 running the latest FlashFloppy firmware. I replaced the 3 digit display with an OLED. Seems to work well after converting the Teledisk images to raw images, then told FlashFloppy the parameters for RX50:

10 sectors per track
512 bytes per sector
Single sided
80 tracks
No interleave

 

[shirsch]

Sounds like you have a Gotek working as a RX50 replacement on a DECmate II? That's impressive. I'm sure I recall recent mention that a Gotek has problems emulating a RX50. Please share which Gotek variant you are using with your DECmate II?

Gotek's work fine as RX50 replacements in a DECmate. The problems you might hear about are relative to probing done by the controller to determine whether a drive is attached. For example, a DEC Pro350 refuses to recognize a pair of Goteks during POST and fails the self-test. I found a recent post that suggests they are expecting behavior that can only occur if both logical drives are stepping simultaneously. There may be a work around for that - still thinking on it. But the pair of Goteks in my DECmate III work just fine.

On the original subject of power supplies: I would strongly advise you to replace all the electrolytic caps AND the RIFA line caps (!!) ASAP.

 

[czunit]

Weird. I've used two GoTeks on my Pro 350/380 systems without issue. It's all in the jumpering, the jumper settings are not the same as when using with an RQDX3.

Also I see OpenCourseware is now hosted on YouTube. Is it possible to download the lectures in MP4 format like one used to, or are we stuck forever with trash tube?

 

[antiquekid3]

On the original subject of power supplies: I would strongly advise you to replace all the electrolytic caps AND the RIFA line caps (!!) ASAP.

Depending on the situation, I typically do one of the following:
1. Replace all electrolytic caps, no questions asked
2. Reform existing capacitors out of circuit
3. Keep the original caps running until they fail

This is a number 3 situation for me. So far, the supply seems quite content.

I did not see any RIFA caps in the unit, unless they are in the sealed line filter.

 

[shirsch]

Weird. I've used two GoTeks on my Pro 350/380 systems without issue. It's all in the jumpering, the jumper settings are not the same as when using with an RQDX3.

Also I see OpenCourseware is now hosted on YouTube. Is it possible to download the lectures in MP4 format like one used to, or are we stuck forever with trash tube?

That's odd. You are able to boot it without diskettes mounted on either Gotek and it doesn't fail the diskette system during POST? I can get past that problem only by putting a properly formatted non-system diskette in the first drive before startup.

 

[czunit]

I seem to recall so, and I never put a floppy in the real RX50's. Maybe there is a problem.

 

[shirsch]

The real RX50 is fine, but there is something the controller expects that the pair of Goteks were not producing. From other comments I'm starting to think it does something like this:

• Select drive 0 and seek to some track N
• Select drive 1 and seek outwards N number of tracks
• If it has the Trk0 signal at that point all is well, otherwise fail.

I am trying to come up with a simple arrangement that will let a pair of Goteks seek simultaneously while blocking the one that is not selected by the host from signaling back to the controller. Shouldn't be rocket science, but will take a bit of thought and a few gates.

 

[thunter0512]

Couldn't a single Gotek implement the dual drive RX50 and simply emulate the seek behavior of the real RX50 dual drive setup?
The firmware is freely available, so it could be modified to make a dual drive RX50 variant.
The source is at: https://github.com/keirf/flashfloppy

 

[czunit]

I suppose, but I still don't think this is a problem with the Pro/350 floppy controller. Unlike the RQDX3, the Pro controller is as dumb as a box of rocks. I've had it working, I installed 20+ floppies of POS 3.2 using a pair of GoTeks.

 

[shirsch]

Couldn't a single Gotek implement the dual drive RX50 and simply emulate the seek behavior of the real RX50 dual drive setup?
The firmware is freely available, so it could be modified to make a dual drive RX50 variant.
The source is at: https://github.com/keirf/flashfloppy

That could be done, but it would be a considerable amount of work.

 

[shirsch]

I suppose, but I still don't think this is a problem with the Pro/350 floppy controller. Unlike the RQDX3, the Pro controller is as dumb as a box of rocks. I've had it working, I installed 20+ floppies of POS 3.2 using a pair of GoTeks.

The Pro controller has an 8051 with masked rom, so to some extent it's a "smart" controller. Do you have any way to dump the 8051 firmware from yours? I'm wondering if we have different versions. I built a little gadget called "8051 Dumper" that cheerfully writes out the internal programming.

 

[czunit]

The Pro controller has an 8051 with masked rom, so to some extent it's a "smart" controller. Do you have any way to dump the 8051 firmware from yours? I'm wondering if we have different versions. I built a little gadget called "8051 Dumper" that cheerfully writes out the internal programming.

Possibly, but I remember it being a stupid track, sector type of thing with each drive being a "head". I'll pull the card and see if there is a rev, then fire it up with some Goteks and screw around again. Been awhile.

 

[shirsch]

The 8051 goes out and pokes at the drives at reset. With the Goteks it simply reports "something is wrong" back to the system, which then puts up that nice graphic to tell you the floppy disks aren't working. That's why I dumped out the 8051. When I get a round tuit I was planning to disassemble the code and try to spot which part of the init is getting tripped up. Remember, this happens only when the drive is empty. If a disk is mounted on drive 0, all is well. I use that as a workaround currently.

 

[czunit]

The 8051 goes out and pokes at the drives at reset. With the Goteks it simply reports "something is wrong" back to the system, which then puts up that nice graphic to tell you the floppy disks aren't working. That's why I dumped out the 8051. When I get a round tuit I was planning to disassemble the code and try to spot which part of the init is getting tripped up. Remember, this happens only when the drive is empty. If a disk is mounted on drive 0, all is well. I use that as a workaround currently.

Ah, maybe that it. I always had the Goteks set to a disk image, so there was never a boot issue. Back to the power supply thread