6
May
'18

So I’m messing around with a Burroughs TD831 terminal which uses a 6800 processor, 8 kilobytes of DRAM and 16 kilobytes of mask PROM.

The PROMs are fairly typical of the era, in that the chip select lines are also programmable. So you program the first one in a bank of four to have two active low chip selects, the middle two ones to have an active low and an active high, as well as the reverse, and the fourth one to have two active high chip selects. That way you can run address lines into the chip selects and four PROMs act like one PROM four times the size, effectively.

How I figured this out: the PROMs have 24 pins, the largest 24 pin PROM is a 2732. Told my EXPRO that’s what they were, not much joy. Went down to 2716s, and that gave data out of one of each bank of four PROMs. I figured that this means the devices are similar to for example the 82S191. So it was time to write some code.

I was lazy and just told the code that the three potential chip select lines were address lines. This gave me a 16 kilobyte per PROM dump, three quarters of which is blank Looking at how the banks were located in the 16 kilobyte address space makes it look like pin 21 (A10 on the 82S191) is an active high Chip Select, while pin 20 is A10 and pins 19 and 18 are the programmable Chip Selects.

I suppose I can rewrite my code to map things that way, but I should be able to paste my dumps together into something that can be disassembled. If ever I am arsed to do that.

But if you are here on a quest to restore one of these things to life, I think I have given you everything you need in order to be enlightened.

Edit: You might notice that I did change the code and re-dump the ROMs in nice neat 2k binaries.





On our recent Kgalagadi trip the water bottle burst because the water level ran low. And the little red light that’s supposed to tell me that the water level was low never came on.

So I took the instrument cluster apart to find the problem

P1150143r

Here’s the gauge on the bench. Apparently if the light stays on it’s the capacitor and you can replace that without taking things apart. In my case however…

P1150144r

I had to drill out the two rivits holding the face plate on.

P1150149r

Lots of electronics to multiplex the analogue temperature and the low water signal from the relay on one wire. The PCB hangs off the two pins, gauge on the right (also goes to the heating element that moves the needle) and earth at the top. The blue wire from the left carries power (regulated 10V).

P1150154r

Hmmm. I think I see the problem.





30
Jul
'17

From a bunch of stuff some other ham wanted to throw away, this ex-SWR meter.

Stripped

I have no idea why the Dreaded Previous Owner stripped it down to this state. The meter movement is fine, 950-ish mV over a 4k7 resistor gives FSD, so it’s a 200uA unit.

inside

The detector components are still in place, and it looks very similar to the Micronta 21-520A except that there’s only one meter. There’s also a little bobbin on the side for an antenna, presumably to make it into a Field-Strength Meter, but that’s a gimmick and won’t happen.

NewSwitches

The junkbox yielded two switches of the right type and size (one selects Power / SWR, and in SWR mode the other selects Forward / Reverse). There will also be a pot to set FSD in Forward mode after which the Reverse mode should give the SWR. Give or take. Don’t expect a lot from meters like this.

Fast forward a bit and we have

IMG_0070r

IMG_0068r

Don’t ask me what used to live in those two extra holes. This setup works for me, for the price of a few junkbox parts and some time.

 





15
Jul
'17

(IMO, of course. And Geek Alert)

MFJ-949E

This is the switch and tuner schematic for an MFJ-949E Versa Tuner II. Great little unit, with a built-in dummy load. It has a switch that selects the dummy load, then the three antenna connectors in pass-through mode, then the three antenna connectors through the matching network, and then the dummy load again… through the matching network.

Which means that if you want to tune into the dummy load, you have to use the setting all the way to the left, or you have to adjust the tuner to match the dummy load to the rig, using the switch setting on the right.

Now why would you need to match a 50 ohm dummy load to your rig? Insane. The dummy load switch setting on the right should connect straight to the dummy load, not via the tuner.

I have a soldering iron, I can fix it.





I mentioned previously that I figured out how to make a new display for an FT-x90R. My own rig is still in pieces ‘cos I’m not quite happy with the software yet (or actually, with my reverse engineering of the display protocol, but it’s the same thing in the end).

But Dave ZS5DF wanted his rig fixed so I fitted a new display, even though it doesn’t have the F key “P” priority display function in firmware yet.

P1130871r

Two filament bulbs, because the replacement display needs light from the right-hand side and the S meter is on the left-hand side.

P1130882r

Component cost is close to R500 which almost puts it in good-money-after-bad territory but I like these little radios.





12
Oct
'16

Younger kid had to build an electric motor for a school project. Having a garage full of stuff, it was no hassle to find everything required to build one.

motor

close

I made her wind the coil.

But the question is, how do kids complete (or even conceptualise) something like this if they don’t have a Wouter around the house?

goes





31
Aug
'16

C1166

This is a C1166 Pulse Tetrode made by English Electric, who also made transistors. Good for switching 17 500 volts at 15 amps. But only for 1 microsecond every 1 millisecond (1T4 and AA battery for scale).

The filament alone needs almost 60 watts (9 amps at 6.3 volts).

Quite useless unless one wants to build a glowbug radar or something. But quite marvelous in a way.





16
Feb
'16

PM2421-cropped

Mahala gratis verniet and for free, nogal.

And it seems to work, that’s indeed a 4.7k resistor.

Look at the ridiculous scale on this thing — not only uA but nA as well. Why would I want to measure a nano-amp?

I think this will become part of my permanent test setup.





Oooh look! Someone threw out this perfectly good Spectrum SPL-603 power supply!

Well, mostly perfectly good…

She who was once the beautiful 723

The 723 had given up all its magic smoke, to the point where the socket is also buggered. Fortunately this is a stock part in my junkbox, as are 2N3055s (the one output transistor was shorted emittor-to-collector, but I can’t explain why this would break the 723). I also replaced the H1061 with a BDW93B because I could.

Truth be told, this is a terrible PSU.

  1. The non-regulated DC is 52V, which means that at 30V output the transistors are dropping 22V and at 2.5V out they are dropping just about 50V. No wonder two 2N3055s are needed for a measly 3A output (A 2N3055 is good for 15A and 115W, so when dropping 50V you can only get a smidge more than 2A out of it, and that with a good heatsink).
  2. The 723 Vcc is connected to the non-regulated DC, but the 723 is rated for 40V max Vcc. This is probably what broke it.
  3. When I set the PSU to about 6V and turn it off, there’s a ~20V spike on the output. WTF?

So why the high non-regulated DC? I thought maybe it drops under load because cheap transformer, but this is not the case. The transformer gives out 38VAC which drops to 35VAC with a 1A load.

The general schematic is similar to http://www.circuit-projects.com/cimg/2V_to_7V_8A_power_supply_by_723_and_7812.gif but with a difference — the base of the current limiting transistor is connected to a voltage divider (R4/R5) to ground. This causes the current limiting point to be higher for a higher output voltage. Again, counter-intuitive.

So I changed things around to how it should be. Fitted a different transformer, from an old UPS — it’s only good for about 18V out but that’s enough for me. Not going to try to make a purse out of this sow’s ear.

 

 

 

 





24
Aug
'15

My new brewing setup. A nice large stainless steel urn with a dual element, and a cooler box with a filter that used to be a braided hose and a tap.

The box under the urn is a PID controller so that I can (in theory*) dial the temperature, have a homebrew, and have water at the right temperature on tap.

It’s not going to win a panel wiring contest, but it works. The PID doesn’t have the switching capacity, so there’s a solid state relay. I also wired diodes to send the positive part of the AC to the one element and the negative part of the AC to the other element to spread the heat.

The switch controlling whether the diodes are in-circuit or not comes from the junkbox. Been a while since one could buy something useful for R3.45.

Grain in the cooler (this is 6 kilos of pilsner, on Saturday), add the water, let it be for about an hour…

..and drain the good stuff (this is Sunday’s brew, an IPA).

The pilsner is all-grain, which means that you end up with quite a lot of liquid to boil.

That’s the next step. The urn, on low, should be able to boil the wort as well. If not, I need to get a 30 liter+ pot, I have quite a few gas burners.

Good news is that both brews started bubbling easily (I added Servomyces to both and DAP to the IPA). The pilsner is my first all-grain and my first lager — I’m using ice to keep it cool.

* The water coming out the tap seems about 5C too cold. Some calibration required, looks like.

 

 





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