I’m not dead!
Some time ago I scored a great deal on eBay for a Data I/O 29A setup with a load of modules and manuals.
Pretty much everything worked great apart from the display was really dim.
It doesn’t look too bad in the picture but it looked much worse in real life
Schematics are readily available for the unit so fault finding was fairly quick.
With the display being dim my initial though was a voltage issue and the schematics have conveniently labelled the area up as “FILAMENT SUPPLY GENERATOR”.
Using the scope I could see straight away the CD4049 hex inverter at U14 was not inverting at all.
Simply replacing this fixed the fault
Some days ago I wanted to test some RAM chips from an arcade PCB using my B&K Precision 560A tester :
For the uninitiated this piece of equipment allow you to do out-of-circuit and in-circuit testing of many ICs like TTLs,RAMs and ROMs thanks to an extensive internal IC library.I often use it during my repairs but in that day it suddenly died, no sign of life, nothing came up on display when I powered it up.For first I checked the main fuse and it was good so I decided to open the unit:
Having no schematics I started my troubleshooting from upstream checking the big transformer just after the AC input, voltages were present on its secondary coil.Following the path I figured out the AC voltages are rectified by a circuit on one of the PCBs.In particular the rectified +12V is then regulated by a couple of ‘LAS1605’ fixed +5V voltage regulators (in TO-3 package) mounted on a big heatsink for providing +5V to the circuitries on the other boards:
I probed the inputs of both and +12V was present:
As well as +5V on the output of one regulator:
But the other one was giving only +1.706V on its output, too few for correct functionality of the logics on PCB:
I looked online for buying a ‘LAS1605’ +5V voltage regulator or a compatible one but found only few at not really cheap price.Then, as a last resort, I asked my local electronics shop, they surpringsly had in stock some TO-3 ‘MC7805CK’ which is a perfect drop-in replacement (although it can deliver up to 1.5A output current against the 2A of the ‘LAS1605’ but this is not really an issue since I don’t think the tester can draw more than 1.5A during normal operation)
I pulled out the suspicious part:
I swapped the spare in and was welcomed by this :
My BK560A rised from its grave! Not too bad for a 2-euro repair! (as much as the replacement part costed…)
I bought this setup from a good friend quite a few months ago now. I knew it needed some attention when I bought it.
On power up I got this most of the time
No response from any inputs from the keypad.
The keyboard generates an interrupt for the 6802 CPU. Using the scope I could see the /IRQ signal was being asserted.
I checked all of the ROM’s and found no issue and also checked to surrounding logic and found no issue.
Looking around the main PCB I found a couple of problem areas.
As you can see, both of these chips had some corrosion. I replaced them but they made no difference to the problem I had so after a while I came to the conclusion the CPU must be bad.
Searching eBay turned up nothing UK based so I fired off an email to my friend, Purity to see if he had a spare I could buy. He had one and said I could have it.
And now I get this
Next issue was with the Unipak 2 itself.
I could successfully select and read chips but the data being read back was a little wrong.
Reading a few carefully selected addresses of an EPROM I found that bits 2, 3, 4 & 5 were stuck high.
Looking at the schematics I quickly came to a potential problem area.
You can see from the schematic above that the LM339 comparator is responsible for those exact bits.
I removed the chip and tested it out of circuit. The chip failed and I ordered some new ones.
With a new one fitted everything was back to working status.
I’m really happy to finally have this in my collection and working.
Massive thank you to Purity for his generosity. He has been very kind to me recently and also a great help. I hope one day to be able to return the favor.
There Is a calculation outlined in the manual around predicting the life expectancy of the tube and ive made a little spreadsheet up to deal with that.
The calculation relies on being able to set the G1 voltage on the pot.
I noticed when I first got this that when I adjusted the G1 pot it was pretty loose feeling and also the reading bounced around everywhere.
Fast forward a year and I’ve come to use this again. I was completely unable to get accurate steady results from the G1 this time so I went to check the pot and found readings like this
I tried cleaning it out with contact cleaner but its had its day.
I ordered a new 250k pot from CPC and it came a few days later
Fitting this gave me solid results from the G1.
One thing I recently noticed about this rejuvenator is it has a switch marked “Soft” and “Strong”. Id not really noticed anything wrong with this before but the BMR 95 doesn’t have this option normally. It is present on the BMR 2005 model though.
Looking at it inside it looks to be a factory mod. The same wiring was used and its all part of the same loom. All solder joints look factory as well. Its either a factory mod or a very well done aftermarket mod. Either way Im pretty sure this give me the functionality of the BMR 2005.
Got myself a new HP logic comparator which had a fault on it.
The fault was with pin 11 on the LED’s.
The schematics for this are available but its so small its pretty easy to trace out just by looking at it.
Using the scope I found all the outputs of the 74L04 hex inverter at location U4 were stuck at around 1.7v. I replaced with with a 74LS04.
While I was poking around I also found the zener diode CR3 had a dry joint so resoldered that too.
Now its all working.
(Ignore the sound, had kids TV playing on Netflix while doing this)