Feb 062019

Some days ago I wanted to test some RAM chips from an arcade PCB using my B&K Precision 560A tester :

B&K precision 560A IC 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:

Its internals

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:

Two LAS1605 fixed +5V voltage regulators in TO-3 package

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)

MC7805CK used as replacement

I pulled out the suspicious part:

The culprit in all its loneliness…

I swapped the spare in and was welcomed by this :

BK560A lives again!

My BK560A rised from its grave! Not too bad for a 2-euro repair! (as much as the replacement part costed…)

 Posted by at 9:30 pm

Data I/O 29A & Unipak2 repair log

 Equipment Repair Logs, Repair Logs  Comments Off on Data I/O 29A & Unipak2 repair log
Jun 072017

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

Problem solved.
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.

 Posted by at 5:17 pm

Muter BMR 95 rejuvenator repair log

 Equipment Repair Logs, Repair Logs  Comments Off on Muter BMR 95 rejuvenator repair log
Oct 032016

Had this for a while now and its been fantastic.

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.

 Posted by at 9:33 am

HP10529A repair log

 Equipment Repair Logs  Comments Off on HP10529A repair log
Jun 112016

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)

 Posted by at 2:17 pm

Polar Toneohm 850 repair log

 Equipment Repair Logs  Comments Off on Polar Toneohm 850 repair log
May 282016

Last week I bought on Ebay an untested Polar Toneohm 850 (picture from Internet) :

For the uninitiated this is a instrument that is able to locate quickly and accurately shorts on PCBs.Device is essentially an audible milliohmmeter which, indeed, produces a tone whose frequency is inversely proportional to the resistance measured across its probes.The more you get closer to the short  and so the lower will be the resistance, the higher will be the produced tone.This is very useful when you come across a PCB with a dead short or small resistance between VCC and GND, using a normal multimeter would have no effect since you will end up to measure always same resistance values due its accuracy (most if times of 0.1 ohm ) which is too coarse.

So, I bought this kind of equipment which arrived me a couple of days ago.When I saw the package, I immediately had a bad feeling :


The box was broken and, once opened, the case of the unit too:


But worst , the unit didn’t power on at all, it was competely dead while the seller clearly stated (sending me also a picture)  that, although untested, instrument turned on showing numbers on display.

Since I needed this piece of equipment I opted for repairing it instead of asking for a refund and send it back.

I opened the unit and PCB was in untouched state, no sign of damage or burn components.DSCN3328

First of all I went to check voltages on the various test points, I could only measure few Millivolts.Thanks to the user ‘Fraser’ (thanks again!) on EEVblog forum I got schematics of this specific model so I could start my troubleshooting more confident.Design of the unit is quite simple : the 220VAC ( unit came with 120VAC line selected so  had to change it accordingly to my country) reaches the main transformer ‘T1’ being reduced to 9VAC then it got rectified by a couple of bridges and then regulated by a 7805 and  couple of 78L05 providing power for all the logic :


Again, I was able to measure more or less +1V on the inputs of the tree voltage regulators so problem was upstream, specifically in the main transformer (‘T1’ on schematics) :


Said simply, a transformer is a device made of a primary winding  and one of more secondary windings.Windings are usually wound around very high permeability ferromagnetic cores.A varying current in the transformer’s primary winding creates a varying magnetic flux in the core which is transferred to the secondary windings by magnetic induction.

So, my problem was clearly located in the windings.I removed the transformer:


I went to check windings with my multimeter and none of them gave me resistance values, they were almost all opened due the shock received during shipping!But luckily they were interrupted just near the pins, see picture of primary ones for example :


A bit of ‘surgery’ was needed using some AWG30 wire to restore all connections (part in excess of wire was cut, obviously)


Once mounted the transformer and reassembled all the pieces, the unit came back to life fully working!


Video below shows a testing on an arcade board where a 100nF by-pass ceramic capacitor was intentionally shorted:

 Posted by at 10:43 pm