Beamex TC-301 Thermometer/Calibrator hangs on bootup

[enut11]

I purchased a Beamex TC-301 Thermometer/Calibrator. The unit does not boot up and hangs on a test screen.
I replaced the  8v Nicad battery but still no luck. I then dismantled the unit and did some basic testing. All supply voltages seem OK but the current draw oscillates from about 100mA to 150mA??
Pix of the unit and main PCB attached.
I have a user manual but no service manual.
I am not into micro processor circuits so could use some pointers on how to proceed.
Thanks
enut11

EDIT: I have since found out that the TC301 has an identical twin, the Omega CL521

[enut11]

TC-301 block diagram and manual attached

[enut11]

The MPU in the TC301 is a Hitachi HD63A03R (8 bit 6800 class).

What I have measured by probing some of the CPU pins:
Pins 2/3 XTAL 2.45MHz sinewave
Pin 4  NMI  +5v
Pin 5  IRQ  +5v
Pin 6  RES  +4.3v
Pin 7  STBY  +5v
Pin 40  E   614KHz squarewave (ie, 1/4 of crystal freq)

So, am I safe to conclude that the CPU chip itself is working OK?

[enut11]

There is a curious problem in powering the TC301 - the current (8v supply) oscillates between 110mA and 165mA.
The capture rate on the chart was one reading per second.

[enut11]

Power-off tests on the HD630A03 MPU on the TC301 main board:

With an ohm meter,
Checking all MPU pins to ground, no shorts detected.
Checking all MPU pins to +5v showed pins 4 (NMI), 5 (IRQ) and 7 (STBY) connected.

Also, all the pull-up resistor ladders (SIL) around the MPU tested ok.

[enut11]

Now testing the display/keyboard to see if this is the area associated with failed bootup.
This has two MM5452N LCD driver chips and little else. I checked continuity in the ribbon cable and this is OK.
The chips have a clean 5v supply and poking around on the other pins seems to indicate that some activity is happening.
My lack of understanding is slowing me down here so more reading required on how this part of the circuit works.

[enut11]

Struggling with this repair. Now suspect ROMs (MBM2764-30). Can anyone suggest how I might check these?

[MDO]

Hi, i saw this tread a couple of weeks ago but wasn't able to log in so i had to make a new account just to reply on this  

Did you ever find the problem with the TC301? I have the Master eproms v1.5 for the TC301, if you want i can make firmware dumps, I can even make copies to "new" eproms in case you don't have a programmer? You just pay shipping costs in that case, i don't mind giving up 2 eproms to rescue a nice Beamex calibrator 

[enut11]

Hi @MDO. This is an offer too good to refuse  .
The 301 has been sitting on the bench waiting for an inspiration and it seems that you are the one.
I do not have an EPROM programmer so, yes please to the offer for fully programmed new chips.
I can transfer funds via Paypal or a method that suits you.
enut11

Contact details sent via PM

[enut11]

@MDO
As requested, a pic of the TC-301 motherboard

[MDO]

Another thing you can check in the time you wait for new parts is the capacitors C6, C10, C27 and C28! I have seen these type of capacitors go half dead, meaning that they switch between shorted and working when power is applied so it does not show up when just checking with a multimeter. I had a Datron 1082 multimeter that could work for hours or days and then some days it would not boot just show random stuff on the display. it took a long time to find the problem but it was one of the tantalum drop caps that randomly half shorted pulling down a 5V rail to about 2V then up to 5V again in just a couple of mS. found the problem when checking the voltage rails with a oscilloscope.

After that i always replace the tantalum drop caps when repairing old equipment The Datron had probably 50+ of those so i replaced them all as i could not gamble that the other ones would be okay another 20 years

[enut11]

 @MDO
That is an odd failure mechanism for tantalum caps! I have experienced them shorting out completely to the extent that they can burn out a transformer.

Thanks for the tip. I will replace them.
enut11