HP 67 Power Supply (not oscillating)

[mbielman]

I have a 67 logic board here that I'm trying to set up as a test unit. At this point, it has no ROMs installed other than ROM0. ACT and CRC are installed.

Problem is the PS isn't working. No oscillation. So Vcc is nearly at Vbat and no Vgg.

But it WAS working! Components measure 'OK' in-circuit. (no shorts) Vcc not shorted, etc.

Wondering... does the PS need the load of the ROMs to run? Seems odd to me. If so, what is the expected load current?

Any thoughts?

Mark

[teenix]

I have one with 2 ROMs missing, it measures 96mA.

I would have thought if the oscillator is idle the transistor connected to pin 3 of the coil might have failed as it would be trying to short the battery through the coil.

cheers

Tony

[mbielman]

Found it! The 1.5k feedback resistor was open. I thought the cap (C6) might be shorted... with ohmmeter measured many megohms. (?) But should be ~1.65k (1.5K + 150 + coil). Pressed on R4 and it easily broke in half. Will replace tomorrow and check but I'm sure that's it.

[mbielman]

Couldn't wait. Replaced the R and it's working now.

Tony (et al) - I'm curious... what is the minimal amount of ROMs needed to get any sort of display?
With only ROM0 I get nothing. Maybe add ROM1? and ROM2? I removed ROM3 once and it sort of works. (so it had ROMs 0, 1, 2 & 4)

Mark

[teenix]

Nice

This is a list from my 67 emulator and shows what ROM pages (256 words) execute at start up to the key wait loop.

ROM 0 $0000 - $00FF 1818-0268
ROM 2 $0200 - $02FF 1818-0268
ROM 4 $0400 - $04FF 1818-0228
ROM F $0F00 - $0FFF 1818-0231 or 1818-0232 (not sure)

I don't have a list of RAM that might be accessed during start up, maybe I'll add that to the emulator when I get time.

This is a list of 67 chip data that I got from my ROM/RAM reader. The last 2 I'm not sure of because the chips were faulty. I didn't want to remove these from a working board in case of damage like what happened with the ROM0 chip from a HP-92 - it fractured.

Code:


                 ROM Start   RAM start
1818-0268        0           NA
1818-0228        400         0
1818-0226        800         10
1818-0232        C00  ?      20 ?
1818-0231        1400 ?      30 ?

cheers

Tony

[mbielman]

Tony - You are thinking firmware and I'm asking hardware.

What ICs are necessary to get a display?

Mark

[teenix]

I'm a little unsure if this is where you would like to go, but delving deeper hardware wise, I suppose in that sense all of the components of the calculator, except for the CRC IC, as they work as a team.

Hardware dependencies down the line would be something like...

The output to the display is generated by the anode driver and the cathode driver.

The cathode driver for digits is dependent on the ACT for RCD, and ROM0 for STB
The anode driver for LED segments is dependent on the ROM0 IC
ROM0 is dependent on information on the Is bus, bits 0 - 7, generated by the ACT
This info is dependent on the ACT A and B register settings and the display ON/OFF control
These settings are dependent on the code from the ROMs

As mentioned, the code in the ROM ICs listed in the emulator output will get the power up display [0.00] and these should be the only ROMs required to get to this point.

cheers

Tony