(last updated: 14-Jul-2017)
The current configuration is in its original 'as acquired' condition. I haven't changed any of the modules/cards or their order in the backplane. The Z80A-based board in the last row had me puzzled for a while, but then I came across it's manual while organizing the large amount of documentation that was part of the pickup, and I see that this Z80 board allows the system to run CP/M 2.2!
The hacked-in SPST switch enables/disables the LTC clock signal. With the LTC clock disabled, RT11 does not count up the TIME clock.
| HEATHKIT H11 | A | B | C | D | |
|---|---|---|---|---|---|
| Q22/Q22 | 1 | M7270 : KD11-HA : LSI-11/2 CPU | MSC 4601 : 32KW (64KB) Memory | ||
| 2 | H-11-5 : Serial I/O (TERM:) | H27 : Floppy I/O | |||
| 3 | H-11-5 : Serial I/O (MODEM:) | H-11-5 : Serial I/O (LPT:) | |||
| 4 | HyDISK Z-11 Z80 CPU | (empty) | |||
04-Mar-2006
This machine has another (like the H27) set of holes in the top cover. Don't know why. It also has a small toggle switch that protrudes through another, fifth, hole in the cover. I had already dismounted this switch at the pickup event to prevent it from being snapped off during shipment.
The two leads from the switch attach on the backside of the power supply PCB (photo). The attachment turns out to be a couple of pins plugging into sleeves that are soldered to the PCB, so I just pulled them out and set the switch aside. Someday I'll check the schematics to figure out what it's all about.
I popped the rear panel connectors out so that I could remove the card cage intact. With these connectors are a set of three DB25 conversion cables, one marked "CON", another "LPT", the third without marks. I recall photographing these in place during the pickup in Longmont.
Like the other H11, the card cage securing nuts are missing from the bottom of the enclosure. I removed the cage and also discovered a strange pigtail ribbon cable with an 8-switch DIP at the end. Don't know what that is. (Baud rate select for a serial card?) Yes! It's labeled "Modem".
The inside of the cabinet had the usual dust, but no spiders or water infiltration. The power supply looks good so I don't think I'll remove it. I vacuumed the inside.
07-Apr-2006
In preparation for pulling the power supply, I disconnected the rainbow ribbon cable from the front panel switch assembly (photo), the power connector from the fan housing, and the connector from the transformer.
I removed the capacitor and power supply PCB assembly by removing the two silver screws that secure the PCB to the fan housing, and the five black screws from the chassis bottom. This let me get at the capacitor. Unfortunately, the capacitor's label wasn't visible in it's mounted position, so I removed the capacitor from it's securing band, and removed the leads from both terminals.
09-Apr-2006
I removed power and let the capacitor discharge. I reconnected the wires to the capacitor terminals and resecured the capacitor into the housing with the strap. I then reinstalled the power supply subassembly in the chassis, using the 5 screws on the chassis bottom, and two on the power supply PCB.
Okay, I'm now ready to power this up, without the card cage installed. I connected the AC and switched on the power. Looks good! The fan started up without any trouble. I read 5.12V on the 5-volt lines, and 12.4V on the 12-volt line. Looks pretty good. The power supply looks like it is in good shape.
Now I need to read more about this board placement and so on, before I try anything more with this system.
18-Oct-2006
After further examination, and a review of the Pictorial sheets that came with the assembly manual, I determined that the backplane is serpentine, but it is a mirror image of the normal DEC backplane. Slot 1 is at the top right, but the boards are to be inserted component-side down. Turns out that each AB/CD slot is labelled on the backplane PCB on the board side. So the board installation sequence is:
D C B A
1 v--2-- < --1--
v
2 v--3-- > --4--v
v
3 v--6-- < --5--v
v
4 v--7-- > --8--
[only Memory or quad-sized CPU in slot 1]
I removed all of the cards from the cage and photographed them.
22-Jul-2007
The final, and very beneficial bit of information that I (at last) discovered, given in the Final Assembly section and particularly in Pictorial 5-1 (and, as I later discoverd, also confirmed in the H27 documentation), is that the CPU card must be installed in the left-most top slot (Slot #2) of the backplane, with the boards going in with the component side down (which is on the left with the component side down, as Heathkit designed it!), and with the first or only memory board installed in Slot #1. Apparently Slot #1 does not have all the signaling required for the CPU or other boards that require interrupts or DMA. This system is just strange enough to make things quite confusing. Even when you think you have a handle on how a Qbus backplane works, this H11A comes along! But, at least, now, I know the proper location of the key boards in the system.
24-Jul-2007
Reviewing the schematic suggests (to me) that the jumper connects a 60Hz clock to the backplane, that is, it is the souce for the LTC signal. With the jumper in place, the backplane LTC gets it's clock from the AC line, and if the jumper is removed, then it does not. I suppose that the switch hacked into the top panel was an earlier owners attempt at putting an accessible switch on the system. My other H11A has a third front-panel LTC switch, versus this one which is home-made. Obviously, Heathkit eventually integrated a real LTC switch into the design.
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