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Yes—the Heathkit H11 is a Heathkit-built member of DEC’s PDP-11 family, based on the LSI-11 and its QBus. It is not simply a standard DEC PDP-11 in a different case: the Heathkit wiring, board set, options and revisions matter during restoration. Before removing cards or applying power, document the system as found and inspect the power supply. A published H11 teardown found a diode that was not properly attached and corrected the supply by replacing it.
What the H11 is—and what “PDP-11” means here
Heathkit introduced the H11 in 1977 as a kit priced at $1,295, according to its historical archive. The original system description specifies an LSI-11 CPU on a fully wired DEC KD11F board, 4,096 × 16 words of read/write MOS memory, a 38-line bus, a fan-cooled switching power supply and a backplane with space for up to six I/O and memory modules. The backplane was described as compatible with standard DEC LSI-11 accessories.
That makes “PDP-11” a useful family-level description, but the more precise identification is an LSI-11 QBus system assembled and documented by Heathkit. Hackaday’s November 22, 2021 coverage makes the same distinction for the H11A: it is PDP-11-class, technically an LSI-11. H11A material describes a physically smaller KD11-HA configuration and compatibility with the DEC PDP-11/03. Do not assume every H11 or H11A has the same board layout or options.
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For restoration, treat the chassis as a particular configuration of CPU, memory, backplane, I/O cards, cabling and power hardware—not as a generic PDP-11 with interchangeable parts. Record those details before changing anything.
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What to document before teardown
Photograph the system in place before unplugging or removing boards. Clear pictures and written notes preserve clues that manuals may not capture, especially if the machine has been modified or is incomplete.
- Photograph the exterior, model and serial labels, rear connectors, card cage, wiring and each board.
- Write down every board designation and its slot order. Mark empty positions and note any missing boards.
- Record jumper and switch settings, connector orientation, cable destinations, and any labels added by a previous owner.
- Identify the CPU, memory and I/O boards separately where possible; note terminal, serial or parallel interfaces and any disk or other storage hardware.
- Photograph the power-supply connections and how they reach the backplane. Keep the backplane and card connectors supported when handling them.
The original H11 backplane routes bus data, control and power connections to module positions. Its physical connectors therefore carry more than just a board’s identity: preserving slot and cabling information helps avoid confusing a bus connection with a power connection during reassembly.
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How to inspect an H11 before applying power
Start with a visual and mechanical inspection
With the machine disconnected from mains power, inspect the mains wiring, ground bonding, fuses, connectors and fan. Look for corrosion, loose or damaged components, heat damage, debris and evidence of earlier repairs or modifications. Check that boards and connectors are seated and supported, but do not reseat cards merely to see whether the computer starts; first record their original positions and settings.
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Evaluate the supply separately
Inspect and assess the power supply before reconnecting it to the backplane. In a published H11 teardown, the repairer found a diode that was not attached properly; replacing that diode corrected the supply. That is evidence of a specific fault in that machine, not a universal H11 failure or a prescribed parts list. The available account does not establish a standard capacitor-replacement schedule, a guaranteed startup procedure for every revision, or a board failure rate.
Use the exact supply and system documentation to determine the appropriate checks for your unit. Do not infer wiring, test points or acceptable readings from a different revision. If a replacement supply is being considered, verify electrical compatibility, grounding and protection, and make the change reversible where practical.
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A careful repair and bring-up sequence
- Document the as-found system. Complete the photographs, board inventory, slot map, switch and jumper notes, and cable map before removing parts.
- Inspect the chassis and supply. Resolve visible wiring, connector and supply concerns before reconnecting the backplane. Consult the matching Heathkit documentation for kit-specific wiring and service details.
- Identify the processor and memory. Match board designations and configuration to the DEC/LSI-11 processor documentation and the H11 manuals. Do not assume a board’s purpose from its position alone.
- Reconnect a known configuration. After the supply has been checked, restore the documented CPU and memory arrangement. Use the console or ODT procedure specified in the relevant manual; there is no substantiated single power-on sequence that applies to every H11 revision.
- Add other hardware deliberately. Treat each added memory, I/O, terminal or storage board as a configuration change. Record what was added and the result before proceeding to another change.
This staged approach preserves the ability to associate a new symptom with a specific change. It also avoids using the card cage as a trial-and-error diagnostic tool when the original board order or wiring is unknown.
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DEC documentation for the processor and bus
The PDP-11 hardware-documentation index lists the 1975 LSI-11 PDP-11/03 Processor Handbook, along with processor maintenance, peripheral and terminal manuals. Use the processor handbook for LSI-11, bus and programming context, and select maintenance or peripheral documentation that matches the boards and equipment actually installed.
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Heathkit documentation for the kit
Heathkit’s H11 assembly and operation manuals are the references for kit-specific wiring, board placement, assembly and Heathkit options. Match the manual edition to the machine’s revision and board set; a general DEC manual cannot substitute for Heathkit’s chassis and wiring details.
Finding replacement parts or reproductions
Heathkit’s parts-information page said in 2024 that it sometimes had vintage replacement parts, with stock on about 1,000 different vintage parts plus a limited boneyard. Availability changes, so check current stock rather than assuming a listed or historically available part can still be ordered. Identify the hyphenated Heathkit part number in the assembly manual before contacting the supplier.
Heathkit also says vintage manuals are sold as bound reproductions and notes that its manuals are copyrighted. For any reproduction, verify that the contents are complete and legible and that the edition matches your H11; do not assume an online scan or another H11 revision is equivalent.
Quick Recap
Choosing a restoration path
| Choice | What to compare |
|---|---|
| Original H11 boards or DEC-compatible QBus substitutions | Bus and electrical compatibility, available software support, historical fidelity and serviceability. Record any substitution and preserve removed original parts. |
| Repair the original supply or fit a replacement | Compatibility with the system, grounding and protection, documentation for the change, and whether the work can be reversed. The published diode repair is specific to the teardown machine, not a general replacement prescription. |
| Original peripherals or modern serial/storage alternatives | Compatibility with the bus or interface, software support, historical fidelity and ease of future service. |
| Original paper manuals or reproductions | Completeness, legibility, copyright status and edition match for the actual machine. |
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