Sinclair ZX80 / ZX81 / Z88 Forums

It's been a while, but today I found time to get the Zeddy out and see if the 16K RAM pack I bought would work.

Sadly not, zeddy runs up fine on its own, but no display at all with the RAM Pack fitted.

This is what I have: Where do I start?

I have a multimeter and a cheapie lcd oscilloscope

Testing pins 1, 8, 9 and 16 of the 4116 DRAM chips. See the link in my sig below to find the pin out.

Mark

1, 8 & 9 relative to 16?, so checking the supplies to each chip?

Yes, these actually run off the +12V supply, but need the -5V supply to correctly bias the internal circuitry. The +5V is used by the output buffer for the data output pin.

If the +12V supply is missing they won’t work. If the -5V supply is missing with the +12V present (even if out of specification) they may self destruct.

Mark

Thanks Mark, I'll have a look soon. I have a scrap issue 4 spectrum pcb with 4116s in lower RAM. Might be good for spares if needed. Cheap enough to buy new in any case

Just a thought though, I did try to fit an internal 16K which did not work in a very similar manner - no display. I have since removed it.

Is it possible there is something wrong with the Zeddy that only manifests itself once the memory goes up?

Unlikely. In the computer, the ULA does the address decoding for the ROM and the RAM. To keep costs down the simplest address decoding is used. This is also known as partial address decoding. Hence the entire address space is split into four:
ROM 0K to 16K (8K of actual ROM followed by a shadow of the same ROM)
RAM 16K to 32K (1K of actual RAM followed by a lot of shadows of the same 1K)
ROM 32K to 48K (two shadows of the ROM at 0K)
RAM 48K to 64K (used by the video system, which actually accesses the same RAM that is at 16K)

The external 16K RAM packs page out the internal RAM (all of it) and perform their own address decoding.

A fault with an address line may cause problems but then the system would work but have less RAM available than expected (as long as it’s at least 1K).

The ROM code does limit the “auto detection” to 16K maximum. But it can cope with any amount between 1K and 16K. And you can POKE a system variable if you have more RAM than this.

Have you cleaned and inspected the edge-connector contacts?

Mark

Thanks Mark, a coincidence I guess. I'm not going to get distracted by the internal 16k module and focus on the copy of my childhood ram pack for now.

I'll get the Zeddy apart and check the edge connector and give the connector a good blow out with switch cleaner. I can get the heatsink fitted the right way round at the same time.

I've jumped the gun a bit and responded to an offer on eBay, I have a modern 16K ram pack kit on the way for 15 quid.

Which works fine. Confirming Mark's observations that the old RAM pack is at fault.

Now to find time to start diagnosing it.