Confirming a diagnosis

[lowrybt]

I recently purchased an Issue/D13 UK Sinclair QL in need of repairs and refurbishing. New keyboard membrane, replaced the MDV rollers, cleaned the heads. Here's the problem. Both the MDV's that came with the unit work great when attached to HBC 2, in other words when positioned as MDV2. When the drives are positioned as MDV1, the best they can do is respond to a dir mdv1_ command by running incessantly. Again, both drive units work great when connected to HBC2. So, I've established that the issue is not the heads and it's not the head board logic.

This points to some kind of component failure on the main board. The QL Service Manual suggests replacing the ZX551(TR7) when MDV1 runs incessantly.

I'd be fine just leaving MDV position 1 empty but -- I've learned -- MDV2 can't be addressed/commanded without MDV1 being plugged in. Leaving a perfectly good but unusable MDV in position 1 seems silly.

So, here is the buried lead question: given the symptoms, is the problem likely to be TR7 and/or is there something I can do (cut leads, run a jumper) that would let me ignore MDV1 (no drive connected to HBC1) and address MDV2 directly?

Thanks,

Tom

[1024MAK]

So, here is the buried lead question: given the symptoms ... is there something I can do (cut leads, run a jumper) that would let me ignore MDV1 (no drive connected to HBC1) and address MDV2 directly?

Yes!

Mark

[Dave]

That helps!

Here is the authoritative answer:

[lowrybt]

That helps!

Here is the authoritative answer:

Dave, the authoritative answer is blank -- is it the TR7? Thanks, Tom

[tofro]

Trying to be a slight bit more helpful:

Yes, TR7 would be worth checking.

If you want to keep mdv2_ operational even if mdv1_ removed, you need to jumper a line on mdv1_ connector - The mdv select mechanism is daisy-chained and needs a continuous chain between first and last mdv_

The signals that need to be bridged on an unconnected mdv1_ connector are COMMS-IN and COMMS-OUT (pins 1 & 2). Note after doing this, your mdv2_ will listen to mdv1_, even if it is connected to the second connector.

Tobias

[1024MAK]

Sorry for the short reply, I ran an update on my system and that caused the web browser to freeze
So this morning I started looking into this... This prevented me from double checking my thoughts with my copy of the schematic, hence the lack of follow up reply

Mark

[Dave]

...and I was just making fun of him ^^^^

[lowrybt]

...and I was just making fun of him ^^^^

Thanks for the guidance all!