Help Required with Cheetah Amdrum

[crayzyian]

All,


One of the peripherals I've attempted to blow the dust off is Cheetah's Amdrum, which definitely worked when I purchased it about thirty years ago!


In short I can get no sound from it.  And currently I am unsure if it is plugged into the expansion port properly on my CPC 464 and therefore "talking" to my unit.  Is there a command I can enter in BASIC to determine if any communication is being had?  Should I be cleaning or poking anything?  The port from Amdrum does seem a bit flimsy...


Assuming it is plugged in, I cannot get any sound out through my external amplifier.


Does anyone have any experience of this or similar peripherals that can offer some pointers.


The software boots up fine BTW; I have the original tape and one of the expansion packs which gives African-based drum sounds.  I remember the output being fairly cool and I wanted to impress my sceptical daughters that 1980s tech can be a surprise!

[GUNHED]

Probably you already know this, but it does emulate the Amdrum too:
https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/new-'next-generation-speech-synthesizer'-for-the-cpc-464/msg144273/#msg144273

[Bryce]

Probably you already know this, but it does emulate the Amdrum too:
https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/new-'next-generation-speech-synthesizer'-for-the-cpc-464/msg144273/#msg144273

And how does that help him get an original Amdrum working again?

Bryce.

[GUNHED]

And how does that help him get an original Amdrum working again?

Bryce.

It's one way to replace it, in case it won't work again.

[Bryce]

It's one way to replace it, in case it won't work again.

So every time someone posts that their CPC has a hardware issue you're going to tell them where to download an emulator?

Bryce.

[GUNHED]

Can you please be productive. I try to help where I can. If you have an better idea, please share it instead of behaving the way you do.

[crayzyian]

Emulation is a solution, but it would be nice to get a physical fix.


Initially knowing whether a peripheral is plugged in and talking to a CPC unit must be a "thing", I just don't know what commands to use.

[Bryce]

The first thing you should do is clean the 50 way connector on the CPC and the Cheetah. Unfortunately, the Amdrum is a purely passive device with just an analogue to digital converter and a small amplifier, so it doesn't have a ROM or any other device that registers with the CPC. The only way of knowing whether it works is to load the software and try to get it to play.

Bryce.

[crayzyian]

Good point - cleaning the connector is probably necessity given 35 years' use!


What do you recommend to use; is there anything that would benefit the connection?  Is their anything to avoid, bar the obvious things?!

[Bryce]

Contact cleaner or Isopropanol is best. Don't try to sandpaper them or similar, you'll just destroy the surface.


Bryce.

[crayzyian]

Bryce - sorry to be dumb; do you mean contact lens cleaner?

[Bryce]

No, something like this: https://www.amazon.co.uk/WD40D-N-A-Contact-Cleaner/dp/B006UCJ5WQ/ref=sr_1_5?dchild=1&keywords=contact+cleaner&qid=1587833115&sr=8-5

Bryce.

[crayzyian]

  Right - with you now!  Thanks.

[GUNHED]

Also you can use some commands of the LambdaSpeak III RSX ROM to check if the Amdrum works.
Link see my signature. 

[RetroCPC]

Be great a fun project to update the original design - improving its Audio performance maybe.

I've been toying with the idea of re-spinning SPO256 DKtronics Speech synthesizer - maybe combine it with the Amdrum... Only because its Retro

Maybe I'm being rather romantic in my old age, but computers where more fun back in the 80's...

[Bryce]

Be great a fun project to update the original design - improving its Audio performance maybe.

I've been toying with the idea of re-spinning SPO256 DKtronics Speech synthesizer - maybe combine it with the Amdrum... Only because its Retro

Maybe I'm being rather romantic in my old age, but computers where more fun back in the 80's...

I think someone already did a 100% clone, ie: no modifications to the original schematic. There's an eagle file of it floating around somewhere on the interwebs.

Bryce.

[RetroCPC]

Yes, I did see a PCB layout. maybe even here on the forum - as you says seems to be a very close copy of the original design..

I've not seen one for 35years, but reading the schematics its based on a 8Bit DAC, clocked directly from the Address Bus with unregulated charge pump powering the analogue stage...

Just out of personal interest, I'd like to know the sonic effects of any design improvements - but maybe any "improvements" will remove the innate charm of the product...

Sure its only 8bits, but its about looking at old designs with 35 more years of knowledge...



I took a risk and purchased what I hoped is an Amstrad DK'Tronics speech Synth from EBay last week:-

https://www.ebay.com/itm/333578421284

The Auction listing described it as "ZXSpectrum speakers"... but I took a chance that it was in fact an Amstrad speech synth...

Arrived today, and can confirm its a DKTronics speech synth for the Amstrad - the early version without internal ROM, and in very clean condition, date code in the SP0256 is the 49 week of 1984.

Single layer PCB - with only 3 jumpers

Now all I need is a CPC computer I purchased a non working CPC6128 yesterday from EBay - it should be here hopefully sometime next week...

[Bryce]

That's a DK speech synth for the Amstrad. The connector would be wrong for a Spectrum.

Bryce.

[RetroCPC]

That's a DK speech synth for the Amstrad. The connector would be wrong for a Spectrum.

Bryce.

Yes, it was the expansion connector that convinced me the listing was in error - Good for me

As I say, it arrived today and its the early version without internal ROM...

I cannot recall how the ROM's where addressed - do you have any info on external ROM expansion boards / interfacing on the CPC?

Also, is it possible to somehow load a software image from Tape or disk into ROM? (presuming that it fits within the ROM Memory size). I guess games that load extra game play during the game would have an issue - but I'm thinking of software like the Amdrum loaded into a ROM for instant load.

[Bryce]

The ROM-less version isn'tan earlier version, they offered both the ROM and ROM-less version the entire time, so that there was a lower cost version. You can download the ROM image from the Wiki and install it in any ROMBoard. The ROM position doesn't matter as long as it's not ROM0 or ROM7.

I created the MegaFlash a few years ago, a ROMBoard that uses a single Flash ROM chip. Since then there have been many similar and expanded versions created. Speak to @TotO I'm pretty sure he still sells a similar device.


Bryce.

[RetroCPC]

The ROM-less version isn'tan earlier version, they offered both the ROM and ROM-less version the entire time, so that there was a lower cost version. You can download the ROM image from the Wiki and install it in any ROMBoard. The ROM position doesn't matter as long as it's not ROM0 or ROM7.

I created the MegaFlash a few years ago, a ROMBoard that uses a single Flash ROM chip. Since then there have been many similar and expanded versions created. Speak to @TotO I'm pretty sure he still sells a similar device.


Bryce.

Do you have any information how the ROM decoding is done, (Rom Address etc). I'll fit a ROM position when I redesign the PCB... for my Advanced O Level electronics project I designed a 4 slot ROM card for the CPC (with speech Synth + lightpen) all on a single layer PCB using a lightbox + track and tape - my minds completely blank about the details... Later today I trace out the DKTronics circuit and upload here...

[Bryce]

I'm not sure what you mean with ROM address. All ROMs have the same address (as the BASIC ROM) and get paged in and out as required. Check out this article here for an explanation of how they work: http://www.cpcwiki.eu/index.php/ACU_Romboard_(DIY)

Bryce.

[RetroCPC]

Bryce,

Thanks the article in the link has everything I need to understand the ROM address decoding / page selection.

"ROM Selection is made using a port decoded with A13=0, A14=1. D3 must be 0, D2..D0 select the ROM, allowing up to 8 ROMs to be selected, but the PCB only has 6 EPROM sockets. Sockets for ROM 0 and ROM 7 aren't fitted. More than one value can select the same ROM (e.g. &11 will also select &1)."

Not sure if you have looked closely at the schematic, but I wonder why the designer added pull-up resistors on the output of the 74HCT137 decoder and the 74LS32... non have Tristate outputs, and the 74HCT series has full rail swing... their outputs can never be floating making the 8 extra resistors redundant unless I'm missing something?

They only make sense if the IC's where removed from the PCB, but then why bother?

[Bryce]

When the CPC is booting, you can't be sure of the state of the 74HCT137 outputs, any one of the outputs could be at 0 (Note that the expansion has no access to the CPCs reset circuitry). This would block or cause bus-retention issues with the CPC's internal ROM as both (or multiple) ROMs could have their CE pin low at the same time, when the CPC needs to access the Firmware ROM). To avoid this issue they added the resistors. I'm pretty sure that if you removed them, the CPC would have a random chance of successfully booting.

Bryce.

[RetroCPC]

When the CPC is booting, you can't be sure of the state of the 74HCT137 outputs, any one of the outputs could be at 0 (Note that the expansion has no access to the CPCs reset circuitry). This would block or cause bus-retention issues with the CPC's internal ROM as both (or multiple) ROMs could have their CE pin low at the same time, when the CPC needs to access the Firmware ROM). To avoid this issue they added the resistors. I'm pretty sure that if you removed them, the CPC would have a random chance of successfully booting.

Bryce.

Bryce,

Yes I can see the case for an undefined condition on Powerup - but the actively driven outputs of the 74HCT137 are "low impedance" in any logic state - so the 2k2 resistors will have little effect verses the say 30 - 50 ohms output impedance of HCT (High or Low logic state)... they can only increase dissipation when the outputs are in logic Low state - but will have no effect on the actual logic state of the Gate outputs...


Lets consider that a few of the 74HCT137 outputs randomly powered up in a logic Low condition - the output impedance of each will be around 50ohms - the respective 2k2 pull up verses this say 50 Ohms ACTIVELY Driven Low impedance - these resistors will not going to have any effect on the output condition WRT logic levels...

I must be missing something here... Non of the gates have Tri-state outputs...