exxos's Atari, Amiga & retro forum

cmorley wrote: 22 Apr 2021 13:04 Sounds like a remote mounted volume control mod. Some people mount a pot at the back of the machine. If the pot is missing then you will get no audio... if CAPS LOCK doesn't work then it is probably "long beep"ing but you just can't hear it (catch all term for not passing the POST).

I've read of them, but definitely missing here. Not sure if caps lock is working or not as the machine is also missing the KBD LEDs, so I'm working on a replacement.

*MOTOR 1
*MOTOR 0
will click the tape motor relay on and off if you blind type that.... I imagine if bits have been cut off then it wasn't booting for the previous owner though!

THe good news is they are generally easy to repair.

New pot ordered, specs from schematic.

cmorley wrote: 22 Apr 2021 13:17 *MOTOR 1
*MOTOR 0
will click the tape motor relay on and off if you blind type that.... I imagine if bits have been cut off then it wasn't booting for the previous owner though!

THe good news is they are generally easy to repair.

It's odd, the KBD is a bit bodged and the LEDs were removed and sockets fitted. Not nice sockets either, crappy ones. I will try your cassette suggestion, thanks.

Ok, so things I've ascertained so far:

The PSU is delivering about what it should.

The speaker is disconnected, so it isn't going to be doing any boo bipping until I can fix that.

The reason for that disconnection is because VR1, a variable resistor package is missing from the board. Gone. It looks like somebody added wires to make it an easier proposition to adjust the voltage. All that remains of those are about 2mm of each snipped off close to the board.

So, replacement is on the way, and I have a new supply of IPA now so will start to clean up the mainboard a bit. At least I have some more documentation now, which is helping me get going.

Baby steps.

Woo hoo, my replacement VR1 just arrived!

:dualthumbup: :cheer: :yay:

cmorley wrote: 22 Apr 2021 13:17 *MOTOR 1
*MOTOR 0
will click the tape motor relay on and off if you blind type that.... I imagine if bits have been cut off then it wasn't booting for the previous owner though!

THe good news is they are generally easy to repair.

@cmorley This did precisely nothing, and I hooked up the composite to the comp input on my TV, and just blackness.

VR1 is replaced and I can hear a slight hiss from the speaker which gets louder/quieter when VR1 is turned, so I am confident that works at least. The other thing that is worrying is that the caps lock LED I fitted comes on, and stays on regardless.

The voltages still strike me as low at about 4.7v-4.8v, -5v is bang on though. The other thing driving me mad is that I'd like to go and test the voltages at all chips. But the schematics don't seem to show VCC and GND for everything, which is where the ST schematics seem to be better.

I can certainly go and get all the necessary datasheets but I'd rather refer to the schematic than a pile of different datasheets, it's just a bit quicker when you're trying to do lots of chips.

Do you know of a good source for datasheets that show all pin connections on the chips?

I also need to ID the jumpers (Sooooo many jumpers) so I'll look through the service manual tonight, and maybe try removing the ULA as @JezC suggested now that I have a video cable.

Definitely missing my LA and oscilloscope at this point as they are in storage, but hey, more of a challenge without!!

If a B doesn't boot then you should get the low drone from the sound chip AKA "long beep". If you don't get that then there is either something wrong with the sound chip/amp or perhaps you are missing clocks - they all come from the ULA derived from the 16MHz crystal circuit. Power, clocks & reset (there are two /RST & /RSTA)... common faults are dead DRAM, IC14, 6522s and 6502. There's a sticky topic for the common faults on the stardot forum in 8-bit hardware.

I don't know of any schematic which shows the power and ground on the ICs.

cmorley wrote: 25 Apr 2021 22:20 If a B doesn't boot then you should get the low drone from the sound chip AKA "long beep". If you don't get that then there is either something wrong with the sound chip/amp or perhaps you are missing clocks - they all come from the ULA derived from the 16MHz crystal circuit. Power, clocks & reset (there are two /RST & /RSTA)... common faults are dead DRAM, IC14, 6522s and 6502. There's a sticky topic for the common faults on the stardot forum in 8-bit hardware.

I don't know of any schematic which shows the power and ground on the ICs.

I've just downloaded a troubleshooting guide which actually describes my problem exactly. Blank screen, no sound from speaker and caps lock LED is lit. I'll have a go at the process indicated there, see how I get on.

Most if not all of my ST schematics show GND & VCC soI was hoping the Beeb ones would be the same, but apparently not.

Checking for clocks is going to be tough right now as I have no scope due to most of my workshop stuff being in storage, so limited to a very basic mutimeter. If it's nothing simple then it may have to wait until I get moved in properly.

For now I guess it's keeping me busy :)