exxos's Atari, Amiga & retro forum

Well, that's good. It means that there is at least some life. Do you see activity on the RAM data pins too? I can't remember if you said that you replaced all the RAM chips with known good ones or not.

stephen_usher wrote: 17 May 2021 12:14 Well, that's good. It means that there is at least some life. Do you see activity on the RAM data pins too? I can't remember if you said that you replaced all the RAM chips with known good ones or not.

The RAM hasn't been replaced, but removing it all makes no difference to the symptom.

What's odd is that over lunchtime I fitted a replacement socket for the serial ULA (Not the video processor) and things are slightly different with the chip back in. If I just plug in the speaker I get a tone that then goes away almost immediately.

If I plug the KBD in as well I get a wavering tone that changes frequency very slightly, up and down. This is new, I used to just get a solid tone before, either with or without the KBD plugged in, now I get 2 different results.

As for data, yes I see activity and also on the address lines. CAS is also pulsing, but RAS is steady, though I suppose since it is potentially one row of a single bank of 16 chips, then it wouldn't change at all since that row would always be selected, but the columns would change.

The serial ULA also does the cassette interface via an opamp. That opamp is the only other chip on the board other than the audio ampifier to use the -5V line, which suggests something there may be bad. Remove the opamp (at the rear of the board) in case that's doing something nasty.

That RAS sounds iffy. Though I guess that if the processor can't access zero page properly the CPU would be crashing immediately and restarting looking at address zero (I think). I would definitely replace the RAM with a known good set otherwise you'll get nowhere.

I did a Tube USB adapter which allows you to snoop the bus via the Tube then feed that in to hoglet's 6502 decoder to get an execution trace on a model B.

I think I have PCBs left. The FT232H modules are about £6-7 including delivery.

Take a look at the stardot threads...
https://stardot.org.uk/forums/viewtopic ... 05#p198905

stephen_usher wrote: 17 May 2021 18:22 The serial ULA also does the cassette interface via an opamp. That opamp is the only other chip on the board other than the audio ampifier to use the -5V line, which suggests something there may be bad. Remove the opamp (at the rear of the board) in case that's doing something nasty.

That RAS sounds iffy. Though I guess that if the processor can't access zero page properly the CPU would be crashing immediately and restarting looking at address zero (I think). I would definitely replace the RAM with a known good set otherwise you'll get nowhere.

I don't suppose you know the IC number of the Op-Amp? I could try pulling it and see what gives. As for RAM I don't have a known good set which is frustrating, hence one of the reasons I would like to get another working Beeb.

Annoyingly, I have a working Beeb, but like nearly everything else, it's in bloody STORAGE :dizzy:

Grrr!

cmorley wrote: 17 May 2021 19:05 I did a Tube USB adapter which allows you to snoop the bus via the Tube then feed that in to hoglet's 6502 decoder to get an execution trace on a model B.

I think I have PCBs left. The FT232H modules are about £6-7 including delivery.

Take a look at the stardot threads...
https://stardot.org.uk/forums/viewtopic ... 05#p198905

Interesting, I shall have a peek later, thanks.

I'd like to get as good at troubleshooting Beebs as I am with STs, but ironically the STs seem slightly less complicated!!

rubber_jonnie wrote: 17 May 2021 19:11

stephen_usher wrote: 17 May 2021 18:22 The serial ULA also does the cassette interface via an opamp. That opamp is the only other chip on the board other than the audio ampifier to use the -5V line, which suggests something there may be bad. Remove the opamp (at the rear of the board) in case that's doing something nasty.

That RAS sounds iffy. Though I guess that if the processor can't access zero page properly the CPU would be crashing immediately and restarting looking at address zero (I think). I would definitely replace the RAM with a known good set otherwise you'll get nowhere.

I don't supposeyou know the IC number of the Op-Amp? I could try pulling it and see what gives. As for RAM I don't have a known good set which is frustrating, hence one of the reasons I would like to get another working Beeb.

Annoyingly, I have a working Beeb, but like nearly everything else, it's in bloody STORAGE :dizzy:

Grrr!

Looking at the schematic the opamp is IC35

@rubber_jonnie if you need some ram let me know what it is and I'll send you some if I have any.

I also have various op amps as well. I think the beeb used a few lm324n's etc

@stephen_usher @PhilC Thanks both very much.

One thing that has kept me going throughout the whole covid thing is the generosity of the retro community 😁

I will have to check on the RAM numbers @PhilC so I'll let you know.

rubber_jonnie wrote: 17 May 2021 21:58 One thing that has kept me going throughout the whole covid thing is the generosity of the retro community 😁

Probably because we're all old buggers.