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General Category => Technical Support - General => Topic started by: bolofski on 19:19, 28 January 22

Title: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: bolofski on 19:19, 28 January 22
i have killed my cpc6128.  the drive had a new belt fitted and was fully operational, i have been using the drive to save a few basic programs and loaded them back up no probs, drive never even stuttered...then...   i decided to link the tape port up to my pc to try loading a game via a cdt converted wav file, problem was as soon as i connected the tape lead to my pc (its the proper lead) the house electric tripped, tried again, same thing, house electric trips. now the bloomin drive wont read any disks, just gives read errors, im presuming ive blown something on the board :( any ideas?


thanks for any advice or pointers :)
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: Nworc on 20:27, 28 January 22
Wow .. before starting to repair the CPC, you should better check the electricity of your house - it could save a life.

The power supply of the CPC monitor is an idiot proof design in that it is galvanically isolated from the mains. That you experience such a thing when connecting the PC tells me that the power supply you are using is not from the original monitor.

Regarding the drive: you could check whether the head moves while seeking track 0 - could be that is got stuck below track 0 or beyond track 39. Does the motor spin? Is the power supply still delivering exactly 5V and 12V? ...
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: bolofski on 23:15, 28 January 22
the power is from the original monitor ctm644, it doesnt trip when i connect that, thats all still working fine, the 6128 turns on fine even now and works fine, the drive is still spinning and the read head moves up and down a bit before telling me theres a read error.  its strange, the lead is the din socket to 3 leads and was with the cpc6128 when i acquired the whole thing (along with a knackered realistic tape player) i can connect the din plug to the 6128 but as soon as i connect the headphone jack to my pc's headphone socket the electric trips and its took something with it :( the drive was working fine before.  i only wanted to play green beret..
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: bolofski on 19:34, 31 January 22
just an update on this, the monitor was causing the problem with the disk drive not reading, so even though motor is spinning i presume theres a voltage issue somewhere, i had another monitor (gt65 green screen) and the drive still works great, not sure whether i should try to link up the tape lead again though
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: Bryce on 19:49, 31 January 22
The power tripping you mentioned in your first post was almost to be expected. The CPC negative is grounded to earth as it should be. Then you connected a device (the PC) which uses a Switched Mode PSU, where the ground is referenced to one side of the 230V phase, so effectively, you shorted the mains. It's 50/50 most of the time with such a setup, which is why I regularly tell people to avoid this setup (although people rarely listen and in some cases even contradict me).

As to the drive issue: The house fuse tripping is unlikely to have damaged the drive, but it could have damaged ground traces in the CPC. Check that the drive definitely has a connection to the negative rail and that it's getting 5V and 12V. My guess is that the 5V rail is no longer being fed to the drive, but there are other things that could have failed.

Bryce.
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: pelrun on 07:34, 01 February 22
Hang on, that doesn't make sense. The CPC can't be referenced to earth, because it doesn't even *have* an earth pin (at least on my unit; other countries may differ, but even then the schematics don't show an earth connection.) It's an isolated supply, so it's ground will be floating at some arbitrary point.

On the other hand, a desktop PC with an ATX PSU definitely has chassis ground connected to earth. This is mandatory for devices that have a metal chassis where a short to the case could potentially make it live. The PC will also inevitably have it's DC ground connected to chassis ground, so it's the PC that's earth referenced.

Assuming the tape cable is correctly wired (it's not guaranteed!) I bet you connected the cable only after both machines were already on and had found their respective ground levels. Connecting them then provides a low-impedance path between the two grounds and suddenly a large current flowed between the two as they tried to equalise. As the PC is explicitly connected to earth, some of that current probably found it's way to your ground-leakage safety switch and tripped it. Connecting the cables before turning the CPC on may minimise that problem, but persistent ground loops are a constant problem for people connecting audio cables between mains powered devices and it may not go away.

You can get ground-loop audio interrupters which go inline with the audio cable and are a 1:1 transformer that breaks the ground loop but allows audio to pass anyway... or you could try another isolated device instead of your PC, like a smartphone or a laptop.

For the smartphone option, I have a webapp that runs perfectly well in a phone browser and plays CDTs, and it has a huge collection of them to choose from: https://pelrun.github.io/PlayUEF/PlayUEF.html (https://pelrun.github.io/PlayUEF/PlayUEF.html)
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: Bryce on 10:17, 01 February 22
He's in the UK, all devices there are grounded and his monitor most likely has a 3 pin plug (because the UK doesn't have 2 pin plugs).

Bryce.
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: pelrun on 10:55, 01 February 22
Just because the pin is present on the plug, doesn't mean it has a meaningful connection internally (given that the case is plastic, it's a bit difficult to see where it would go.) This is why I mention the schematic, which clearly shows an entirely isolated mains supply section. Binding ground on the isolated side to earth is just as possible as it would be on the PC side, but given there actually is a potential difference I'm more likely to suspect the CPC is the one that's floating.
There could also be a legitimate fault.
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: Bryce on 11:45, 01 February 22
The secondary side of the CRT transformer is definitely connected to the internal metal chassis. If the CPC and Monitor were completely isolated, there's no way the fuse could have blown.

Bryce.
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: GUNHED on 16:44, 01 February 22
Exactly right Bryce!  :)
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: bolofski on 17:27, 01 February 22
ok so its is definately a problem with the monitor as the green screen works, i also plucked up the courage to try the tape thing again, no trip with the green screen, ive also converted some dsk files and managed to rebuild the disks on the cpc with the help of dsk2cdt / tzx2wav and now have fully playable games on disk, green beret, commando, ghost n goblins, all the games i wanted when i was 12 but couldnt afford.  so now i have a ctm644 colour monitor that powering the cpc6128 but the drive doesnt read disks (motor is spinning and head is moving but i get read error) however drive works fine when using the green screen, so next question :) i will be checking the voltages given out by the ctm644, am i just looking for 12v and 5v?
Title: Re: more cpc6128 disk drive woes :( drive not working after tape port caused trip!
Post by: GUNHED on 19:53, 01 February 22
Glad you get ahead with that topic.
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