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I have a Dragon 32 with an issue 3 power board - (is there any way of checking which issue of the motherboard it is?)

I presume both power regulators should be firmly attached to the heatsink - one was sticking up in the air, no sign of the rivet used to hold the other power regulator - and it looks as though it has partially melted.

On the main board capacitor C11 was smoking (presumably linked to the above issue, but hard to tell).

Can anyone advise of the correct replacements for these and I will repair the board.

Rich

The power regulator is REG 3 - a 7905CT if anyone has any spare - I only have 7805 regulators

I can't read the C11 capacitor

One up and one down for the regulators is normal - you should *NOT* connect them both as they have different potentials.

If you take a pic of the motherboard I can let you know which PSU modules are compatible...

admin wrote:One up and one down for the regulators is normal - you should *NOT* connect them both as they have different potentials.

If you take a pic of the motherboard I can let you know which PSU modules are compatible...

Oh well that resolves that - I presume it may just be the capacitor C11 which is blown - the power module still seems to work, as the capacitor on the motherboard smokes when switched on - it is an issue 2 board.

RWAP wrote:The power regulator is REG 3 - a 7905CT if anyone has any spare - I only have 7805 regulators

I can't read the C11 capacitor

I believe you should be able to get the 7905 from the local Maplins (if you have one near you).

I think the scans of the circuit diagrams for the various Dragon boards are in the archive, though they are not very good quality, they should be good enough to determine which pins of the PSU spit out which voltages I think there should be (from memory) :

Dragon 32 : +12V, +5V, -5V
Dragon 64: +12V, +5V, -12V

I believe the main difference between a Dragon 32 PSU board and a Dragon 64 one is that the 32 board has the 7905 -5V reg, and the 64 has the 7912, -12V reg....

One thing I did think of, is does the main board have a row of jumpers at the back near the power connector ? As these route the power lines to the RAM chips if someone has played with these without knowing what they are doing that can cook things as some 32's had 16 1x16K 4116 chips which use +12V, +5V and -5V. However some used 8 (half good) 1x64K chips (like the 32K bank on a spectrum), which only use +5V, obviously if you feed +12V and -5V to these bad things can happen and the magic smoke can escape......

Ok that's more info than you needed but never mind

Cheers.

Phill.

You will all be pleased to know that I have managed to obtain some 1st generation copies of the Dragon 32 schematics (for all types) and will be doing some hi-res scans of these shortly.

admin wrote:You will all be pleased to know that I have managed to obtain some 1st generation copies of the Dragon 32 schematics (for all types) and will be doing some hi-res scans of these shortly.

Ohhh Excelent, those wouldn't be the A1/A0 size ones would they ? If so how are you going to go about scanning them ? As I also have some original Husky computer schematics that I'd like to get scanned.

.....Now all we need are the Alpha and the Beta schematics......

Cheers.

Phill.

prime wrote:

RWAP wrote:The power regulator is REG 3 - a 7905CT if anyone has any spare - I only have 7805 regulators

I can't read the C11 capacitor

I believe you should be able to get the 7905 from the local Maplins (if you have one near you).

One thing I did think of, is does the main board have a row of jumpers at the back near the power connector ? As these route the power lines to the RAM chips if someone has played with these without knowing what they are doing that can cook things as some 32's had 16 1x16K 4116 chips which use +12V, +5V and -5V. However some used 8 (half good) 1x64K chips (like the 32K bank on a spectrum), which only use +5V, obviously if you feed +12V and -5V to these bad things can happen and the magic smoke can escape......

Phill.

Many thanks Phill - there is a row of jumpers - they are all in the same position (closed from memory as the Dragon is not in front of me at the moment) -

I really think it may just be the one capacitor on the main board which has failed (C11), as there must be power, to make it smoke!

I just need to find out what the capacitor is, so I can get a replacement!

RWAP wrote: Many thanks Phill - there is a row of jumpers - they are all in the same position (closed from memory as the Dragon is not in front of me at the moment) -

I really think it may just be the one capacitor on the main board which has failed (C11), as there must be power, to make it smoke!

I just need to find out what the capacitor is, so I can get a replacement!

K looking at a similar board that I have it looks like C11 (next to the power connector), is a 4.7uf 16V tant.

It looks like a decoupling cap so the machine may work with it removed, well well enough to test it further anyways

Cheers.

Phill.

I checked on issue 2 and 5 boards and they are both rated at 4.7uF and 16V.
I also checked a copy of Dragon data drawing SA2120 sheet 1 of 3 and this also shows a 4.7u capacitor for C11.
Hope this helps, Gareth