Dragon 32 Repair

[sixxie]

After variables if followed by an operator that could be mistaken for part of the variable name, e.g. AAND1 needs to be A AND1

[martinjharvey]

I've now socketed all the remaining 4116 ram IC's and in doing that and testing them I did manage to turn up 1 possibly 2 bad ones! Now when I've re-inserted known good chips ( tested them in the lower 16k of a spare speccy 48k ) I'm getting a black and white display, with the same garbage appearing some times and then nothing others. I've tested the voltages and where I was getting the 12v,5v,-5v and 0v, the other voltages are still there but I'm now only reading -2.1v where there should be -5v. Testing the voltages at the pins of the power connector turns up the same readings.

I've thoroughly checked and double checked my work for continuity, broken tracks, solder splash etc. around the newly fitted sockets but as far as I can see and have tested with the multimeter it's all good.

Guessing there's an issue with the power board but has anyone an idea to what to look for now in terms of the missing -5v? Bad voltage regulator perhaps?

Cheers

Mart

[sorchard]

I would suggest disconnecting the power board from the main board and check if the -5V is OK unloaded. Then see if it can maintain -5V into a 100 Ohm resistor. Probably fine if it can. Resist the temptation to fix the -5V reg down to the heatsink unless it has an insulating kit as the tab is not at ground potential and things won't be happy.

Also worth unplugging all the DRAMs to see if the -5V comes back.

[martinjharvey]

I would suggest disconnecting the power board from the main board and check if the -5V is OK unloaded. Then see if it can maintain -5V into a 100 Ohm resistor.

The -5v is there when the power board is disconnected and is still good through a 100 Ohm resistor.

Resist the temptation to fix the -5V reg down to the heatsink unless it has an insulating kit as the tab is not at ground potential and things won't be happy.

I've not messed with any of that One regulator is attached to the heatsink and the other is unattached. I'd assumed that's how they were supposed to be, so left them alone.

Also worth unplugging all the DRAMs to see if the -5V comes back.

With all the 4116's back out of the sockets, the 5v,12v and 0v are still there but the -5v at both the IC Sockets and the power connector has dropped back to -2.1v again.

I suppose it rules out the power board being faulty, but where does that leave me?

Cheers

Mart

[prime]

With all the 4116's back out of the sockets, the 5v,12v and 0v are still there but the -5v at both the IC Sockets and the power connector has dropped back to -2.1v again.

I suppose it rules out the power board being faulty, but where does that leave me?

The other place that the -5V goes on a Dragon is to one end of one of the variable resistors, it is possible that this is dragging it up to -2.1V. So it might be worth taking this out and measuring the voltage with it disconnected. If you get your -5V back I would replace it.

You will of course also want to check very carefully for solder splashes and shorts, make sure you check the top of the board too (as far as you can between your sockets).

Cheers.

Phill.

[tormod]

My Dragon 32 that I bought broken on eBay showed this screen pattern. I thought it was the 6847 but it turned out to be a poor -5V supply. The culprit was the capacitor C5 on the power board, the one between the R5 resistor which gets pretty warm and the row of power diodes. Make sure you get the polarity right if you replace this capacitor!

[martinjharvey]

The culprit was the capacitor C5 on the power board, the one between the R5 resistor which gets pretty warm and the row of power diodes. Make sure you get the polarity right if you replace this capacitor!

I'm willing to give any suggestion a try so I've ordered some replacements for C5. The screen shot looks slightly different to what I'm getting but as I'm clutching at straws it cant hurt to swap it out. Ordered a replacement 7905 regulator to swap as well.

The other place that the -5V goes on a Dragon is to one end of one of the variable resistors, it is possible that this is dragging it up to -2.1V. So it might be worth taking this out and measuring the voltage with it disconnected. If you get your -5V back I would replace it.

Are we talking about the 10k variable resistor next to TR4 on the MKII Issue 6? Think its labelled RV1, just below the power ribbon cable.

Don't want to chuck silly money at replacing the 6847 or the SAM unless It looks like theres no other option. Start with the cheapest probable fixes first I reckon

[sorchard]

The MKII Dragon schematic is slightly different to the existing ISS2 schematics.

viewtopic.php?f=5&t=5308

There are a few components on the -5V line, that might be pulling it up, mainly in the video circuit, for example: C10, IC9, IC10, TR4

These would be noticeably hot if there was a problem.

[martinjharvey]

Ive swapped out the 7905 reg this evening and as expected it made no difference to the -5v. But at least I can rule that out

As pretty much most of the main chips on my motherboard are now socketed I removed them all and then tested the voltage at the RAM sockets and surprise, surprise, the -5v is back!

So now Ive gone through inserting one chip at a time back into its socket and reading the voltages at the RAM sockets. Everything is groovy until I stick the 74LS783 back in. The the -5v comes up to -2.1v as before.

Does that indicate that this is maybe where the fault lies?

If so where am I likely to get a replacement at a reasonable price?

[Rolo]

Hi Mart,

I bought two last November on ebay from a guy called "hifiic ".
I payed EUR 14.60 plus 3.19 EUR for shipping.

"1PCS SN74LS783N SYNCHRONOUS ADDRESS MULTIPLEXER"
( 221204846169 )
Anzahl: 2

Last year, there were plenty on ebay. I just checked. They are all gone This is not a good sign. I fear somebody bought all the stock and we will see those circuits reappear on the internet finally, for a fantasy price...
I hope not!

Maybe you can contact hifiic in China and ask, if he has some SAMs left.