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Master 128 (No1) - Short Circuit

Posted: Sat Feb 01, 2020 7:51 pm
by KenLowe
Hi folks,

I'm trying to get an old Master 128 up and running. The battery pack is needing replaced, but that's the least of my problems right now...

Unfortunately the bigger issue is that, even with all external devices (PSU, keyboard, battery pack & speaker) disconnected and removable ICs (IC15 & IC24) removed, I have a direct short between PL19 (+5VB) and PL20 (GND). I've had a look through the circuit diagram and see that this +5VB rail feeds a lot of the components.

As an initial check I removed capacitors C63, C69 (thinking it was C67; which doesn't actually exist) & C72 but still have a complete short.

Before randomly removing other components, are there any specific components powered by this rail that are prone to failing to a short circuit that I should focus on?

Re: Master 128 - Short Circuit

Posted: Sat Feb 01, 2020 8:53 pm
by johnkenyon
What do the ceramic decoupling capacitors look like?
If they are coated in a dipped blue covering, that's where I would look first.
On another product with the same issue I would power up the board, monitor the 5V supply and then cut each decoupling capacitor in half with the power on until the short disappears. Replace each one with a 100nF polyester.

John

Re: Master 128 - Short Circuit

Posted: Sat Feb 01, 2020 9:23 pm
by KenLowe
Most of the decoupling capacitors are brown ceramic disc. I can see one rectangular capacitor that is blue in colour. Good plan about cutting out the capacitors and testing. I'll need to order up a replacement batch first, though.

Any other thoughts from anyone?

Re: Master 128 - Short Circuit

Posted: Sat Feb 01, 2020 10:12 pm
by cmorley
I'd hook up power from a bench supply and see if I can work out what is getting hot to locate the short.

Re: Master 128 - Short Circuit

Posted: Sat Feb 01, 2020 10:15 pm
by KenLowe
cmorley wrote:
Sat Feb 01, 2020 10:12 pm
I'd hook up power from a bench supply and see if I can work out what is getting hot to locate the short.
The short is enough to shut down the Master PSU. Would a bench supply work differently? In any case, I don't actually have a bench supply, but I have been considering getting one, and this might be the justification for it.

Re: Master 128 - Short Circuit

Posted: Sat Feb 01, 2020 10:34 pm
by cmorley
Yes. With a current limit on a bench PSU you can control the power going in to the short and heat it up while minimising the risk of doing damage (e.g. burn traces). You only need a Watt or two to get something warm...

Re: Master 128 - Short Circuit

Posted: Sat Feb 01, 2020 10:56 pm
by 1024MAK
Better yet, with a bench supply in current limit mode, if you have a sensitive multimeter with a good 200mV range (especially if you have a 20mV range or a 3¾ digit meter), you should be able to use the resistance of the PCB tracks to find out where the voltage drop levels off. Do this from both the positive supply and the 0V/GND side. The short will then be somewhere close to where the voltage levelled off.

You do need to take lots of readings (and maybe record them) to be certain that you are actually narrowing down the fault.

So at normal room temperature, all conductors have at least some resistance. Start with one meter probe connected to the positive incoming supply point (from the bench PSU for example). Now use the other probe to take readings. Start not too far from the incoming supply point and follow the tracks around the board. If the voltage is the same as an earlier result, and continues to be the same as you move away from the supply point, that track is not going to the short circuit. But if the voltage recorded by the meter is going up, and keeps going up as you move away from the incoming supply point, that’s the track going to the short circuit.

Then do the same, but working away from the negative incoming supply point.

Mark

Re: Master 128 (No1) - Short Circuit

Posted: Wed Apr 15, 2020 8:06 pm
by KenLowe
Getting back to this today, and I've made a bit of progress. I noticed that one of the ceramic capacitors had a very small chip in it, and when I looked closer, I could see that the disc had a hairline crack. Pulled this capacitor, and measured a direct short. This capacitor sits underneath the plastic speaker / cartridge surround, and it looks like the surround has been sat on top of the capacitor, which had been bent over and was lying flat on the PCB.
20200415_201352.jpg
Broken Capacitor - Hairline looks worse now that I've bent it slightly.
Anyway, powered the board back up again without this capacitor, and LED on the keyboard lit, and the relay clicked. No sound at all, though. Connected it up to a monitor, and no video output at all. Then there was a big crack, and magic smoke appeared from the PSU. PSU is still working fine, so I'll come back to that later and replace the caps.

I've replaced the keyboard with a known working one, and measured voltages (both +5v and -5v) at various locations, and everything seems to be in order. Pressing R on power up doesn't make any difference. I'll get the scope out to see what's happening around the 6512, but wondered if anyone has any initial pointers. Should I be hearing the uninitialised beeeeeeee sound, similar to the beeb? Or is the Master different?

Edit: Right, I'm getting 16Mhz clock at IC43, pin 2, but I'm not getting anything at the VC2069 (IC 42), pin 8. Investigations continue...
Edit2: Jumpering between the 2, and it's sprung into life! Now just need to track down where the track has broken. I'm suspecting a bad solder joint at the VC2069.
Edit3: Link 60 missing! Now installed, and we're all up and running! Pity the keyboard's in a pretty bad way. Actually, I think I put this bad keyboard into this faulty Master No1 when I was trying to get Master No2 up and running. I'll swap them back, since Master No1 (now that it's running) is in better condition. I'll also repair the bad keyboard, but it's likely going to need a lot of patch wires.

Re: Master 128 (No1) - Short Circuit

Posted: Thu Apr 16, 2020 6:11 am
by Kazzie
Well spotted: good to see you're making progress :D

Re: Master 128 (No1) - Short Circuit

Posted: Thu Apr 16, 2020 11:41 pm
by KenLowe
Kazzie wrote:
Thu Apr 16, 2020 6:11 am
Well spotted: good to see you're making progress :D
Thanks. All sorted now, I hope.

Having sorted the motherboard short and 16MHz clock issue, I spent the remainder of yesterday evening trying to fix the keyboard, as there were a lot of damaged tracks. Most of them seem to have failed where the track meets the thru hole via. I was originally just trying a simple solder bridge between the track and via, but that wasn't very reliable. As soon as I started punching the keys, the bridge would break. In the end, I ran a number of patch wires between key pins. It isn't the neatest, but it works. I also had to clean a few of the pins on the key switches to get reliable key operations.

I also started to have issues with the PSU late last night. Earlier in the evening there was a loud crack, and puff of smoke as one of the caps let go. I was still getting good voltage at the motherboard, so I persevered with it. However, later in the evening I began to intermittently lose power to the motherboard, with the power led and cassette relay starting to chatter. Whilst trying to measure voltages on the motherboard, a second puff of smoke came out of the PSU, so I switched it off and left it. Today I replaced C1, C2 & C9 in the PSU, and it seems to have been running reliably on soak test since then.

Hopefully I'll have no further issues!

Re: Master 128 (No1) - Short Circuit

Posted: Fri Apr 17, 2020 12:55 am
by Wheel_nut
KenLowe wrote:
Thu Apr 16, 2020 11:41 pm

Having sorted the motherboard short and 16MHz clock issue, I spent the remainder of yesterday evening trying to fix the keyboard, as there were a lot of damaged tracks. Most of them seem to have failed where the track meets the thru hole via. I was originally just trying a simple solder bridge between the track and via, but that wasn't very reliable. As soon as I started punching the keys, the bridge would break. In the end, I ran a number of patch wires between key pins. It isn't the neatest, but it works. I also had to clean a few of the pins on the key switches to get reliable key operations.
I don't know the Master but the Beeb keyboard is a single sided PCB without plated through holes. If the Keyswitches are not absolutely tight flat to the board when soldered, the keyswitch pins punch through when hammered and detach the pad from the board, cracking the copper track. When repairing the tracks, I use a smalll loop of (prefeably stranded) wire to allow for some movement even after ensuring that the Keyswitch is firmly bottomed on the PCB.

I hadn't picked up on your problem with the short on the board earlier. I have a low impedance squealer tester which emits a increasing pitch squeal as the impedance between the probes reduces. I have used it to detect failed Tantalum Beads on boards which used to be a common failure. I haven't used it for over a decade but I (think I) know where it is! 8)

Robin

Re: Master 128 (No1) - Short Circuit

Posted: Fri Apr 17, 2020 1:07 am
by KenLowe
Wheel_nut wrote:
Fri Apr 17, 2020 12:55 am
I don't know the Master but the Beeb keyboard is a single sided PCB without plated through holes. If the Keyswitches are not absolutely tight flat to the board when soldered, the keyswitch pins punch through when hammered and detach the pad from the board, cracking the copper track. When repairing the tracks, I use a smalll loop of (prefeably stranded) wire to allow for some movement even after ensuring that the Keyswitch is firmly bottomed on the PCB.

I hadn't picked up on your problem with the short on the board earlier. I have a low impedance squealer tester which emits a increasing pitch squeal as the impedance between the probes reduces. I have used it to detect failed Tantalum Beads on boards which used to be a common failure. I haven't used it for over a decade but I (think I) know where it is! 8)

Robin
You're absolutely right. Thinking about it, they're not plated thru holes; they're just pads. Most of the cracks between the pads and tracks were not caused by removal / reinstatement of the switches. There seems to have been some form of corrosion at these junctions. It almost looks like battery acid corrosion, but it's obviously not that. Perhaps just moisture or dissimilar material?

Hopefully I won't have any need for a low impedance tester now. But I'll know who to ask if the issue ever crops up again :lol: :lol:

Re: Master 128 (No1) - Short Circuit

Posted: Sun May 03, 2020 12:17 pm
by KenLowe
KenLowe wrote:
Thu Apr 16, 2020 11:41 pm
I also started to have issues with the PSU late last night. Earlier in the evening there was a loud crack, and puff of smoke as one of the caps let go. I was still getting good voltage at the motherboard, so I persevered with it. However, later in the evening I began to intermittently lose power to the motherboard, with the power led and cassette relay starting to chatter. Whilst trying to measure voltages on the motherboard, a second puff of smoke came out of the PSU, so I switched it off and left it. Today I replaced C1, C2 & C9 in the PSU, and it seems to have been running reliably on soak test since then.

Hopefully I'll have no further issues!
Spoke too soon on this one. Unloaded, the PSU is providing a steady 5v. However, as soon as I stick any load on it (eg Gotek via the auxilliary connector) the voltage drops significantly and hunts about a fair bit (but always below the 5v). I'm struggling to find any circuit diagrams for this PSU, so was wondering if there's anyone out there with experience on these PSUs that might be able to point me in the right direction (other than to replace with a Meanwell, which I may yet do!).

Edit: Right, pretty sure I've sorted it this time. When powering up the PSU on the bench, I could hear arcing on initial power up. Looking closer I could see sparking at the solder connection for one of the large high voltage DC capacitors. Looks like a dry joint, where the leg of the capacitor wasn't making a good connection to the PCB. Resoldered this connection, and all the other connections in that area, and I've got a steady +5v again.

As per previous post, hopefully I'll have no further issues!