Interesting scope repair: Philips PM3315 early DSO

[MrBungle]

Well I found it interesting anyway

I obtained a non-working PM3315 from another forum member (thank you) as a project. It's a rather interesting piece of kit. Latest date code I can find in it is 1985 and the service manual indicates 1983 as a possible start date for this line so this is a very early Philips DSO and it's actually quite a cool piece of equipment for the age.

Fundamentally it's pretty low spec by today's standards i.e. 125Msps, two channels, no cursors or anything fancy like that but it has some very useful features: firstly a trigger that is measured in div and you can set trigger events into the very far future. Also it has 4 capture memories and each of those can display both captured channels so you actually have a nice 8 channel scope if you fancy a bit of arguing with captures and the user interface and swapping probes around. Great for digital debugging! It also has a sweep rate that goes down to one hour per division so it has lots of possibilities for watching PLLs lock and nice things like that you can't see on a normal analogue unit. Rise time is around 6ns which is pretty handy and equates to around 60MHz analogue bandwidth.

But alas it was broken. Powering up, greeted by the dim LED of doom and it shut itself down after 5 seconds.

After finding a copy of the service manual (marked PM3311 if anyone else comes across this thread) I had a look at the power supply schematics and description and it was evident that if it couldn't start and get decent feedback from the reference rail, it would shut itself down. I measured the rail voltages with a DMM but none came above about 400mV. Hypothesis: short somewhere. Time to dismantle. This is nicely designed with 4 screws on each side. It's a VERY complicated piece of equipment. There are two sides completely stuffed full of parts.

I proceeded to disconnect boards one at a time from the power supply and power up the unit until it came up. Turned out the main analogue board was the culprit which is the largest board in the unit annoyingly.

I then proceeded to pop a DVM across each large electrolytic on the board and measure resistance. If this creeps up, the rail should be ok i.e. no shorted caps on it. Within 5 minutes I found that the -12.6v rail was shorted. Sticking the DMM on the lowest resistance range and measuring across all capacitors on the rail referring to the service manual and looking for the lowest resistance pointed to one of the smoothing electrolytics. I snipped this out as far away from the board as possible so we have some legs to solder a new part to without having to extract or damage the board, reconnected the board to the PSU and powered it up. Bingo it worked.

Left it to settle for 10 minutes and sat down to eat a curry. This was interrupted half way through by fizzing and a large bang. The mains filter had blown out. Spent half an hour cleaning this out and bypassing it for now.

Spares ordered from RS arrived today. Fitted new electrolytic, new mains filter. The CRT was rather blurry looking so I tweaked the focus, astigmatism and intensity trimmers and it's nice and sharp now.

There's a RIFA X2 hiding in the AC side switching power supply so this will be replaced early next week when the parts arrive.

For reference, this has a relatively sluggish 5MHz 8085 powering it. To get the capture bandwidth required it pours the digital data into a CCD after the ADC as it was the only tech fast enough to devour 125Msps at the time. Impressive design!

I couldn't see a modern DSO lasting this long or even being repairable!

Some photos - it's a bit of a beast!

I love the LED sub displays and of course the proper CRT. Is it digital at all frequencies or can you switch to analogue (or be forced to) for high frequency work?

[MrBungle]

Yeah they are rather cool. DIS1416 displays I think.

It's actually always in digital mode which was a surprise. I was expecting a wall there. Very impressive bit of equipment considering the vintage. A modern DSO runs rings around it but it's actually very usable.

Edit: it has a dots mode so you can see the samples being taken. This is 20ns/div running off a crystal colpitts oscillator.

[wd40addict]

I used a similar Philips scope at my first job, but it could be either analogue or digital!

Brilliant: in analogue mode but, can't quite see some slow moving thing or a one-off event, then switch to digital. In digital mode, but can't work out what's real and what's an alias, switch to analogue!

It also taught me how much easier a pen plotter could be instead of a Polaroid camera for screen shots!

[MrBungle]

The aliasing is a pain I will say. Sometimes you wonder why it won't trigger and realise you're 5 timebase settings above the actual frequency and watching one of the aliases zoom past and it's triggering quite happily! This has a plotter output and a plot button. I would buy a plotter if I saw one for it just to play with it

I mainly use a PM3217 analogue but this is handy for slow scan/capture and digital stuff. For example I can trace 8 channels of digital if I have a reliable trigger point and some patience. Great for timing diagrams!

The killer for this is I was a complete idiot at using it until I had actually read the manual. I usually use that as a last resort but it is required to even understand the thing!

[kellys_eye]

If you ever need spares I have a box full of old plug-in boards that may come in useful.

[MrBungle]

Fantastic. I'll let you know if it dies in the future

[MrBungle]

Power supply has been properly tested and the X2 capacitor replaced now. Quick photo for reference:



Small bit of jitter in the timebase. Will look at that another day.

I really like how this is constructed. Unlike Tek they designed it so you could get to all the bits!