View Single Post
Old 19th May 2017, 2:44 pm   #1
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Interesting scope repair: Philips PM3315 early DSO

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!
Attached Thumbnails
Click image for larger version

Name:	scope.jpg
Views:	491
Size:	71.1 KB
ID:	142800   Click image for larger version

Name:	scope2.jpg
Views:	360
Size:	76.7 KB
ID:	142801   Click image for larger version

Name:	scope3.jpg
Views:	255
Size:	66.0 KB
ID:	142802   Click image for larger version

Name:	scope5.jpg
Views:	246
Size:	80.3 KB
ID:	142803  
MrBungle is offline