One of those repair days.  - Page 1
Forum section: electronics / repair

author: Vtile
author_rank: Super Contributor
timestamp: August 30, 2017, 08:08:37 pm
content: I have Fluke PM3390A scope, the problem is the typical illumination incandescent light bulb is burned out (I suspect). I finally found the service manual from my storage HDD, read that 450 page behemoth like an 2½ hours to find out that the type of the bulb is specified with one line in somewhere between pages 100 and 400 as 28 volts type... great... Now I need to open the scope to see which type of socket it is using.I found also the opening proceduce. I realised that the unit should have a hard front cover which is supposed to be in place while pulling the case out. Oh, well I opened the screws tried to pull and jerk the sheetmetal case with different styles without that "face on the floor method" described on the manual. After 15 minutes without success I did put the srews back in and the scope back to its place. I do not need a stinky reticle.PS.If you need to went, be go for it. Let see your bad repair day.

author: Armadillo
author_rank: Super Contributor
timestamp: August 30, 2017, 08:38:30 pm
content: PM3082 would look like this;

author: Vtile
author_rank: Super Contributor
timestamp: August 30, 2017, 08:51:02 pm
content: Hmm.. MF6S socket? Now I only need to figure out how to get the case open without damaging the frontpanel knobs etc. or other components without the frontcover.Some day.

author: HighVoltage
author_rank: Super Contributor
timestamp: August 30, 2017, 08:58:29 pm
content: 1. Remove the plastic feet from the back2. Remove all screws from the back3. Bring handle in an angle position so it is pulled out a little4. Slide off the metal caseIt is very easy to do.If you need a replacement bulb, I might still have a original new one.

author: Vtile
author_rank: Super Contributor
timestamp: August 30, 2017, 09:03:46 pm
content: Quote from: HighVoltage on August 30, 2017, 08:58:29 pmThank you for your kind offer, but in local shed they seem to stock whole line of different signal lamps with minimal cost.What comes to the opening the unit. I did just like you did describe, except the stage 4 were only in a level of a mental excercise. Physical execution did end up to internal cursing.

author: texaspyro
author_rank: Super Contributor
timestamp: August 31, 2017, 02:52:01 am
content: I always replace lamps with an LED/resistor... no more dead lamps... ever.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 31, 2017, 04:17:57 am
content: Quote from: texaspyro on August 31, 2017, 02:52:01 amMe too. I have several old bits of kit where I simply took out the glass part of the bulbs and installed the resistor+LED into the old bulb base. Including a few of those exact 28 volt Philips bulbs.

author: Bushougoma
author_rank: Regular Contributor
timestamp: August 31, 2017, 11:15:01 am
content: Not to mention you can get 5mm and 3mm LEDs in warm white (3000K) to match the original incandescent color I just bought a bunch on E-bay to put in some old Tek gear.LEDs are long lived and shock resistant can't say that about an incandescent lamp.

author: Vtile
author_rank: Super Contributor
timestamp: August 31, 2017, 12:09:01 pm
content: Quote from: Bushougoma on August 31, 2017, 11:15:01 amLeds do have one problem in some circuits, they are rectifiers and highly-nonlinear. I'll became to solution that I'll throw in indascent bulb and forget it. I'm not interested to go and measure all the sourcing and controlling circuitry if the non-linear properties of led could somehow upset the system in some cases, especially as the illumination power is drawn from the EHT transformer secondary and new indascent bulb cost like €1.5 and serves propably next 20 years on that scope.Also indascent bulbs do have many favourable fail case properties, like ability to withstand short term extreme over-voltages etc.

author: Bushougoma
author_rank: Regular Contributor
timestamp: September 01, 2017, 04:55:59 am
content: I was referring to forward facing indicators graticule illumination which usually uses some type of diffuser can be hit or miss unfortunately.As a side note if the lamp supply is not regulated and I'm concerned about over voltage I just tack on an LM317 or similar adjustable linear regulator in a TO92 package to act as a constant current source.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: September 01, 2017, 07:28:58 am
content: You might be able to get these or similar bulbs from truck or RV parts suppliers. Commercial vehicle bulbs are actually run on 27.5v, not 24v.

author: Vtile
author_rank: Super Contributor
timestamp: September 01, 2017, 10:00:47 am
content: Quote from: Bushougoma on September 01, 2017, 04:55:59 amI weren't trying to argue your method and success and overall thing which makes LEDs good, but just kind of pointing out that in some cases the LED might not be the ideal "drop in" replacement.Like this PM3390A case the control circuitry is not just potentiometer and constant voltage source, but some form of transistorised CPU output (PWM propably) with positive and negative source voltages from flyback transformer windings (parallel for EHT windings), so it propably would just work as led and series resistor.. But I don't want to risk my scope as it is my best and only traditional unit and I'm only interested on "drop-in" solutions in this.Quote from: IanMacdonald on September 01, 2017, 07:28:58 amThanks, that is actually a good to know information as the bulbs are becoming niche.

author: Vtile
author_rank: Super Contributor
timestamp: September 01, 2017, 05:35:32 pm
content: Ok. Got the case open and quillotined one electrolytic capacitor that were on the work table, collateral damage. Lack of faith it were, but the amount of force needed were surprisingly big.The bulb were indeed with MF6S socket, in the attached picture one can see the markings of the original bulb (written top of the zip-lock bag). I can report that 40mA rated illuminates enough.

author: Armadillo
author_rank: Super Contributor
timestamp: September 01, 2017, 06:15:44 pm
content: Quote from: Vtile on September 01, 2017, 05:35:32 pmShould have oiled and clean that fan, its on the right side.Looks like there will be another day of Schwarzenegger.

author: texaspyro
author_rank: Super Contributor
timestamp: September 02, 2017, 04:21:16 am
content: Also, running lamps on DC causes them to die much more quickly than on AC (look up "filament notching"). And AC on an LED is generally a bad thing. If the lamp was being driven by AC, it's best to include a series diode with the resistor (or in some cases back-to-back LEDs).A little sandpaper on a clear LED makes for a good diffuser. I've also used thin paper... ask your stoner friends for some rolling papers.I've put LEDs in over 100 types of equipment and have never noticed any (negative) issues. I've seen some (very) minor improvements, probably due to lower heat and current. In those cases the lamps were close to some sensitive circuitry.