Philips PM 3215 oscilloscope seems to have a dead channel

[tedybear315]

Hello,

First time poster here.  Hope someone can offer some basic help for a new oscilloscope user.  Picked this scope up dirt cheap at a local flea market.  Which makes sense, as it is covered in a large amount of dust and dirt.  I think it came out of a barn.

Right out of the box?  It blows the RIFA capacitor to about 19 chunks of stink.  I've got a different replacement of the same value(s), but it's a polyester capacitor.  Seems the factory ones are known for going up and out in a blaze of glory.

Anyhow, I start playing with it again, and note it's dead with that cap out of circuit.  (schematic shows it should still fire up with it out of circuit)  I traced it back to a blown out thermal fuse on the transformer.  Have a fast blow fuse rigged into it for the moment--and I can not locate any sign of a replacement thermal fuse for that transformer at all.   Google left me high and dry on that one.

Fast forward to the reason I stopped in.  Channel 'b' seems to have full function.  Position goes up/down and all that good stuff, and if I change the time on the knob--I get a bit of static on the screen which seems to be normal.

Channel 'a'?  Position does not have any effect on the trace.  Turning the function knob while it should make a bit of static?  Totally nothing.  Just a line at dead 'zero' (center).

While I used to be pretty trained in electronics, I'm about as brain dead as it gets with oscilloscopes.  I'm fair with a soldering station and multi meter--and actually repair flat panel TV's on the side--but that's more of a research what usually fails..and replace parts.  I'm trying to get back into the diagnostic work again.

I have the test leads coming in next week, and I'm confident channel 'b' should be okay.  So I'll at least be able to use that channel for the intended purpose.  But I would like to get channel 'a' functional.  Has anyone had experience working on this type of issue?  I can of course supply photos of any parts and info as requested.  There is a service manual online as well that I've already downloaded.  It's pretty much Greek to me trying to understand it.  (25 years ago when I did electronics back in school?  No problem.  That part of my brain is covered in more dust then this oscilloscope.)

Many Thanks!!  I'm pulling double duty at work so my response time might not be the greatest.

S-

[oldway]

Hello,

Welcome to the forum eevblog.

First, it is good practice to provide the link where you download the service manual to make life easier for those who want to help.
For me, no problem, I already had this Service Manual.

Given your knowledge of electronics, it seems to me that you have only two options:

- Use the oscilloscope as it is, with only the channel B working.
We can do many things with only a single channel.

- Search near you for help from someone who has more knowledge in electronics than yours

However, I can tell you that this is probably the V501 BAV45 diode which is shorted or FET BFS21A V504 which is defective.

This FET is very sensitive to ESD and I recommend you do not try to change it if you are not aware of ESD precautions.

[samnmax]

I also had a faulty channel A in my PM3218 oscilloscope. I posted about the repair here: https://www.eevblog.com/forum/repair/philips-pm3218-repairs/.

[oldway]

Open V508 BF450 transistor is not a common failure and may give a wrong direction to the diagnosis.
In most cases, the failure was caused by a too high voltage at the input of channel A, which causes the failure of the diode and the FET.
One of the causes could also be a defective SL3145C (D501) .

[tedybear315]

Oddly enough I am decent with ESD precautions.  My job for the last 21 years has been managing a video arcade and I'm also the store tech.  Most of the time it's figuring out which modular part failed and then we call in for "AV" replacements.  (Advance Replacements.  Swap and send back the defective modular part where the bench techs at our home office get the fun of figuring out the part on the modular part..and then replacing it.  And then back on the shelf it goes until the next location has that part fail.)


I'll poke around a bit and see what's available for parts--I'm chomping at the bit to get the leads in so I can start working with this.

I figure even with one channel going it should be fine as a starter scope.  For the price-  I'm not into it for more then about $26 dollars with the capacitor for the power supply transformer. 

Someone has this one for sale and while I love old school test eq?  For a doorstop it's a bit to pricey:

http://syracuse.craigslist.org/ele/5156975389.html

Thanks for the help and replies!!

S-