Testing for shorts: How to??

[coondog]

I posted earlier about the lamp problem I was having with my 2252B. I
thought the lamps were blown after replacing them but they weren't. I
still have a problem and need help. My stereo works great except
the dial and meter lamps will not light. The dial needle lights up,
the am/fm/phono/aux/tape red lights all work however the names in bright white don't light up. The bright white station lights don't work either (the little white dots). I have two fuses on the board to the right as you are looking down from the front. With the dial lamps in the lamp holders, the 2amp/250volt back fuse blows when I turn the unit on. I take the 5- 8v/200ma and the 2- 8v/250ma meter lamps out and put a new
2amp/250volt fuse in, turn the unit on and the fuse does not blow.
I checked the lamp holders for ac voltage and all seven checked 8.18 volts. By the way I buy all my lamps from Vintage Electronics and have never had a problem with them with the other 5 Marantz stereos I've worked on.
Someone suggested there is an actual short from
one of the lamp sockets to some other circuit or the frame that is
causing the fuse to blow, so in fact the fuse is doing its job
perfectly, and you need to figure out where the short is. He also suggested: Try looking for a short first with your meter, with fuse lamps in but power OFF. I checked the continuity on each lamp holder with the lamps out and I got a beep on all of them. Is there suppose to be continuity there without a lamp being in the lamp holder? I also did the same thing to the 250v fuse holder, checked continuity with the fuse out and got a beep. How exactly does one check for shorts?? I'm pulling my hair out, what little I have left...James

 

[EchoWars]

Continuity test won't work with the fuse in...the DC resistance of the 8VAC transformer winding is very low.

Measure the resistance of each fuse lamp. Cold DC resistance should be about 2 or 3 ohms. I've actually bought fuses that were shorted.

You can also install the lamps one at a time...:scratch2:

 

[Musicalme]

You will probably find your testing back through the rectifier and through the transformer which is causing your meter to "beep"
If you take all the lamps out and put the fuses back in and they dont blow and you have voltage at the lamp holders then you dont have a short so forget that.

Put the fuse in with the lamps out and one by one put them back in and try it. When it blows then look closely at that last bulb and holder to see if anything looks strange. If they are the bulbs that look like fuses then maybe that one lamp is actually a fuse and sold to you by mistake that would definitely get you a short. Also could be the bulb is faulty from the manufacturer, not likely but not impossible. Try a different one in its place and see if you still get a blown fuse.
This method will probably cost you in fuses but its the simplest way.
Another way would be to disconnect the output of the power supply that feeds those bulbs and measure across the those wires with your meter and with the bulbs out. You should get nothing. Then put the bulbs back in one by one and measure with your meter on the 200 Ohm scale (or whatever the lowest scale is on your meter) you should have about 2 Ohms roughly. By any means you should measure something other than a dead short or an open circuit. Also you could test across each bulb you should get 2 to 3 Ohms if you get all zeros (assuming its digital) then its shorted.

Hope this hasnt confused you.

 

[Musicalme]

Doh!
beaten by the KING!

 

[coondog]

Thanks for your help. I'll do what you suggested and get back to you with the results later today...James

 

[coondog]

I did a process of elemination and got everything working. I took all the fuses out. I checked the 200ma for shorts and did an ohm check on them and they all checked 4.5 ohms. I had a set of 250ma and they checked out at 3.0 ohms. I decided to use the 250ma's because you told me the ohms should read between 2 and 3 ohms. ??? I started with all the lamps out and installed the first lamp to my right and turned it on and it blew. I put the next lamp in and it lit up, 3rd in and lit and so on thru #7. I went back to #1 and moved all the wires around making sure nothing was touching and put a new lamp in and it lit. All the lights on the dial work as they should except for the stereo on the left and dolby on the right. Hallaluya!!! Thanks for all your help...Jam

 

[coondog]

I'm a little concerned about using the 200ma lamps that have the ohm reading of 4.5. I have a full set of seven of those. Do you think they would give me a problem?? James

 

[EchoWars]

The cold DC reading don't mean much, other than to tell you that the lamp isn't shorted or open. 200mA lamps are fine, as long as they're bright enough for you.

 

[Musicalme]

Yeah 2, 3, 4, so long as it wasn't 0.
Glad you sorted it out now you can get back to listening to it rather than fixing it.:yes: