2252B - No Dial Lights

[Flip69]

I recently replaced the dial lights on a 2252B with LEDs. The LED kit was advertised as suitable for the Marantz 22 series, with appropriate resistors built into the bulbs. The lights worked fine for a week or two. This morning, I turned on the unit. The lights lit for about a second, then went out. I replaced the fuses on the power board (one looked blown, I could not really tell with the other). That did not fix the problem. I found a thread posted by someone who had pretty much the same problem with a 2230B. The replies included suggestions on where to test on the power amp board, and the dial light board to troubleshoot the problem. I compared the 2230B schematic to the 2252B schematic to figure out where to run the same tests on the 2252B. I am not confident that I have the test points figured out for the 2252B. I don't want to start poking around unless/until I am sure where to put my probes. Can someone please help me figure out how to troubleshoot this problem? Thanks!

 

[rBuckner]

I will assume that the unit otherwise still works fine. The power source, a winding from the power transformer, connects to the power amp/supply board at terminal J712, passes through fuse F702, and goes out J711. The other end of the source winding is grounded. So with the meter set to measure AC voltage and on a range that can measure 8+ volts (or autorange), place one probe on ground which is any convenient chassis point. The other one is used to test for the 8 volts at the board's J712 (before fuse), the J711 (after fuse).

 

[Flip69]

Thank you for this info. The VM showed 7.85v at J712. However, J711 showed only 3.35v. When I tested the fuse, I got 7.85v on one end and 3.35v at the other, i.e., I was losing voltage through the fuse. I switched it again, and the lights are back on.

I note that when I took a close look at the fuse there appeared to be small bubbles on the coil/wire. I don't know if the fuse came that way, or if it got that way after being in the receiver. Could it have gotten hot enough to bubble the coil/wire and reduce the voltage, but not blow the fuse entirely? I will be watching to see if the fuse blows again.

What would cause the J702 fuse to blow? Are there any other tests I should be doing to make sure there is no ongoing/underlying problem?

Everything else on the unit is working fine. I replaced the dial lights as part of a larger restoration project that included new electrolytics, and some new diodes and rectifiers as recommended in threads I found here and elsewhere. I have checked and adjusted the bias and the DC offset.

 

[rBuckner]

Strange but not impossible on the new fuse being resistive! Fuses sometimes just fail with age and many power cycles stressing the wire element. On "globe" style bulbs I have seen them blow in such a way as to short out the connections thereby blowing a fuse but that's kind of not possible with a fuse lamp. The only test I can think of is to measure the AC current being drawn, requiring disconnecting either a wire or one of the lamps to see if something is goofy with one of the new LED units. Better yet do an inspection of the new fuse lamps under magnification to see if there's a near solder short or the like that may have made brief contact shorting out the supply. Possibly it left a small burn mark but not likely.