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60MB needs some love - Printable Version

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Pages: 1 2

RE: 60MB needs some love - klondike98 - 03-15-2015

Well...not really sam...I got in the shower after posting and was thinking and realized....  Icon_crazy  I was holding the caps in the correct position to get the 1 & 3 and 2 & 3 wiring...sheesh... as Rosanne Rosannadana would say...'nevermind'  Icon_biggrin  I crossed out the misstatement...shows how easy it is to get 'cross-wired'  Icon_lol

RE: 60MB needs some love - klondike98 - 03-17-2015

Finished the bakelite block recap and then changed out a few of the dogbone resistors.  Some were surprisingly close to spec so I left them in.  I haven't shown an under chassis "before" picture so here it is:
[Image: https://www.dropbox.com/s/1i3nkg97fqwzjrf/60MB_underChassis_before.jpg?raw=1]
The next pic is after the recap and I've started remounting the power transformer and the can capacitor (#20 ).
[Image: https://www.dropbox.com/s/yjf2ntsdzapooiu/60MB_underChassis_halfwaydone.jpg?raw=1]
The tone switch was new for me but not as difficult as I expected.  I may cover the caps in the tone switch with some black cardboard rather than fill it with black hot glue or wax, easier to fix for the next fellow that way.
[Image: https://www.dropbox.com/s/40fbqisrj79v0b6/60MB_toneSwitch1.jpg?raw=1][Image: https://www.dropbox.com/s/enl4to4yjbvoxcu/60MB_toneSwitch2.jpg?raw=1]
I was all set to mount the two electrolytics that were originally missing from the set only to realize that the mounting bracket was missing as well.  I found the cans on John Kendall's site but had not thought about the bracket Icon_sad  until now...but after looking at some photos of them online I was able to reproduce one from some aluminum ducting I happened to have.  
[Image: https://www.dropbox.com/s/zlrnco412tcitvb/60MB_electrolyticMount1.jpg?raw=1] [Image: https://www.dropbox.com/s/lgfx7cq3y39ecug/60MB_electrolyticsMount2.jpg?raw=1]

Now to finish reconnecting the power transformer, can and electrolytic caps and put in new wires routed to the speaker.

RE: 60MB needs some love - klondike98 - 03-19-2015

Added the power cord, put all the tubes back and wired in the speaker.  The dial scale needs some work to flatten it out but that's for later...here's the finished under chassis and the topside.
[Image: https://www.dropbox.com/s/s9bza5iscoi7pg3/60MB_underChassis_after.jpg?raw=1]
[Image: https://www.dropbox.com/s/58nm69se3irr1ld/60MB_chassis_after1.jpg?raw=1]

Dim bulb showed no shorts but ramping up the voltage brought nothing except the pilot lamp and the 42 filaments ....hmmm.  Deoxit and pipe cleaner on the tube sockets and some cleaning of the tube pins gave better results...some crackling and humming of the speaker.   Voltages readings noted below:

[Image: https://www.dropbox.com/s/1x78i2qhume537p/60MB_voltages.png?raw=1]

The 75 tube failed when tube tested on the grid cap so one is on order.  Thought I'd try it anyway.  Is that causing the negative voltages on the CG to K line?  (Power input is at 110v ac)

I thought I had checked the coils but decided to check the antenna coil and when I measure ANT to GND I get an open .   Icon_sad   (rechecked other coils and they ohm out OK )  Maybe I damaged the coil during derusting, recap etc.?
[Image: https://www.dropbox.com/s/7rh0trk69vm8kt4/60MB_antennaCoil.jpg?raw=1]

When I measure from the ANT lug to one side of the #1 (10k) resistor located on the Band Switch I get 11ohms but from that point to GND  is open so the 0.7 ohm section of the coil must be what is open.  Does that sound right to you guys? 

RE: 60MB needs some love - Mondial - 03-19-2015

Bob, the voltages you measured look correct. There should be - 1 V on the grid of the 75, which is developed across the wirewound bias resistor. The other negative grid voltages you measured on the other tubes are also correct.

As to the antenna coil, it does measure as if the lower .7 ohm section is open. This part of the coil is only used for the SW band, so for testing purposes on AM broadcast, you can short it to ground for testing. Connect a ground wire to chassis to the end of the 10K not connected to the antenna terminal.

Have you checked for proper oscillator operation by bringing a transistor radio tuned 455 Khz higher than the dial freq and listening for a carrier as you tune the dial?

RE: 60MB needs some love - klondike98 - 03-20-2015

Mondial..thanks for the info on the voltages! I grounded the end of resistor#1 and am now picking up the broadcast band, at least the strong local am @ 790! Thank you for the advice.

Guess I need to rewind a portion of the antenna coil. That will be a new experience for me. First I'll pull the coil out and see if I can see the problem.

RE: 60MB needs some love - Ron Ramirez - 03-20-2015

It should be an easy fix. I seem to recall that the primary for SW (the 0.7 ohm winding) is only a few turns of #37 or 38 gauge wire on the outside of the coil, while the AM primary is a couple hundred turns on a bobbin mounted inside the coil, near the top. In which case, you are lucky. I've had several of those coils cross my bench where the AM primary was bad. Icon_thumbdown

RE: 60MB needs some love - klondike98 - 03-20-2015

We'll see...appreciate the help!!

RE: 60MB needs some love - klondike98 - 03-20-2015

The antenna coil SW section was pretty easy to rewind.  Ron's post showing a coil diagram was very helpful as well, as was Chuck's service tip on rewinding RF coils. Icon_clap  Here's a pic of the coil after I got it out of the chassis, then one of the new insulation (made from a piece of packaging plastic) underneath the pic of the old one and then the rewound section with a coat of shellac to provide a little protection.  
[Image: https://www.dropbox.com/s/uolh8cd3bi228lk/60MB_antennaCoil_before.jpg?raw=1] [Image: https://www.dropbox.com/s/nokxmmiat9bwm84/60MB_antennaCoil_insulation.jpg?raw=1]
[Image: https://www.dropbox.com/s/w9mal1otf3uocm2/60MB_antennaCoil_after.jpg?raw=1]

Radio seems to be working OK now so after an alignment and some warmer weather it will be time to work on the cabinet.  See that thread in the Cabinet Restoration section (I'll add a link when I start the thread).

RE: 60MB needs some love - Radioroslyn - 03-20-2015

Nice  work!!!!
Terry

RE: 60MB needs some love - sam - 03-21-2015

nice work   Icon_clap Icon_clap
sam