40-180 Questions - Printable Version
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40-180 Questions - GaryM - 07-22-2012
I began my first restoration project a few weeks ago and have recapped it, replaced most of the resistors and completely rewired it. I brought the input voltage up slowly with just the 84 tube and was happy to see I hadn't done anything catastrophically wrong!
With all the tubes in place all filaments are glowing nicely, B+ seems a little high at 205V and a few random checks at plates and screens seem reasonable.
I am getting dead silence from the speaker however. The field coil is good at 1105 ohms and I am confident the field coil is wired correctly at the + side of C58 and C61. I also double checked the audio transformer primary wiring against the schematic and it's good. There are no other transformer wires and no sign of any other wires that may have been cut. I think I'm missing a secondary winding wire.
The speaker voice coil wiring makes no sense to me. I rewired it exactly how I found it. The chassis however was in suspect condition when I started. For example, there was no connection at the 84 cathode! One voice coil wire is connected to the audio transformer ground strap and the other is wired straight to ground.
Assuming I need a new audio transformer, I also need the correct connections for the speaker voice coil.
Can anyone point me in the right direction for a replacement transformer and the correct wiring?
Thanks much.
Gary
RE: 40-180 Questions - Radioroslyn - 07-22-2012
Well the simplest think to check is the resistance of the voice coil and the output transformer secondary. Disconnect the secondary form the voice coil before measuring it. Both should read a very low resistance like a few ohms tops. Also if both wires from the secondary are connected to ground unshort them as this will cause your problem too.
Terry
RE: 40-180 Questions - GaryM - 07-22-2012
Since I am among others who all struggled through their first restoration, I will sheepishly admit that I mistook the secondary lead for a grounding wire. There is a bare wire coming out the bottom of the transformer and from the angle I've been looking at this chassis for the past few weeks, it looked like it was the same bare wire which is spot welded to the transformer case. I unbolted the transformer to take a closer look at the underside and realized I was actually seeing both secondary leads. Ugh. Wouldn't have expected to see bare wires for transformer outputs.
So I properly connected the secondary lead to the voice coil wire and I have barely audible static and only at the highest volume setting. Progress but I would appreciate any suggestions on getting normal volume levels.
Gary
RE: 40-180 Questions - Radioroslyn - 07-22-2012
There are not really bare there are coated with lacquer. How's the rubber wire? All the tubes good? You should be able to touch the center connection on the volume control and get a blast of hum if all is well with the audio stages.
Terry
RE: 40-180 Questions - GaryM - 07-22-2012
Well miracles happen. I checked the plate, screen and grid voltages at the 41 tubes and all seemed good and I had a very loud hum when touching the center tap of the volume pot. I haven't rebuilt the top of the chassis yet except to mount the tuning cap so I turned the chassis right side up and tried tuning by hand with the wheel. I wish I had a picture of the look on my face when a local AM station came in blasting loud and clear! The 7G7 RF amp tube that came with the radio tested very weak so I didn't have it installed. Ironically, reception deteriorated badly when I installed it.
The chassis was in rough shape when I got the radio and the previous owner(s) made several incorrect wiring/component changes. I can't believe it's actually working. I still have the rest of the chassis reassembly to go and I have a stripped cabinet awaiting but I think the hard part is done. I've been taking pictures along the way and I'll post some when I finish.
RE: 40-180 Questions - gary rabbitt - 08-01-2012
Hi Gary,
Good to hear your radio is playing fine.
One thing that may not have been mentioned is that those Loktal tubes are bad for getting corrosion on the stubby pins. Causes bad contact in the socket.
I've had more than a few sets with dirty pins cause the set not to play.
The 40-180 is a good larger set to learn on.
Take care, Gary.