[tfonzart]

Drive Cord Question - So I noticed my tuning cord is shot.
I think I've read in other places that a waxed linen cord might work for a replacement? 0.8mm - does that sound right?

What does everyone else use?

Also, any idea how to estimate the correct length of the cord?

Thanks for the advice!

[RodB]

I use 20 lb nylon casting fish line.

[Arran]

Fly line backing for fly fishing, which is often woven, not to be confused with the monofilament fish line which is nearly useless for dial cords.
Regards
Arran

[radiorich]

Hello Arran,
I never thought of that thanks for the tip !
Sincerely Richard

[tfonzart]

Thanks for the recommendations Rod and Arran!

[tfonzart]

Bakelite question -

So after digging out the internal caps etc. and reusing the bakelite (now three isolated terminals), the one terminal I thought was attached to ground via a bolt is no longer ground (not sure if when I took everything out inside, I lost a connection from the one terminal to the ground connection. This happened to anyone else?)

Regardless, before I realized this had happened, I used that terminal as a ground when reconnecting everything and after taking a measurement from it to the chassis, its open. So I guess I'll wire this terminal now to the chassis where everything else is grounded. Just wanting to make sure I'm not missing something obvious (or doing something stupid... or both for that matter)

[RodB]

Did you take photos before you started?

[tfonzart]

Rod
This!
I looked back at my initial
Photo and found what I did wrong
Thanks for the reminder !

[tfonzart]

Status Update -

I replaced the electrolytics. Replaced a couple of other caps that looked pretty bad on the 41 tubes.
Powered up with variac (without tubes) to check for any issues, shorts, etc. All seemed good (even had the tuner dial lamp working, so there's that).

Put in the 84 rectifier tube - slowly increased variac.
But, hardly any output from the rectifier (essentially nothing versus the 270 V needed).
Checked the plates center tap voltages - all that was good. The DC to the lamp is good. Ground looks good.

I disconnected the electrolytic coming off the recitifier output just to make sure that wasn't doing anything strange and measure rectifier output again - like millivolt range output (basically nothing), even with the filament illuminating.

So, my conclusion (I think) is the 84 Tube is the issue.

Anything else I should be testing? (New tube on the way).

Thanks.

[tfonzart]

Variac is a great purchase. Received new rectifier tube and found i finally have a rectified output now - hooray!
But, at 60VAC is where the output stopped climbing. Still have a few caps left to replace so off to finish those. Will see what we've got after that

[morzh]

1. It is generally not a very good idea to run a rectifier tube without any B+ load. If only because the DC voltage at the full input voltage (117V) could exceed your electrolytic capacitors' WV.
And then it stays there without anything to discharge it.

2. It makes VERY little sense to check the DC voltage (B+) before the full recap. Do the recap, check for shorts, put at least the output tube and the speaker in, and then check. Actually if your transformer is good, put all the tubes in.


3. Before you do anything, have you made sure your transformer is good? It should be some 600-700V AC end-to-end (between the yellow wires) of the Hi-V transformer winding. Convenient way of measuring is just sticking the Voltmeter's probes in the Plate (smaller ones) holes in the 84 tube's socket while ramping up the Variac to 117V.

[MrFixr55]

+1 on Morzh's comments. I speak from experience. No load voltage could exceed 500V. Was working on an Arvin Rythm Baby chassis and was measuring B+ voltage with no speaker (and no B+ to output tube screen grid, another thing to ensure if there is no voltage on the plate). I measured 550V just before the input filter cap blew up.

[Arran]

Some sets have a disconnect built into the speaker plug, but I don't think the 40-180 has one of those, it uses a Bakelite socket on the end of the speaker cable, so the output of the rectifier is connected directly to the input filter cap with the speaker removed. The typical old radio power transformer has a 350-0-350 layout on the high voltage winding, since these sets use an #84 tube as a rectifier the plate voltage is lower then most, I think 270 volts, and the rectifier tube shares the same winding as the rest of the tube heaters.
Regards
Arran

[RodB]

I just worked on a set with an 84 rectifier tube and the power transformer had 2 six volt windings, one for the 84 and the other for the rest of the tubes.

[morzh]

It is actually a much better arrangement if you have a separate winding for 84 and the rest of the tubes, as in 40-180 they grounded the filaments, and so the potential between the filament and the Cathode of 84 is full rectified B+, which could be well over 250V before the load kicks in (these radios use lower voltages, the output after the field coil filter is just 185V). Still. In other radios it could be higher.

Gosh forbid that F-K short occurs, I think it could be pretty catastrophic for the power section. Even though from what I read, this is not a very common thing to happen; still better safe than sorry.