1947 Marconi restauration help neede

[Trio]

Replacing the wiring is not a bad idea- it looks better in the long run. Spagetti on the terminal leads is often done in many radios for insurance against components getting shorted out. I would usually follow the lead of the manufacturer- if they used spagetti- then use it on those leads. If not- it depends on how much lead is showing and how much chance there might be of somponents settling over time due to gravity or vibration. You do not hurt anything by doing it, but it might not be needed.

When replacing the wires- do one wire at a time so you do not mess up connections. If the wires go up into an IF can, you need to take the outer cover off the IF can.

 

[scuba]

While replacing the rotten insulation on the rubber coated wires, I came across two wire that went nowhere (shown in pics below).
The first pic shows the green wire from the gang condenser C2 which if I am reading the schematic correctly, was and should be wrapped around the wire coming off the gang condenser C1. It also shows the second wire (blue arrows) and the point where I think it should be connected (green arrow).

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The second wire I came across runs from C5 which is the short wave oscillation trim pot and was wrapped around the yellow wire as shown in the pic below. The green line shows where I think it should be connected, but I am in no way sure of this. this disconnected wire might go a long way towards explaining why there is no shortwave reception at all.

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This pic shows the path I think the wire (green) should follow. The red line show the path of the wire coming off the gang condenser C2

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After replacing the dual cap with two new axial caps (which tested very well) The squealing at higher volumes has completely gone away though the hum is still present. Once I have finished replacing all the rubber insulation, I will attempt the tuning steps.

Fingers crossed.

 

[toxcrusadr]

On the schematic, why does that red wire wrap around the other one but does not actually seem to connect?

 

[scuba]

On the schematic, why does that red wire wrap around the other one but does not actually seem to connect?

I have no idea.:headscrat This is how it is shown on the schematic.
This is my first serious attempt at reading and understanding schematics, therefore I may very well be misinterpreting it.

 

[Pete B]

I have no idea.:headscrat This is how it is shown on the schematic.
This is my first serious attempt at reading and understanding schematics, therefore I may very well be misinterpreting it.

Nice project, lots of good suggestions. If that battery eliminator is very old you should check the caps.
There are carbon comp resistors in that unit and you should measure their values with your meter and
replace those that are far off. They are known for going way off value.

Obviously if you've checked for wiring errors, etc. and the other band still doesn't work you could spray some
contact cleaner in the band select switch.

It is probably for about 1 pF or less (maybe much less) of capacitance between the two wires. It is called a gimmick:
http://en.wikipedia.org/wiki/Gimmick_capacitor

My dad told me about these many years ago and he probably learned in Navy Electronics school, from the ARRL book, or from friends in college.

 

[scuba]

Nice project, lots of good suggestions. If that battery eliminator is very old you should check the caps.
There are carbon comp resistors in that unit and you should measure their values with your meter and
replace those that are far off. They are known for going way off value.

Obviously if you've checked for wiring errors, etc. and the other band still doesn't work you could spray some
contact cleaner in the band select switch.

It is probably for about 1 pF or less (maybe much less) of capacitance between the two wires. It is called a gimmick:
http://en.wikipedia.org/wiki/Gimmick_capacitor

My dad told me about these many years ago and he probably learned in Navy Electronics school, from the ARRL book, or from friends in college.

The battery eliminator is brand new. I bought it for this project.

I am trying to confirm all the wiring as best I can but due to my inexperience, it is an uphill battle. Lots of fun though.

Gimmick capacitor! Whoda thunk it. Thats cool
Still trying to figure out that other wire though.
Thanks for the link and your help.

 

[scuba]

I built a 90V B battery and hooked that up to the radio instead of using the battery eliminator and the hum went away. It seems the problem is either with the house current or the eliminator itself. I will try several other outlets in the house to see if that makes any difference. If it is with the house current, what if anything can I do about it?

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[Trio]

Nice project, lots of good suggestions. If that battery eliminator is very old you should check the caps.
There are carbon comp resistors in that unit and you should measure their values with your meter and
replace those that are far off. They are known for going way off value.

Obviously if you've checked for wiring errors, etc. and the other band still doesn't work you could spray some
contact cleaner in the band select switch.

It is probably for about 1 pF or less (maybe much less) of capacitance between the two wires. It is called a gimmick:
http://en.wikipedia.org/wiki/Gimmick_capacitor

My dad told me about these many years ago and he probably learned in Navy Electronics school, from the ARRL book, or from friends in college.

No probable about it, it is. It was fairly common among some manufacturers. It does allow for capacitive coupling.

With some radios you could increase the "Q" of the tuned circuit by adding a well placed wire. Heat AR 1-3 radios among them. I have the details written down somewhere and will dig them out.

 

[scuba]

After shelving this project for a while due to an inordinately long Blue Job List, I have finally been able (allowed) to resurrect it. After following Trio's tuning instructions, I have succeeded in getting pretty darn good reception most of the way through the AM band. Unfortunately, I have yet to get the SW section to work at all. Although I get the occasional hint of a carrier wave, i cannot tune in any station whatsoever.
I have approximately 50 feet of speaker wire as an antenna leading out from the radio, across the second floor balcony and dangling over the edge. I have also connected the ground antenna wire to the ground in a wall outlet. In theory, this should be ample for most SW frequency reception yet I get bupkis other than the occasional bit of hiss.
Does anyone have any idea where to look next?

 

[toxcrusadr]

I assume your antenna is insulated and isn't grounding out on a metal balcony railing or anywhere else. If that's all OK, you should be getting some reception. From what little I know about radios, here's a couple ideas beyond the band switch.

- Weak oscillator tube. Find the tube that provides the IF oscillator frequency. Sometimes if it's weak, it may oscillate with enough power at lower frequencies but not higher ones, so AM will work and SW will be weak. You can't always tell by testing the tube as the tester doesn't necessarily apply a high freq signal. Replacing the tube might be a good way. If it's a cheap one, use NOS.

- Check any coils or interstage transformers that are in the SW circuit but not the AM circuit and make sure nothing is open. If you're getting hiss then probably not but something to watch out for.

- Bad caps or resistors in the SW circuit. Usually best not to tinker with the flat ceramic pf range caps, but if there are any paper foil caps you haven't replaced, that can be a problem.

Someone correct me if I'm wrong on any of this.

 

[toxcrusadr]

Also, what are you using for the filament supply? I once did the 9V battery thing and used a bunch of NiCd cells in parallel for filament power. The radio didn't work very well. Turns out NiCds are 1.2V instead of 1.5 and that was enough to reduce filament output enough that the radio was weak.

I think the 1A5 is the oscillator/mixer, aka converter. The 1N5's following that in the circuit are the IF amps to boost the signal once it's been converted to the IF frequency.

 

[scuba]

Also, what are you using for the filament supply? I once did the 9V battery thing and used a bunch of NiCd cells in parallel for filament power. The radio didn't work very well. Turns out NiCds are 1.2V instead of 1.5 and that was enough to reduce filament output enough that the radio was weak.

I think the 1A5 is the oscillator/mixer, aka converter. The 1N5's following that in the circuit are the IF amps to boost the signal once it's been converted to the IF frequency.

I was using alcaline batteries which tested at 1.47v. I removed the 1A7GT and reseated it and tried again and was able to pick-up the universal talking clock at approx the 30m band. Not a strong and clear signal, but way more than the nothing I was picking up before. I will try rearranging the antenna this evening and see if that changes anything. I have replacements for all the tubes except the 1A7 (go figure) and will try to find a strong replacement for it. with a little luck, that will also help to strengthen the reception.
Even as it sits right now, I'm totally stoked just to get anything at all.

Thanks for all the help guys