1947 Marconi restauration help neede

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1947 Marconi restauration help needed

About a year or so ago, I bought a non functioning Canadian Marconi farm radio model 229 at an antique store. I have since replaced all the caps with the exception of the red dry electrolytic Minicap and with the help of a battery eliminator, put power to it. I have also tested all the tubes and replaced a couple which were borderline or no good. So far no sparks, smoke, flames or other sundry no-goodness.
The radio receives Broadcast Band (AM) and Short Wave. The radio puts out a pronounced hum on both waves and receives only on the Broadcast Band.
While pressing on each tube to make sure that it was well seated, I noticed that when I touched to top connection on the 1N5GT which was connected to the IF transformed #1, the hum disappeared. The hum also disappeared when I disconnected the clamp from the top. As long as it was close enough for induction current to flow, the sound and reception was decent, and the closer to contact it got, the louder the volume/reception was. When connected, I also got a hight pitch squealing at loud volume as well as a “heterodyning?” or “doppler” squeal as I moved the tuning dial around. This squeal also went away with the top connection disconnected.
Also, there is no SW reception period.
I am attaching several pics and schematics in the hopes that someone can explain to me what may be going on and perhaps what to do next as I am at the bleeding edge of my ignorance at this point.
From the looks of some of the connections, it would appear previous repair attempts have been made (somewhat ham fisted too)







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I can provide more pics and info if that would help.

Thanks Akers
 
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How good is your battery eliminator? Do you have AC noise on it?
I'd definitely change out that large filter cap that's original, after all, that's it's job to filter out any AC.

Glenn
 
vfr800fiman said:
I assume you running 1.4v heaters, and 90v plate?
Click to expand...

1.5 volts heaters and 91.3 volts plate current confirmed with a multimeter.
I haven't replaced the large cap yet, but it is on my list of things to do. As far as I can tell, it is not original to the radio as the parts list calls for two 10 mfd 90 volt dry electrolytic caps
 
This is the battery eliminator that I am using. As far as I know, it is a good one.
It was designed specifically for use with battery powered tube radios but I don't know how to test for AC noise.


 
Would an oscilloscope help in tuning/setting the frequency alignment etc?
If so, could anyone suggest an inexpensive yet effective model. I have scoured eBay and have come up with so many models that I have no idea what would do the job.
 
I don't have any experience with that model of battery eliminator, but at least it's one that was designed for this purpose. I thought you might have been using something that was a home brew unit.

Probably still doesn't hurt to check how clean the DC is coming off the power supply. Have you used this on other radios with success?

You will need a scope to check for noise. You should have one if you are going to do more radio work in the future.
 
The usual reason for hum on a battery set is power supply related. There just isn't anything in there to cause a 60 or 120 cycle hum. If you can, try stringing together a bunch of 9v batteries and using that for the B supply temporarily to see if anything changes. If not, try a couple of D cells in parallel for the A supply. That would let the radio run without the power supply, and it would tell you whether or not thats the issue.

Also, that red capacitor is a 2 section cap near as I can tell. If it is bad, you'll get poor decoupling between the stages of the B+ supply and that may do silly things. Would be worth swapping that guy out first just to eliminate it as a possible issue. You'd need two caps to do it.
 
g'day. I don't advise a digital scope for your HV tube work, to easy to fry making a mistake.

Check out scopes on DIY and tube forums.. or just post a new thread on either one and ask for links if you can't use the forum search. You can use google and include audiokarma in the search.

wish I could come up with a good one for you but not knowledgeable on cables and or adapters needed but I'd choose a scope with basic cables included then maybe pick up on home made adapters for specific measurements?
 
pfcs49 said:
put a DVOM across DC supply, with meter set to AC V.
Click to expand...

I did this and get 203 volts from the 90 volt taps and 2.3 volts from the 1.5 volt taps. Unfortunately, I have no idea what this means. If you have the time, could you or someone else explain this to me? Why would I get aa AC reading off of a DC source?
 
The caps in the eliminator filament supply are probably bad. or non-existant.

C10 and C11 should be 20 microfarad 200 WVDC each.

These tube radios are very sensitive, so if you have flourescent lighting, CFL's, or dimmable LED lighting on the branch circuit or in close proximity- you will pick up hum from those as well.

If you run on batteries, "C" is not an absolute must have voltage. It is just there to reduce plate current draw when on batteries.

Other supplies that work, can be fabricates easily, a number of links are on this page:
http://members.tripod.com/Art_Deco_era_radios/farm.html

Replace the large cap. The one that is there is a daul section capacitor (2 caps one container. This is your C10 and C11. Refer to note I made above.

(I wish people would not use Photobucket, it makes it more difficult to download an image to review it.)

You do not need a scope for this at all.

You can also do this more quickly without a signal generator than with one.

On AM, locate a tation near the top end of the dial, use just enough wire on the antenna to pick up a few stations. You want a weak station. Then there are 2 trimmer caps on the tuning gang- the forward one is an RF trimmer. The rear one will change the local oscillator. Trim for best reception of that station- leave it on that station. Peak the IF's for loudest signal. They are slug tuned so you have one on top and one on the bottom of each IF transformer.

Once peaked for loudest signal, check the tuning dial to make sure you do not have any images next to each other while tuning- if you get an image, the IF's are not peaked. You should hear everything clearly and an "S" should sound like an S not a hiss. These are the 2 brass screws sticking out the side of the IF cans. If these are out of adjustment, these will cause your howl. As can an open capacitor in the vicinity of that stage.

Once those are peaked, go back to the tuning gang and check the RF trim- it should be okay, but sometimes sensitivity can drift a little.

Then, If you have a strong station that is broad in tuning compared to the rest of the stations on the dial, adjust the wave trap (C12) to make that one station as narrow as possible on the tuning dial.

Next, switch to short wave, set the SW trim, essentially as the image is oriented with the drawing identifiy the adjustments, work from the front of the chassis back peaking for the strongest signal you can find on the upper end of the SW band. You can then use the SW Oscillator trimmer to adjust the radio to the dial, Use WWV at 10,Mhz for lining up the dial. You can use one of the other signal standards as well- but you need to know which one you are listening to and it's frequency. The US has them on 5, 10 and 15 MHz, Canada has them on different frequencies.

I have worked on many radios of this type, and I can do this aspect in my sleep. Too many people make it much more complicated than it needs to be. Once you get the hang of doing it this way, and develop an understanding of the interactions, then you can make the dial tuning as precise as you want to make it.

Lastly- do this late at night for best Short Wave reception. If you have questions, you can certainly PM me.
 
JURB said:
Anyone notice the output tube has no cathode ?
Click to expand...

These are directly heated type tubes, they generally draw less current when designed for battery powered sets.

Sent from my SM-N900V using Tapatalk
 
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Thanks for all the answeres and suggestions folks. I now have a clear path to follow to get this radio up and working as it should.
I' m sure that there will be many more questions to come and hope that you folks will bear with me as I work my way through this restauration.

BTW. This is way more fun and challenging than I had thought. :tresbon:
 
Only thing I can add is to make sure you don't reroute any wires around under the chassis. Try to keep things where they were originally. Sometimes sensitive parts of the circuit can pick up interference from being too close to other wiring. It's probably worse with AC operated radios but something to watch out for.

The ARBE III is a well known (at least in my book) battery eliminator and as long as it's working OK it should be just fine. I don't know why you'd be seeing that much AC voltage on the outputs but if it works OK on other radios, the problem is probably in the radio and not the power supply.
 
toxcrusadr said:
Only thing I can add is to make sure you don't reroute any wires around under the chassis. Try to keep things where they were originally. Sometimes sensitive parts of the circuit can pick up interference from being too close to other wiring. It's probably worse with AC operated radios but something to watch out for.
Click to expand...

I have started putting heat shrink on as many of the bare leads from the caps and resistors that I can. Is that a bit of over-kill?
I also plan to replace much of the original wiring since the insulation on them is seriously degraded in many places and will take care to keep the original routing.
Thanks toxcrusadr.
 
It's not overkill but isn't always necessary either. I tend to do it when wires are crossing each other or running close, just to make sure they don't touch.

If the wire is OK and it's just the insulation, you can lift one end of the wire and put spaghetti over it, then reattach. BTW, spaghetti is just hollow tubing that doesn't shrink, made specifically for this purpose. Shrink will work also but might be more pricey. Anyway this might be less work than replacing all the wiring. But it's entirely up to you what you want to do.
 
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