Marconi Model 137 Power Transformer

When you figure out what you need, give a posting here in the Buy-Sell-Trade section.

  Many of us have piles of transformers sitting "Just in case" we need one.

A new one from Hammond will likely will run you 100-200$, based on what I have seen.

Now if you want, you could give a try on rewinding it. Not an easy project, but do-able.   I have done a couple, and it is not fun, but then again my dexterity sucks.

If no one here has what you need, you might try posting on Antique Radio Forums: https://antiqueradios.com/forums/index.php 
Also Mark Oppat may have what you need also:  moppat at comcast  .com-- just remove all the spaces. I have bought several items from him. He is a square guy to deal with, based on my interactions. 

Keep in mind if there is room on the chassis and cabinet you can change mounting hole positions and such if there is physical room for a larger size transformer. Also one can use separate plate and filament transformers if room too. I did that a few times with success.

Thank you again all. Any recommendation as to where to purchase replacement components whether new old stock or used?

Generally you will see a dim glow if the transformer is OK, a brightly lit bulb- say 60W indicates a short.

Thank you very much to you both for taking the time to read my post and assist with my questions!

I have been doing a lot of Googling since Frank's response trying to familiarize myself with these topics and fundamentals. 

I did manage to build myself a dim bulb tester as recommended which was a very interesting project. I tested the tester with a known good circuit as well as a purposely shorted circuit. The tester functions as expected!

I did not power on the radio once in possession incase of a potential short. I removed and isolated the power transformer to inspect before my initial blog post. After hooking up to the DBT, the bulb does indeed illuminate in which I am assuming means the PT is dead. The PT seems to have gone through a lot of heat as it has a dull dark glossy residue look. The secondary leads are also quite loose feeling within the laminations. 

My next step is to study the circuit to determine the voltage ratings as Frank suggested. 

I will keep you posted on my progress and thanks again!

Put Shrink tubing over the wires as previously noted by Frank and note the colors. There are clues as to which wires perform which voltage. 

[You must be registered and logged in to see this link.]

Marconi went through many manufacturers with businesses changing hands. A lot were produced in Great Britain. Schematics are few and far between. You can test wires to find which are from the same windings.
If you can correlate resistances then you can calculate windings ratio and then appropriate voltages.

There are many resources for determining voltages and/or color standards to determine windings by color.

[You must be registered and logged in to see this link.]

Welcome to the forum. 

Please remember that we all had to start somewhere.


 Step # 1  Is the transformer actually bad? Does it smell burned?   Have you tested it since you removed it from the radio? Is the transformer center tapped? The more information you can give us, the more we can help.

 The insulation on the wires on old transformers is often brittle. Just put heat shrink tubing over the wires. Typically that will solve that problem.

Most often it is bad filter capacitors causing the transformer to hum a lot, over heat, or blow fuses. Bad filters can also destroy the rectifier tube.

Replace the filter capacitors before putting in a transformer.

You really need a "Dim Bulb tester" and there are numerous postings on this forum and others on how to make one. Basically a NON-LED light bulb, a socket, switch, and a box to put it on.

That vintage set will likely need to have all the capacitors replaced, to insure reliable working, plus it is a good idea to have the tubes tested, and any shorted/weak ones replaced.

If the transformer is bad you will need to know the voltages and current rating.  Knowing the physical size and mounting of the transformer is important, plus the cross sectional area of the laminations. 
Other ratings can be determined by adding up the filament currents for the rectifier tube- usually a separate winding, and the other tubes. Plus I typically add up the plate current for the tubes and the screen current to get a ball park idea of the ratings needed for a replacement if I don't know the data. The voltage can be roughly determined by looking at the voltages printed on the filter capacitors. Typically the  voltage of the transformer will be .707 of the rated voltage of the filter capacitors, on a good design.
   Some companies did push it tight though, and the filters can prematurely fail from too much voltage. If the filters are rated at say 450VDC, likely the transformer put out from 275-350 volts.

 Remember you can always use higher rated capacitors than what is in the set. If the filters were say 20 mfd, you can safely use 22 to 33 mfd at the same or higher voltage rating. For 40 mfd, 47 mfd is just fine. Some sets used 10 mfd, and either 10 mfd or 22 mfd is fine.

So here is a start for you. Please post how the transformer tests out.

Hello,

My knowledge is extremely limited with radio repair. I recently came into possession of a Marconi Model 137 radio which is from around the late 1930s I believe. After disassembling the chassis, it seems quite clear that the power transformer has scene its day and would need a replacement. The wires are extremely brittle and loose their jacket when touched. I was looking for some assistance in matching a replacement if possible. I purchased a parts/schematic sheet for the unit but do not seem to see any voltage specifications on the secondary windings of the transformer. 

Please advise and I can supply any requested information to aid with this question.

Thank you again. [You must be registered and logged in to see this image.]