[Chas]

Radiola 28 AC with resistor strip attached, more info when I am back at my keyboard.

They are high rez so they can be blown up for detail.

I did not melt out this cat nor do I know if it has problems.

It appears unmolested after melt out, so things like OEM caps are still in place...

Note that two caps pass over the internal steel support beam, it they are too close they can short to the beam which is at B-. Should have had a cambric covering, may have shorted when re-filled...

There are several ways to go here.

Best option since your transformers are O.K. as well as other coil/connections. Take this cat w/resistor strip. Determine which resistor strip to "gut", soak in solvent and get an Exacto under the wires, cut them off, avoid scratching the phenolic that would weaken it. May, have to re-order the one or both of the triple tabs to satisfy the jumper connection scheme.

Use your favorite hookup wire and either drill tiny holds through the straps or surface solder all the required jumpers.

Alternative is WTB in any forum for a 28 DC catacomb or its jumper strip.

If, indeed there is a shorted cap, I, doubt, but the cat has been monkeyed with. You could use the 28 cat I have and not have to bother melting out.

I have no 28's in my collection, I don't need this cat for any future project.

BTW the 'short" could be in the criss/cross of whisker wires under the cover. See the photo...

I have seen these wires pop off their soldered connections to the brass #4 screws.

I also have a few of those long brass screws & nuts if you need some.

If you have to work with the caps, do NOT bend the exiting leaves from the body of the cap, that WILL break one or more of the leaves and decrease the effective capacitance.

   
   
   
   

I checked the audio transformers on this cat, the last audio has an open winding, that explains the melt out...

Let me know if you need this cat...

Chas

[Rob Quay]

Thanks Chas. I’ll take you up on the offer for the cat. Let me know how you want to work out the transaction. 
Something that may be helpful would be for you to take a couple of pictures with important components marked or labeled on the cat before sending. That could take away some possible confusion later on as we progress. 
BTW- I’m not familiar with the term OEM. We didn’t get to that term in radio repair classes at the Bowie Md. radio museum. ….or I wasn’t paying attention!?
Terminals 17 and 19 were connected under the battery harness when I took it off FWIW.

[Chas]

O.K. I will send a PM. for the transaction.

Actually, look at pix that has cat showing audio transformers on the right. The only way I could identify components other than memory is to refer to the RCA notes or to Gernsback for the Radiola 30 as Gernsback did not do a simplified drawing of the 28 (same as 30) with last audio blocked. You have access to both via World Radio History.

Someone may have done a re-draw of the 28 in modern times, however, I would not be comfortable with any of their work.

Left to right are the RF input and mixer coils on their core with associated caps. Next are the IF group with a couple of caps on their core, next in a narrow space vertical are the grid leak and the grid cap both in cambric sleeves and electricians friction tape. I did not check the leak but if it is at or about 8.2 meg it is fine. Then the AF transformers 1st audio on the left and last audio on the far right. In between the sides is a pair of fiber board slotted panels holding an array of 4 bypass caps. All the rest is criss/crosses of jumper wires. Soldered to the #4 screws. Note that some of the fine coil wires are soldered to a larger cambric covered bare bus that is soldered to a #4 screw.

Pay strict attention to the wires connecting the external oscillator coils, if they get mixed it will be a number of tries before it works right.

Do NO touch any caps across any of the RF or IF unless they are known shorted, check to be sure they are not in contact with the center steel reinforcement.

OEM = Original Equipment Manufacturer.

The rosin is soluble in isopropyl alcohol, use 91%, do not use de-natured as the methanol tends to react and leave a white ugly deposit. To remove an audio  transformer: De-solder leads, but do not move them around as the cold brittle rosin may grab the copper and break the wires if flexed cold. Note they have colored cloth covering, warm the area with a heat gun to soften the remaining rosin, extract the mounting screws while warm, then carefully pull away the transformer. Do note the start/finish of the windings, the colors should conform to convention. The OEM transformers have a high plate impedance to properly load the proceeding stage. Only the Hammond transformer will work properly. If a transformer has been installed with a any one of its coils reversed the audio will uncontrollably oscillate. Reverse that winding or any other one winding to fix.

This is a Radiola 28 schematic re-drawn by Gernsback.

It has info on the DC and the AC, but no internal component values. Can clearly see the jumper strip and the resistor strip connections. The values of the resistor strip are in the text.

You could try this if one of the IF resonating caps are defective:

Need RF oscillator for 40khz, a 10k isolation resistor to input signal and a 1 meg to take output to a 'scope. The scope or a DVM if DVM can deal with 40kz as an output indicator. Turn on generator and look for signal increase on meter or scope vertical. Manually change generator and look for resonance at 40kz. If resonance is elsewhere considerably remove cap or shunt cap with a new value to achieve 40khz.

Do this only if there is bad IF resonating cap. It is possible that resonant tuning coul make the cat oscillate and may have to de-tune.

Must have the can on when operating as cat may oscillate...

I have not had to do this "static alignment" as the problem has never come up. Allow 5 pf on plate and grid for the tube or plug tubes in but not powered.

Use COG silver mica to resonate if none of the OEM caps are in range from either cat.

   

The UV-877 safety lamp can be ignored when using the ARBE-III.

Chas