RCA Radiodyne 28 - Printable Version

RCA Radiodyne 28 - Printable Version

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RE: RCA Radiodyne 28 - Chas - 06-06-2024

This is an important follow up....

It appears the volume control for DC operation only controls the first IF, there is some convoluted routing for the filament connections to do that. So indicated, the resistance cannot be some 200 some odd ohms but must be around 30 ohms.

I cannot find any other clear notation of the original value...

To save hinting for reference schematic here is the Gernsback for DC with jumper strip, note AC resistance strip is off to the lower right as a group of resistors, unconnected:

PDFsam_AC operation of Radiola 25 & 28RCA-Victor-Service-Notes-1923-1928.pdf Size: 778.88 KB Downloads: 67

One has to wade through the conversions for the Radiola 25 (ignore) to get to the Radiola 28 info.

Reverse engineer the change over for AC. Note the mention of a connection. In particular, page 14 which describes a re-positioning of Whisker 16 to terminal 19 from terminal 17. You will move it to terminal back to terminal 17...

You will also have to consider what isn't there because the AC mods were for the Receptrad power supply...

I will scavenge through the parts I have. I have a lot of chassis parts for Radiola 25 (DC) I believe I have the low resistance cartridge and relatively sure I have the caps. Please allow several days for me to get this done as I have other Summer (household) projects going...

In mean time be ABSOLUTELY SURE that the filament circuits have no accidental connections to the B+ via the new jumper strip and by the battery harness. Do not remove any wires on the terminals at this time as they maybe the extension leads to the bypass caps. Once those caps are in hand the wires if not needed can go away.

What is the spacing of the two filament test jacks and are they horizontally placed?

I may have a meter...

Take a look at this uniquely finished 28 done back in the 20's. Download the image so it can be blown up to see the detail...

Best!

Chas

RE: RCA Radiodyne 28 - Rob Quay - 06-06-2024

The filament test jacks are horizontal and are 1 1/4” center to center. I’ll double check my work for shorts.
Wow- that 28 is a good looking piece of furniture. The 1920s were something else!
I’m busy with music performances this weekend so we can slow the pace down. I’m pleased with where things stand and don’t want to rush. Thanks. - Rob

RE: RCA Radiodyne 28 - Chas - 06-07-2024

Rob,

I have two Radiola 25 frames cut down for test panels. I only need one test panel. I also have a 25 in a phono panel, but both rheostats are of a high value (AC). For the 25 DC as in the 28, only one tube has filament control for volume and they are around 60 ohms.

However, one control is a flat-pack resistance and the other is the round cartridge.

>>> I must know what type control flat or round resistance for the volume <<<

I also found two 2 mf Stromberg-Carlson bypass caps. But no brackets FWIR these were from the AR-812 v-cat of which the bracket was integral with the cabinet... Bypassing the detector and the B+ 90 with these should be fine. There may be pairs of holes in the 28 frame where these should be mounted, a flat strip of metal with two holes and long screws/nuts would be fine.

Please advise resistance type...

Chas

RE: RCA Radiodyne 28 - Rob Quay - 06-07-2024

Hi Chas,
Thanks for the update. The resistance type is round.

RE: RCA Radiodyne 28 - Chas - 06-09-2024

I have got all the parts together this Sunday afternoon...

Found the #4 internal cat screws too.

The meter face is deliberately dark so not to be intrusive into the living room, works...

Save the old cartridge resistor. It can be re-wound to a lower value for another DC Radiola.

For the volume rheostat, the old "track" for the wiper may not align and be "scratchy", so use 400 paper bit, under wiper and stroke just a couple of times. Too much removed will increase the overall resistance and weaken the resistance wire. Clean the rotating contact under the hub of the wiper too...

Try to mount the caps onto the frame, so they come out with the radio and not be screwed to the cabinet. A help when servicing the chassis on the bench. Locate the terminals out of the way less some errant hand contact the B+... Stay clear of the RF and oscillator coils... If, you see a shadow and screw holes in the cabinet that agree with the caps then mount to cabinet and put lugs on leads to reach the terminal strip connections for B- and the B+'s. There may be leads within the battery harness for that purpose, taped off. Often, the three wires for the protective lamp are joined and taped off...

The OEM finish on the caps is a bitumen (tar) tinted lacquer...

I'll pack these and try to post tomorrow.

Regards,

Chas

RE: RCA Radiodyne 28 - Rob Quay - 06-09-2024

Thanks Chas! Good news! I’m still looking for where the caps are on the schematic so I will be asking some more questions. Send a PM with what you want for the parts and I will get that to you.
We ought to be getting close with the radio. I’m anxious to hear what Aimee Semple McPherson’s been up to with her broadcast! Thanks again. - Rob

RE: RCA Radiodyne 28 - Chas - 06-09-2024

The caps are C12 & C15...

In other Radiola catacomb data these are shown to be 2mf, each.

The schematic seems to indicate that they are not part of the frame but part of or an extension (wired) from the terminal strip. So, may be in the battery box below if need be...

Think, if the ARBE is going below, you can extend a pendant switch to control it, the caps could go across the ARBE and for that matter could even be modern caps as they are out of view even if the the lid is opened. The ARBE provides a noise bypass only on the 135v connection...

Save the ARBE receipt to validate the warranty...

Much of power supply control can be screwed to the floor of the lower cabinet, up to you.

The 2 mf caps are to provide RF bypass.

>>> In this transaction, the parts are FREE. I do not need them to restore a Radiola to OEM, I have a round meter for the test panel and a flat pack rheostat for the volume for the one Radiola 25 test panel.

Forward me reimbursement for the shipping when I PM the USPS receipt.

Not seen in the image, there is a very very faint ghost price of 25 cents on the celluloid bezel of the meter Icon_biggrin

>>> When the replacement audio transformer was installed, did the correct leads, start and finish of each winding get connected to correct locations? This is important as a transformer can invert the signal and in some designs this can cause a feedback howling that cannot be controlled. FWIK the Catacomb is conventionally connected to the transformers, observing the known color code at the time, but if it howls and the volume control does not work, then one pair of wires needs to be reversed. Did you note colored silk covering to the transformer leads?

Chas

RE: RCA Radiodyne 28 - Rob Quay - 06-14-2024

Hi Chas,
Thanks for the parts! It looks like cap C12 goes inline between 45 v. B+ to A+/B-.
Cap C15 is a bit more confusing. Lamp V9 - is it normally part of the circuit or does it serve as a safety device? Since it’s not in the radio as it stands does cap C15 run from 45 v. B- to 45 v. B+ or to the 90 v. connections? Inquiring minds want to know! I’m sending a couple of pictures showing where things stand. -Rob

RE: RCA Radiodyne 28 - Rob Quay - 06-14-2024

Here’s the second picture.

RE: RCA Radiodyne 28 - Chas - 06-14-2024

Cap C15 goes inline between `90 v. B+ to A+/B-.

There is no cap bypassing the 135 volts...

All the wires going to the UV-877 lamp should be connected together, soldered and taped. That same connection is to C12///

The lamp was an early solution to careless battery replacement.

Meter looks good Icon_biggrin

Has the 60 ohm +- volume resistance cartridge been installed yet?

Moving forward confirm connections for the "A" battery first, be sure there is no cross connection to "A" except at the A+/B-. DO the same for "C" be aware that the jacks may be problematic.

In a previous post I suggest two options for the "A" voltage. Try the "A" with "B" disconnected (all three voltages). Battery rheostat should affect all tubes, volume just one tube.

Be SURE "B" is NOT connected to the "A" voltmeter, be shame to take that out...

If all is well ,connect "B's", insert loop and give it a go...

GL

Chas

RE: RCA Radiodyne 28 - Rob Quay - 06-16-2024

The volume cartridge is installed as well as the caps. Ready to start checking voltages as well as my workmanship. I also need to see how good the tubes are and address that as well. Stay tuned……..

RE: RCA Radiodyne 28 - Chas - 06-16-2024

I know I asked about the speaker...

But I will again.

The plate current (B+-135) for the LAST AUDIO tube must flow through the speaker coils or some sort of audio coupler in the speaker. So measure the DC resistance using a analog meter. must be something less than 2K and more than 500 ohms. A cone speaker will not have polarity to be concerned with. Side note the last audio only gets the -22-1/2 volts "C"

A diaphragm speaker will have polarity, it should be connected such that plate current draws the diaphragm inward. That, action re-enforces the internal magnet and makes the speaker more sensitive. Long term in the incorrect polarity will weaken the internal magnet. The panel jacks can be problematic, be sure they are clean and that no wire strands are shorting at the terminals.

Bias to the RF tube "flows" through the loop, the connections must be clean, teasing the blade connections on the bottom of the loop and in the loop socket will help. If the signals are "scratchy" when the loop is turned the connections are not secure.

If you get regeneration oscillation the loop trimmer must be adjusted. Follow the RCA instructions. If that clears it up, great. If not then there is a poor connection somewhere.

Good Luck!

Chas

RE: RCA Radiodyne 28 - Rob Quay - 06-18-2024

I checked and installed tubes, hooked up a speaker and made connections to the ARBE. A slow power up showed partial progress. The A voltage shows 3 v. and there’s static in the speaker. Tapping the tubes produces a ringing sound that slowly fades (microphonics?).
This is generally what I expect at first power up. I see about.1 amps at 110 v. Will continue to check continuity and run things down with the RADIOLA manual. Getting closer but no home run! - Rob

RE: RCA Radiodyne 28 - Rob Quay - 06-18-2024

BTW- There’s no trace of the V9 lamp in the radio. That must have been cleaned up when the battery eliminator was installed.