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
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
(This post was last modified: 05-23-2024, 01:20 PM by Chas.)
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.
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
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
(This post was last modified: 05-23-2024, 09:44 PM by Chas.)
I set both catacombs side by side and as often happens there are differences between the two. I’m still trying to figure things out. I tacked the “new” cat into the chassis and found that terminal 22 to G7 shows cond. No. 2 shorted like the original cat did. That could be a coincidence but I’m wondering if there’s something else in the circuit that I’m missing.
As an aside I did find and correct a second short in the original cat. P2 to G2 had shown cond. No. 5 shorted. What I’m trying to figure out is which cat is the easier fix. Fortunately I enjoy this game of “cat” and mouse.
In order to sort things out I looked at the continuity test for the AC RADIOLA 28 and it shows 22 to G7 closed which is different from the battery radio. Should I be following the AC instructions and ignore the battery info? Are there two different wire resistance strips? Inquiring minds…….. -Rob
There is but one 8 tube catacomb, they were built in FWIR two different facilities so the internals may not appear alike but hookup internally is the same.
The SAME catacomb can be for AC or DC.
The RCA notes are for the DC catacomb with a jumper strip attached...
This is an extracted test list for the 28-AC-Catacomb:
The test list above when done on the existing catacombs with the resistor strip, performed as described should be valid.
The EXTERNAL resistor strip for the Radiola 30 is different as the last audio tube is not used and output routed to a PA in the power supply. But the 8 tube cat is internally the same!
I CAN tell you the cat I provided has a bad second audio, I read the resistance to be open on one winding with a Simpson 260...
Look carefully at the RCA schematic that has the jumper strip. Then reference the terminals in the continuity chart, follow the circuit through the jumper strip schematic to the whisker numbers. I showed the whisker numbers in one of the images.
Since you do not have a jumper strip, the chart is not valid for a check list with a resistor strip...
As said in an earlier post, an ARBE will NOT support series filament operation (resistor strip) of the Radiola 28. The filament voltage is 34 volts at 60 ma. when in series.
The resistor strip purpose is to connect all the tube filaments in series AND provide bias, as the 104 supply does not provide the bias.
With the jumper strip the filaments are wired in parallel and connection brought out to provide bias via battery via the whiskers to the terminal strip...
With the resistor strip removed, referencing the terminals follow through to the whisker number, then complete the continuity test.
Pleas do not use a DVM for the continuity preferred is a low resistance VOM as low as 1000 ohms/volt. OR use the battery and headphones "click" test. Fair warning a click test of the audio transformers could result in a inductive jolt if handling bare conductors for testing.
If a certain CAPACITOR fails short. trace the wires to that cap internally, carefully de-solder the easiest end to get at. Do NOT push or swing the cap around as that WILL break the foils and ruin the cap and test in-place. If it is truly shorted it will show there, but it may be tinkering has been done to the whisker wires where they crossover in the top of the catacomb that is not potted and accessible.
Once your satisfied then decide which resistor strip to cut up and create a jumper strip. You may have to fabricate or cut at least one spade lug on the former resistor strip to be sure it makes the correct connections.
Note, the Gernsback Radiola 28 schematic has a internal wire error for the filaments inside the cat at tube #V6 & #V7. It shows the tubes permanently in series, that is not true, there is a missing connection "DOT" (connection) and not a "Semi-circle" (no connection)...
Useless information, there is a continuity error in the RCA test list for the 6 tube Radiola 25 catacomb.
So errors DO exist, but I have not seen any others for the Radiola 28...
If investigation proves the resistor strip is too "messy" to convert to a jumper strip. Post a WTB to ARF classifieds. The Philco Phorum may not have "Radiola" readers...
I note that some common jumpers on the resistor strip would have to be split to make it work as a plain jumper strip, It seems now tedious as well as messy...
Oh, the component numbers are not the same in the RCA vs the Gernsback schematic
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
(This post was last modified: 06-02-2024, 11:16 AM by Chas.)
Thanks for the info Chas. It looks like my next moves should be to take off the resistor strip and make up a jumper strip. I have a sheet of sintra that I can use for that.
Also, it seems like the Gernsbach schematic should be the source of information from now on. I apologize for my confusion. The style of many of the components is foreign to me. Especially the capacitors.
Quote:I apologize for my confusion. The style of many of the components is foreign to me. Especially the capacitors.
I understand completely...
IMHO I am sure that the RCA's desire to have a uniform radio "module" fit a number of radios/phonos is the result of the "rat's nest" that is the terminal and resistor strips. I go further to say that is the principle reason that the value of a 28 and it "brothers" is artificially low. Very difficult to understand, as well as somewhat costly tube requirements.
The 8 tube catacomb when correctly operating is extremely sensitive for a triode 8 tube radio. Even the 6 tube catacomb is nearly as sensitive but comes with "images" which can be both a liability and an asset depending on how the radio fan uses it.
Yes, the relatively large IF coils on steel lamination's and open clinched mica caps are a bit primitive. You will note that the grid leak is buried inside of a tube of varnished cambric. FWIR it is painted red and has no other color coding, before the application of the BED code.
RCA use the open clinched micas in the 16, 17, 18, 20, 21, etc up to the 66. Generally, unless these are poked, prodded twisted they are fine.
As far as the unknown values, in the AF amp these values, still relatively low are to stabilize the amp and prevent feedback via the common B & C circuits. as well as shunt any 40khz signal.
The micas in the RF are intended to resonate the IF transformers.
There should be at least one or two 1mf or 2mf large paper block bypass caps around the battery connections, being an AC set that never got the large bypass block array you will probably will need these caps. Since I do not have a complete 28 DC to reference for where these caps are physically located There may be missing brackets and such involved. As well as extension wires in the harness...
I DO have several of the caps and possibly mounting brackets but they are attached to harnesses form a Radiola 30 or a 25... I only need enough caps to provide for ARBE-III operation of the test chassis, there are three of these but only a 25 is near completion.
These caps if they are leaking can be easily stuffed with flat-pak metalized, but if they are good and I suspect, they are Stromberg-Carlson caps...
>>> the volume rheostat in an AC or even an AC (original conversion) is different for the DC version. Those series filaments... I am not sure what the correct value is but I may have the correct value, The rheostat is serviced as a cartridge. So only the resistance element cartridge is changed out. We can get to that as the last step in completing the 28 DC...
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
This is my attempt to replace the wire resistor strip. It’s not finished yet but the main idea is there. If this works I plan to move on to replacing the open transformer.
Replaced the open transformer in the cat and ready to install in the radio. Then I plan to make a new “battery” harness. The Gernsback schematic shows two C voltages (4.5 & 22.5). Never saw that before. Also, the battery harness that came with the radio uses 8 connections instead of 7. So many questions for one radio!
Quote:The Gernsback schematic shows two C voltages (4.5 & 22.5). Never saw that before. Also, the battery harness that came with the radio uses 8 connections instead of 7. So many questions for one radio!
The -22-1/2 is for the 120 output tube. That tube has a more open grid structure to keep the plate current in check ,a greater negative voltage is required...
The -4-1/2 is the other tubes, it also goes or should, to the center-tap of the loop. Then to the grid or the RF tube. The loop connections must be clean or there will be noise when manipulating the loop.
With the ARBE-III setting the "A" voltage is permanent and regulated. The BATTERY setting rheostat is used to follow the wear-down of the "A" battery. There should be a reference plate "still" even on an "AC" that recommends where to set the battery rheostat, there should be a cast-in "tick" at 3.3-3.5 one the setting escutcheon. RCA took sets in production and also sent kits to dealers, removing old "DC" data plates was not done, I suspect too "messy"
Those are the facts so, one can choose to bring the battery setting into play, by setting at 3.3 or the "tick" on the BATTERY setting escutcheon with the ARBE it would never be set again. OR dial out or jumper out the battery setting rheostat and set the ARBE-III for 3.3 to 3.5...
The overall filament voltage can be taken from the two jacks on the panel apron. There is an accessory meter that plugs into those jacks and actual supports the entire meter. The correct meter has a pair of swivel contacts, insulated that are rotated for either vertical or horizontal position of the meter. The black meter face has the obvious "RED LINE" at 3.3...
A reminder about the "VOLUME" rheostat... For series filament "AC" that rheostat is considerably higher value. The cartridge is changed. As I said in a previous post I do not know what the resistance is for "AC or "DC" However depending how many tubes when in parallel will determine the correct resistance. For only one '99 is should be around 30 ohms, for two 15 ohms and for three around 10 ohms. If you find you need a cartridge resistance, I'll see what I can find. Failing all else, the cart can be re-wound OR search the WTB.
I did place an WTB in Antique Radio Forum for a jumper strip.
IMHO if one traces the filament circuit and includes the battery setting rheostat, the resistance of the battery setting provides an additional voltage drop and additional bias in doing so. That bias may have a dramatic effect on the sensitivity and overall volume of the set... If you have small children that my twiddle with the controls, choose the safest method that provides results. No harm short term will come to the tubes if operated at 4.5 volts IN THE SHORT TERM, overall long term use (weeks) the emission will fall off at a faster rate than "normal".
The "AC" "battery" harness with the extra wire is to access a portion of the catacomb for the "AC" mode. Should not be in use when operated on "DC".
I took a second look at the "DC" jumper strip ,NO nothing changes in making the one you made, KUDOS.
It has occurred to me that the entire strip could be a PC board fabrication, double sided so circuits can pass with through hole jumpers to combine multiple connections. Holes through the board with contact rings for the screws. There is ample clearance to make the strip wider to account for filament current flow... Since the strip is unseen in normal installation, the "modernization" would not be noticeable.
I kinda wish I had thought of this sooner, like 20 years ago. As it would be inexpensive to fabricate multiples. The circuits could be sealed or not with a conformal coating. Seems all too slick, but my minds eye says it would work a treat. What a blessing for those who find 28 AC and no power supply
So many projects, so little time...
Oh!
Any problems extracting the open audio transformer? Readers would like to know. Especially dealing with the "nest" of mica bypass caps. All stuck together with rosin...
Do post pix along the way...
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
(This post was last modified: 06-06-2024, 12:39 PM by Chas.)
Sending a photo with the strip installed. I used #6 spade terminals and they are narrow enough to keep from shorting.
The transformerectomy and subsequent installation wasn’t bad.The most delicate part was getting the cloth insulation off the transformer leads. I ended up touching the wire with the soldering iron and then carefully cleaning the bare wire. If the bolts attaching the transformers to the frame are stuck just touch them with a soldering iron for a few seconds.
Your idea for a PC is a good one. That would be a nice time saver.
BTW what did you get for a grid leak resistance, 8 to 8.2 megs is fine, much higher say 10 megs and volume may be too low...Or the detector will cut off and the radio will go Putt-putt sound.
#6 O.K. but the screws were #4, so long as there are no shorts...
That's a great technique, a little heat and the rosin gives way...
"We" are moving along, let me know as soon as you can if you need one of more of the large bypass caps and a volume rheostat cartridge.
BTW its O.K. for the bypasses on the B+ to leak a little as the ARBE-III can take the load... If it were battery's the leak would kill them...
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
The grid leak reads 5.7 Meg. The volume rheostat cartridge is at 248 and I don’t have bypass caps so that’s where things stand.
If I would make any more connector strips I’m sure I could simplify and neaten up the end product. Not planning on doing that though!
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