Majestic model 90B....no audio

[rca2000]

I have to restore(electronically) a Majestic radio model # 90B. This is a VERY heavily made radio!! 16GA metal all over the thing. Anyway...It seems to be a TRF set. 4 or 5 '27 tubes, a pair of 45 outs and an 80 rect. Tere are NO normal types of capacitors in this set, all are VERY large, oil-types or such. The power supply is mainly choke-filters, with a couple of low-value caps-2-4UF in a "pi type" filter, ALL in metal boxes.

This is NOT my radio I am fixing it for a friend's boss--who "wants it fixed" and pretty much "no matter what". It was his fathers radio or such, and he does NOT want it "modded" or "rigged" with some sort of "hidden SS radio" taking place of the original 1930 circuitry. He wants it done RIGHT--and I am trying to help that get done.

I looked it it yesterday. I seem to be getting plenty of B+, over 350 to the plates of the 45 tubes, and around 55 or so on each side of the filaments to ground. NOTHING on the grids--is this normal?? I have a transformer coupled amp here--the grids of the 45" go to the input tranny, and it is driven by a '27. I CAN get weak audio, if I inject it at the plate of the 27 from an external (isolated for safety, even though the radio is "cold"), source....but I get NOTHING if I try and inject audio at the grid of the '27 driver tube. I get weak, slightly bassier audio on the cathode of this tube, and this had about 25V. or so on it....--but again, NOTHING from the grid, and NO voltage there either!! The driver tranny SEEMS to be good, ohm-wise. The ONLY schematic I can get is VERY poor, with NO voltage readings or such. I have to go "by instinct" on this so far. I do NOT feel I am getting proper filtering, since if I put extra filtering on the b+ line right out of the 80, say, a 22/450v. cap it WILL raise the B+ to about 500 volts there. But it does NOT "change anything" as far as the operation goes. I suspect an open resistor or coil, one of the big" copper colored can's, under the chassis, probably. I ran out of time, having to remove that big metal shield over the tuning cap, to get to the coil leads to check them, and the one I THINK goes to the grid of the driver tube shows to be SHORTED, and is NOT so in the schematic!! There is a cap and resistor to ground here.

SO anyway....am I on the "right track" here--suspecting that I SHOULD be able to inject a signal into the grid of the driver-and get DECENT audio out of the speaker?? a pair of P-P 45's should give GOOD, POWERFUL sound, if they are working right, true?? And shouldn't there be some negative voltage on the grid of the '27 driver tube??

Any help is appreciated.

[David Roper]

That's not the driver, it's a biased detector. It's biased nearly to cutoff so that only the audio portion of an RF signal will get through it. When you applied the signal to the cathode you must have succeeded in effectively changing the bias. I guess you could clip in an appropriate resistance from cathode to ground on the last 27 just to confirm the output stage is good.

[droptop]

I have a few tips for your Majestic project. I’ve restored many of these beasts and they all have similar problems. One version has a separate power supply in a metal box with a cable connector. The later version is a single chassis design but either one is bad on your back when heaving upon the bench.

The first thing to do is replace all 4 of the filter caps. Each one goes from its particular B+ bus point to ground. Use 600 volt replacements. These sets run much higher B+ than most sets of the era. As you’ve discovered electrolytic caps aren’t for use here. The first one off the rectifier will run warm and raise the B+ usually. The first one I restored some 30 years ago actually blew the cap. I’ve used regular caps ever since.

The next thing to do is check the large black 20 watt wire wound resistors on the terminal board where the power transformer windings are connected to the rest of the chassis wiring. There is one underneath this board that is connected to the center tap of the 45 filament supply winding. This one will usually be found open removing the grid bias voltage on the #45’s causing the dead audio section you have. I believe its 1800 ohms. Once the power supply is operative check the plate voltage on each #45 tube which should be around 400 volts and each #27 tube which should be around 180 volts but it’s not too critical. Open RF plate coils are fairly rare but I have found a couple. If all is well and remove the cover from the tuning condenser. With one exception I’ve found shorted plates in every one of these sets when the condenser is rotated due to the swelling of the pot metal. Disconnect the stator connection of each section starting with the one nearest the front which is the detector condenser. A short here will kill any audio if it is present up to this point. Connect a continuity meter between the stator and ground. Rotate the condenser and watch for shorts. My meter has an audible buzzer that indicates shorts. Bending both sets of plates to clear shorts is the only fix I’ve found. Also check the thin lead connected to the movable bell in the antenna tuning box. Many times it will be found broken rendering the control inoperable.

Once all the bending is done and all shorts are cleared put a drop of oil on the front and rear condenser shaft bearings, re-connect the stators, re-install the cover and you should have some sort of reception. If the set picks up locals well and there is no oscillation when peaking the antenna trimmer control or trimmers on the tuning condenser neutralization is probably ok. To peak the tuning condenser trimmers use a local station at the high end of the dial or signal generator set at 1400 peak each trimmer using a ¼ socket on an extension. You will have to remove tube shields one at a time to get to the trimmers. At high volume control settings the set may go into oscillation when peaking trimmers. Just back the volume control off as needed. If it is impossible to peak any trimmer without causing oscillation the set needs neutralizing. Once the set is playing as well as possible you can sometimes improve reception by disconnecting the lead from the control attached to the rear of the condenser and connecting it to ground. Try it both ways after the set is playing. BTW dial calibration is a happy medium as with most TRF sets of this era.

The trimmers located between the tube sockets and visible from underneath the chassis are the neutralizing condensers. Leave them alone unless you plan to neutralize each stage. If you need information on the process of neutralization let me know. It can make a big difference in performance and has to be done before aligning the tuning condenser. You’ll have to exercise a little judgment as to how well it performs and how much more tweaking might be necessary. Leaving well enough alone is always a good thing. I’ve found that they perform pretty well with plenty of volume on even the low power local stations here.

Hope some of this is helpful,
Rick

[Gary T. Lane]

Hi Rick, Do you have any info on a Majestic 130-A?
Are all the cap replacements the 630V Mylar type and NOT electrolytic?
This set don't have a speaker YET..A Majestic electro-dymanic is on the way.
Thanks for any help, Gary