The Masterpiece V receiver chassis. Note the chrome grid-cap shields on metal tubes. This radio has 5 bands with a range up to around 65mhz. It has 20 tubes counting the eye tube. There is a BFO designed for CW. The audio output is two 6L6s into what was the 18" Jensen, gone now but I am using two Jensen A12s like in the Masterpiece VI, and added a separate tweeter.

What is under all of those chromed shields? Better take a look if you are going to do a restoration - a lot of paper caps.

The underside of the chassis resembles a communications receiver such as a Hallicrafters.

Almost all of the original resistors were well within spec. This was a big contrast with the Strat chassis that I was working on simultaneously in which almost all of the carbon comp resistors were way out of spec.

The power/amp chassis is also chromed. Not quite as nice as the main chassis but still presentable.

Back side

The 2 Jensen A 12 speakers were not original equipment (according to my friend Norman B.) but were included in the exchange as was the Oxford cabinet. This cabinet is most often seen with the later 15-17 model but since all cabinets were optional, any cabinet that fits is appropriate. A note, that it has been reported that the Masterpiece V will not fit in cabinets made for the 15-17 without removing the chromed cover is puzzling since the chassis fits very well in this example.

The shelf was drilled for a pedestal mount speaker, probably also an A12. The front baffle is new and all 3 speakers are mounted to it.

The two Jensen A 12s have different FC DCR. One being about 250 ohms and the other being 5K ohms. This is good and similar to the FC values on the original Jensen Super Giant. An additional 50W 1K ohm resistor is placed inline with the two field coils which drops the B+ to just the right value and keeps the FCs from passing too much current and getting hot. A pair of Jensens with 800 ohm and 7 or 8K ohm field coils would be better but very hard to find. A pair like these is still a challenge to locate and not cheap.

I  think that the Masterpiece V is one of the best radios sold prior to WWII. Especially among those that were available to the general public (omitting some of the high tube count Scotts).

It will receive up to 65mhz, a real advantage over the Strat. This is probably a credit to the tube choices and the fact that the metal tubes were superior for RF circuits to the G type glass tubes in the Strat.

I set the 0-beat on the BFO while adjusting the IFs. It is very useful for CW signals. While the level of the BFO is not adjustable, a little careful tuning will allow SSB signals to be demodulated. This is also an option the Strat does not have.

The audio is good, really good for 1935, yet in a comparison with the 25 tube Zenith it is not quite as impressive. It does have separate bass and treble controls (tone only on the Strat) but the treble is not quite as crisp and max bass is not quite as strong as the Strat.. In this respect the 8 45 triodes with separate bass and treble interstage transformers works well for the Zenith. Keep in mind that the 18 inch Jensen would probably have helped the bass somewhat, but I am comparing apples to apples since the Zenith was stock with two A12s and a tweeter - the same configuration I am using in the MSM-V.

Tuning is always on the fast ratio until you reach the desired spot and then you can slow tune back, using the same knob. Fast and slow tuning are  separate controls or manually switched on most other radios.

There is an audio expander circuit. I'm sure it ads dome dynamic range to some music, but speech is a little weird with the volume going up and down. The expander can be switched off.

The same control will give you narrow and wide IF control. The wide IF setting would probably be used on strong broadcast stations whereas the additional 2 stages of IF selectivity are a real gain for shortwave listening.

The dial is easy to read and fairly accurate if aligned properly.

I would rather have the on of switch on either the tone or volume control but McMurdo used a separate switch mounted on the side of the cabinet.

If there is any interest, I could do a more complete comparison between the MSM V and the Zenith Strat and maybe the Scott 16 and/or the Philco 37/116 (though it is 2 year newer) and the GE M-125 (also from 1935) just for fun.

Well I couldn't wait for the tubes to get here, so I substituted some used ones in both chassis and began testing. It WORKED on the first try. Yeah, even I would not have bet on that but I'll take it.

My bench is not all that large so it take a few additional carts and such to hold all of the parts.

I did not get off with no issues though. Actually there were two.

Below you see the control/signal cable that runs between the chassis. There are three conductors that carry line voltage. Depending on the plug polarity (it is not polarized) and whether the set is on or off 120V can be on any - or 2 of the three wires. These 3 wires are twisted within the armored cable. Over years of rubbing and becoming brittle, there were many abrasions on these wires. The result could have been a short but what I saw was 120V AC on the chassis intermittently.

I knew that this cable was a problem. I had hoped that some repair to the end was going to suffice since an exact reproduction was going to be hard to produce. But now I have given into that reality.

The other issue was less significant. In the comments on the earlier Strat post below John mentioned a problem with the "Q" relay not working. Well this one - the one I rewound - didn't work either. So -

I resisted the urge to take it apart and rather measured the source voltage. It was around 300V with a short warm-up. I measured the connection to the plates of the 85 tube. It was at around 280V. So I had continuity across the relay coil and an expected voltage drop meaning that the 85 was conducting.

I then checked the grid voltage generated by the AVC tube. Something close to 0 volts allows the 85 to conduct. A signal as high as 10V will switch the 85 off, deenergising the relay. I was seeing from -.4V to about -7V. So my AVC signal was good. (when the relay is energized the closure shorts the secondary of the high frequency interstage transformer which mutes the audio until another strong signal is tuned).

So what was the issue? Checking the less than deluxe relay (see pictures in the earlier post) I found that a little nudge closed the relay. The simple solutions would be to lessen the spring tension or move the stop so that the magnetic coil was closer to the throw. I chose the latter approach and all was well.


Below is a short video after alignment. The radio is only on a short antenna so short wave reception is limited. However, when I switched on the upper band I did receive several stations, WWV and even CB transmissions. This is notable since the oscillator section of this receiver is a bit challenged above 22mhz. So I was pleased.

So this concludes the chassis restoration. Once the NIB tubes arrive (42, 5Z3 and 45s) the chassis will be reinstalled into the cabinet.

Since this project was a trade, labor less parts for a unrestored McMurdo Silver Masterpiece V, I had declined the cabinet work, instead preferring to do the fun part. The cabinet was refinished by a friend, Gary, who lives a bit north of here. The cabinet is finished, but I have only seen pictures, so far. Probably within the next two weeks you can expect to see pictures of the reassembled 1000Z.

While this project was running I was also restoring  the MPV. It is also finished - a very impressive 20 tube radio in itself. When I get some time I will post some pictures of it in the next blog.

Russ

While waiting for the arrival of the NIB replacement tubes for the Stratosphere project I have had time to work on the Atwater Kent 768Q first mentioned two posts below.

I had been lucky enough to have completed the AK 559N project a short time ago. This radio is also of that vintage with similar cabinet design as well as chassis design along with the associated common issues.

The  768Q is a 8 tube "farm" radio meaning that it was battery powered. In most circumstances, this was an effort to provide radio receivers to a large portion of the country that had yet to acquire commercial electric service in the mid 1930s.

This radio shares a cabinet with other AK models such as the AK 447. Other than the power source, significant differences between these models include  - tube count, the lack of a shadow meter and the use of a pin-type speaker using a natural magnet rather than a field coil. Most of these differences reflect the considerations that must be made to improve battery use economy. The addition of an additional tube also reflects this consideration. The 8th tube is in the audio output, the 768Q having push-pull 30s driving the speaker directly through a center tapped "magnetic" coil. This coil energizes the speaker accomplishing cone movement as compared to the voice coil in a dynamic speaker such as in the 447.

I had to use 3 separate power supplies to generate the necessary voltages. 2V for the tube filaments, 180V for the plate voltages and -22V for a C voltage (bias). I did experiment with lower plate voltages. Except for a slight decrease in volume the radio performed well down to about 135V. (decreasing the C voltage accordingly).

Above, the before and after view, noting that the cabinet had been stripped by one of the previous owners and had incurred damage including the loss of veneer as well as some structural damage.

Besides restuffing all of the capacitors and building 3 replacement resistors (note the metallic end caps of a slightly different grey/silver color) most of the rubber insulated wire was also replaced. See pile at bottom of picture above.

This chassis had survived storage very well, maybe better than any other AK of the period that I have worked on. Most of the original resistors were OK and the rubber wire was remarkably well preserved - estimating that only around 1/3 was cracked or had hardened insulation. None the less, I replaced all of the wire on the IF and RF transformers and the interstage transformer even if it appeared ok. Having the radio apart for service made replacement of the wire more practical now rather than waiting for it to fail and I did not want to take the transformers off again later. So all in all , I replace about 90% of the original rubber insulated wire with new neoprene rubber insulated wire of the same colors - or at least close.

As is often the case, the rubber insulation on the black wire was always bad and failure of other colors also tended to be similar in all examples. It seems that the pigment has something to do with the failure rate. Also degradation by mold/fungus seems to be a factor so wire that is isolated in a can or elsewhere  usually is in better shape than the mass under the chassis.

The original speaker had been removed - unplugged. It probably went to some hi-fi project, for which it would not work (well) or maybe another radio since the original owner had some interest in restoring this radio and probably had other projects as well.

Other than the cabinet being stripped/sanded, it looked like somebody had done a bit of poking around under the chassis, cutting a few wires, but nothing else.

Above: The radio in operation on the bench. Note the dial lamp. Dial lamps were often omitted from farm radios to reduce A battery consumption. It is nice that AK decided to include this option probably because it was a high end set, even for a farm radio. If the user wanted to conserve power he could have just removed the bulb.

The pin type speaker is of the same size as the original (there were two different models of speaker used in this radio). It is similar in appearance to one of the original speakers. It is not a dynamic speaker rather one that uses the lever action attached to a pin that exerts force on the cone. The "permanent" magnets are not permanent and often need repolarization. It can reproduce midrange frequencies pretty good but is very poor at reproducing low frequencies.

To make this speaker work  ,I had to remove the winding (coil) and rewind it with a center tap which goes to B+ with both ends going to the plate of one of the push-pull number 30 tubes. A number 30 is a triode similar to a 01A but with a 2 volt common cathode filament. They are efficient but don't produce a lot of audio, so using 2 in PP was a good option.

I had never noticed before that 2 of the 5 knobs have serrated edges, probably for better grip.

Above is a demonstration of the cool band selector. The 2-speed tuning is on the upper right. The knob on the lower left is a multiple position tone control and the one in the lower center is a local/distance switch.

The local distance switch is useful since it is easy to overdrive the front end increasing distortion. Testing using my AM transmitter required the switch to be in the local setting for best reproduction.

The radio has 3 SW bands with the upper band going to about 22.5 mhz. Reception on the shortwave bands is good.