Motorola B&W Portable 1974

[maxm]

Doug recently posted the original 1965 version of this portable design with the side handle.

This one appears to have been built in 1974 or maybe 75, model BP3105LR. Made in Taiwan. It must have been one of the last tube B&W sets, only two tubes left amid transistors and ICs. The back cover has the "interlock" warning message, while the bottom of the set has the solid state "no user-servicable parts" message. I believe I've also seen a solid state version of this set. The cabinet can be opened with no tools, the plastic clips that secure the power cord slide off and the back cover can be opened.

Too bad the antenna is broken off, and a chip on the side of the cabinet. Otherwise in OK condition. I bought it at a flea market many years ago.

[maxm]

Additional photos:

[drh4683]

Thanks for posting this set with the nice photos, Max. This is a very unique set and one I have not seen before. It's definitely a 1974 built set. Motorola serial numbers starting sometime in 1968 include a Julian date code in the first four digits which represent the final production date of the set. Yours is "T4062" which translates to the 62nd day of 1974, and the "T" means "Taiwan" plant. I've never seen a two-tube hybrid. Very neat!

[WISCOJIM]

A couple more of these are on ebay right now.

http://www.ebay.com/itm/262292321861?

http://www.ebay.com/itm/111668289088?

.

[dieseljeep]

Originally Posted by WISCOJIM

A couple more of these are on ebay right now.

http://www.ebay.com/itm/262292321861?

http://www.ebay.com/itm/111668289088?

.

There's a bid on the newer one already!
As much as I like Motorola and their engineering, I dont think, I could part with a C note for a set that cost that new.
I bought a Porta-Color in great condition, for less.

[dieseljeep]

Originally Posted by drh4683

Thanks for posting this set with the nice photos, Max. This is a very unique set and one I have not seen before. It's definitely a 1974 built set. Motorola serial numbers starting sometime in 1968 include a Julian date code in the first four digits which represent the final production date of the set. Yours is "T4062" which translates to the 62nd day of 1974, and the "T" means "Taiwan" plant. I've never seen a two-tube hybrid. Very neat!

There's a lot of information on the tube location diagram!
Full-wave bridge rectifier, tube heaters run on DC, CRT on separate supply, push-pull audio output and a fuse lamp, for the vertical output stage, etc.
Rather impressive!

[wa2ise]

Looking at the heater string, it looks like the two tubes adds up to 81V. One could avoid the power dropping resistor by using an unfiltered diode off the powerline to heat these heaters to their correct temperature. I'd use a separate diode connected between the grey wire feeding the bridge rectifier and its cathode to pin 12 of V1. Disconnect the dropping resistor. The extra diode would allow only half the AC waveform thru to the heaters. As heat is power, and power is volts squared over resistance, you'd want a heater string of 120V * (1/square root of 2) = 83V. Not 60V. The diode divides the delivered power by 2.
This would cut waste heat inside the cabinet. Today a diode would be cheaper than a power resistor.

[zeno]

Must be the last real Motorola B&W. I have seen one
but probably only one. The antennas broke out of almost
all of these sets for the many years they built them. Made
a kit to fix them. Good rare catch.

73 Zeno

[dieseljeep]

ay

Originally Posted by wa2ise

Looking at the heater string, it looks like the two tubes adds up to 81V. One could avoid the power dropping resistor by using an unfiltered diode off the powerline to heat these heaters to their correct temperature. I'd use a separate diode connected between the grey wire feeding the bridge rectifier and its cathode to pin 12 of V1. Disconnect the dropping resistor. The extra diode would allow only half the AC waveform thru to the heaters. As heat is power, and power is volts squared over resistance, you'd want a heater string of 120V * (1/square root of 2) = 83V. Not 60V. The diode divides the delivered power by 2.
This would cut waste heat inside the cabinet. Today a diode would be cheaper than a power resistor.

I looked high and low and couldn't locate a 58GY7. The only tube from that familyw is a 50GY7. I know that the TLG shows 53 or 58, maybe a typo?
Anyway, it's always an open discussion, why certain manufacturers did things the way they did! That's what makes this hobby a little more interesting, than some others.

[rca2000]

I think it is 50GY7. A number of them are available on ebay now !!

http://www.ebay.com/itm/VACUUM-TUBE-...h4rQdvlRnfuSdQ

http://www.ebay.com/itm/VACUUM-TUBE-...4TRRVNLDu0xJIA

http://www.ebay.com/itm/One-Tested-N...IAAOSwbqpT9lST

[dieseljeep]

Regarding using a diode dropper, Sylvania did that in one of their earlier series string color sets. The diode would have a tendency to short and run the tubes at excessive voltage, until one burns out.
The factory recommended fix was to replace the diode with a power resistor.

[wa2ise]

Originally Posted by rca2000

I think it is 50GY7. A number of them are available on ebay now !!

Looks like you're right, 50 not 58.

The diode would have a tendency to short and run the tubes at excessive voltage, until one burns out.

With the tubes now adding up to 75V I'd have about 8V to burn off to get to 83V, so with the diode I'd use a deliberately maxed out in wattage resistor to drop that voltage. Idea being if the diode shorts out, the resistor would then burn out, like a fusable resister.

[KentTeffeteller]

Originally Posted by zeno

Must be the last real Motorola B&W. I have seen one
but probably only one. The antennas broke out of almost
all of these sets for the many years they built them. Made
a kit to fix them. Good rare catch.

73 Zeno

I am reasonably sure this was the last model Motorola B&W Portable built. Then the Matsushita/Panasonic takeover. A fine, rare catch. Very nice little set.

[tom.j.fla]

Looking at service data SAMS #1493-2 the heaters run on dc to get the 20 volt reg. supply for the solid state circuits less heat generated then a big honken 20 watt power resistor. All the best, Tom