A Little Nugget of Joy!

Here's a service story about one of the more challenging repairs I've faced in recent memory.

It's about a Hoffman Nugget portable radio. It was manufactured in the mid-50s by Hoffman of Los Angeles CA. Here's more info on the radio.

It uses sub-miniature vacuum tubes! Wow. These are the earliest 'pocket-size' portables based on vacuum tube technology. A few years later, pretty much all portables started using transistors. It's easy to see why. 

As a result, these portables based on sub-miniature tubes are a marvel as well as a curiosity. And, they are very collectible. But, they are difficult to work on because they are so small and prone to failure since they operate at relatively high voltages in a compact package.

This little radio is 6.25" (L) x 3.5" (W) x 1.5" (H) and weighs about 1lb 2oz. It uses two batteries, 45V industrial dry cell 415, and a regular C-type 1.5V. 

Here's what it looks like:

I first received the inquiry from the customer back in Nov 2017. If you've been curious about sending me your radio for repair and restoration, here's what the process looks like. Folks have been sending me their radios for repair for years. I have been thinking about representing these repairs on my Sold pages, but haven't gotten around to doing it yet.

Followed by my response within a few hours:

Months later, customer places his service order. Here's the order confirmation sent by the website. As soon as I receive the order, I reply back with my shipping info:

This repair was one of the more challenging ones I've dealt with this year. I wasn't able to find a schematic for this exact model. Someone on Antique Radio forums said the Emerson Model 747 was very similar.

The tubes weren't always easy to find either. My Heathkit tube tester wasn't able to test these tubes. 

The first thing I did was buy the 415 45V battery, which wasn't cheap. The radio didn't make any sound with fresh batteries. I checked to see whether the switch was corroded. I checked whether the earphone jack contacts were open. I verified voltages were getting to the circuit.

I decided to replace the 8uF electrolytic capacitor first with a 10uF which I had on hand, before I started swapping out the tubes which were $12-$20 each.

The replaced 10uF capacitor did not fix it.

I started ordering replacement tubes one at a time. I replaced the 1AG4 first. It did not fix the issue. Still no sound. The voltages across the circuit were within tolerance except for the 1AJ5 which was reading +1.5V across the grid. Somewhere there was a short in the ceramic capacitor network, or so I thought. I spent several months checking resistances and voltage drops in this area of the circuit to no avail. The capacitor circuit is that tan looking rectangular component at the center of the chassis pictured below:

I still couldn't find 1AH4, so I replaced 1V6. No difference. 

After I replaced 1V6, I noticed 30V volts across the grid 1 on 1AH4. Maybe I didn't notice it before. That's not good. I also replaced the 10uF electrolytic back to the original 8uF. I ensured the polarity of the electrolytic was correct.

I measured 500K across the 5mmf capacitor that connects pin 2 of the 1st stage IF transformer and it measured 500K or so. That seemed low to me. I replaced with a 5mmf capacitor.

Then, in late July I found the 1AH4. When I plugged it in, I heard sound! I was able to tune into stations across the dial.

Bascially, the 5mmf capacitor had shorted which put 30V across Grid 1 which was meant to be just -3 to -3.5V according to the tube spec sheet:

The 30V across Grid 1 basically destroyed the previous 1AH4. 

After proper alignment, the radio sounded great! Here it is on Youtube playing after almost 4 months in my shop. Usually, repairs take no more that 4 weeks shipped back to the customer.


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