Mcmurdo Silver 15-17

Restoration of McMurdo Silver 15 17

Author: Edgardo Castro Bruse

Introduction

This radio was made by McMurdo Silver to the order of Don José Velazco, probably in 1938. Don José inherited the radio to his son Gabriel Velasco (YS2JV). He in turn, gave it as a gift to Dr. Fernando Alcides Melgar (YS2AFD), who kindly, gave it to me (YS1ECB) as my 70th birthday present. Please see pictures.

Initial Observations

The radio had been partially dismantled: the power transformer, filter choke and electrolytic capacitors had been removed. The split secondary audio driver also had been removed. And the output transformer was missing.

To make tests and to finally operate the radio, I used my homemade all purpose power supply. It provided a B+ of 160 volts and 6.3 VAC. The radio operates absolutely well with these voltages. Please see pictures.

Brief Description of the Circuit

This radio has a number of refinements which have been described in a number of internet places. So, I will make only a general description, such that, the reader gets an idea of the magnificence of its design.

- This radio tunes from 0.53 to 32 Mc. in four bands.

- The radio uses a 6L8 G simple conversion stage, preceded by a 6K7 G radio frequency amplifier.

- It has two 6K7 G stages of IF amplification tuned at 472 KC.

- It also has an L/C filter that provides variable selectivity of 3Kc. 6Kc and 16Kc.

- The radio has a 6C5 detector of the infinite impedance type. This type of detector eliminates the harmonic distortion common to diode detectors. It also provides additional selectivity to the IF amplifier.

- Finally the audio amplifier carries two 6L6 s in a PP 16 watts class A amplifier. These tubes are driven by a 6C5 tube using a split secondary transformer.

Please see diagram. Additional information can be found here.

Disassembling the Front End of the Radio

In order to thoroughly clean the Tuning Mechanism, and its very original vernier type fine tuning system, The Dial’s assembly had to be removed. I have outlined this process in seven steps. Please see pictures.

Re-assembling the Front End of the Radio

I present here four pictures that represent the Re-assembling process. Please see pictures.

Pictures Before Restoration Process

Repair Work

The repair work consisted of:

a - Exhaustive Cleaning.
b - Tracing the circuit to determine the connections points for the filament voltage and the B+ voltage.
c - Adapting a similar split secondary audio driver transformer.
d - Adapting a similar PP audio output transformer.
f - Repair of one 472Kc. IF transformer.
g - Adapt a 6BA6 RF amplifier to replace the original 6K7 tube.
h - Modify the BC antenna transformer. Please see pictures.

Pictures After Restoration Process

Final Remarks

Due to the requirements of a 6L6 PP amplifier working in Class AB1; it is reasonable to assume that the original Power Supply provides 360 volts for the plates and 270 volts for the screen grids and the RF circuits.

I found that with my home made Power Supply which provides 160 volts under load and 6.3 VAC /7 amperes the radio works great. The radio shows great RF sensitivity in all bands.

It also has a rich tonality and a sufficiently strong audio power.

The volume control functions in a unique way. For a short part of its initial rotation the sound is likely to benefit speech (voice range). Past this point sound evolves into truly high fidelity sound.

The radio has a special vernier for real fine tuning.

As the tuning knob is rotated, searching for stations, and you suddenly stop because you found one. It immediately engages a speed reduction mechanism, such that, if you move forward or backward about this frequency a vernier action is executed for a short length. Past this length, normal tuning action is resumed! Until another station is tuned and the cycle repeats!

Finally I would like to say that I feel very happy to restore and posses and enjoy this radio.

This work is also a humble tribute to the memories of Don José Velasco and his son Don Gabriel Velasco. Hijos meritísimos de El Salvador. And my appreciation to Dr. Alcides Melgar and the owner of this site who kindly uploads this work!