National HRO-60

Restoration of National HRO-60

Author: Edgardo Castro Bruse
Author: EDGARDO CASTRO BRUSE
San Salvador October 2022


Introduction

I obtained this radio back in 2003 from my friend José Antonio Guevara, YS1JW.
I gave him a pair of Hi Fidelity speakers in an enclosure made and designed by our mutual friend Ing. Ernesto Chan Peña, YS4EP. He gave me the HRO-60 in a non working condition and with only one set of plug in coils (plug in box with coils) AB. It covers from 25 to 35 Mc.

National HRO-60

Initially I repaired this radio in 2003, and at a later time, I was able to get two other plug in coil sets. Thus I was able to cover from 25 to 35 Mc. with Box AB. From 1.7 to 4 Mc. With Box D and from 50 to 54 Mc. With Box AD. It becomes evident that a major section of the short waves was left out. Please see pictures.

Box AB Box D, Box AD, Box AB

Description of HR-60

This radio is able to tune from 50 Kc. To 35 Mc. And from 50 to 54 Mc. By means of 13 plug in coil sets or plug in coil boxes.

Above 7 Mc it uses double conversion. The first IF is 1990 Kc. And the second IF is 455 Kc. It has a crystal filter with variable selectivity in 6 steps. It comes with 2 steps of RF amplification and a push pull of 6V6 s in the audio amplifier. It receives AM, CW and SSB in the CW mode. Please see here
Please see pictures for external description of my HRO.


2022 Mayor Repairs Steps

For almost 20 years I had postponed the repair of the contacts (inside the radio) that connects the coils in the plug in boxes to the circuitry of the radio. Each band has its own plug in box. These copper strip contacts were much deteriorated, but the radio operated miracously well. Please see pictures.

contacts that connects the coils in the plug in boxes Each band has its own plug in box copper strip contacts were much deteriorated Contacts need to repair

A - Reaching the Band Change Contacts.

There are 4 porcelain boards. Each one carries 5 copper contact strips. These boards are bolted under the chassis and become visible and functional trough 4 openings in the chassis. To take off these boards, in order to repair the copper contacts, one must first, remove the band indicator cylinder, then the huge variable condenser and finally the metal frame under this condenser. Note: carefully mark the wires of this condenser before disconnecting them! Please see pictures.

inside the case AB box installation chassis band indicator cylinder

B - Repair of the Copper Strips.

This project was possible because, finally I was able to find and buy in eBay a porcelain board with excellent copper strips.

I removed the porcelain boards one at a time, and carefully marked each wire.
Each board was left overnight in vinegar, then vigorously brushed with UNOX detergent and thoroughly washed with water. Finally it was dry with a hair dryer. At this moment the bad copper strips were removed and replaced by the new ones. Immediately the board was installed in place. The process was repeated with the other 3 boards. The complete process took me 4 days, with rewarding results! Please see pictures.

rivets contacts screw before all contacts were deformed dirty and with some rust after

C - The Variable Condenser.
Besides the general clean up and oiling, the most important work was to remove the collars that connect the rotor to the body of the condenser, in order to provide a proper ground path. I brushed them with UNOX and then washed them with plenty of water. This made a noticeable improvement to the performance of the radio. Please see pictures.

Brush before cleaning Contact brush Brush after cleaning Cleaning and oiling Variable Condenser

D - The Short Wave Converter.
An alternative solution, to the lack of the plug in coil boxes in the HF range, was to design and build a crystal control converter. I used my AB plug in coil box, from 25 to 35 Mc. as a variable intermediate frequency IF. Please see diagram and pictures.

Diagram Crystal control converter Inside converter

To help the reader visualize how the variable IF works, I have prepared a table of data considering three different crystal frequencies and box AB.

You can easily see that, with Box AB, a 17.00 Mc crystal would provide a tuning range from 8 to 18 Mc. Please note: the IF frequency is the HRO reading minus the crystal frequency. My friend Dr. Andres Goens YS1AG had a 17.23055 Mc. crystal which was close enough and my tuning range was slighted changed from 7.77 to 17.77 Mc. Consequently the RF stage of the converter must tune from 7 to 18 Mc. Please see table of data.

HRO frequency table

E - Conclusion.
The uses of a short wave converter, based on the plug in coil box that one has, is a very viable way to extend the HF range of the radio. With this method you can also cover the regular broadcast band and even the Low frequency bands!




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