Vintage

Radio & Test Instrument

Restoration 1

Updated 04/11/2021

The information presented here was authored by Robert W. Krueger (AB7CQ) copyright (2021).

For those of you who enjoy restoring old vintage radios and test instruments, I'll be adding various documents that will help you in that process, they are primarily intended for individuals who are new to the restoration process, those with little experience, or who have never attempted restoring any vintage electronic device.

First and foremost I will address the issues that will focus on the precautions which are necessary regarding the safety hazards which are part and parcel of restoration, and the necessary tools and equipment involved.

Several years ago I got involved with refurbishing old vintage radios and test instruments, during which time I became well aware of the dangers associated with working with these types of devices. I've come to the conclusion of the following:

Regardless of all the varying opinions about such gear and the potentially serious safety hazards involved, the following should always be observed when working on these types of devices to prevent personal injury, or death, to yourself, or to someone to whom you may give, sell, or donate these types of devices (remember the word liability).

1. Do not plug in the device(s) until you've thoroughly inspected it to be sure it’s electrically safe to use;

2. Before you switch the device on, replace all old capacitors, even if you find that a few aren't leaky (highly unlikely), after 50 - 60 years, or longer, capacitors have generally exhausted their working life, they're going to eventually fail, chances are, they’ll cause damage to other components ..... if electrolytic, there's a strong chance some will eventually self-distruct spewing residue all over the inside of the chassis and your work area. DO NOT depend on any ESR tester, they do not validate the entire working condition of a capacitor, they do not test the capacitor up to it‘s rated working voltage that may be encountered when operating, you must know if the capacitor is capable to operate within the working voltage requirements of the device in which it will be installed.

Also so keep in mind that old vintage radios and test equipment can produce working voltages up to 1600 volts or higher, serious injury or death can occur;

3. If the device is transformerless and the device is equipped with a 2-wire power cord, it's recommended that the two wire power cord be removed and discarded, install a new 3-wire grounding type power cord, this is necessary to help prevent the possibility of a "HOT CHASSIS" which could have leathl voltages present resulting in personal injury, or death. If you properly ground the metal chassis and cabinet enclosure you’ll lessen your chances of electrical shock, injury, death, or perhaps even prevent an electrical fire that could burn down your property. Besure that the device is properly fused.

Manufacturers of old 4, 5, & 6 tube household radios often referred to as a American 4, 5, or 6, often encased their radios within enclosures of plastic, wood, and Bakelite for this specific reason, they knew their was a high potential risk of electrical shock/hazard and possible injury or death because the chassis could be hot. Some older vintage electronics/test instruments were enclosed within metal cabinets/enclosures and most often weren't grounded;

4. Even if the device is equipped with a transformer, install a fuse block with the appropriate fuse (same reasons apply), don‘t rely on the transformer as a fuse, transformers are not fuses, they were not intended to function as a safety device, besides ....... old vintage replacement parts are expensive, even if you can find a substitute, chances are they‘re going to be pricey. I would suggest a fast blow fuse for sensitive test instruments;

Forty five or more years ago, a great many manufacturers of both radios and test instruments like Heathkit, Knightkit, PACO, EICO, Jackson and others, had a bad habit of routing the neutral through the on/off switch and they failed to install a fuse block with the appropriate rated fuse. On top of that, they used a 2-wire non-polarized power cord, so you also had a good chance of reversing the hot and neutral when plugging the device into the wall receptacle.

5. The accepted method is to route the hot line through the on/off switch and for it to be fused. You wouldn’t route/wire your home wiring neutral into your home panel circuit breaker (hot connection) and wire the hot wire into the neutral/ground bar.

6. Make sure you have the appropriate test bench protection equipment when restoring vintage devices, including amplifiers, audio or RF power amplifiers. Devices like guitar amplifiers present additional safety hazards.

If restoring these type of devices, you should have an Isolation Transformer, Variac, and Dim Bulb Tester (current limiting device). There are different types of isolation transformers, one is known as a hospital grade another is known as Tech Grade, the Tech Grade will have a floating neutral conductor, not grounded ...... once again you have been warned.

Be sure to check the wiring of any isolation transformer, before making it part of your test bench setup, the majority I've seen advertised that claim to be TECH grade isolation transformers are not what they claim to be. It's easy to modify a medical grade isolation transformer to a TECH grade.

The correct chronological order is:

1. First you plug in the isolation transformer, you never want to overload your isolation transformer, it’s ALWAYS connected to live mains first, never, ever, assume otherwise, the chassis of the isolation transformer is connected to earth ground.

You do not want to power your isolation transformer with the Variac, if the isolation transformer is operating at a voltage lower than what it's rated for, it can heat up and over time damage your expensive isolation transformer, isolation transformers always come before the Variac.

2. Next you connect the isolation transformer isolated output to the Variac (Note: The isolation transformer output has no connection to earth ground).

3. Next you connect the Variac output to your dim bulb current limiter, (Note: The Variac output has no connection to earth ground).

4. Finally, you connect/plug the device under test into the Dim Bulb Tester, (Note: The Dim Bulb Tester has no connection to earth ground).

BE ABSOLUTELY SURE THAT YOUR ISOLATION TRANSFORMER IS A “TECH GRADE ISOLATION TRANSFORMER.” Do not touch the chassis of the Variac, the chassis of the Dim Bulb Tester, or chassis of the device under test (DUT) with one hand and touch anything connected to earth ground with the other hand, if the chassis is hot, you’ll could be completing the path of hot to earth ground, many older devices only had a 2-wire plug, thus hot and neutral could be reversed and you had a 50/50 chance that the hot line was connected to the chassis.

REMEMBER: there are two types of isolation transformers:

1. Lab/Hospital Grade

2. Tech Grade

Tech Grade isolation transformers have a true floating neutral. A Lab/Hospital Grade isolation transformer can be easily modified to a Tech Grade isolation transformer.

The important thing here is that your test bench isolation transformer output has to have a floating neutral, there‘s no connection to earth ground.

There's an ongoing debate on whether the output receptacles of a Tech/Bench grade isolation transformer should be grounded or not grounded to earth. The output grounds of my isolation transformer receptacles are not grounded, as most are, this is the accepted and safest approach when working with devices under test, however, rather than trying to debate the issue here, I'm providing an excellent link on this subject so you can make your own personal decision on whether to ground, or not ground the output receptacles on your isolation transformer, here's the link:

Watch the entire video, you'll learn a lot about working with isolation transformers, the differences between medical and tech grade isolation transformers, and devices under test (DUT's).

Lastly, a lot of folks have been mislead into believing that Variacs (Auto Transformers, Variable Transformers) are isolated devices, that's far from the truth, most aren't isolated, especially those purchased on eBay and other venues. Can the Variac, Auto Transformer be modified and used as an isolation powering device ..... YES, but be sure you know what your doing, there can be serious consquences if you do it wrong.

Testing Capacitors & Leakage-

Right off, there's no way that you can be 100% sure if a capacitor is any good (leaky) unless it's tested at the rated working voltage. You can perform some rudimentary tests with an analog VTVM, digital multimeter, and a ESR tester, but none of these test instruments are going to provide you with a definitive YES or NO if the capacitor is leaky unless it's tested at the maximum working voltage that it's rated for. A capacitor may check OK at lower working voltages, however, if it's leaky it's telling you that it's on it's way out, and it's eventually going to breakdown totally.

The net result of using such capacitors is that they may cause damage to the device, worse yet, if you've ever seen a leaky electrolytic capacitor explode, you're in for a great surprise, here's a link for an actual electrolytic capacitor explosion and the result ..... dangerous and messy......Stupid Is As Stupid Does

https://www.youtube.com/watch?v=4ivvObWij1g

If your recapping a 50 to 100 year old electronic device (radio or intrument), do it right the first time, don't install suspect, or capacitors that show signs of not being able to perform to their rated specification, why take a chance that you'll have to go back and repair the device a second time.

This document is not all inclusive, there are varying approaches/opinions regarding working with grounded and ungrounded devices under test (DUT's), those require further explanation and safety precautions depending on how one approaches the testing of DUT’s. Suffice to say, BE EXTRA CAUTIOUS WHEN WORKING ON ELECTRICAL POWERED DEVICES, THERE CAN BE LETHAL VOLTAGES PRESENT.