Vintage
Radio & Test
Instrument
Restoration 1
Updated 04/11/2021
The Information Published Here Is For The Noncommercial Use Of
Radio Hobbyists
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.
Restoration Safety
Very Important Safety
Information
Restoration of vintage radios and test gear can be
an enjoyable and rewarding experience, however, it also involves a high risk of
exposure to high voltages which can cause serious injury or death if one is not
experienced with working on these types of electrical devices. You have been
warned, if you work on these types of devices you do it at your own risk. I know
this safety document is somewhat lengthy, but it’s worth reviewing.
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.
There's another good reason to use a 3-wire
grounding type power cord:
1. It provides a safety chassis ground;
2. It helps eliminate the possibility of ghost
voltages;
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:
Service mains to isolation transformer, to variac,
to dim bulb current limiter, and finally to the device under test
(DUI).
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'' 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 receptacle grounds of the isolation
transformer are not grounded, 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:
https://www.youtube.com/watch?v=5SjVm-rF39A
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-
Capacitor leakage needs to be measured at the
full rated working voltage of the capacitor.
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, the first thing this guy should have done was "NOT PLUG
IN, OR TURN ON THE OLD RADIO OR INSTRUMENT", and knowing the age of the
device he should have replaced all the old capacitors before powering up
the device.
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.
Vintage capacitor test instruments are becoming
harder to find, those that have been refurbished and are guaranteed in
working condition. Prices have been driven exceedingly higher due to the nature
of the economy and availability. Suffice to say, if you can find one, at a
decent price, I would recommend the Heathkit IT-28. I wouldn't pay more than
$225 - $250 for it. Lately, the hijackers on places like eBay are charging up to
and over $600 for one, strictly greed driven.
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.
04/01/2021
Bob Krueger,
AB7CQ
Web
Administrator
RPTR 1:
146.920/146.320 PL 123.0 (WIRES-X Room 28187)
RPTR 2:
444.600 / 449.600 PL 100 (LAN Linked)
Simplex IRLP
Node: 7515 PL114.8 (146.540)
Email: ab7cqradio@ebidpal.com
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