[Hallicrafters] HT-37 Questions - Roy's Diatribes

[Roy Morgan]

At 02:22 PM 6/2/03 -0500, "John T. Mudd" <w0tlo at juno.com>, wrote:
>How about posting further info to the list for everyone's benefit?
>
>Enquiring minds want to know!

John,

Ok here goes:  I include the following diatribes, in order:

1) Variacs, Just Say NO
2) Roy's Diatribe on Fused Line Cord Plugs and Line Cords and RF Bypassing
3) To Re-form electrolytic capacitors:

I hope this saves someone's life, or even a radio's

Roy


<Soap Box Mode ON>

VARIACS, Just say NO!

Your variac CAN produce up to ONE HUNDRED AND FIFTY TWO* volts output, IF:
  -  it is not re-wired to accept modern line voltages ** and
  -  it is wired for overvoltage operation, and
  -  you JUST HAPPEN to turn it all the way up.
*(this is assuming that your variac is set up for 115 volts input and 140 
volts output.)

Do you want that to happen?
Even for a minute?
I don't think so.

Note: The SP-400 and SP-600 power transformers have a 125 volt input tap. 
Use it.  Most other radios do not have such a tap.

You can set up a voltage bucking transformer to run your boatanchor safely 
at its rated voltage.  See the following link for details:
http://www.r-390a.net/faq-HiVolt.htm

Sola or similar constant voltage regulation transformers can be had with 
115 volt output, and they are self -current-limiting.  They are noisy, hot 
and inefficient, but do have many advantages.   It may be the case that 
some of those set up for 120 volts output can have a few turns removed from 
the output winding to lower the output voltage, although the Sola 
transformers I have seen are varnished thoroughly.  You may also be able to 
ADD a few turns in the opposite direction to lower the output voltage - a 
built in bucking winding.

If you really want to take the risk of applying overvoltage with your 
variac but want its dial to read correctly (that is, tell you the awful 
truth when you smell smoke from your radio), read on:

** Briefly, to correct your Variac so that the dial reads correctly for 
modern line voltages, do these steps CAREFULLY:

1) Measure your nominal house current line voltage.
2) Make sure the thing is UNplugged.
3) Move the input tap of your Variac to the winding the wiper rests upon 
when the  dial is set to indicate that voltage.  Now your dial will read 
correctly most of the time.

Careful use of an Exacto knife will loosen the one winding you need.  Then 
lift it enough to strip the enamel and solder the input tap.  Use a piece 
of insulating tubing under the lifted section, and apply some coil dope, 
finger nail polish, or varnish to hold it all in place.  If the original 
input tap is made on the inner side of the windings, just tape it off and 
make a new tap on the outer side of the winding.

One Superior Electric variable transformer I have has a center-off switch 
for 0-115 and 0-135 volts.  The dial plate shows 0 to 100 percent.  THAT 
one is dangerous for sure.  There is no way you can turn that thing off in 
a panic situation without moving the switch to the opposite ON position.

While you are at this, check that there is a fuse in the OUTPUT of the 
variac, not just the input, install a three-wire cord AND outlet if it 
doesn't have these.

Of course, the thing will still produce overvoltage, so a better method is 
to move the input connection to the end of the winding and locate a new 
dial plate or re calibrate the original one.  Some variacs had two-sided 
dial plates with the different calibrations.  Be careful, because some 
two-sided plates are meant to be used for panel mounting or case mounting, 
where the dial is either fixed on the panel, or is mounted on the knob and 
rotates with it. The numbers increase in opposite directions for the two 
mounting methods.

Good luck, and long live your radios.



Roy's Diatribe on Fused Line Cord Plugs and Line Cords and RF Bypassing

Fused Line Cord Plugs:

Under no circumstances should you ever use a fused line cord plug, 
period.  It can kill you in a variety of ways.  The Johnson company put 
them on Rangers, Valiants, and other equipment.  I have a Heath VHF-1 6- 
and 2-Meter transmitter with one on it.  That will be the first thing to go 
when I start returning it to serviceable condition.

Some time ago I wrote imaginative but quite serious descriptions of some of 
the many ways fused line cords can make a widow out of your wife.  I want 
to re-write that thing and put in back into circulation, but that will have 
to wait for another day.

In summary, however, the way to make your wife a widow is as follows:
1) The equipment with the fused line cord plug suffers an internal short 
such as in a transformer or RFI bypass capacitor, with the short circuit 
more or less to the chassis.
2) ONE of the line cord plug fuses blows.
3) You unplug the thing, move it to a work bench, and the ground you 
*might* have had on the chassis is removed.
4) You plug it back in and haplessly insert the unpolarized plug so the 
intact fuse puts line voltage on the chassis.
5) You reach for the power switch, the current kills you and your wife 
becomes a widow.


This is a sure-fire topic to generate much traffic on any mailing 
list.  People's attitudes seem to fall into four groups:

1) "Originality forever." To hell with the fact that it may kill me or 
someone else, I will use the original deadly fused line cord.

2) "But of course."   Safety in line cords is easy to understand and worth 
paying attention to.

3) "Hmmm..."  I'm glad to know about all this (but I may not DO anything to 
prevent my death or that of any other hapless and innocent person.)

4) "Problem?  What problem?  There's no problem here." Duuuhhhh!

Be safe, live long. Do not use fused line cord plugs.
Install a three-wire grounded line cord.



Line cords and how to install one safely:

(in US standard line cords):
GREEN is safety ground, and should be tied directly to the chassis.

BLACK is "hot" or "line" - it goes directly to the rear of the fuse holder, 
the terminal farthest from the outside of the fuse holder where the cap is 
installed.  The terminal on the fuse holder nearer the chassis or panel 
goes off to the switch and should go nowhere else.  If you use an open 
clip-style fuse holder under the chassis, use either end of it.

WHITE is "neutral" and goes UN fused to the system, e.g. power transformer 
primary.  Do not put a fuse in both power cord lines. This can lead to a 
dangerous situation, though it's less likely to be dangerous with a three 
wire grounded line cord than with the deadly fused two-wire un-polarized 
line cord plug.

On the line cord wall plug,
  - the round, longer pin is green
  - the larger flat pin is neutral
  - the narrower flat pin is line or hot.

Note that European color coded line cords have different colors.. Green is 
still ground, but there are brown and blue colors used.  Below is 
information about the standard.  Carefully test with an ohmmeter.

(begin extract from message by Bob Nickels)
From: "Robert Nickels" <w9ran at oneradio.net>
Subject: [Johnson] Fused Plugs
...
A good source of replacement 3-wire power cords are those used by computers
and peripherals, but most of them use the European color designations   For
years, the colors of individual conductors in cords for use in North America
have been black for line, white for neutral, and green for earth (ground).
But in order to harmonize worldwide standards,  two major UL equipment
standards, UL 1950, Information Technology Equipment, and UL 2601, Medical
and Dental Equipment, started several years ago to require the more
traditional European conductor color coding: brown for line, light blue for
neutral, and a  combination of green and yellow for ground.

The color conversion is as follows:

LINE            (US) Black  =  (EU)  Brown
COMMON   (US)White  =  (EU)  Blue
GROUND    (US) Green     (EU)  Green/Yellow

(end extract from message by Bob Nickels)

Older HP and other test equipments were equipped with an oval line cord 
connector and matching cord.  The Belden/Volex 17280 power cords are 
apparently the normally-connected cord.  In all of them, the offser 
(center) pin is chassis ground.  There is a version with reversed line and 
neutral. When working with these equipments and line cords, do take time to 
sort out hot from neutral so you retain the safety aspects of the fuse 
connection.


RF Bypass caps should be installed as follows:

One from Line to Neutral, after the fuse.
One from neutral to chassis.

Do not install one from line to chassis and neutral to chassis as was 
normally done years ago.  This causes a danger of fire should the line to 
chassis cap short but not draw enough current to blow the circuit breaker, 
and this arrangement also causes the chassis to be at half the line voltage 
if the safety ground is not present (such as in a two-wire outlet used with 
an adapter, or in an outlet improperly wired or faulty.)

There are currently available "Safety" capacitors meant for line bypass 
applications. You can tell them from normal caps in the catalogs because 
they cost about 5 times as much as normal caps.  If you take apart computer 
power supplies or junk TV sets, you will wind up with one or two from each 
unit.

Fuses:

The topic of fuse installation and choice of fuse type and rating is a 
complicated one, but here are some points to consider:

  1) For normal equipment, put one fuse only, in the Hot wire of the line 
cord as near as practicable to the point where the line cord enters the 
equipment.  The black, Hot wire of the line cord runs directly to the tip 
of the fuse holder.

In equipment such as the Valiant transmitters that are made with extensive 
RFI filtering on all leads exiting the case, you may not want to add a rear 
chassis skirt mounted fuse holder. In other radios that never did have a 
fuse, such as the Hallicrafters S-20R, adding a fuse holder would ruin the 
originality of the set.  In these cases, mount an open fuse clip below the 
chassis, possibly using an existing screw or transformer mounting bolt.

Some military equipment was built with a fuse and possibly a switch section 
in both wires of the line. This was done  where the equipment was to be 
used aboard ship or in other places where the power circuits were floated 
from ground for safety and reliability reasons.  (This situation also 
applies generally in Sweden and some other European countries.)  If the 
equipment is in good condition you can leave it as it was built but make 
sure you have a good three wire grounded line cord and that your outlet 
grounds are properly connected.

Note: the three-terminal MS series, "Amphenol" type connectors used by the 
military on such equipment as the CV-591 and CV-89 were installed with (at 
least) two different pin arrangements.. Be very careful that you dope out 
what you have when working on equipment of this sort. Mis-matching the cord 
to the equipment could lead to trouble.

2) The Hot or line wire of the power cord goes directly to the rear of the 
fuse holder, the one farthest from the outside of the fuse holder where the 
cap is inserted.  The terminal on the fuse holder nearer the chassis or 
panel goes off to the switch. This reduces to the absolute minimum the 
amount of conductor inside the equipment that is not protected by the 
fuse.  And it reduces the chance of a shock when you are installing or 
removing the fuse.

3) The "cold" or neutral side of the line cord should NOT be fused.  The 
Green, safety ground wire should never be fused.

4) Follow manufacturer's specs for the type and rating of the fuse.  Beware 
of low voltage fuses that may fit the fuse holder you have. Fuses rated at 
32 volts may not properly protect you and the equipment in normal line 
voltage situations.  If you don't know the ratings needed, make a guess at 
the rating and use smaller and smaller fuses until they blow from time to 
time.. then increase the rating a bit.


GFI and Hot Chassis Troubles:

The R-390 series of receivers, among others, was built with robust line 
filters. The arrangement of the capacitors places half the line voltage on 
the chassis if the chassis is not grounded.  This is NOT because the line 
filter capacitors are leaking or shorted, it is normal voltage division 
behavior of the circuit as installed.   If the chassis is grounded, enough 
current flows in the safety ground wire to unbalance the two line currents 
and trip many ground fault interrupter devices.  Old style methods of 
bypassing both wires of a two-wire line cord to the chassis can do the same 
things.  Steps to correct these troubles include:

1) Remove the line filters or line bypass capacitors connected in the old 
style and either leave them out or install bypasses as above.

2) Use an isolation transformer or a "Sola" type constant 
voltage  transformer (most of which have isolated secondaries).  Note: most 
"Variacs" or variable voltage transformers do NOT provide any 
isolation.  Some that do apparently exist but they are rare.  Further, 
fuses in these things may be in the input wire only and not in the variable 
voltage output wire.  This can lead to overloading the low voltage turns of 
your transformer.  Both input hot and output hot should be fused.

3) Use properly grounded three wire line cords with non-GFI protected power 
circuits.


Outlet Safety Testers:

Most home stores and electrical supply houses can sell you a small gadget 
to test your outlets with.  They have a number of neon lamps that indicate 
proper functioning or various fault conditions. The cost is less than $10 
and is one of the best investments in your safety you can make.  Even 
"licensed professional electricians" can and have made mistakes wiring up 
houses, and time and wear can open up safety grounds that were installed 
properly in the beginning.  You owe it to yourself, your family, and later 
occupants of your house to buy and use one of these very helpful gadgets.


Think safety.
Install proper grounded line cords.
Live long.


To Re-form electrolytic capacitors:

In brief, supply the capacitor with a current-limited voltage source
higher than the working voltage of the cap.  WATCH what happens.

With the "patient" set off, set the external supply at the rated voltage
of the cap(s), and feed the old set at the input to it's B+ filter through
a 100K, 2W resistor.  (Use a higher value if it dissipates more than
2 watts.)  The old caps will slowly come up to voltage as
their electrolytic layer re-forms after long storage.  You may want to
unhook bleeders or screen voltage dividers, and remove any VR
tubes,  if present in order to get no
dc load other than the caps.  Once re-formed up to nearly the cap rating,
increase the external supply voltage to the point where increased voltage
only increases the current drawn (the electrolytics begin to "leak".)  You
can vary the series resistor depending on the voltage of the cap you're
trying to reform.

If you have no external supply, remove all tubes except the rectifier, place
the high resistance in the circuit between the rectifier and the filter 
capacitors
and use a variac to raise the voltage slowly.

If the final cap(s) voltage is high enough, it doesn't need to be
replaced.  If it's too low, put new one(s) in (leave any original cans in
place for appearance, and substitute new axial lead ones under the
chassis.)

Some caps take only a few minutes to re-form.  Some take a day or so!  Be
patient.  Your Adjusta-Volt or Variac can be well-used for this if your
external supply is solid state, or has a separate hv supply transformer.
I have one good for 900 volts no-load having 5R4's and separate filament
transformers.  This lets me re-form 500 volt electrolytics if I need to.

With a 500 volt supply, and a number of 100k or 200k resistors, you can
re-form a number of caps all at once.  Measure the voltage on the caps as
time goes on with a high-input-resistance meter (VTVM or solid state
DVM).  Allowing an electrolytic to idle with a small leakage current of 1
to 5 ma won't hurt it, so if the thing re-forms to it's limit during the
night after you've left it on the re-former, no harm is done.  If it quits
increasing in voltage at, say, one half its rating, still no harm is done.

Most electrolytics in good health will leak at a voltage from 125 to 200
percent of the continuous rating.  If the final leakage voltage is low, 
only a little
(below about 110 percent of the cap's rating),
then you can expect it to not live too long.  New axial
lead caps are fairly cheap, and are for good peace of mind in my opinion.

PAPER COUPLING CAPS:

Test interstage coupling caps (e.g. from an audio driver tube to the grid
of the output amp tube) by measuring the dc voltage at the grid  (across
the grid resistor if it's not going to ground).  Use a high-impedance
voltmeter like a VTVM or DMM.  If it's above zero, you need a new cap!
The vast majority of paper caps from the 30's through the 60's are at
least moderately leaky now.  Your tubes will thank you with long life for
replacing these caps.  Most disk ceramic caps have indefinite life expectancy,
as do good quality modern film caps.

Fixed bias supply to audio output tubes require you to think through the 
circuit:
compare the bias you expect at the grid with the measured voltage.

High power tubes such as the 6550, and KT-88 are especially manufactured to
reduce grid current, and maximum grid circuit resistance limits are often 
approached
by designers.  Worn-out tubes of this type will show increased grid 
current, especially
if they are being run at high plate dissipations, or high cathode current.
Carbon resistors in such grid circuits are among the most likely to have
drifted high in value, thus exceeding the design limits of the tube. The result
is a tube that will run away in cathode current.  Watch for red plates.

You can do this kind of testing while you are re-forming the filter caps
in-circuit.  The tubes are off, and will not be harmed by excessive plate
current while you find all those leaky paper caps.  The voltages across
them will be higher than  normal running conditions, because the driving
stage is not drawing any plate current through the plate load resistor.

SCREEN BYPASS CAPS:

With B+ applied and the tube pulled or set off, the voltage at the screen,
again measured with a high-impedance voltmeter, should be the full B+ or
value at the other end of the screen dropping resistor.  If not, the cap is
leaking.

LOOSE CAPS:

Set your high-impedance voltmeter to a range at or above the B+ supply you
intend to use,  and clip one end
of the cap to the DC probe and connect (carefully) the other end to a B+
supply corresponding to the rating of the cap.  The meter will jump up
briefly and then settle down toward zero.  Analog meters (VTVM's) are good
for this because you can watch the movement of the needle.  Once the
reading settles, any indication much above zero indicates leakage.  A
quick ohms-law estimate with the input resistance of your meter will give
you a value for the leakage.  DVM's are often 10 megohms, and so the
leakage will be indicated at about 10 volts per microampere.  Some few
VTVM's such as the HP-412, I think, have an effective dc input resistance
of 100 megohms.

Here's an exercise for all those who want to "just bring the set up slowly
on a variac":
  - get an old electrolytic cap
  - get a variable B+ supply, with no current limiting
  - put a current meter between the two and a voltmeter across the cap
  - bring the supply up and watch what happens.

Then think about this: You have a radio with a three -section filter 
capacitor. One section is going to leak seriously at a moderately low 
voltage, but you don't know it.  You just "bring it up slowly on the 
variac" and as soon as the rectifier tube begins to operate, your capacitor 
is drawing excessive current. It gets hot and blows its guts all over you 
lovely radio.  Or, your power transformer high voltage winding gives up the 
ghost after only a moderate amount of the smoke escaped.  You had no idea 
this was going to happen because you did not have your hand on the can 
capacitor to feel the heat and did not monitor the B+ current as it 
heated  up the cap and the transformer.


<Soap Box Mode OFF>



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --