Re: FS: Marantz 10B tube tuner $2250
- From: "Bret Ludwig" <bretldwig@xxxxxxxxx>
- Date: 4 Sep 2006 18:57:02 -0700
More:
reform.txt
From K1LKY
(Updated Jan '06: last paragraph.)
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 possible situation:
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 escapes. 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. Now you are looking for both a
replacement capacitor AND a power transformer.
---
Variacs.txt
From K1LKY
Updated:
July '05: Sola information
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.
The following is from the maker of SOLA (TM) transformers about their
CVS, MCR and MPC Series units at:
http://www.solaheviduty.com/products/powerconditioning/pdfs/opchars.pdf
"Overload and Short Circuits
When the load is increased beyond the regulatorís rated
value, a point is reached where the output voltage
suddenly collapses and will not regain its normal value until
the load is partially released. Under direct short circuit, the
load current is limited to approximately 150-200% of the
rated full load value and the input watts to less than 10% of
normal.
A constant voltage regulator will protect both itself and its
load against damage from ex ces sive fault currents. Fusing
of load currents may not be necessary. The actual value of
short-circuit current varies with the specifi c design and
rating. Units may be operated in defi nite ly at short-circuit.
This characteristic protects the unit itself as well as the
load and load circuit being served. ... "
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.
.
- References:
- FS: Marantz 10B tube tuner $2250
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