[powerdoc]

I recently purchased a Fisher 800B receiver and want to test the audio
output transformers for good/bad status and wanted to know if there was
an out of circuit (or in circuit) way to do same before I start to
retube and spend a good bit of dollars.

[robert casey]

powerdoc wrote:

I recently purchased a Fisher 800B receiver and want to test the audio
output transformers for good/bad status and wanted to know if there was
an out of circuit (or in circuit) way to do same before I start to
retube and spend a good bit of dollars.

See if you can smell any burnt order. But proceed anyway with the ohmmeter.

First I'd check with an ohmmeter for continuity of the primary and the
secondary. If it's a push-pull amp, each section of the primary should
be roughly similar resistance. One side being 10 to 20% higher
resistance would not be unexpected. See if both transformers yield very
similar resistances.

Next step is to test for shorted turns. Odds of a shorted turn vs it
being blown open circuit are fairly low, though. Especially if you don't
smell a faint burnt odor when sniffing around the transformer where its
wires come out (with the power off of course!) you can probably skip the
following test, and assume that the transformers are probably good
(however, if the transformer is heavily potted, this lack of burnt odor
test won't be valid). You'd need an audio test generator that you can
measure voltage and current on. Or a variac *AND* isolation transformer
(variacs do not provide isolation from the powerline). And an AC
milliamp meter and AC voltmeter. With no load on the transformer under
test secondary, you want to measure any large amount of current draw
when you crank up the variac voltage. Any shorted turns will cause
large current draw. At some point when cranking up the variac voltage
you may see the current suddenly go up. Back off immediately. What you
are seeing if this happens is the core going into saturation. Normal.
Both transformers should behave the same. Try loading the secondary
with an 8 ohm resistor, you should see the current draw go up. Both
transformers should produce the same current draw when doing that.

If you pass the above, the transformers are probably as good as the day
they were made. Of course if the design was poor, this test won't tell
you that, but the above test should reveal any abuse that might have
happened.

[Phil Allison]

"powerdoc" wrote in message
oups.com...

** Groper alert !

I recently purchased a Fisher 800B receiver and want to test the audio
output transformers for good/bad status and wanted to know if there was
an out of circuit (or in circuit) way to do same before I start to
retube and spend a good bit of dollars.


** First connect a sine wave oscillator set to around 400Hz to the 16 ohm
secondary and check the primary side gives the same voltage ( on a scope or
DMM ) across each half winding and that the two trannys test the same. Each
primary half should have about 10 times the voltage imposed across the 16
ohm winding.

Also check the DC resistance of each half is the same +/- 10%.

Insulation may have deteriorated inside the tranny but this will only show
up under high voltage conditions. Driving the 16 ohm secondary with the
output of another amp, at say 400Hz and 30 volts rms, will check for
insulation break down. Be very careful, doing this generates very high
primary voltages.




........ Phil

[David R Brooks]

robert casey wrote:
powerdoc wrote:

I recently purchased a Fisher 800B receiver and want to test the audio
output transformers for good/bad status and wanted to know if there was
an out of circuit (or in circuit) way to do same before I start to
retube and spend a good bit of dollars.

See if you can smell any burnt order. But proceed anyway with the
ohmmeter.

"Burnt ordure"? I know some kit is sh*t, but really

[Patrick Turner]

powerdoc wrote:

I recently purchased a Fisher 800B receiver and want to test the audio
output transformers for good/bad status and wanted to know if there was
an out of circuit (or in circuit) way to do same before I start to
retube and spend a good bit of dollars.

The other guys have all offered good advice.

The burnt smell is not a reliable indicator of a stuffed OPT
and many such fused or problem OPTs have no smell.

With the amp turned off and cold the primary halves should both measure
near equal dc resistance
between anode connections and CT for the B+, and be the same as in another
amp with equal circuitry.

If the digital meter won't read the dcr due to a lot of inductance of the
primary, place a
shorting link across the secondary.

One may use a signal gene with at least 20Hz to 200kHz range connected to
one primary anode connection.
Mine produces 6.4Vrms at about 4k7 output source resistance and
the response signal at the secondary should be a small voltage of about
1/10 of the signal across the 1/2 primary.
An oscilliscope should be used to monitor the secondary signals across the
AF range.
The F response should be -3dB at say 10Hz and 30kHz, but if the LF -3point
is say above 1kHz,
then you have a shorted turn in the primary.
Also betwen 20Hz and 50kHz the same voltage and bandwidth with opposite
phases should
appear at both anode connections.

Insulation breakdown can be seen using Phil's method, and yes, it is
potentially dangerous
because if you have a sec to pri turn ratio of 1 : 20, then with 30Vrms
applied
across the secondary, there will be 600Vrms from anode to anode.
But under real useage conditions there will be say 350V dc PLUS the ac
signal voltage
of say up to 1,000pk ac V under fault conditions, so primary to secondary
voltage could reach +/- 1,500pk V easily at each anode connection
with respect to 0V.

The source resistance for the signal input to the OPT sec under test
should be less than 4 ohms, so another power amp with a 3.3 ohm buffer
resistance
can be used so that as the input voltage signal is raised then if there is
a an arc
from the primary to ground or groundeds secondary the short won't damage
the
other amp supplying the test signal.
If there is an intermittent interruption of the signal input to the OPT
under test as the signal is
raised then it indicates a short due to insulation break down.

An alternative way to test P-S insulation is to simply apply
+2,000V dc to the CT. with anode and CT connections disconnected from the
amp.
This requires a HV supply above able to make the 2,000V, and 6 x 220k 1
watt resistances
in series between the HV supply and the CT so that if there is a short the
current flow
is limited to a harmless amount. Careful HV measurement techniquiqes are
required
since a shock from 2,000V is quite deadly.

The dc V test need not be carried on for longer than a minute, lest the
presence of the HV
cause a corona to form and ionisation and an arc over a longer time.

New transformers should all sustain dc HV for a minute, 4,000V being used
typically.


But usually such shorts from primary to something earthy can be fatal to an
OPT
since an arc started by a high ac voltage will sometimes
then be sustained by an arc of dc current of B+ supply to 0V.
A fuse might blow but many amps don't have a fuse to protect against such
an arc.

I have repaired one amp with such problems and I could hear and see where
the arc would
form, and I was able to scrape out resulting burnt area carefully and soak
the OPT in wax.
Its been in use for the last 6 years without any more trouble, and probably
it was
moisture in the ancient 1950s OPT which caused the problem to start.
And this was in a Geloso 15W PA amp B+ = only +320V for a pair of 6BQ5.

But usually arcs due to failing insulation ruin an OPT, and don't occur
where they can be seen.

If an OPT is possibly damp, ( a Katrina victim? ) then the only way to
dry it out is to heat it up to about 110C and if possible place it in a
vacuum chamber while still hot
and suck the air off with the water vapour.

Patrick Turner.

[powerdoc]

Thanks to all replies; will get on it this w/e.