Hello. I'm fixing up a Harvey Wells T-90 transmitter and have a
question. Perhaps another T-90 owner would be so kind as to help.

The tube socket resistance table in the instruction manual states that
the resistance from V9 pin 8 to ground should be 400 ohms. Same for V10
pin 8 to ground. These are the grids of the modulator tubes, fed in
push-pull from the secondary of driver transformer T1 which has a center
tap to ground. In my unit, the resistances are only half that much (211
ohms and 223 ohms). I'm suspicious that the 400 ohm value in the
resistance table might have been meant to pertain to the *total*
resistance between the two grids.

It would seem an unlikely coincidence for the transformer to have failed
with shorted turns and end up with exactly half the specified resistance
in both halves of the secondary. Of course, that's always a possibility.

Would someone else with a T-90 please help by checking these resistances
for me? I would appreciate it very much. (Disconnect the transmitter
from the power supply while making this measurement.)

73 & TIA,
Joe K9LY

Joe,I have a H/W T-90 The mod driver shows 400 Ohms one side and 390
other side. On these older rigs it is not uncommon to find un-balance
in the windings and many have had the driver xfmr replaced,,windings
are extremely small. gauge wire..I know the people that built them
never expected us to be using them over 50 years later...hi..A great
little rig... 73 Harold W4PQW
Joe L. wrote:
Hello. I'm fixing up a Harvey Wells T-90 transmitter and have a
question. Perhaps another T-90 owner would be so kind as to help.

The tube socket resistance table in the instruction manual states that
the resistance from V9 pin 8 to ground should be 400 ohms. Same for V10
pin 8 to ground. These are the grids of the modulator tubes, fed in
push-pull from the secondary of driver transformer T1 which has a center
tap to ground. In my unit, the resistances are only half that much (211
ohms and 223 ohms). I'm suspicious that the 400 ohm value in the
resistance table might have been meant to pertain to the *total*
resistance between the two grids.

It would seem an unlikely coincidence for the transformer to have failed
with shorted turns and end up with exactly half the specified resistance
in both halves of the secondary. Of course, that's always a possibility.

Would someone else with a T-90 please help by checking these resistances
for me? I would appreciate it very much. (Disconnect the transmitter
from the power supply while making this measurement.)

73 & TIA,
Joe K9LY

Thanks Harold for the quick response! Darn, that must mean my driver
transformer *is* fried. It's the original one because the
factory-applied red varnish on the solder joints is intact. So, the hunt
begins for a replacement. It's got a Thordarson part number stamped on
it (T-54296) but it's not listed in either of the two Thordarson
catalogs (dated 1942 and 1980) that I found on the Web. Maybe it was
custom made for H/W.

Also found an open 27K resistor in the clamp circuit. Finding a
replacement won't be as tricky as finding the transformer, hi.

It's true, no one expected us to be using these things half a century
later. But the T-90 was so well crafted, it would be a shame not to
bring it back to life. I've got an R-9A fixed up and waiting for its
mate.

Thanks again for the help!

73,
Joe K9LY


In article .com,
" wrote:

Joe,I have a H/W T-90 The mod driver shows 400 Ohms one side and 390
other side. On these older rigs it is not uncommon to find un-balance
in the windings and many have had the driver xfmr replaced,,windings
are extremely small. gauge wire..I know the people that built them
never expected us to be using them over 50 years later...hi..A great
little rig... 73 Harold W4PQW

Hello Joe:

I question whether there is a problem with the driver transformer. What are
the symptoms, other than the different value of resistance? Is this just a
general check of everything or is the audio section having a problem.

There are a number of possibilities. First, the factory could have
substituted a different transformer from that specified. This happend more
than infrequently, in order to keep the rigs flowing. They are not
critical. Secondly, the ohmmeter might be something other than a pure dc,
in which case the resistance might not be correct when inductance is
involved. Another possibility is that the factory put the wrong transformer
in the case and it got mismakred - but it was close enough that it passed
factory specifications.

A shorted driver transformer could cause excessive current draw by the
driver tube, or distorted audio.

If the winding ratio was different than the one specified, that would not
cause any problem for the modulator, other than possibly lower audio because
there was less drive to the modulators.

The 1958 thordarson catalog (which is about the time the T-90 was made)
shows a number of driver transformers. None of them have numbers close to
the number you cited.


73, Colin K7FM

wrote:
Joe,I have a H/W T-90 The mod driver shows 400 Ohms one side and 390
other side. On these older rigs it is not uncommon to find un-balance
in the windings and many have had the driver xfmr replaced,,windings
are extremely small. gauge wire..I know the people that built them
never expected us to be using them over 50 years later...hi..A great

The resistive measurements will not be the same because one half of
the winding is closer to the core and therefore has a shorter length
per turn. So that may have perfect AC balance, even though the DC
resistances don't match. The DC resistance is proportional to the length
of the winding, which is not directly proportional to the turn ratio.
In the case of the rig with the 390/400 resistances, it's probably balanced
better than you think.

But the origininal poster's rig... that 200 ohm number doesn't sound good
to me. Put 6VAC 60 Hz into the primary and measure the ratios... if there
is a shorted turn, the DC measurements may change a little or a lot, but
the AC measurements will be totally off. On the other hand, you may have
a different transformer than was original; if so the DC measurements will
be completely off but the AC numbers will be fine.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article . net,
"COLIN LAMB" wrote:

I question whether there is a problem with the driver transformer. What are
the symptoms, other than the different value of resistance? Is this just a
general check of everything or is the audio section having a problem.


Hello, Colin. This is just a general check before applying power to it
for the first time. I wanted to fix any obvious faults before applying
power. (Building the power supply will be the next part of the project!)

I've decided to leave it in for now and (carefully) watch what happens.

A shorted driver transformer could cause excessive current draw by the
driver tube, or distorted audio.


Will put a Simpson 260 across the cathode resistor to watch for
excessive current.

If the winding ratio was different than the one specified, that would not
cause any problem for the modulator, other than possibly lower audio because
there was less drive to the modulators.


Good to know that. I see that Antique Electronic Supply sells a Hammond
124D driver transformer that looks to be pretty close, in case the
original really needs to be replaced.

The 1958 thordarson catalog (which is about the time the T-90 was made)
shows a number of driver transformers. None of them have numbers close to
the number you cited.


Thanks for checking. That's exactly the right time frame. I'd bet it was
a custom part, and won't appear in any catalog.

I appreciate your comments, Colin.

73,
Joe K9LY

In article ,
(Scott Dorsey) wrote:

But the origininal poster's rig... that 200 ohm number doesn't sound good
to me. Put 6VAC 60 Hz into the primary and measure the ratios... if there
is a shorted turn, the DC measurements may change a little or a lot, but
the AC measurements will be totally off. On the other hand, you may have
a different transformer than was original; if so the DC measurements will
be completely off but the AC numbers will be fine.

Hi Scott. Yes, I'm suspicious of that 200 ohm number too but have
decided to leave it in for now and carefully see what happens. Your idea
sounds good and I may give it a try. Can actually check it at ~1 kHz too
with an AF generator.

73,
Joe K9LY

Sorry I'm late to this thread but just for interest I checked my T-90. I
don't think you have a problem at all. I get 183 and 202 ohms. It works
fine with excellent modulation. All original parts other than the caps I
replaced and a few out of spec resistors.

Chuck...K1KW



"Joe L." wrote in message
...
In article . net,
"COLIN LAMB" wrote:

I question whether there is a problem with the driver transformer. What
are
the symptoms, other than the different value of resistance? Is this
just a
general check of everything or is the audio section having a problem.


Hello, Colin. This is just a general check before applying power to it
for the first time. I wanted to fix any obvious faults before applying
power. (Building the power supply will be the next part of the project!)

I've decided to leave it in for now and (carefully) watch what happens.

A shorted driver transformer could cause excessive current draw by the
driver tube, or distorted audio.


Will put a Simpson 260 across the cathode resistor to watch for
excessive current.

If the winding ratio was different than the one specified, that would
not
cause any problem for the modulator, other than possibly lower audio
because
there was less drive to the modulators.


Good to know that. I see that Antique Electronic Supply sells a Hammond
124D driver transformer that looks to be pretty close, in case the
original really needs to be replaced.

The 1958 thordarson catalog (which is about the time the T-90 was made)
shows a number of driver transformers. None of them have numbers close
to
the number you cited.


Thanks for checking. That's exactly the right time frame. I'd bet it was
a custom part, and won't appear in any catalog.

I appreciate your comments, Colin.

73,
Joe K9LY

In article ,
"Chuck - K1KW" wrote:

Sorry I'm late to this thread but just for interest I checked my T-90. I
don't think you have a problem at all. I get 183 and 202 ohms. It works
fine with excellent modulation. All original parts other than the caps I
replaced and a few out of spec resistors.


Thanks for taking the time to check your T-90, Chuck. Interesting
result. I hope to report that all is well with mine once it's fired up.

73, Joe K9LY