Ten-Tec 1254 receiver
 

I just got finished building one of these things. It is not quite stock. I
used film caps in the audio chain and the synthesizer loop filter, silver
micas in the RF filters, a Murata CFR series filter in the I.F. strip,
matched hot carrier diodes in the 2nd mixer, and 1N5767 PIN diodes instead
of the stock attenuator diodes.
At 15MHz, MDS is .06uV. No overloading was noted on any band, including 49
Meters. System gain is quite high.
MW performance isn't too hot, but it could be the input filter. I might use
a design of my own and see how it works out.
I had seen this unit in the past, but I didn't want to pay 200 dollars for a
kit..............this radio kit was part of a trade I made a couple of weeks
ago.
Knowing what I know now about this radio, I would say it is worth the kit
price.

Pete

On 25/02/2005 3:09 AM, Pete KE9OA wrote:
I just got finished building one of these things. It is not quite stock. I
used film caps in the audio chain and the synthesizer loop filter, silver
micas in the RF filters, a Murata CFR series filter in the I.F. strip,
matched hot carrier diodes in the 2nd mixer, and 1N5767 PIN diodes instead
of the stock attenuator diodes.
At 15MHz, MDS is .06uV. No overloading was noted on any band, including 49
Meters. System gain is quite high.
MW performance isn't too hot, but it could be the input filter. I might use
a design of my own and see how it works out.
I had seen this unit in the past, but I didn't want to pay 200 dollars for a
kit..............this radio kit was part of a trade I made a couple of weeks
ago.
Knowing what I know now about this radio, I would say it is worth the kit

Nice. Are you getting the birdies below 7MHz that some people think are
caused by IF from the LED display?

The AGC is reported to be a bit harsh on AM. Have you noticed this, or
is it as bad as the report of eham.com suggests? I wonder if your
tolerance tweaks in the IF stage helped in this regard.

What kind of antenna do you have hooked up to this? I understand
Ten-Tec advises using a long shielded hookup to the antenna. I might be
tempted to replace the phono jack with BNC or coax to make this easier.
Though, I suppose shielded cables meant for audio applications are a
dime-a-dozen.

Not yet..............I tried it with an outdoor antenna. Hooking it up to
the RF generator, though also poses no problems. I do notice one
problem.........when you peak the 455kHz I.F. transformers that feed the
1350 I.F. amps, the NF suffers. I notice that the circuit, as designed,
applies V+ to the high side of the primary instead of using the center tap.
This connection is also bypassed to ground. I think that if I feed V+ to the
center tap, that this problem should go away. I have used these devices in
the past, with no problems, but I did feed V+ through the center tap of the
primary.
I also noticed the the PIN diodes in the front end are being biased at 20mA.
PIN diodes should be biased at 75mA or so, for low "on" resistance. I am
going to reduce that 220 ohm bias resistor to around 47 ohms and see what
happens.
I haven't measured the Q of the front end coils, but I would surmise that it
isn't too high. I've got some magnetically shielded units with the same form
factor that have a Q of 50. This should improve the front end filter loss.
I did remove the 1k swamping resistor from the primary of the 1st 455kHz
I.F. transformer. Gain came up quite a bit, so the radio sounds more like a
651S-1 or a 75A-4.
I'll let you folks know how things turn out, once I finish with the radio.
It really is a cool radio.

Pete

"clvrmnky" wrote in message
...
On 25/02/2005 3:09 AM, Pete KE9OA wrote:
I just got finished building one of these things. It is not quite stock.
I used film caps in the audio chain and the synthesizer loop filter,
silver micas in the RF filters, a Murata CFR series filter in the I.F.
strip, matched hot carrier diodes in the 2nd mixer, and 1N5767 PIN diodes
instead of the stock attenuator diodes.
At 15MHz, MDS is .06uV. No overloading was noted on any band, including
49 Meters. System gain is quite high.
MW performance isn't too hot, but it could be the input filter. I might
use a design of my own and see how it works out.
I had seen this unit in the past, but I didn't want to pay 200 dollars
for a kit..............this radio kit was part of a trade I made a couple
of weeks ago.
Knowing what I know now about this radio, I would say it is worth the kit

Nice. Are you getting the birdies below 7MHz that some people think are
caused by IF from the LED display?

The AGC is reported to be a bit harsh on AM. Have you noticed this, or is
it as bad as the report of eham.com suggests? I wonder if your tolerance
tweaks in the IF stage helped in this regard.

What kind of antenna do you have hooked up to this? I understand Ten-Tec
advises using a long shielded hookup to the antenna. I might be tempted
to replace the phono jack with BNC or coax to make this easier. Though, I
suppose shielded cables meant for audio applications are a dime-a-dozen.

Pete KE9OA wrote:
I did remove the 1k swamping resistor from the primary of the 1st 455kHz
I.F. transformer. Gain came up quite a bit, so the radio sounds more like a
651S-1 or a 75A-4.

Considering you're not getting any overloading on any band, why do you
suppose they put it there in the first place? I can only think of
reducing primary Q, which doesn't seem terribly worthwhile.

I figured that out earlier this evening...............It looks as if this
receiver has a single loop AGC system. This causes some problems, because of
the high gain of the 1350s themselves. The I.F. gain is so high that AGC
starts on signals as small as .1uV. This means that the PIN diode attenuator
is active at very low signal levels; the result is a degraded NF as I.F.
gain is increased. It appears that the designer's solution was to
intentionally lower the system gain so that this wouldn't happen. That
didn't cut it for me, so I delayed the RF AGC by inserting a series resistor
between the PIN diode attenuator and the AGC control voltage source for the
shunt diode. For the series PIN diode, I bumped up the current to 75mA, a
much more realistic drive current for this type of device.
The I.F. strip was not optimally designed, in that V+ was applied to the
Pin2 of the 1350 (OK here), and Pin1 at the same time (not good), allowing
the voltage to flow through the primary of the I.F. transformer to Pin8
(other pin of differntial output pair, also not good). What I did was break
the V+ connection from Pin1 and rerouted this connection to the center tap
of the I.F. transformer primary. This is good, because instead of having
Pin1 of the 1350 I.F. amp at AC ground, the center tap of the transformer
can now be grounded, providing a balanced load for the MC1350. This is how
the chip was intended to be used, so this is also good. Now, when I peaked
the I.F. transformers there was a definite peak instead of a very broad
peak.
This, in combination with the RF AGC loop changes and the removal of the
high pass filter from the preselector gives the receiver a .05uV MDS over
the whole tuning range. Because of the delayed AGC, the receiver I.F. strip
clips at 15mV instead of 25mV. Once you tune the strong signal out of the
I.F. bandpass, no overload is encountered. There was some crossmodulation
from the series PIN diode, until I bumped up the diode current from 15mA to
75mA.
A 1N5767 PIN diode is ok up to 100mA, so the higher current isn't an issue.
Because of the very high AGC action, you cannot really align this receiver
by ear. What you need to do is look at the AGC voltage at the output of the
emitter follower with a VOM (analog meter), and then you have a nice signal
strength meter for doing the alignment.
I am very happy with this receiver; I will give the folks at Ten-Tec a call
about my findings.

Pete

"clifto" wrote in message
...
Pete KE9OA wrote:
I did remove the 1k swamping resistor from the primary of the 1st 455kHz
I.F. transformer. Gain came up quite a bit, so the radio sounds more like
a
651S-1 or a 75A-4.

Considering you're not getting any overloading on any band, why do you
suppose they put it there in the first place? I can only think of
reducing primary Q, which doesn't seem terribly worthwhile.

Pete KE9OA wrote:
I figured that out earlier this evening...............It looks as if
this
receiver has a single loop AGC system. This causes some problems,
because of
the high gain of the 1350s themselves. The I.F. gain is so high that
AGC
starts on signals as small as .1uV. This means that the PIN diode
attenuator
is active at very low signal levels; the result is a degraded NF as
I.F.
gain is increased. It appears that the designer's solution was to
intentionally lower the system gain so that this wouldn't happen.
That
didn't cut it for me, so I delayed the RF AGC by inserting a series
resistor
between the PIN diode attenuator and the AGC control voltage source
for the
shunt diode. For the series PIN diode, I bumped up the current to
75mA, a
much more realistic drive current for this type of device.
The I.F. strip was not optimally designed, in that V+ was applied to
the
Pin2 of the 1350 (OK here), and Pin1 at the same time (not good),
allowing
the voltage to flow through the primary of the I.F. transformer to
Pin8
(other pin of differntial output pair, also not good). What I did was
break
the V+ connection from Pin1 and rerouted this connection to the
center tap
of the I.F. transformer primary. This is good, because instead of
having
Pin1 of the 1350 I.F. amp at AC ground, the center tap of the
transformer
can now be grounded, providing a balanced load for the MC1350. This
is how
the chip was intended to be used, so this is also good. Now, when I
peaked
the I.F. transformers there was a definite peak instead of a very
broad
peak.
This, in combination with the RF AGC loop changes and the removal of
the
high pass filter from the preselector gives the receiver a .05uV MDS
over
the whole tuning range. Because of the delayed AGC, the receiver I.F.
strip
clips at 15mV instead of 25mV. Once you tune the strong signal out of
the
I.F. bandpass, no overload is encountered. There was some
crossmodulation
from the series PIN diode, until I bumped up the diode current from
15mA to
75mA.
A 1N5767 PIN diode is ok up to 100mA, so the higher current isn't an
issue.
Because of the very high AGC action, you cannot really align this
receiver
by ear. What you need to do is look at the AGC voltage at the output
of the
emitter follower with a VOM (analog meter), and then you have a nice
signal
strength meter for doing the alignment.
I am very happy with this receiver; I will give the folks at Ten-Tec
a call
about my findings.

Pete

"clifto" wrote in message
...
Pete KE9OA wrote:
I did remove the 1k swamping resistor from the primary of the 1st
455kHz
I.F. transformer. Gain came up quite a bit, so the radio sounds
more like
a
651S-1 or a 75A-4.

Considering you're not getting any overloading on any band, why do
you
suppose they put it there in the first place? I can only think of
reducing primary Q, which doesn't seem terribly worthwhile.



Hello Pete

I just picked up a used 1254 and I would like to improve the
performance. I read though this thread and I wonder if you could list
your improvements and any more suggestions you might have? Thank you.

Richard