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Author Topic: Tough Swan 250C problem  (Read 3760 times)

Posts: 344

« on: July 02, 2013, 01:46:48 PM »

Hi. I hope WB2WIK is readin' this.... Hi Steve.

I have a nice example of a Swan 250C (I know, NO such thing, but I love these old room heaters).....
Here goes. I purchased this radio as a 'parts' radio. No rcv, no xmit. It had a couple of bad tubes, and a dead xtal in the marker.
After a cosmetic refurb, rcv is fine, no xmit. The 'manual' is pretty slim on transmitter alignment. I next acquired a second 250C and PS. This one is also a very clean unit. After some work, it's fully functional.
BOTH radio receive EXACTLY the same. Both were aligned using an HP 606A and counter. Perfect receive and very sensitive.
Now I have both of these radios on their back. I did the voltage checks as listed in the manual. One was way off (on the 'bad' radio). It was caused by a 47K resistor going into the Meg ohm region. Still NO output. No carrier balance. No carrier.
I have swapped every possible tube from radio to radio, changed PSs, no change. BTW, all the tubes were checked on a Gm tube tester, as well as substitution. NO change....

Today, I was in them using a 'scope. I noticed that the xformer that feeds the balanced modulator had very low RF voltage compared to the 'good' radio, when ptt was actuated.
Next, I measured the in-circuit resistance of the xtal filter. The 'good' radio measured 300K ohms, the 'bad' radio measured 36 MEG ohms. oh no.....
Since I can't get a spare filter this side of Mars, am I on the right track? It still puzzles me why both receivers are identical. The block diagram shows both xmit and rcv paths through the filter.
I was going to measure every resistor point-to-point when I found the anomaly with the filter.....
Any ideas?Huh

thank you!!


Posts: 15272

« Reply #1 on: July 05, 2013, 10:51:35 AM »

Quote from: N4UE

...Since I can't get a spare filter this side of Mars...

That's the 8-pole crystal filter at 10.7 MHz?  Something like 1" wide and 3" long with
4 pins on the bottom?   Does it have a manufacturer's marking on it?  I don't have
a Swan "Too Drifty" to compare against, but I might have a filter that fits.

But if I recall the input DC resistance of the filter should be nearly infinite, so the
difference you are seeing would be due to the circuitry attached to the filter rather
than the filter itself.

Posts: 344

« Reply #2 on: July 05, 2013, 11:18:58 AM »

Hi. It's marked:

Model 10.9 - 2.8  (10.9 MHz - 2.8 KHz)
C-F Network Division

It's 2 1/2" X 1"

As far as my resistance check. Someone also mentioned that the surrounding circuitry would explain the difference. I don't know how, as I haven't pulled the leads off, yet. However, the schematic shows one end of this filter to be 'isolated' by 3 caps. Mica dipped types.
So, if the 'bad' radio's filter had resistance through these caps, it's resistance would be < 36 Megohms. The 'good' radio at this point in the circuit, measures 300Kohms. Just the (logical) opposite.....
See why this doesn't make sense?
I still feel it is an xtal or alignment issue. The Manual is devoid of instructions for xmitter alignment. Evidently, loss of xmit is not rare on the 250 series.
Thanks for the assistance.....


Posts: 15272

« Reply #3 on: July 05, 2013, 11:47:13 AM »

10.9 MHz, huh?  That is rather odd.  I have some 10.7 MHz filters that I was told
would work in the 250, but it appears that is not the case (unless the BFO crystal
and frequency calibration are changed accordingly.)

I had a similar case with a Ten-Tec Triton, where there was a bad crystal in the filter,
but that affected both TX and RX.  Even though the filter was soldered onto the board
I was able to demonstrate the difference by swapping boards with one from out of my

Is the balanced modulator also used on RX?  Since that is before the crystal filter, then
any problem with drive to that stage likely wouldn't be due to the crystal filter, and would
not affect RX.  I'm assuming that there is a common BFO for TX and RX, but taking
separate paths to the modulator / product detector (perhaps through some RX/TX switching
circuitry).  If so, that would be a good place to start.  You also might have a mis-adjusted
or broken winding in one of the coils in the modulator.

Posts: 6

« Reply #4 on: July 13, 2013, 03:44:17 AM »

Probably like you I got a 250C, non working, off e-bay.  I do not have a VFO, so have been working on that issue.  Receive is working now, that I have jumped pin 3 and 4 on the accessory plug and pin 1 and 8, since the antenna was really not connected, hi hi. 
Have checked all tubes, had a pair of 6146's installed them, and neutralized, but it is really touchy.  Receive is good, hardly any drift, but similarly on transmit nothing.  I can hear the carrier on my other 6 meter rig, but won't load into an antenna.  Into a dummy load (60watt bulb) it will load, but not into antenna.  Like you, the transmit alignment instructions less than helpful.  Meter readings on transmit, I show the bias, but that is all.  Since I don't have the VFO, it looks like pin 6 and 7 have same voltage and from checking, they do.  With no load they are supposed to be +215 volts, I read 246 instead.  Thought I had traced it to the relay, but it is working.  Any thoughts about jumping any different pins?  Stu, with Swan users group was looking into it, but have not heard any thing since July 4 from him.

Posts: 344

« Reply #5 on: Yesterday at 10:31:07 AM »

Hi. Well, I was having one of those rare 'good weeks' where everything was going good. I thought I'd look at the Swan 250C that had been giving me problems.
Got all the test equipment ready and while the HP 606A was warming up, went through checking all the tubes again. All ok.
When I fired up the Swan, I noticed the s-meter wasn't working like it should. Long story short, the s-meter zero adjust pot was open. I had a spare 1K pot and everything is fine.
I'd be ashamed to admit how much time I chased this, but being a stubborn Frenchman, I never give up!
So now I have a pretty nice station consisting of:

250C (2)
117XC power supply (2)
Noise blanker for the 250C
VOX unit (2)
250C remote VFO
TV-2C Transverter

Back in the 'day', I had the 250C and TV-2C combo. The TV-2C uses FETs instead of the Nuvistors in the TV-2B.

Now to get my HP 8640B going again....


Posts: 131

« Reply #6 on: Yesterday at 01:06:14 PM »

Possible there is an impending problem with the "good" unit since most crystal lattice filter designs measure open circuit. Might be one of those three capacitor starting to leak.
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