Tram D201a swr meter

[389]

My D201a swr meter indicates 3:1 swr on a 1:1 swr setup, tested with quality revex ext meter.
I have set swr cal correctly. Also tubes related with the meter have been replaced.

Is there a calibration point somewhere for this ? The Manual does not mention any.

73
Adrian

[2600]

Dec 15, 2022 19:28:50 GMT -5 389 said:

Is there a calibration point somewhere for this ? The Manual does not mention any.

73
Adrian

Nope. What comes up from the dice roll is what you get. The SWR meter's directional coupler is the small circuit board against the rear panel between the final tube and the relay. There are two 47 ohm resistors, R700 and R701. They serve as terminations for the parallel foil traces that make this board a directional coupler. To make it show minimum reflected at exactly 50 ohms calls for extreme mechanical accuracy in this component. Lacking that, making R700/701 variable with a trimpot is a trick that Black Cat used in their SWR meters to correlate them to 50 ohms. My luck with that trick has been up and down, both.

Built-in SWR meters are better than nothing, but frequently not by much. The trimpot method first sets the trimpot for minimum reflected reading with a 50 ohm dummy load. The meter is then unhooked and reconnected in reverse, so the "forward" side is now reading reflected power. The trimpot for that side now gets set for minimum reading into the dummy.

Don't quite see how to get the "cal" side terminating resistor in the D201 checked. No easy way to hook up the built-in coupler backwards.

When a customer asks how much it will cost to make a radio's built-in SWR meter work right, I tell them "enough to buy two accurate external meters".

73

[389]

Thankyou, I thought maybe D701 may be faulty. Thanks for explaining how it works, I did checkout the schematic.
How does the V701 final tube plate load section effect the swr at detector ?

The thing I am looking at now is random squeaks and sound effects in the rx audio, still monitoring.

73
Adrian

[tubefan]

Check the bottom of the board where the BA board plugs in. Look for cold or cracked solder joints. That causes the sounds you are describing in the receive audio. Along with many many other known problems this is the easiest to give a quick look. Re-solder all the pin connections on the BA board as well.

[389]

Thanks, I had already re-soldered all of that area when fitted a new beaut 'NomadRadio' BA upgrade custom board.
Great  results then. This noise seems to have happenned more recently after I replaced some receive and audio section small tubes,
and I suspect one of them is not the best, when toughed or rocked it has an effect on it.

The v701 final tube shows purple haze on TX, is that a sign of being gassy ? I also see flashes in the relay, and a white flash behing it occassionly.
Looks like a high voltage are effect or something. This radio will keep me occupied untill the end of trime I am sure. Have ordered a new relay.

[Sandbagger]

Dec 15, 2022 19:28:50 GMT -5 389 said:

My D201a swr meter indicates 3:1 swr on a 1:1 swr setup, tested with quality revex ext meter.
I have set swr cal correctly. Also tubes related with the meter have been replaced.

Is there a calibration point somewhere for this ? The Manual does not mention any.

73
Adrian

One other thing that bears mentioning, if your radio is putting out excessive out of band spurs or harmonics, it will show up as an increased SWR. It should also show up on an external SWR meter as well. So if the external SWR meter (with the D201 straight into the antenna) shows 1:1, while the internal meter shows 3:1, then that's probably not the issue. But if both meters show a high SWR, but another radio shows 1:1, then I would suspect excessive spurious emissions, which probably requires the final tube to be neutralized.

[2600]

A blue glow on the surface only of the glass is no problem. But a purple glow from inside the tube means that the vacuum is contaminated with gas. That tube is no good.

The neutralizing adjustment capacitor C706 is frequently twisted in a desire to get a bigger wattmeter reading. This adjustment is set to make the final tube stable, so it won't oscillate and put out extra unwanted frequencies. Setting it is finicky, and involves a back-and-forth with the tuning slug in T700. When one of these two adjustments is changed, it throws the other one off its proper setting. And if someone turned it for max wattmeter reading, the adjustment screw is probably pretty loose. If it feels tight, maybe it's not part of the problem. That's not the same as adjusting C706, but it's worth finding out if the screw is at the loose end of adjustment.

I would start with a new final tube. A gassy tube can cause harmonic frequencies that fool a SWR meter.

73

[389]

Thankyou, Yes I was looking at it today, The am and ssb power had dropped to less than 1 watt.
I fitted a NOS tested supply V703 final. Now the C706 adjust I gather with T700 is set for a null or dip in power output when adjusting, stop at that dip yes ?
Dean do you have a neutralising procedure better than the manual ? I have am deadkey at 2w with a 6w swing and ssb is worse 3-4 w. went of T500 and the Bal mod slugs,
plate load etc, for not much improvement.

The relay is smoked, I have a 2nd D201a, and grabbed the realy from that, seems ok. I have a new one on order from Barketts, with a few other xtals etc.

The relay is a TF154-CCC-CCC-115VDC and these are not easy to find, whereas 24 or 48 vdc versions are easier and less expensive .
As 9000 ohm relay coil deals with 115 vdc , would a 8600 ohm VD resistor placed in series allow a 24vdc version with 430 ohm coil ?
Obviously measuring the vollt drop and adjust to get 24vdc to finish...

I am leaning more to fitting the dds-vfo board in this( UI have 2 sets coming), then I can set the Bal mod am-lsb/usb Xtals lower at their spec frequency instead of
running them high to allow for the 
xtal board xtals which are all low to various extents, and the 16 and 4 mhz trimmers combined do not have enough needed range, so the 6.25 LO compensates.
I was hoping to restore this one d201a in original condition, and this one does have an ok black ch switch, which I did service and lubricate internally.

The Bal mod xtals set high may be retarding the power available through the filter. I am enjoying the journey and learinng a lot along the way.

73
Adrian

[Sandbagger]

Dec 18, 2022 7:56:38 GMT -5 389 said:

Thankyou, Yes I was looking at it today, The am and ssb power had dropped to less than 1 watt.
I fitted a NOS tested supply V703 final. Now the C706 adjust I gather with T700 is set for a null or dip in power output when adjusting, stop at that dip yes ?
Dean do you have a neutralising procedure better than the manual ? I have am deadkey at 2w with a 6w swing and ssb is worse 3-4 w. went of T500 and the Bal mod slugs,
plate load etc, for not much improvement.

The relay is smoked, I have a 2nd D201a, and grabbed the realy from that, seems ok. I have a new one on order from Barketts, with a few other xtals etc.

The relay is a TF154-CCC-CCC-115VDC and these are not easy to find, whereas 24 or 48 vdc versions are easier and less expensive .
As 9000 ohm relay coil deals with 115 vdc , would a 8600 ohm VD resistor placed in series allow a 24vdc version with 430 ohm coil ?
Obviously measuring the vollt drop and adjust to get 24vdc to finish...

I am leaning more to fitting the dds-vfo board in this( UI have 2 sets coming), then I can set the Bal mod am-lsb/usb Xtals lower at their spec frequency instead of
running them high to allow for the 
xtal board xtals which are all low to various extents, and the 16 and 4 mhz trimmers combined do not have enough needed range, so the 6.25 LO compensates.
I was hoping to restore this one d201a in original condition, and this one does have an ok black ch switch, which I did service and lubricate internally.

The Bal mod xtals set high may be retarding the power available through the filter. I am enjoying the journey and learinng a lot along the way.

73
Adrian

It is not a good idea to compensate for synthesizer crystal error with the carrier oscillator crystals. It is important, for proper audio frequency response on SSB, that the carrier oscillator frequencies are set properly. Otherwise you could sound like you're holding your nose, or like you have a pillow over your mic. And if it is way off, it could affect SSB power output.

[389]

Yes, well the dds-vfo it is then. To get the xtal board on at each range matched, would involve fitting parallell adj inductance on each xtal. As it is I can get 100Hz up towards target by shunting the trimmer cap,
but it is still not enough. I also did the Nomadradio vfo/clarifier mod, and reset the claridier knob 12 oclock position to near 'up' end-range, which was very useful.
I discussed that mod with Greg at Barkett, and he mentioned one in which the clarifier works on vfo/manual. I see the advantage of that now in fine tuning, as the vfo, even in vernier reduction is still coarse.

I have seen videos of Tram D201a fitted with pll 40 ch switch setups, but do not know the details. It is a shame Tram did not look at pll when going to 40ch.
It should have been cheaper to produce than the massive xtal board, Isn't that why the other cb producers go that way ? Yes restoring a D201a is a labour of love..

The pll method retains the original look, and if anyone knows the method, equipment used, I would love to know please ?

Edit update, Found the thread on pll here ; grumpy.proboards.com/thread/4971/tram-d201a-pll-mod  

What cound be possible if the 40ch wheel could be mounted on the dds-vfo encoder shaft, and mount the dds-vfo display inside the d201a (lift lid to use, interlock defeated), then
The radio would work / look as per original. The manual vfo setup should be able to be retained with the correct 16mhz (ch9 xtal) oscilator/mixer kept or built on a small pcb.

That pll looks a lot of work with now hard to find components.

73
Adrian