Re: [drakelist] R4C xtal bank bad

[Eric Webner]

Garey,

 

Several weeks ago when I discovered the play in one of the bandswitch wafers, I set the wafer where it should be and used adhesive  on the inner wafer and shaft. It appears to work very well, and with no binding. I wouldn't recommend that for all the wafers, but just one seemed ok.

 

Regarding my low output, I may be ok. First of all, I was not loading it to full power, since one of the errata sheets that Drake put out said to load it for only 340 mA in order to avoid any spurious output. After reading your latest message, I loaded 80 and 40 meters to full power, obtaining 135-140 watts measured on my Bird wattmeter. Loading it to full power ran the plate current to 500 mA!

 

I also took your suggestion and checked a few components. R44, the 680 ohm resistor, is good. The cathode resistors are good and very well balanced. The screen resistors checked out as well. However, the resistor across the meter measured 5.6 ohms. I disconnected one end from the circuit board and the resistor snapped in half. I couldn't come up with another one, so I hope to try a 2.2 ohm unit (measures around 2.6) so I can get on the air this weekend.

 

Do you recommend that I vary the other resistor in order to achieve accuracy on the meter, or can I play around with values on the shunt resistor (the 3.3 ohm resistor)?

 

One other observation: my low power output could be attributed to this meter shunt resistor being off, as that would cause me to back off on the loading quite a bit. This is also why I didn't want to load it to full power (500 ma???) for fear of melting a couple sweep tubes. It also makes me wonder about my bias. Is there a voltage one should shoot for, or is getting the bias (idling) current still the preferable approach? Do I need to realign everything if I need to reset the bias?

 

OK, enough questions for now. Thanks for all the good responses!

 

73,

 

Eric KA8FAN

Garey Barrell <[EMAIL PROTECTED]> wrote:

Eric -

There isn't a "good" way to correct the wafer "play" once it occurs,
short of wafer replacement. I have seen some with very thin Teflon?
tape wrapped around the shaft that seemed to work. I saw one where
someone had cut small pieces of brass shim stock, carved out the hole,
and epoxied them to the wafer. Inexpensive, but highly labor
intensive! :-)

The Load control should be in the 2.5 - 4.5 range for a non-reactive 50
ohm load. The Plate control should be in the 3 - 8 range. As long as
a "dip" can be achieved, it doesn't really matter.

Your power output is very low. If you load for maximum output, you
should get 130 plus watts out on 80 - 20 in Tune position, 150 -160
watts in CW. Power that low is most likely from weak finals. With
"new finals", you probably have an alignment problem. If you have done
t! he alignment, check the 680 ohm resistor at R44, (located under T7,
just at the end of the final feedthrough panel under the chassis,)
especially if you did the alignment with a metal screwdriver. Shorting
the alignment caps to the chassis, hard not to do with a metal
screwdriver in those little holes, will damage this resistor. With
regard to the plate current, this is most often a result of a meter
shunt resistor that has gone up in value. This is a 3.3 ohm resistor on
a small board under the chassis near the relay. The 'best" way to
correct this is to replace it with a 1W (the original is 1/2W) carbon
film resistor. At the same time, check the 15 ohm 1W cathode resistors
for the final tubes. They should be close to 15 ohms, (+/- 5%) and
closer to each other, (2%). Best replacements are Ohmite OY ceramic
composition, although the carbon film types seem to work fine. If you
haven't already, check the screen resistors for the! finals as well.

After you replace the 3.3 ohm resistor, it is best to measure it's
actual resistance and the voltage drop across it with an accurate DMM to
determine the "actual" plate current, then use this information to
verify that the Plate meter is measuring accurately at 100 mA. The
meter is not great, and not highly linear. The 100 mA point is most
important since that is what you use to set the bias for the final. If
it is NOT accurately reading at 100 mA, you will have to reselect the
resistor marked (select at test) which is mounted on the slide switch
linked to the meter switch between the Plate and Load controls. It's
usually around 200 ohms, but can be anything from 47 to 360 ohms.

The 50-123 knob will work, but you'll be amazed at how it's "difference"
sticks out. Of course it will bother YOU much more than anyone else!
Same phenomenon as that little paint slip on the wall in the living
room. No one else e! ver sees it, but its the first thing you see every
time you walk into the room!! :-)

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual




Eric Webner wrote:

> Garey et al,
>
> I appreciate your comments. Perhaps I did a little too much work on my
> T-4XC.
>
> I just finished restoring my transmitter, and during my work I found a
> few problems with wafer misalignment and too much "play" on certain
> wafers. Can you comment on the best ways to correct these problems?
>
> I would also like to know what typical settings for plate and load
> controls should be when loading into a dummy load. I have never owned
> a working C-line before, so I am not sure what "normal" is. Suffice it
> to say, I believe my plate control is turned relatively high. I just
> about "run out of room" on 10 meters.
>
! > Also, what kind of output power should I be getting. When I follow the
> tuning procedure given in an errata sheet indicating that I should
> limit my plate current to 340 mA in order to meet FCC regs, I only get
> about 85 watts out on 80 and 40 meters. When loading to full power,
> what is the maximum plate current I can load to without melting my new
> finals?
>
> For those of us that were discussing how to obtain knobs for the
> Drake: I called Caltronix today and they confirmed that the 50-133
> knob is only available from the manufacturer if they buy 500 pieces.
> They are looking for another vendor who will sell in more reasonable
> quantities. In the meantime, they have available the 50-123 knob,
> which is 1.04" in diameter vs. the 50-133, which is 0.93" diameter.
> Has anybody had any luck with other vendors? I still need to replace
> one knob.
>
> I hope ! to chat with you guys this Sunday during the CE Contest.
>
> 73,
>
> Eric KA8FAN
>
>
>

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