[Drake] T-4XB--no output

[ed brooks]

If you are getting proper bias idling current, everything in the final 
stage is OK.  Voltage readings check OK on the driver stage, so that 
pretty well eliminates that.  You have eliminated tubes as the culprit. 

The symptom is no drive to the finals, and possibly, not to the 12BY7.  
Of course, that means a loss of drive in earlier stages.  One 
possibility is the grid coupling capacitor to the driver.  That is a 1.5 
pf, c33.  By tuning the Plate, Load and Drive controls, and seeing and 
hearing a difference in the R4B, you are simply tuning those circuits, 
but there is no RF power being applied to them. Only a small signal that 
may not be reaching the grid of the driver., and almost certainly is not 
reaching the grids of the finals. 

If the driver is working, but there is no drive to the finals, take a 
look at  c47, coupling cap.  It is not shorted or the bias to the finals 
would be positive - plate voltage of the driver, (and your finals would 
go kaput very quickly!) but it could be open.  Using the R4B as a probe, 
as someone else suggested (be sure the end you probe with is insulated  
- no bare wire!) check the grid of the finals, versus the grid of the 
driver.  If the signal on the final grid is much larger than the one on 
the driver grid, then clearly the driver is working, to at least some 
extent.   If the driver has output, and the finals have no input, not 
much place to go except c33 and T6, mentioned below.   I'm thinking you 
will find the Driver is OK, too, and the problem is before that, in the 
mixer stage. 

Pull the Driver tube only.   Now listen on the R4B.  Did the signal drop 
noticeably  in level from what you have been hearing?  If so, the driver 
is working.  If the signal did not drop, or dropped only very slightly,  
the the loss of signal is ahead of the driver.  Since the mixer is 
obviously working, (or the signal would not be at the operating 
frequency)  that leaves very few possibilities.  One is the c33, already 
mentioned.  Another is T6 (mixer plate tank circuit) being detuned.  
Another is contacts on the Mode switch.   And, of course, the mixer 
itself, the 6AU6, which is a fairly common problem anyway! 

Remember, if you have proper idling current on the finals, as you 
indicate, everything in the finals compartment is ok.  With the possible 
exception of the coupling cap from the driver. 

Have fun

Ed



Paul Ellis wrote:

> Frank,
>
> Thanks for the note!  There is no plate current beyond bias being 
> indicated.
>
>> You did not mention anything about plate current.  Is there any plate
>> current?  Can you dip the finals?
>>
>> Now, before you try that I would suggest removing the RF cage and
>> inspecting the parts before doing any more tests under power.
>
>
> I have it apart on the bench and have checked for stuff touching in 
> the RF cage. I haven't checked the output cap yet, though.
>
>> Clearances inside the cage are pretty close.  Make sure nothing is
>> shorting to ground and that everything is mechanically solid.  Nothing
>> broken or flopping around.  Use an ohm meter and check all the points of
>> HV and RF to make sure there are no shorts.  Check the plate RF choke to
>> see if it is open.  Make sure the output capacitor is good (not open or
>> shorted).
>>
>> Then do the same under the chassis for the drive circuitry.
>>
>> Connect the ohm meter across the loading capacitor and tune it through
>> its range to see if there are any shorts.
>
>
> Still need to try that.
>
>> Check the transmit relay coil and contacts.  If the relay does not
>> close, there will be no Rf output.
>
>
> I need to check that more closely. I did apply DeOxit to the relay 
> contacts and tried rubbing a De-Oxit soaked piece of paper across the 
> contacts.
>
>
>> You might also check the wattmeter to make sure it is working properly.
>
>
> Check.
>
>> >From your description it sounds like the driver may be okay.
>
>
> I have the feeling that it is, too.  I need to go back and check the 
> circuit between the driver plate and the final amp grids more 
> thoroughly. The resistors all around the finals and driver check out 
> within spec with the DMM, but that's in-circuit. I'll try unsoldering 
> resistors next to try verifying that those resistors are all right.
>
>> Regards,
>> Frank Kamp
>
>
> 73,
> Paul
>
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