Thanks
Mike
:I just recapped my 38-7 and replaced bad tubes and I'm getting everything above 1000kc very loud (only need about 1/4 volume) and clear. My problem is that I'm not getting anything below about 1000kc. I'm not even getting enough to try to adjust the IF transformers to bring in clearer. I dont have a signal generator. Any suggestions?
:Thanks
:Mike
Mike
The static then silence would be a good sign that the parallel plates of the tuning cap are shorting. A close inspection or an ohm meter across the the rotating and stationary plates (UNPOWERED) could reveal touching plates. The cap is an air dielectric and rotating plates should not touch the stationary plates.
marv
:How would I check for shorting of the variable cap? At about 980 I get some static and then nothing all the way down. Thanks for any help here,
:Mike
Mike
Good job!! Now that you've been there/done that, next time will be a snap.
marv
:Upon closer inspection of the tuning cap, I found that one blade was slightly bent to one side which was probably causing the shorting. Works good now. Thanks for the help!!
:Mike
{ I see that your problem is already solved.... with it being the easiest of the multiple situations.. I dug up my reply made to a person on a HAM forum back in the late '90's and am just re-utilizing its text with a minor modification to encompass your situation. Read for further info should you find a much harder situation in the future...or others might find this to be equally as useful to them }
Your initial detection of the loss of reception was being due to the onsetting of a contact / shorting action between a rotor plate(s) and an adjacent stator
plate .
That now brings you down to the precise zeroing in on as to the area where it is occurring. First of all, if you observe the actual action which is
taking place when you are tuning across the band you will associate the fact that as you are tuning towards a lower frequency that there is need
for more capacitance from the tuning condenser, therefore more of an inter meshing action of the plates are required, whereas at the very low freq end of the band they require a total inter meshing.
That means that in your type of situation that you have encountered, one would expect very little chance of its onset at the high end of the band
where less plate area is being inter-meshed.
Once you rotate onto where that initial shorting has JUST onset, hopefully you can find that it is
merely being due to either of the two easiest accessible.... outer rotor plates... having accidentally been bent inward. Possibly by having been bent
accidentally by a hand reaching in across it while trying to reach some distant internal tube or possibly trying to wrest a pilot light from its socket.
Also the possibility of having an exposed chassis out of cabinet for servicing / examination, with its vanes being left in open position and the vane(s) gets bent when the chassis is upside down.
Possibly by now, you have already found the contacting area, with it being blatantly apparent, with it merely being one of the outer plates and you
have now counter bent it to restore it back into its original alignment.
In case the situation might have been a bit more confusing and you might now have, or are finding even more side companion plates out of line, with them now being involved also.
I will throw in my two preferred techniques that I use . Initially one would want to shed any inductance that is shunted across the particular tuning
condenser section that is to be worked on, be that the respective oscillator coil or the antenna transformer / loop antenna coil.
A lifting of the wiring to the stator connections should accomplish that. Then one would want to connect some type of circuit continuity indicator between the frame connection of the rotor and the other connection being made to the relevant stator section. Two or three options are viable for the
accomplishing that, one being an ohmmeter, a test light or an aural indicator.
Two of those features being available in a DVM in the respect of the tone being available in the diode test function, but the ohms test function is less than desirable in the respect of the typical time lag from the shorting action taking place and the digital display ratcheting thru its ranges and ending up with a stable display read out. A quick display response is definitely more in order in this situation and could be served best by the utilization of the old sytle VOM with its analog meter readout.
A simple option that would serve both the initial short detection and the later refinement that might be utilized in the location of a very minor scraping action might also be well served by the utilization of a simple incandescent test light set-up. This can be built up via a quick jury-rigging of a 6V flashlight lamp and 4 "C" or "D" cells in series. Solder tack interconnecting wiring between the cells and a a couple of turns of bared copper
wire , with a final intertwining twist around the shell of the lamp makes one lamp contact, with the other being made to the lead tip of the lamp with solder. Two longer leads are brought out for interconnect, such that their shorting out acts to complete the series ckt in order for the lamp to light in the presence of a short.
That test lamps external wiring leads is then put across the condensers terminals and a slow meshing of the plates is started until the lamp just lights and
indicates the onsetting of a plate shorting action. That is the time when I pull out a standard round toothpick and use its tip at the crossing axis
[Refer to the green line shown on the pictorial] of the rotor and stator plates position where that short has just occurred, initially using the toothpick tip for swinging the two
extreme outer rotor plates outward [See red markup on pictorial] to see if they were at fault and the shorting condition cleared by observing the test lamp. Should that not have pinpointed an individual plate, complexity rises a notch with the possibility of inner plate(s) being the culprit. In that situation I merely reestablish the condition of having that initial shorting action and then the using of that toothpick in the manner of a guitarist gently using a pick on a string.
Starting at one end and moving easily in one direction such that each rotor plate is ever so slightly moved aside by that action, constantly keeping an eye on the test lamp.
If that displaced rotor plate is then located, swing the rotor to its max unmeshed position such that a counter bend tensioning of that plate is done with a final eyeballing in to confirm it now centering in between its companion stator plates.
If that testing sweep across all of the plates with them all being slightly shifted ajar in that one direction and it not yet reveaing the suspect plate, it is then time to start at the other side with the process repeated with them all being now slightly shifting ajar in the OPPOSITE direction. This should have revealed the culprit by now by the end of that series of plates.
Should you have not had luck in precisely finding the short or still having a very slight scraping action you can now move to phase two where you
reestablish that condition of having found the very initial onset of the shorting action and the lamp lighting up then you back off (un-mesh)the tuning just a bit until the short opens and then you short across the lamp in the circuit and you move the tuning condenser back to that shorting onset where you should now see a sparking action occurring as the two plates touch, move the condenser back and forth to let you visually zero in on the the junction of the two shorting plates. For no error assurance you can take a blue micro tip SHARPIE and mark the offending stator plate and remember on which side that the arcing is occurring and then recheck to see if your marking was correct by trying another arcing test. Then you un mesh the cap and work on the alignment of the offending plate and open the the test lamp shorting to then confirm that your plate no longer is shorting out at that mechanical point. Then you run the condenser through a
complete meshing to see that no additional shorts are occurring.
Should you have the VERY worst case situation of MORE than one plate sequentially shorting, get the unit working with the test light just coming
on as the plates touch, then perform the shorting out of the lamp to have raw DC coming in to make an arcing and then its backing off to remove the short to the batteries. Then you zero in on the offending plate for marking and then correct that plates centering alignment and move on through that testing sequence in finding all of the other plates in order, until the set is free of any further shorting plates. If you are having real problems in seeing any minor arcs merely darken ...or wait until you can...the room so that any arc will be readily discernible while you work with a flashlight in its pushbutton mode such that you can additionally and immediately light up the offending plate to be SHARPIE marked as mentioned before. If you happen to slightly miss-mark an adjacent plate on the unit, you can at least easier differentiate between your suspect plate being one plate off to the right or left from your marked one, rather than guesstimating between multiple identical unmarked plates.
[Addenda, should one have a Lab type of power supply with voltage and current metering, just initially set the Current Limiting adjustment to an ~1-2 amp value and then clip in the supply in place of that complete test lamp rigging and then you can watch your metering for the detection of onset of a plates shorting condition]
PICTORIAL:
73's de Edd
:Mike,
:Good job!! Now that you've been there/done that, next time will be a snap.
:
:marv
:
::Upon closer inspection of the tuning cap, I found that one blade was slightly bent to one side which was probably causing the shorting. Works good now. Thanks for the help!!
::Mike