[Boatanchors] audio input notes

[telegrapher at q.com]

I've been working on this HB 4-250 transmitter now for some time figuring out what the heck is going on with it.  Comments in the past have described  it partially and the fun i was having getting to know things about it.  First of all no wiring information came with it.  So i've been tracing wires between the different chassis units along with pulling the different units out of the rack for inspection etc.  Replaced bad components when found and drawing pictures that sometimes look like a chicken has wandered around on the paper.  

Well this past winter season, early spring anyway i got a 220Vac line hooked to the contactor for HV and away we went.  350-400W out into the dummy load was easy to obtain in the AM mode.  I  knew it was AM mode because of what i found in the circuit tracing and the fact that when i set the mode switch to a certain position the filaments on the 100TH modulators come on.  Don't do that in any other position of that switch.  However, i could never get any audio thru the modulator.  I finally figured out that i needed some other source of driving voltage and that pure audio from an audio amp was not going to fill the bottles  with sound. It so happened that on the LV power supply deck there was ample room for a 6SJ7-6SN7 speech amp and i had spare terminals on the back of the chassis for attaching wires to.  I got a bit ahead of myself here.  I hadn't looked at the DC resistance of the driver transformer on the mod deck but it in electrical pictorial style sure looked exactly like what i was dealing with.  The thing was specified as having two secondaries, one for each 6L6 driver.  As that portion of the circuit, along with the assorted caps and resistors were all in place i went ahead and built the associated previously mentioned speech amp.    

Time to hook it all up and see what happens.  But first i had to pull the mod deck and haul it into the shack work room to check out a few more circuitry items i was questioning before applying power etc.  Retraced the input wiring, rechecked the operation of a couple of pots as well as what the two jacks on the front panel were actually doing in the circuit.  Here we go.  I'm finally going to get some sounds thru this thing.  Well maybe.  Before i do i think i'll slap the 360 volts on the primary just to see what kind of results i get.  Ya know, just to make sure there aren't any problems in this area that can't be seen with a VOM.  First attempt, wow look at the B+ current.  Wonder what caused that.  VOM out and remeasure things to ground.  Nothing there.  How about across the winding.  WHAT!  180 ohms?  Lessee now, according to ohms law,  no wonder i drew a lot of I.  Well according to the schematic i built this from, page 271 of the 1951 ARRL handbook it should handle 300+ volts on the primary.  No specs in the article of course and me being a non-experienced, non pro electronic guru like so many others have a learning curve to go thru.  Anyway the short of it is that the operating voltage specified of 360V was not going to happen.  No joy in Mudville tonight.

Evidently a lower voltage was going to be needed but from where?  The leads going to the transformer input on the back of the chassis were both shielded but had been cut off about 20 feet from the chassis end.  What did he use to drive that modulator unit?  Looking in some storage closets in  the garage i found a pair of Bogen C35C PA units and on the back it said they could supply either 25 or 70V to a speaker line.  Well maybe.  Into the shack, where am i going to stack this thing so its close to the mod deck, aha?  on top of the Motorola bench supply of course.  I disengaged the "ground to common" link as i was using a balance line into the mod deck.  One side of the line went to the common terminal and the other to the one marked 24V/4.5 ohms.  To make a longer than necessary story shorter, it worked.  The Bogen amp had a pair of mic input terminals and some AUX inputs as well.  I finally hooked the tape deck line out to one of the AUX inputs so i could use the mic that came with the tape deck for a much better tool than a 1 Kc tone from the audio osc.  

My end question then is this.  What would be a correct voltage to see driving the 6L6 grids.  Of course with the control on the Bogen amp i can set it anyplace but i'd bet that it's probably driving the pants off those tubes if its set very high.  I want to make sure this thing is working right before i haul it back out to the garage and reinstall it in the rack.  After taking it out and putting it back in the rack 3-4 times i'm beginning to build up some muscle but its getting to be a bore of a job and i'm tired of doing so, just for fun.  What i probably will have to do, because i don't have the test gear to do a lot of tests, is put it back in the rack, get it all wired up and then put the thing on the air doing the necessary audio adjustments while watching the output on a scope.  Have to work with what i've got.

Hope you all had a good laugh at this because when i think about all ive gone thru to get this far, i've had to set back and laugh a few times myself.  believe me, there is more to this story than what i've written here.

Larry
W0OGH