[Amps] Filament monitoring - LOOKING FOR ELMER

Thu Apr 7 22:49:24 EDT 2016

Hey there,

Ok, before we go exploring the circuitry a necessary statement, this 
amplifier contains voltages that will kill you dead if you put yourself in 
contact with them.  If you do not have the experience with these multi kilo 
volt beasts main power supply, the bias voltages or even the mains AC that 
runs around inside, find a local ham with more experience to help you, 
preferably in the same room.  Or bring it to someone who will do the service 
for you.  We have lost too many people to the electricity monster.  And 
believe me you get to watch all the way and feel every second of the pain 
that you can not escape from.

Ok now that I am done doing the 'DANGER Will Robinson' thing.

I read through what you had for a description and I think you are pretty on 
target with your understanding.  But, so that we are both on the same page, 
I will walk through it, but I will skip over the maths for the moment.

I took a quick look at the schematic and the 0.3 ohm is in series with the 
tube filaments.  This does a two fold action, It gives a little limiting 
action on the inrush current on power on to help out the tubes from shock 
stress and allows a means of sampling the current of course, mostly the 
latter.   That way, they can get a proportional sample of the current, the 
voltage drop on the 0.3 ohm resistor.  The sense circuitry in parallel with 
the resistor.  They take the signal from that resistor, which is AC, and 
rectify the the signal to give a dc voltage for the comparator to use, D19 
and D20 and associated capacitors.  Then U8A  compares that to the fixed 
value in the other input and flip states from -15 to 15 volts at the 
appropriate value.  Then, the circuitry on the output of the op amp, D25 and 
D24 converts that signal to a TTL level signal for the logic board to use. 
Basically, U8A and the voltage divider of R23 and R22 are where the decision 
of good current is made.  After that is is just an on and off signal.

Now, with your local elmer ham to help you ensure that you don't get 
yourself in trouble, and with the power disconnected and the supplies 
confirmed discharged (TWICE) measure the loop resistance of the circuit from 
J1 pin 1 and 2 with the transformer unplugged, we will be looking at a very 
low value here.  You will be watching for a shift in the value for lower if 
you find a poor connection.  Try all the simple actions like reseating the 
connectors J7 on the LVPS and J3 on the tube deck to clean the connections 
of any possible oxide which would add additional resistance to the 0.3 ohm 
part of the power to the tube sense circuit and the sense line.  Also the 
tubes could benefit from a reseat to clean the tube connections which would 
increase the current a little.  While the tubes are out, check the value of 
the sense resistor R7 and see if it is still within the 0.3 ohm 1% limits, 
0.303 to 0.297 ohms.  Being that it is sinking a fair amount of heat, it may 
have drifted.  If that resistor is good, then the sense circuit could be 
giving false loss of current indications, active device most likely but one 
should scope or multimeter the signal voltage values to isolate the 
intermittent component, however to see these signals this would be a LIVE 
test and fraught with risk.  Robby the robot would be twirling his arms by 
this point.

Ok, I will stop.  I must be getting old, you may not even know the original 
Forbidden Planet series, even in rerun.  Mind you there was the movie 
remake.....

Chris VE3CEA

-----Original Message----- 
From: Jerry
Sent: Tuesday, 5 April, 2016 16:34
To: amps at contesting.com
Subject: [Amps] Filament monitoring - LOOKING FOR ELMER

I am taking every opportunity to learn more and working on my Alpha 87a
always provides a good lesson.  I have been getting intermittent faults on
the filament monitor for the 3CX800A7's showing the filament current is low
- suggesting that one of the tubes has an open filament.  I know this is not
the case as it is intermittent fault and when not faulted I can get full
legal power.  I figure it's an issue with the filament monitoring circuitry.
I understand the basic concept of current monitoring using a parallel
resistive shunt load and feeding that resistive voltage drop into an op amp
to provide a corresponding proportional voltage that could be read by a
microprocessor analog input.  The 87a filament monitor per manual is looking
for 3 states, less than 2 amps or more than 4 A and anything in between so
it seems the current sensing method would not need to be too precise.

The transformer's 13.6 vac go via lines labeled 'FIL' and "FIL RETURN"
through the Low Voltage Board, these are passed through to the Tube deck
along with a 3rd line called  FIL_SENSE GND.    On the Tube Deck,  FIL is
connected to one side of the filaments and the other side of the filaments
are grounded.  FIL_SENSE GND is connected directly to ground  and FIL_RETURN
is connected to ground through a series 0.3 ohm, 5W voltage dropping
resistor.  If my take on the filament monitoring circuit is correct, it is
'reading' the current through this 0.3 ohm dropping resistor.   First I
don't know why there is a dropping resistor since I read from EIMAC that
these tubes are designed to run only +/- 0.6v ( or better) from 13.5 but
that's a different story I guess.

I cannot figure out how the monitoring circuit is working and would
appreciate some advice.  The following link has copies of the tube deck and
LV board schematics.  I hope someone can volunteer to be an Elmer for me on
this issue.

https://www.dropbox.com/sh/qlih0j20xsv76dx/AAAY2tsr3Mnc2FG7N46kQ7Z7a?dl=0

73 Jerry

NY2KW (ex-K1JOS)

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