Jim
Good to hear from you. I was hoping you would have comments.
I had already made the change on the keying going to a 100K 5w and
series string of 1N5408 in ct for bias. The relay shorts the
resistor. Just like my ES4L4B kit includes with the soft start and
soft key. I also don't like putting the 120v on the ct and running
zero bias on keyed.
I had also done the diodes HV- to gnd and the one across the grid meter.
Of course I made sure along the way the changes didn't make any
difference to the problem
Interlock was checked first off and has a stiff bit of pvc cable
jacket in there to keep it defeated while I work on it.
Feel like only thing left in the equation is something is wrong with
that transformer. I have some lv high amp industrial transformers
that with a little adjust from a variac I can run at least one tube on
the bench and I'll disconnect primary and secondary of the Drake
transformer. Don't need the relay power just to test so that should
say yay or nay and I can send the transformer to Heyboer for rewind if
it shows that IS the culprit. I'll let everyone know how it works out.
Any notes or history would be appreciated
Paul K0UYA
-----Original Message----- From: Jim Thomson
Sent: Friday, December 09, 2016 12:05 AM
To: amps@contesting.com
Subject: [Amps] L7 problems
Date: Thu, 8 Dec 2016 20:52:20 -0600
From: "Paul Kraemer" <elespe@lisco.com>
To: <amps@contesting.com>
Subject: [Amps] L7 problems
Group
I have an L7 that was sent to me for repair and I am getting nowhere
with it.
The problem is the grid meter has a small negative reading in standby
even with NO tubes installed.
If the antenna relay is keyed the reading goes to zero. The amp does
work if you ignore that pesky off scale negative reading in standby.
The amp does same thing on three different supplies so it is NOT the
power supply. I have an L7 of my own that is fine on the supplies so
it is the L7 at fault.
Here is an A/B comparison of the bad amp (customers) and a good one
(mine)
The ONLY things I have been able to observe between bad and good is:
(1) Bad the center tap of the filament winding is off center by about
0.2vac. In other words, it should be the center and instead measures
like
2.6vac one side to center and 2.8vac other side to center whereas good
will
measure EXACTLY the same either side to center
(2) Bad there is a measureable 25mv (dc plus 60hz)to ground on the
negative connection
of the HV supply. Since that voltage is across a 1 ohm resistor it makes
the amount of negative grid current. If the amp is keyed the voltage
goes
nearly to zero as it should be all the time. Good amp is ZERO all the
time.
Same supply, same location, just swap the linear.
I hope someone can give some insight to this.
Thanks
Paul K0YTA
## I had the exact identical problem on a hb 4-1000 GG amp years
ago. Neg grid current on RX..... then rises to zero when u hit the
PTT (no drive applied).
With drive applied, it operated normally. Lemme dig out my old notes
as see if I can find it. But it only happens when I cranked the B+
way up to 5600 no load..on RX.
I also own 4 x drake L4Bs...with mating 4 x L4PS power supplies. L4PS
is identical to the L7PS. I have not had the neg grid current issue
on the drake amps.
I did have one of them with a fil to cathode short..that resulted in
idle current on RX. One bad tube, thats it.
## IF the CT is not dead center on the fil xfmr sec, you will hear 60
hz hum over the air. A buddy re wound a fil xfmr for a hb 2 x 3-500Z
amp. The loaded voltage was a
bit high, so he took turns off of one end only. Voltage was back down
to 5.0 vac..... but the CT was no longer dead center, but
offset....since he removed turns from ONE end
only. You could hear the slight bit of hum over the air.... but in
his case, it was a lot more off, vs ur .2 vac offset.
## I had another buddy with the neg grid current issue on RX...on a
drake amp, dont recall if it was an L4B or L7. Again I will attempt
to dig up notes.
Check the B+ interlock circuit. On the L4B...with top lid removed, it
just doesnt short the B+ to the chassis. It also shorts the B- to the
chassis..and does that
1st, a split second b4 the B+ gets shorted to chassis. You can see
that on the spring loaded plunger rod assy. Rls it by hand in slow
motion... and you will there is
2 x sets of contacts.... with the lower set grnding the B- to
chassis...then followed by grnd the B+ to chassis....last.
## On my l4bs.... I tossed the pair of 50 watt bleeder resistors in
the HV supply. The no load B+ didnt budge...2650 on RX. 2500 under
load of 800 ma.
Ok, oem deal was to use a 7k 5 watt resistor in series with the
former bleeders. The V drop across the 7k was + 130 vdc..used to cut
the tubes off. On CW position,
its +90 vdc to cut the tubes off. Its a dumb fubar method imo, since
the CT goes open when TR 3PDT relay is in mid air. With big
bleeders removed, you no longer have any
90/130 vdc for cut off bias. No problem just use a 100k at 3 watt
mof..or a pair in parallel for redundancy... in the ct lead. Then re
wire the center tr relay contacts so they
just short out the 100 k..when on TX.
## RVS connected 1N5408 / 6A10s wired between B- and chassis.
Then the meters are protected. B- cant wander more than +/- .7 vdc.
stay tuned.
Jim VE7RF
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