Joe,
As was mentioned earlier, U.S. AM broadcast tolerance is +/- 20
Hz. Many of the LPB low power transmitter were used for TIS/HAR
service. The tolerance for that service is +/- 100 Hz. I've
maintained several of these stations, including the 100 watt system
on 530 kHz at LAX Airport. The transmitters were made by LPB, were
all solid state, and stayed within 5 Hz for years without any
adjustment. It's been a real long time since I worked on any of the
tube version of these, but >200 Hz sounds like something has gone
wrong. It may be a defective crystal or some component in the
oscillator circuit. To your knowledge, is the crystal that's off
frequency a LPB supplied crystal? The LPB's were also used for
broadcast transmitters, particularly at daytime stations that had
very low power nighttime authorization. Used that way they had to
maintain their frequency within the +/- 20 Hz tolerance, which was
never a problem with a good crystal. They are actually pretty good
transmitters and are capable of 125% positive peak while keeping the
negative peaks at 95% or less when driven with a real audio
processor. I have one of the 30 watt solid state transmitters and
one exciter board. I also have one that is an amplifier only that
I'm able to drive to about 15 watts out with my bridge generator.
LPB went out of business a few years ago and left a lot of customers
who had pre-paid for their equipment holding the bag. I was one of
them. It's a shame because they really are good transmitters.
Burt, K6OQK
At 09:26 PM 6/21/2013, time-nuts-requ...@febo.com wrote
>> I have an old AM transmitter that has three 6AL11 compactrons. The crystal
>> is a fundamental, cut for 660 KHz. I don't have a schematic for
this thing,
>> but I believe that one half of one 6AL11 is used for the oscillator.
>>
>> The problem is, the frequency decreases as the rig warms up. It will
>> eventually stabilize, but the final frequency is over 200 Hz low. Not as
>> good as it should be. I think the original specification was well under
>> half of that.
>>
>> I have replaced the electrolytic caps. The others are mostly silver/mica
>> with a few ceramics. I checked all of the resistors and only
found one that
>> was out of tolerance (I replaced it).Three NOS tubes were installed. There
>> are no tunable components in the oscillator section. Only the output
>> section has anything tunable.
>>
>> I know that there are many Amateurs on the list and I'm sure
many know more
>> about old tube rigs than I do. Does anyone have a suggestion as
to what the
>> trouble might be?
>>
>> Joe Gray
>> W5JG
Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
b...@att.net
www.biwa.cc
K6OQK
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