[BC] Tips to repair a CCA AM 1000D

Mon Dec 5 12:33:35 CST 2005

That transmitter tends to have problems with it's carbon resistors as it
ages. 
In particular, check R41 and R42, cathode return resistors for the 4-400
modulators- 2 ohms at 2 watts. 

Each of the 2 ohm 2 watt resistors tied to the overload relays should be
checked as well, their failure causes the transmitter to do strange things. 

The resistors in the feedback ladder, located in the rear RF compartment,
left-hand side are probably bad.
 Located right behind the tubes in a small unventilated conpartment, they
get quite hot and fail regularly. Judging from the healthy imbalance
between V1 and V2, I'd guess that this should actually be your starting
point-

Finally, with audio removed, can you get current through the modulators by
decreasing the bias voltage?  Momentarily turning the Idle current controls
clockwise should get you 150-200 mA of current if the tubes are good and
the supporting circuits are OK.
If not, the screen voltage dropping resistors, located on the left wall
near the feedback ladders are suspect.  

Good luck!

The AM1000D doesn't fail very often, it's a tank of a transmitter. When it
does, it's almost always either a bad tube or a failed carbon resistor. 

Chuck G. 
Quicksilver Broadcasting of WI and MI

Original Message:
-----------------
From: wernerl at tutopia.com reader at oldradio.com
Date: Sun, 04 Dec 2005 18:20:08 -0700
To: broadcast at radiolists.net
Subject: [BC] Tips to repair a CCA AM 1000D

I just want to get some tips on the reparation of one CCA  AM 1000D. 
as possible for your knowledge ,, I'm new in the tube technology.. my 
field is on the Transistors, Fets on  FM and AM Power transmitters.

Today  I just found no modulation at all in my transmitter .The power 
still looking fine but not the modulation at all.

I checked the tubes on the front of the transmitter and I found the 
rear left to be smoked inside. I change that tube , but I really 
didn't know which are the Modulation tubes from the front .. I 
supposed that the rear right is the V3 ,,  I don't know where is the 
V4 but I suppose that is rear left as Schema didn't show crear.

The handbook said that when is 16mA on the V3, V4 meter the state is 
Idling,, I just have 10mA on both scales.

The V1 and V2 are wrong too ,, the V1 is on 16mA and V2 is on 42mA 
,,  when its value has to be between 20-30 mA

At the point that I increase the Low power Input audio Control , I 
became to get some modulation around 5 to 10% ,, but is all.

I checked the resistors on the 1M network and still fine ,,, some of 
them are around 600K Ohm  , others around 800K,, but that's it.  I 
test all  resistive components around the modulation circuit and the 
two 27K resistors that sum the audio modulation on the 4-400 tube.

Please Let me know which side of the modulation circuit I will check.

Thanks very much for your attention.

Sincerely,

Werner
Sorry my english, my native is spanish.

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