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VK3XX > HELP 27.04.08 04:46l 65 Lines 2735 Bytes #999 (0) @ WW
BID : F81599VK3XX
Read: GUEST DG1VV DF2NZ
Subj: TS850S fault
Path: DB0FHN<DB0MRW<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<DB0EEO<DB0RES<DK0WUE<7M3TJZ<
HS1LMV<CX2SA<VK4TRS<VK3API
Sent: 080427/0217Z @:VK3API.#MEL.VIC.AUS.OC #:12338 [Lilydale] FBB7.00g
From: VK3XX@VK3API.#MEL.VIC.AUS.OC
To : HELP@WW
This is one for the "boat anchor brigade" with servicing experience of old
rigs.
For some time now I have been investigating a fault on my trusty TS830S.
The radio works perfectly on receive whether the PA heaters are on or not.
The transmitter works fine for about 20 minutes of accumulated
transmissions (sometimes less). After this period the PA standing current
drops to half and then shortly to zero. There is no output or PA cathode
current and the receiver ceases to work too. If this happens during an SSB
transmission the monitor continues to output the speech. After a period of
switching the rig off for about 10 minnutes everything works OK again on
switching back on.
Here is a list of what I have done so far:-
1) I suspected a grid to cathode short circuit in one or both 6146B tubes
and replaced them with new ones.
2) Initially all the voltages around the 6146B tubes are quite normal.
Measuring them after the fault occurs shows some reduction of screen and
bias voltages but nothing dramatic. Just about 20% or less.
3) I suspected that the smoothing capcitors for the bias, PA and driver
screen and driver plate might have failed but they respond OK to normal
electrolitic capacitor tests.
4) I have removed the PA circuit board and resoldered every joint
particularly those of the tube holder which is of the circuit board
mounted type.
5) When the fault occurs there is some slight smell from some component
that is getting warm but I cannot locate it. Everything checks out as
normal value. The components surrunding the 6146B tubes are only
accessible after the board has been removed and none show any sign of
having been hot.
After all this the progress in finding this fault has been precisely nil.
I would be pleased to hear if any radio service man or other amateur has
ever experienced a similar problem. From the internet I have found
comments on something very similar happening when a QRO "modification" had
been made to screw a bit more output from the 6146B's which would do
nothing more than produce a few more watts but negligible db's at the
other end at the expense of intermodulation products. Up to this problem
appearing I have had the rig in regular operation since new and the only
repair has been to replace a driver 12BY7A tube which failed last year.
Any help on this issue such as other suggestions to try would be
gratefully accepted from the amateur fraternity.
Best regards to all. Please reply by bulletin as many personal mail
messages never reach here but bulletins do.
Cheers from
73 - GORDON, VK3XX @ VK3API.#MEL.VIC.AUS.OC
Message timed: 12:14 AEST on 27 Apr 2008
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