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Old 13th Jul 2019, 5:47 pm   #1
ben
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Join Date: Aug 2006
Location: Madrid, Spain / Wirral, UK
Posts: 7,484
Default Tandberg series 15 - hummin', bumpin' and thumpin'!

Slightly OTT title I know!

Basically this unit has the classic symptoms :
-hum at switch on, which decreases considerably after a few seconds
-motor boating with flashing of pilot lamp in sympathy if volume set to position 4 or higher, backing off the bass control lessens this.
-Hot smell
-The heat sink on the power amp pcb gets very hot quickly. Since the audio output is otherwise okay at low volume, I suspect the AD131 regulator (Q112).
-There is a 130 ohm NTC (R144) which is in very poor shape.

There is some 24v on the rectifier output terminals, which is too low (spec; 40v!).
There is 21v on the 24v line with the machine in stop.

I have changed the following;
-rectifier bridge
-two 1000uf reservoir caps

Also checked all earths, connectors, rec-amp sw, soldering etc.

Anything else to check before I change Q112 ? I wonder if the 24v zener is doing its job?
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