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Old 27th Apr 2009, 2:48 pm   #37
stonehopper
Tetrode
 
Join Date: Apr 2009
Location: St Albans
Posts: 50
Default Re: Roberts RT7 - problem

Update: Screwfix were on form - arrive 07.15 this-morning.

Battery before switching on - 10.1v. Connected up and switched on, got a channel on Medium wave (just one mind) but carrier wave only.
Checked voltage between the new OC81's; collector to emitter 9.6; collector to base 9.6; emitter to base 9.6. The other was the same. I could smell something getting warm, and it wasn't the transformer which stayed cool to the touch. Twiddled the dial a bit more, but failing to get even a carrier now, re-checked the voltage at the OC81's. This was now down to 3.4v all round on both - and getting very warm. Switched off, disconnected, and checked battery volts - 9.1 and recovering. Stabilised at 9.6 after two minutes.

Something is shorting out. I'm happy to keep at it, though the thought of trying to find a replacement transformer is disconcerting if none are available! I am guessing I would need to know the correct output voltages at the various terminals to discover if it is duff or not, though as it's no longer getting warm it could mean it's either dead, or OK?

Or is the next step change the electrolytics?

Might I have cooked my new (and somewhat pricey) OC81's?

Does any of this help you distant Doctors?

Note: My description of 'carrier wave' might be wrong. As I turned the dial onto and past the station point, a high pitched signal was heard which reduced in note to inaudible, then increased back up through high pitch to inaudible.

Last edited by stonehopper; 27th Apr 2009 at 2:58 pm.
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