UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Roberts RT7

Herald1360 · 30th Apr 2009, 9:36 pm

Good, sounds like the transistors are probably OK. I tried a few (including an OC81, much to my surprise) and found that the meter seems to work, but the socket is a bit iffy and readings could vary quite a bit with finger pressure etc. I think just doing the diode checks c-b, e-b and c-e is probably easier to prove you still have a transistor rather than a blob of fused semiconductor in the can.

I suspect smiler may be right about the 80 ohms causing the problem, especially if the OC81s are reasonably high gain, though the calculations aren't quite right- the open circuit base voltage is Batt volts * 80/(80 + 2530) so it will be a little lower than stated at 0.276V for 9V batt volts.

If you reassemble everything, and connect to the battery via the meter on 200mA range, short out R27 and switch on, current should be low- a few mA. briefly take the short off R27, if the current now rises much above 10mA, R27 IS the culprit. In which case if you've got a suitable 47 to 56 ohm resistor, replace R27. (Or connect about 120 ohm in parallel).

Chris

30th Apr 2009, 9:36 pm	#105
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Roberts RT7 - problem Good, sounds like the transistors are probably OK. I tried a few (including an OC81, much to my surprise) and found that the meter seems to work, but the socket is a bit iffy and readings could vary quite a bit with finger pressure etc. I think just doing the diode checks c-b, e-b and c-e is probably easier to prove you still have a transistor rather than a blob of fused semiconductor in the can. I suspect smiler may be right about the 80 ohms causing the problem, especially if the OC81s are reasonably high gain, though the calculations aren't quite right- the open circuit base voltage is Batt volts * 80/(80 + 2530) so it will be a little lower than stated at 0.276V for 9V batt volts. If you reassemble everything, and connect to the battery via the meter on 200mA range, short out R27 and switch on, current should be low- a few mA. briefly take the short off R27, if the current now rises much above 10mA, R27 IS the culprit. In which case if you've got a suitable 47 to 56 ohm resistor, replace R27. (Or connect about 120 ohm in parallel). Chris