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Old 30th Apr 2009, 5:26 pm   #87
Join Date: Apr 2009
Location: St Albans
Posts: 50
Default Re: Roberts RT7 - problem

Hmmm. I've been reading about testing transistors and see that the advice is to use the diode test facility on a multitester. I have been using the resistance range in 20k ohms - was I wrong?
Because, using the diode setting and testing the now removed rubber jacketed OC31's, they seem to meet the requirements of a sound transistor in all six directions; BE; BC; EC.

Right, that's one thing.

I have removed the C32 capacitor and tested with the multimeter using 29k ohm range. (This is the one getting quite warm, is adjacent to TR6 & 7, and visible in Graham's post 86.)
pos to red cap, neg to black cap - no reading.
But, if I change over;
pos to black, neg to red cap - I get a minus figure that goes through zero and up to around 15 then no reading, and no subsequent reading when repeating the test in that same direction.
If I then go back and reverse my action again, and put pos to red cap, neg to black cap - I get minus figure, going up through zero to around 17 then no reading. Subsequent testing gains no reading.
Am I polarising the capacitor by doing this?
What does it tell me about the capacitor?
And should I not do that?

I have also lifted one end of both the resistors using a croc clip at first, but then thin nosed pliers to pull its tail up through the board as heat was applied.
Tested between the track beneath and the lifted end:
R26 reads 2.53 ohms (20k test range both)
R27 reads .08 ohms

When speaking of 'bias resistors' which ones are they?

The layout of wires on the transistors I fancy might be a red herring, as the original OC81 has its wires in a line - no pattern to match the holes in the base. The newer ones have a triangular pattern, but I fancy they being manufactured forty odd years after the set was made might well be a simple coincidence that they were in a triangular pattern also - albeit the other way. But not being knowledgeable on radios and their possible historic patterns - possibly wrong . . . anyway -

One other point - Heat sinks. TR6 & 7 were mounted in copper clips riveted to a frame support. Surely this is for a reason?

Edit - PS: Fixing the shower would be a doddle!

Here are the old OC81's. The insulation has welded together at the top most ends:
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