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| Chassis ready to test, with temporary connection to Gram socket |
It didn't burst into life initially, so a bit of basic faultfinding was needed.
All the voltages on the EL84 output valve looked OK, and a test signal injected into the grid of this valve gave a nice clear tone from the speaker. Injecting the same audio onto the volume control gave a very weak and intermittent audio, with lots of crackling. The output from the volume pot feeds the grid of an EBC90 double diode triode, which performs the triple role of AM detector, AGC detector and audio preamplifier.
The anode and grid voltages looked OK, but the valve seemed to be very microphonic and gave a terrible noise from the speaker when handled. A quick swap with a new valve was tried, after which the set was completely silent!
After a few minutes poking around the valve base with a meter I began to suspect the socket was the problem. A couple of shots of de-oxit cleaner spray didn't appear to help much. Pulling out the valve again, I had a good look at the socket from above with a magnifying glass and....mystery solved.
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| B7G valve socket after cleaning & adjustment |
Most of the contact pins seemed to have been bent, so that the contact with the valve pins wouldn't have been as good as it should be. With the aid of a very fine pair of tweezers I closed the two fingers that form each the seven contacts together, in the hope that this would grip the valve pins better. I then put the original EBC90 valve back in.
The bodge worked, and the test tone applied to the volume pot was loud and clear from the speaker. At least the audio stages are working!
It's now playing music loudly from an MP3 player connected to the gram input. It remains completely silent on both AM and FM, so more investigation will be needed.
Keep listening!
