Diamond Triplexer - High SWR on 23cm

 Diamond are a great company for producing products ideal for Ham radio enthusiasts. When I say "great", I mean great value for money. Their triplexer is one of those products.

For around 70€, this product splits out three amateur radio bands, 1.6 to160 MHz, 350-500 MHz and 850 to 1300 MHz into one UHF jack. This allows three antennas to share one cable into the shack.

The owner brought this one in, saying that 23cm showed an off the scale SWR and asked me to take a  look at it. So the case was opened.

From top to bottom, we see the filter stages for low to high bands, with 23cm being at the bottom. I have outlined a circle in red at the bottom right of the image above. It shows one coil very close to the stand off, that is used to mount the base plate. When I opened it, this coil was touching the brass stand off and thus shorting the signal to ground. I very gently pried the coil away from the stand off and then hooked the triplexer up to my tracking generator. A sweep of all bands showed that the triplexer was back up and working on all bands.

It's a pity that something this simple caused the triplexer to fail.

The owner was back on the air again and a happy camper. Job done!

Braun P3 Record Player - No Audio On Left Channel

The Braun Atelier P3 was built by Braun Electronic between 1982 and 1984 and, at that time, it sold for around 1,000 DM in Germany. Back in that time, it was around the equivalent of 980 € today (2021). So it was fair to say, that it was on the upper end of the market.

Today, you can pick one up a used one on eBay for around 270-300 €. This turn table had a quartz locked, direct drive. This ensured that your record always turned at the correct speed. A stroboscope display in the middle of the front panel showed a stationary pattern when the rotation speed was just right. The speed could be finely tuned using the wheel on the front left of the panel. 

This device came in with the owner complaining that one channel had no audio, so the case was cracked open and using a multi-meter, the signal path from the cinch plug was traced back through the circuitry. The circuit diagram was pulled up on the computer, so I could follow along. The signal was being lost on the left channel contact (shown in red):

Although the circuit diagram shows this as a electro mechanical relay, it is actually a relay plus a mechanical set of contacts with are actuated by the turn table mechanism. The problem was with the mechanical contacts, which we can see here in the red circle. The electro mechanical relay is on the green PCB on the lower left.

These contacts are designed to short the left and right channel while the arm is moving and the needle is being lowered down onto the record (picture provided by Jörg on the Brau Hifi Forum).

The contacts were adjusted by manually bending them slightly, so that the mute function was deactivated, when the needle is on the record. A re-test, showed that the contacts were working once more and an audio signal was back on both channels again.

However, now we had a new problem! When the start button was pressed, the arm would not move from the stationary position onto the record. Geez.... not my day. So a little bit of adjustment was required on the mechanism that automatically indexes the arm to the first track on the record.

This mechanism is a mechanical brake, which is activated by the turn table to push an activating lever against a brake mechanism, which rotates the arm.

The picture above was provided by the Hifi Forum and shows the component G, which contacts with the mechanical brake F which is pushed up onto the surface of G and provides rotational force to the arm to move it to the first track. The screw S can be adjusted to take up wear that happens with time on any friction based components.

Once the adjustment was set correctly, the turn table was ready to go back to it's owner. Hopefully, for years of use yet.