2020-01-18 More Zenith Royal 200 Conversion to Silicon

Continued from post 2020-01-15

I dug through my drawers and boxes of transistors and found a bunch of MPSA56 PNP general purpose on tape, and put them into the LO and mixer. I think they helped slightly – they are lower minimum fT than the 2N3906s.

I adjusted – optimized – the trimpots I had in some locations, then I unsoldered and measured the resistance. I then put in a regular resistor. The original carbon composition resistors had changed to more than 20% higher values, it wasn’t good, so replacing them has helped some, too. I still have others I should measure and replace if necessary. So far almost all of the 10% carbon composition resistors have drifted more than 10%. Some of the resistors had no tolerance band, they were 20%, but they too had drifted too much.

Update, later, in January. I tried a germanium PNP transistor in the local oscillator, and it put out a clean sine wave without the parasitic oscillation. I didn’t leave the germanium transistor in the LO, I put the MPSA56 back into the LO. The performance of the radio doesn’t seem to be any different.

I tried the 2N5142, a PNP TO-105 in the LO and converter stages. They work okay but the ringing is still visible on the o’scope. So I changed back to the MPSA56s. I’ve been thinking about trying some BD136 PNP power transistors in the LO but the pinouts are different, so I have to make a spaghetti mess with shrink tubing to get it to fit.

I spent some time tweaking a 1k trimpot in series with the 100 ohm and collector of the LO. If I go high enough to get rid of the ringing, the LO level drops too low. If I increase the LO level the ringing comes back. I replaced the trim pot with another resistor of about the same value. The 73 mix ferrite bead helped.

The radio is working okay. I need to stop experimenting and find a speaker for it. Then get all the pieces back together. It will never be the way it was when new, it’s still missing the tuning knob, so I used a pot knob. It has numbers but no frequencies. It’s better than turning the bare shaft. The battery compartment is okay.

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