Continued from post 2020-01-15<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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. <\/p>\n
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.<\/p>\n","protected":false},"excerpt":{"rendered":"
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 <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[311,7,262,335],"tags":[],"class_list":["post-13198","post","type-post","status-publish","format-standard","hentry","category-am-band","category-radio-frequency","category-receiver","category-repair-alignment"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/13198","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=13198"}],"version-history":[{"count":5,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/13198\/revisions"}],"predecessor-version":[{"id":13236,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/13198\/revisions\/13236"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=13198"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=13198"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=13198"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}