with the sig gen set to 200khz deviation (if the sig gen doesn't have a deviation meter, you can use an RTL-SDR to measure it), and the y axis gain adjusted to take up the whole 10 divisions across, you now have a way of measuring the bandwidth. you can use sine wave modulation if you are adjusting the RF and IF, but for measuring, you want a linear sweep) driving the X axis, and the output of the probe driving the Y, axis. the scope is used in XY mode with the modulating audio (preferably triangle wave audio. you would use an RF demodulator probe (consists of a germanium diode voltage doubler and a BNC cable) connected at the input of the radio's FM demodulator, and the sweep generator connected to the antenna input of the radio. If you have access to an FM signal generator and a oscope you can measure the actual selectivity of the receiver. Initially it was only the audio ceramic filters you had to change, plus the notch filter on the video - but with the advent of SAW filters for the main IF you had to replace that as well, as it was sharp enough to reduce the audio carrier. SAW did replace transformers in TV IF circuits, due to their complex nature, but radio IF's (and the audio IF in those TV's) used ceramic filters as they did everything required, and are probably cheaperĪs far as TV's go, I modified a LOT of grey imports (with 5.5MHz audio IF instead of 6MHz) over the years - in fact I modified them both ways (imports and exports). However, for practical purposes, Ceramic filters and SAW are the same thing anyway. I never even considered IF transformers, although I thought ceramic filters rather than SAW - I've no idea if SAW's were used in later radios?, but ceramic filters replaced IF transformers a great many years ago. Looks like an OVRA auction item.Neither did I, which was why I said 'perhaps' - as it could be a difference between the two radios, one with a tuned frontend, and one without. Now what.? I really don't need a 1967 transistor table radio taking up space. speculation, no schematic! So, I rewired the new B+ filter to include an RF chokes in both the ground line and B+ line. There seems to be one or two tiny RF caps from the PT to the FM RF/converter stage. I had inadvertently by-passed these spirals on the PCB. it looks like they are used to isolate the Power line from ground and B+ at 100 MHz RF so that the power cord could be a decent "long wire" FM antenna, not grounded at RF. Then I realized that the spiral coils on the PCB were RF chokes. By touching various points on the PCB with a 3 ft wire the FM came on strong! This radio uses the line cord as an FM antenna. but I forgot to note some PCB connections! Neither did I take a photo! Mea culpa! I put it all back based on intuition and first principles, and it worked. In so doing I unsoldered the PT and cap wires from the PCB to make the new "air circuit". I did a "Q&D" B+ volts drop calculation - still OK since we have 125 VAC mains and the radio was designed for 117 VAC. Decided to put in a pi-section B+ filter with two caps, 1000 MFD after the diodes and the 2,200 after a 27 ohm series resistor. Replaced it with a tested 2,200 MFD from the spares box, but still some audible hum. there was a quite audible 120 Hz hum! The PT is a 6 - 0 - 6 VAC with two diodes to a 1000 MFD capacitor - took it out and tested it. I re-strung it and it worked, of course.īut, new problem. even though it is a cheap radio, I could not just toss it in the trash. I have to confess to being an "obsessive compulsive fixer" of junk. she simply bought a new "Tivoli" radio from Bay Bloor Radio in Toronto (no comment!) I was given the radio by a "civilian" friend who said the dial was stuck at one end. here's the abbreviated story, not reflecting all that well on me! I think I've fixed it without a schematic.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |