Will definitely be worth checking with the folk over at ft817@groups.io | Messages if you don’t find a solution here.
Regards, Mark.
1 Like
I’m not familiar with that radio, but based on your description, it seems the problem seems to be in the IF chain (amp, AGC, NB, etc.).
When you select FM broadcast band, are the signals 50+dB weaker than before, or is it just the meter driver faulty? Your block diagram says the separate FM receiver chip outputs AF signal rather than IF. Is that for sure? Because the signal goes into a switch that gets mixed and then filtered, so I suppose it is an IF signal there.
The 68.33MHz filter is shared with the TX signal pathway as indicated by the dashed line, so the failure point is to the right of that point. That path goes into another switch, IF amp, ATT. I think the problem is in that area. After that, second mixer to 455kHz, and then buffer amp, switch to one of a few 455kHz ceramic filters (mode-specific). The fact that you can hear something at the expected dial frequency indicates to me that the second mixer is working.
Another possible problem area is the amp and buffer amp after the 455kHz switch. AGC and meter drive come from the signal after them.
Thanks for all the advice / help on this one. I have an offer of a signal generator and scope to check this out properly. But with some big work projects coming up needind planning, and an already busy schedule up to xmas I’ve had to admit I can’t afford to spend any more time on this right now.
I’ve will park any further troubleshooting until the xmas shutdown and get back to this then with the full complement of test gear.
The radio is indeed an FT818. So only a <5 years old, but it’s done a lot of kilometers in a backpack - about 550 vertical kms just for SOTA before I got the TX500. And I’d guess the horizontal kms are well into 5 figures when you include the traplines, etc its come along on.
1 Like
At 1 shock per metre…
What’s the serial number? Just so I know what not to buy on TM
1 Like
Signal generator is nice, but do you have a very nasty, noisy power supply, charger or Christmas light LED driver or something? It can also be a “spread spectrum” type (many recent chips) class D audio amp board. Your computer screen generates a lot of noise. You could couple those noise to a probe through a small capacitor and use it as a test signal of broad spectrum. Then you can start with the later 455kHz stage and move to the left in the block diagram.
When I was a little kid I built a low frequency blocking oscillator with very rich harmonics to generate this kind of wideband noise (conveniently AM modulated by the fundamental) to troubleshoot radio receivers. It was super handy when working with superhet radios. Most of the time I didn’t have to touch anything, just bring the inductor close to the board and move around.
1 Like