This thread was about a 148...
I'm impressed - love the freq counter -
Ok, are you probing the DC realm - when you select the various modes, that's either the switch, or the power steering within the radio - you gotta' DVM those lines - that wafer is not perfect.
I'm curious if you've tried scoping' the line in DC to catch it using your triggered sweep - I'm thinking, now after seeing your vid - you have a bad switch. The trigger sweep should show it right away as a "jitter" or simple glitch.
I've seen this problem as a bad S403 - that is a ganged switch - and the logical portion of your problem points to the fact that a mechanical failure in the ganged switch for the 5th set of MODE contacts is most likely your problem. Why? Votlage drop - throw your scope on the line and try to see it's trigger. The 148 used a single layer wafer - the one you need is a dual. There is a SECOND wafer in there and if it's even warped - pow - no power or low power and you lost your AM. Since it's mechanical - there is also the issue of the physical alignment of the two wafers - one set not quite right and you have a resistor in line - not a contact.
Why its so important - the amount of power that particular line needs to sink thru it. Find the S403 and locate that 5th set of contacts locate the common and put a ammeter on it - I wonder in the MB3756 may have a rough time trying to keep the voltage up while that line is powering all those toggles.
Locate R152, check this "trigger" on both sides of this resistor - it's the one that goes to TR 31 - DVM it or whatever you need - if TR31 is having issues, you'll see it in this part. DC and Scope it to see if it "pulses" as it tries to come up - may indicate a failed part that transistor is fighting against while it tries to power up - heck even the R152 if it's as old as I think it is can drift in value.
Remember the R112 and R113 - the ones the route audio and bypass the AN612 - the 2000's show this as a 1.5K - I worry that when you see age in the switches - they tend to act like dead shorts and suck current away from where it needs to go.
It may not come as a surprise - but I wonder if someone "yanked" that knob in AM mode - damaging the wafer? It feels solid or does it have a but of play?
Wow - nice shop!
Ok, I could not see the 2000's freq counter when this Out-Of-Lock on the 7.8MHz occurred - I'm guessing you were on the standard 40 - so that PLL out of lock - should not have been "so wishy washy" there was one frame of your vid that showed 7799.9 - which was at 0 a moment before in AM mode. So, yes, that's a power feed issue - once the line and the switch contact in the wafer -stabilized - you got something - but only after a pretty hefty delay.
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My apologies, I thought you were fixing a 148 - not a 2000 - but now that I see it - check S403 and make sure the wafers ganged together - are not too lose or sloppy as they may need alignment - this can cause that delay in the 7.8MHz uptick - TX otherwise seems ok as far as all the others.
Thanks for sharing this!
:+> Andy <+:
