Royce Sealed PLL Schematics

[Mud Duck 486]

If TP1 looked good, and TP3 looked good (as it should only be active during TX and it had the right freq), something associated with only TP2 seems to be the the issue..

For there to be the proper signal at TP3 during TX, the VCO should be functioning as its signal is mixed with the 10.695 before leaving TP3, and you had that. During RX however, the 10.695 osc and mixer to TP3 is turned off and the VCO signal goes to the now active VCO amplifier that provides the external first RX mixer with its LO signal. If your notes are accurate, I would think the problem is around that VCO amplifier stage. You could start by testing the voltages associated with it during RX and see if things look good.

BUT FIRST>>>Pin 22 should have 9v on it during RX (this is the signal that tells the PLL unit whether it is supposed to be doing RX or TX stuff). Check to see if you have 9v on pin 22 (RX) first. If you do have 9v, then see what the voltage is at the emitter of (what I call) Q97, the VCO amplifier.

Also a correction, I have the 648 not a 641.. was hoping a switch of the PLL's, to at least see if RX came alive on 678 was my plan but once inside realized they are diff because of the delta tune the 678 has so that put a stop to that thought!

 

[brandon7861]

The 641 is a sideband radio and the 678 is an AM radio. I believe the AM radios share a common PLL unit and the sideband radios share a common PLL unit.

Back to the 9v situation.
The sams schematic (for the 621) shows the 9v comes from the DC source, through L1 (RF choke), then through R63 (68Ω) to a pair of zener diodes (D9A and D9B). This is then filtered by caps C53 and C79 before going off to pin 22 of the PLL and the tune circuit. Normally, this 9v signal is pulled down to .7v by passing it through D19 to pin 3 of the mic jack. When the ptt is pressed pin 3 on the mic jack is pulled to ground and that diode D19 is why you get the .7v instead of 0v. See the areas I marked in red.

But thats the 621 schematic. I dont have the 678 schematic. I would guess they are similar

 

[brandon7861]

If you can, suck the solder off pin 22 and slide a slit coffee straw over pin 22 to isolate it from the 9v pad. Next, check if that pad holds its 9v. This will tell us if the PLL unit is dragging it down or if something on the main board is. If there is 9v on the pad with pin 22 isolated, while keeping pin 22 isolated and after turning the power off, see what resistance you get between pin 22 and pin 20 (gnd).

 

[Mud Duck 486]

If you can, suck the solder off pin 22 and slide a slit coffee straw over pin 22 to isolate it from the 9v pad. Next, check if that pad holds its 9v. This will tell us if the PLL unit is dragging it down or if something on the main board is. If there is 9v on the pad with pin 22 isolated, while keeping pin 22 isolated and after turning the power off, see what resistance you get between pin 22 and pin 20 (gnd).

Will do.. I'll let you know what I find. Thanks!

 

[Mud Duck 486]

Will do.. I'll let you know what I find. Thanks!

Well some quick good news! I was able to get the 1-648 finally TX-ing, so I can concentrate my efforts directly on the 1-678. So after reviewing your sams with red lines drawn, my unit is not the same... it looks like there is 8.78v on pin 22 and zero on TX. I am reading 9.37v and 55mV when keyed. Appears that is operating correctly at least. The voltage to test on the VCO output from the scratched resistor on PLL for Q97 has no volts. So definitely not working. The 1k resistor (R62) for the 36MHZ had 1.57v across it. But, where can I take a sample of the freq to see if that is actually putting out before mixer? The 10.24Mhz works fine.. so maybe like you had, my 36 is dead which is killing 10.695 too? I want to buy a crystal tester to test these bad boys so at least can rule them out.. Your thoughts? Also how can I attach the whole Zip file of the 678 sams here, I don't see an attach option? Thanks!