IRET PRC247-B

[IK0MOZ]

Hello Chris. The board you are testing is the DAC9. This board is the Digital to Analog converter means it converts the digital signals according the tuning position in a DC voltage that drives the Varicap in the VCO O3 circuit. My suggestion is to undestand if the problem is in the DAC9 or in O3. The first simple test is disconnect the wire that connect the two boards and check if in output of DAC there is the variable voltage. If yes the The DAC is good and the trouble should be on connection or in the O3. Take care that in the connection line there are some by pass capacitors with the possible well known problems.
Ciao de Mario

[SV1TDZ]

Hi Mario.
I have already done this and it looks like the problem is with the DAC board.
I have 3 FS4 units, 2 do not work at all with the common symptom of no hiss sound (noise) while in AM mode but i have noise in the other modes and one that works fine above 25 Mhz after changing the 776 opamp with a 741 which was probably burned , i even checked the Mc fractions and they work fine in Rx and Tx.
I also found leaky tandalum capacitors on all 3 fs4 units so the next step is to slowly change them all in a unit to see if this will cure the problem.
Today i got the 776 opamps i ordered and i will try with them also.
I am on vacation for this week only and that's why i didn't answer more quickly, i just returned home today only to get my trusted laptop and collect the 776 opamps from the post office...
I will be back next week.
Chris

[IK0MOZ]

Ciao Chris, OK all. Take this suggestion like a joke..... disconect the DAC and supply the OC3 with a dry 9 V battery and see wath happens. Yes the tantalum capacitors are a problem specially after 50 years of service but recap completely the set it is pratically impossible.

[SV1TDZ]

Hi Mario.
Maybe i didn't understood you well but your idea is not bad at all, i also thought of connecting the 9 volt supply pin1 to the varicap to see what will happen in the spectrum analyzer.
I will try and let you know!
Chris

[SV1TDZ]

Btw if anyone has the service manual of PRC247B and can send it to me by email or another way please do!
Scribd has it but it wants me to become a subscriber so i avoid downloading it from it.
Chris

[SV1TDZ]

OK!
The first synthesiser is repaired.
It had 3 faults, the first was the dreaded vco tandalum capacitor in the 0-3 vco as in the pictures i posted, the second was on the upper side in DAC board where the lm776 (the venerable 741 opamp also works fine) was burned AND the two blue tandalum capacitors used for integration of the 776 output (smoothing) where leaky and the third was the tandalum capacitor on the bottom side of the synthesiser that is used for the 0-1 vco varicap voltage smoothing.
This blue tandalum capacitor (sorry my mobile phone camera quit working but i will take pictures somehow) is located in the middle of the vco labeled 60 or 70 Mhz, exactly at the center of the vco can and about 3 cm towards the frequency knobs.
I imagine that the two blue tandalum capacitors in the upper side vco shorted or were terribly leaky and that burned the opamp's output in the DAC board.
NOW I HAVE AN EXCELLENT PRC247 WITH SOCKETS FOR BOTH THE DAC ICs, in the near future i will design a new dac board that will be removable because now it is a PITA to repair.
I also have to say that from all SSB transceivers i have used and they are A LOT the PRC247 has the best modulation quality of them all.
Chris

[IK0MOZ]

Wonderfull Chris, at the end you win the VCO battle.... please if possible take and post the photos of the area where the fault parts were. This for future help for who will have same or similar problem that seems to have this set. 73 de Mario

[SV1TDZ]

I will post tomorrow.
In the mean time look at post #42188 and note the two blue tandalium capacitors in the upper right corner of the DAC board, they created all this mess together with the blue tandalium capacitor in the other side of FS4 synthesizer....
I need to save money to buy a decent mobile phone, rugged of course...
Update
Here are some photos, the changed capacitor is the small black electrolytic next to the bigger blue one in the 0-1 vco side (bottom side of FS4)
On the upper side of the FS4 (0-3 vco side) and on the DAC bord (Look at post #42188) the two tandalium capacitors are actually in series so i only fit one electrolytic 2,2 to 4,7uf @ 63v there but the strange thing is that the polarity must be specific otherwise the circuit does not work.
The + terminal of the electrolytic goes to the upper right corner of the DAC board and the - terminal towards the trimmer potentiometer.
This is the second FS4 i have, the other one is working fine so i installed it in the PRC247B i have, i am repairing the second one as a spare FS4.
Chris