Yaesu FT480R

[Chris 1155]

Hello,
I have a Yaesu FT480R all mode 2m transceiver.
It works OK except for the following fault.
When tuned to any indicated frequency ending in 5 ie 145.525 the transmit and receive frequency is 5Khz low.
This seems to be a synthesizer fault but I do not know where to start.
Any help would be appreciated.
Thanks.
Chris G4BYZ

[G6Tanuki]

I'd be checking the lines to the synth divider for dry-joints on the 5KHz control-line, along with its associated pull-up/pull-down resistor.

[Chris 1155]

Thanks for the advice, I will have a go and post results.

[Jon_G4MDC]

It is probably a bit more subtle than that. That has a 3 loop synthesiser which I am still trying to get my head around.

It would be useful to know which frequency plan it has been set for.
D4004,6,7, D4011 and D4051-53 determine frequency range and S,M,L step sizes.

[Chris 1155]

Thanks Jon,
I will post these details this evening.

[Jon_G4MDC]

Another question

Noting that it can actually tune in 10Hz steps is this problem the same for SSB/CW and FM modes?

For the frequency plan question it is probably good enough to know if the medium step on FM is 12.5kHz or 25kHz and if it presets to 145 or 147MHz please.

[Jon_G4MDC]

I have studied that synthesiser for a while now.
Loop 1 is tuning in 10kHz steps so that is not likely to be the problem.

The mix down local oscillator used in Loop 1 is tuned over 9.99kHz in 10Hz steps and it is likely to be here the problem lies - somewhere in Loops 2 and 3.

I am having trouble with the circuit in that area. It looks like Loop3 produces 50Hz steps which are then doubled to 100Hz before they feed Loop 1. VXO Q3009 in Loop 2 looks like it would be responsible for the 10Hz steps but tuning that affects both Loop2 and Loop 3. It is a bit confusing there.

Another strange thing is Loop3 is supposed to divide 4.075 - 4.57MHz down to 5kHz comparison frequency. That needs division ratios from 815 to 914. It is clear that Q3005, a TC9122, is only able to be programmed by BCD between 800 to 899. That is odd.

Anyway I would be looking closely for dry joints or short circuit whiskers on the PLL board around the shift register 4094s Q3031 and Q3032 and the TC9122 Q3005.

[Chris 1155]

Thank you Jon for all the advice.
It will take a few days for me to get the time to investigate but will let you know how I get on.

[Jon_G4MDC]

OK Chris - good luck with it.

I hope the boards have some silk screen component identifiers you can work with.

From the online manual scans I have not found anything that is good enough to identify component locations and sorry to say I have never had one of these. It is interesting all the same.

[Chris 1155]

Hello Jon,
When I switch the radio on it displays 145.00 Mhz.
The steps when set to FM are :-
S 1.0 Khz, M 12.5 Khz, F 25.0 KHz.
The problem of transmitting and receiving 5 KHz low compared to the digital readout indicated frequency is evident on FM, CW and SSB.
I have located Q3031, Q3032 and Q3005 on the PPL unit and will start looking for problems as you suggested.
Thanks for the help and I will post my findings.
Chris.

[Jon_G4MDC]

Hello Chris - thanks for additional info.
It was expected the problem would be there on all modes.

One more thing - does it really happen only when the kHz digit is showing a 5 -
meaning there is no offset for any other kHz digit 0-4 and 6-9?

I hope you find something simple to explain it. I still can't reconcile the frequencies given for Loop 2 in the PLL Block Diagram with the apparent wiring to the BCD counter Q3005. They just don't seem to agree with one another.

I wonder if you have the service manual for it? I'm working with a downloaded one and I begin to wonder if it has an error or that the frequency lineup was changed somewhere down the line. Can you see a crystal marked 63.9151 MHz maybe? That would help.

Best of luck.

[Chris 1155]

Hi Jon,
I have looked very carefully around Q3031, Q3032 and Q3005.I have resoldered all the pins and there is no evidence of dry joints or surface debris. On switch on, the original problem still exists.
I have checked the transmit frequency on the rest of the KHz digits with interesting results as follows.
Indicated frequency Transmitted frequency
145.520 145.520
145.521 145.521
145.522 145.522
145.523 145.523
145.524 145.520 ****
145.525 145.520 ****
145.526 145.520 ****
145.527 145.520 ****
145.528 145.520 ****
145.529 145.529
145.530 145.530
There is a crystal marked 63.9151 MHz.
I also have a downloaded copy of the service manual but I do not have sufficient experience understand the PLL functioning. I am happier with my 1155 and 1154 am setup.
As a separate issue the squelch is now permanently on ie continuous receive. I think this is a wiring fault and should be easy to resolve.
Your advice is greatly appreciated.
Chris G4BYZ

[Julesomega]

Remind me what the manual has to say about the freq generation system. All I can remember just now was there is a VCXO which is used for fine freq interpolation, with the varicap voltage set by a preset resistor chain. This was normal for that generation of synthesised transceivers.

I think you will find one of the oscillators increments in something like 2.5kHz steps over a 25kHz range. Or maybe it has only two steps, separated by 2.5kHz. See if you can find one oscillator that tries to do this but fails.

[Jon_G4MDC]

That is all very useful info Chris 1155. Will be thinking on it in due course.
It is most strange that the manual appears to disagree with itself - but that is how it is.

Julesomega you are not far wrong. The finest steps (of 10Hz) are indeed by VXO.

The 100Hz steps are actually done at 5kHz but then divided by 100 to 50Hz before being multiplied by 2 for eventual use.

Chris - can you identify Q3005?

Do you think you could measure the Voltages on the pins that correspond with your table? It would need a multimeter or oscilloscope of some sort. I think (offhand) there are about 8 pins to be measured.

If the binary going in is correct then it must be a divider chip fault.

Otherwise, if the binary input does not follow the right pattern, then it must be the shift registers (4094) or else the microprocessor which generated the serial data stream. (That one is highly unlikely I think since the display is correct).

[Jon_G4MDC]

After thinking a little more - that group of 5 frequencies which are in error is very odd.
In BCD code which is being applied to the divider there is no run of 5 "1"s together or 5 "0"s.

If a group of 4 consecutive frequencies were in error we would have a theory - bit C0 is stuck low. 145.524 to 145.527 giving 145.520 would be explained.

It is all undone by 145.528 giving 145.520 yet 145.529 is correct...very strange!

[Jon_G4MDC]

30min edit time limit expired - so adding more info separately.

Do you think you could complete this table by measuring on the pins of the TC9122 Q3005? 1= 8Volts here I think.

Function____________A0_B0_C0_D0_A1_B1_C1_D1_A2_B2_ C2_D2_A3_B3
TC9122 Pin#_________3__4__5__6__7__8__9__10_11_12__13_14_ 15_16
145.520 145.520
145.521 145.521
145.522 145.522
145.523 145.523
145.524 145.520
145.525 145.520
145.526 145.520
145.527 145.520
145.528 145.520
145.529 145.529
145.530 145.530

Seeing what happens there should help a lot. There should be no need to TX and count frequency again - it is just the binary states we are interested in.

[Chris 1155]

Thanks for the further information and thoughts Jon.
I do have the equipment to carry out the measurements and will do so in the next couple of days. I want to be sure of what I am doing and not cause further problems.
I will post the results a soon as I have them.
Thanks also to Julian for your contribution.
73 Chris

[Chris 1155]

Pin No ------1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

145.520 ----5 2 5 0 5 0 5 0 0 0 0 0 0 5 0 0 0 0
521 ---------5 2 5 0 5 0 0 5 0 0 0 0 0 5 0 0 0 0
522 ---------5 2 5 0 5 0 5 5 0 0 0 0 0 5 0 0 0 0
523----------5 2 5 0 5 0 0 0 5 0 0 0 0 5 0 0 0 0
524----------5 2 5 0 5 0 5 0 5 0 0 0 0 5 0 0 0 0
525----------5 2 5 0 5 0 0 5 5 0 0 0 0 5 0 0 0 0
526----------5 2 5 0 5 0 5 5 5 0 0 0 0 5 0 0 0 0
527----------5 2 5 0 5 0 0 0 0 5 0 0 0 5 0 0 0 0
528----------5 2 5 0 5 0 5 0 0 5 0 0 0 5 0 0 0 0
529----------5 2 5 0 5 0 0 0 0 0 0 0 0 5 0 0 0 0
530----------5 2 5 0 5 0 5 0 0 0 0 0 0 5 0 0 0 0

[Chris 1155]

Hello Jon,
The previous post is the results from measuring the pin voltages on Q3005 TC 912.The chart did not send as I typed it the information is there although not well formatted.
The actual voltages measured were 4.9 (5), 1.9 (2) shown as 5 and 2 for ease.
These readings were taken in the receive state but I did notice that the 0 volts rose to .2 volts on transmit.
I hope the chart will give some clue to the problem although the logic is beyond me.
Thanks once again for your time and interest.
Chris G4BYZ

[Chris 1155]

I have attempted to reformat the table, hope this is an improvement.
Chris.