when locked on a (strong enough) signal on band 3, the IF sits at roughly 127.9MHz instead of 125MHz as described in the manual, for which the troubleshooting tree concludes that "VCO or LOCK CIRCUITRY on A108 BAD".
127 MHz is correct, EIP has changed IF frequency in later A203 revisions, all 'B' models are 127 MHz.
You can look at the A203 internal IF amplifier, if I remember correctly at right side looking from front, sometimes I've found some leaking SMD ceramic capacitors. Please DO NOT touch the small wires coming from YIG filter through a hole, these are folded as a kind of inductance during factory calibration.
Check also if the screws holding the YIG filter to A203 box are tighten, sometimes loss of performance happen for these loose screws.
There are a few of the on eb, but none have exactly the same part number. For example there is a reasonably cheap 2010241-08 Rev. D available, is there a chance it will work (after recalibration)?
Should I try to stick it back to the holder? With what kind of "glue"? For the record, these 2 balls are very small, maybe 2 or 3/10 mm, so it will not be an easy task...
Ok I'll be extra careful! How leaky are we talking about there? like they begin to behave like a resistor on DC?
[mode miracle]Don't you have a copy of the 545B service manual somewhere? It would be so much easier! [/mode miracle]
If you have the possibility to measure the input RL (VSWR) setting the YIG coil current for a specific tune frequency, you can determine if the coupling between input RF link (that is critical) and the YIG sphere is compromised or not.
I have some in paper form, unfortunately NOT in pdf, another member has already asked me for a manual, at the moment don't have much time to do long and tedious scanning sessions...
The YIG sphere is typically mounted on the end of a thermally conductive rod (normally beryllium).
This is done for two reasons: 1) the rod acts as a “tuning stick” for orienting the YIG sphere in the resonant circuit and
2) YIG has best performance when it’s temperature is kept constant.
What is the best way of setting the YIG current to match a given frequency (bypassing the automatic search mechanism)? I don't see any TEST mode for that. Could the min/max frequency parameters be used to achieve this?
but using TEST10 (memory read/write). According to the SM, it seems possible to directly write into the YIG frequency controller (at addresses 0x1840 and 0x1842); for a desired frequency F (in MHz), just have to write F/2 as hex value in these registers.
The programmable VCO signal (370-500MHz) need to be raised in power to have sufficient level to drive the SRD (Step-Recovery Diode), to obtain a wide comb of carriers (multiplies of VCO carrier), after this there is another circuit with the microwave mixer.
In some occasions, the old variant of A203 MW converter (2010137-xx) may have problems with two trimmer capacitors around the VCO power amplifier, turning these usually resumes SRD RF drive power level and consequently the sensitivity of counter.
Where is this SRD located? on the A201-A (VCO) board or in the A203 YIG assembly itself (on the ceramic board embedded in the YIG filter assembly)?
BTW, on my unit, the big black resistor near the output 2SC2283 power transistor, can turn very hot, around 150°C; not sure it is expected or a sign of leakage somewhere. The resistor does not seem to suffer, however, from this (it looks physically and electrically OK).
The SRD is obviuosly part of the microwave circuit, I assume is located on LO side of ceramic microcircuit at opposite side of directional coupler, I barely see, on your photo, a complex microwave circuit with inductances, stubs, capacitors, the SRD diode is located here.
Coupling side of the coupler receive the comb signal from LO and is terminated at ground side with a resistor, probably 50 ohm and a laser trimmed capacitor.
LO signal is directionally injected to main side of coupler toward the mixer diode, also on this main side of coupler pass the YIG filtered input signal to mixer.
Just after the mixer there are two small wires for IF output signal (127MHz).
This resistor is connected in series with a inductance to the RF power transistor collector terminal to feed supply current from the +12V power rail, heating of resistor is NOT normal at 150 degrees level, measure the voltage drop on resistor and the resistance value, it should be 10 ohm.
Try to short the base terminal of 2SC2283 to ground and verify if the voltage drop on resistor goes to zero volts, shorting base to ground removes BIAS voltage from resistive divider, if the voltage drop on resistor remains high the RF transistor is probably damaged.
I've had a look at EIP 575 Service Manual and found Error 40 code means DAC table Error.
Thus, if this Service Manual can be used for my 545B I'll be able to repair it. At least I have to dig a bit more onto A107 board now.
douardda, Did you get the YIG working in your counter? If not, I have a 545 that I got in pieces and I have an extra YIG oscillator and it appears to be in good condition. I think it may have option2 in it as well.
douardda, Did you get the YIG working in your counter? If not, I have a 545 that I got in pieces and I have an extra YIG oscillator and it appears to be in good condition. I think it may have option2 in it as well.
douardda, Did you get the YIG working in your counter? If not, I have a 545 that I got in pieces and I have an extra YIG oscillator and it appears to be in good condition. I think it may have option2 in it as well.
Hi, yes I have the power meter working. I made some hardware modification on A107: add an AD7524 (U12) , removed R39 and installed a detection diode (I used a QSCH1245 instead of the original FH1100).