Good to hear from another TR4310 user, Jan:-) I reckon I have both sizes of those key here - somewhere!
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With the synthesiser in the TR4310 now working well (and the VCXO / rubber-crystal alignment for the 100s & 10s Hz tuning steps complete), this morning, I turned my attention to finding the reason the radio would often refuse to tune in one direction.
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My workaround to this problem while de-bugging the synthesiser was to remove U32002, the up/down direction setting flip-flop on the VRTO Digital Board, tie TP-5 (normally the output of the flip-flop) high with a resistor and also connect TP-5 to a push-button, the other side of which was connected to ground. The pull-up meant that the radio would normally tune up the band. Pushing and holding the button would allow it to tune down. Hardly a slick user interface, but adequate for getting around!
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I had previously installed a DIL socket in the U32002 position, which had the benefit of being able to easily insert and remove the required IC (CMOS MC14027 / 4027 type) to aid testing. Plugging the IC back in and removing my temporary pull-up / button circuitry, the erratic tuning was still present. Tuning down the band / from high to low frequency was reliable. Attempting to tune up the band / from low to high frequency would work occasionally, but then get stuck at positions where no further low to high tuning could be made. The alignment section of the manual talks about adjusting the (movable) opto switch to optimise accuracy of tuning direction sensing decisions. While in the bowels of the radio sorting the synthesiser, I had previously taken a detour to check and adjust the opto switch mechanical position. Once again, using a dual-channel 'scope, I checked the squared up outputs from the opto switches at TP-3 and TP4. Their output signals were as close to being in quadrature (90-degrees) with one another as I thought was ever likely to be needed.
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Attempting to tune around some more on the 'VFO', I noticed that the frequency readout (still only reliable for 10s & 1s kHz and 100 Hz digits) would appear to get stuck at various 'round number' values. This suggested that the circuitry thought that the radio was reaching it's upper frequency tuning limit, without actually being there. The TR4310 incorporates circuitry to detect the lower and upper limit of the, nominally 500 kHz, tuning range. (Actually, the radio will allow you to tune up to 200 kHz below the nominal 500 kHz range and up to 50 kHz above - these are the detected 'hard' limits.)
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Checking the 'R' pin (12) on the flip-flop with a 'scope, I could see no 'high' input that corresponded to upper limit reached, and no pulses when tuning. The logic level looked to be a good '0'. Then I noticed that, with the 'scope probe attached, the radio was perfectly happy to tune low to high in frequency. Remove the 'scope probe and it would readily get stuck again! So, there really were pulses / logic '1's there, but with so little current behind them that the slightest load (and we're talking very slight from a scope probe) was enough to reduce them to zero Volt / make them ineffective.
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Pin 12 on the direction setting flip-flop is driven directly from the output of another IC on the board, U32009(A), a quad-input NAND (CMOS MC14082 / 4082). A search of my parts draw yielded a Philips HEF4082, near enough for jazz. After the now somewhat tiresome removal of the faulty IC, cleaning the board to remove traces of brown 'mud', and installation of an IC socket, I plugged-in the new IC and re-installed the board in the radio. Success! The radio now tuned in both directions freely and reliably. I've not yet checked that the tuning limit checking circuitry is getting its calculations right - it's somewhat hard to do when the 100s kHz digit is broken - but this can wait.
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On the subject of said digit, doing some further poking about, the cause of this fault would appear to be the BCD adder IC, interspersed between the 100s kHz BCD counter outputs and the seven-segment display driver inputs, to ensure the display digit reads correctly regardless of whether a range starting at 0 kHz, or 500 kHz above the MHz edge is selected. Once again, a UK eBay seller happened to be selling lots of two IC (type MC14560) for little money - should be here in a day, or two.
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Looking ahead to when the 100 kHz digit issue is fixed, that will leave only one faulty area to go - that of the MHz display digits and range selection from the band switch, including the up/down buttons. The MHz display digits are currently mostly blank, or just plain wrong and the up/down buttons inoperable. I've been able to make contacts on the radio by virtue of the installed AUX7 card which, in addition to the usual modules for LW and MW BC, fortunately also has modules for 80 - 10 metre pre-WARC amateur bands. Standby for further tails of brown mud, failed CMOS and the rest!
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Regards, G4FPH.
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