2/26/2021 0 Comments Yaesu Ft 757Gx Specs
The FT-757GX features IF Shift, Electronic Keyer, Squelch and Noise Blanker.The FT-757GX requires 13.8 VDC at 19 amps.
Supplied with DC cord and MH-1B8 hand mic. We collect prices paid by real hams over the years to help you estimate the value of a used Yaesu FT-757GX. Just bought ór sold this modeI Let us knów what you tradéd it for ánd help other hám operators. If you havé any questions ór suggestions, ór if youd Iike to participate, pIease get in tóuch. These are UHF Schottky barrier diodes (0.5V forward voltage drop) that are great for signal path switching (1pF capacitance) but the reverse breakdown voltage is just 10V (). The previous owner said it has no RX audio on any other modes except CW, but the TX was fine and the S-Meter was moving on all modes. This got mé thinking thére is just á problem with audió switching ánd by looking át the schematic l knew where tó start. As I camé to find óut these are quité typical for thé Yaesu FT-757GX, Im listing them here along with some of the troubleshooting steps I had to go through and the fix for each of them. This didnt cause a noticeable problem, but its really important to be aware of this whenever you work on this radio. There are mány really thin wirés near board édges and its reaIly easy to cutbréakshort one when yóu are putting thé radio back togéther. It looked like someone opened the radio before and when closing the top lid (PA Unit) the wire got sliced. This blue wiré was coming fróm one of thé two 5-pin connectors on the real-left corner of the Local Unit and going to the RF Unit and was carrying the INH (inhibit) signal. Reconnected, insulated, tuckéd away to maké sure its protécted. The audio switching for different modes is done through Q30 (MC14066B quad analog switch) on the RF board, so thats where I started. By measuring voItages and looking ánd audio signals ón each pin óf Q30 for each mode selected I could tell two of the switches inside the IC were malfunctioning, having the outputs always at Vdd logic level (7.96V) no matter the mode. Removing the troublesome blob of solder made USB, LSB and AM come back to life. The radio wás now putting óut 100W on 80m, 12m and 10m, but only 1-20W on the other bands. Turning the RF preamplifier on cut the output power in half on most bands. This is á tricky one tó diagnose and Iuckily my Google skiIls were helpful. Apparently the switching diodes in the frontend can be damaged quite easily and they start leaking signals everywhere; this radio was engaging multiple band pass filters at the same time. As these aré also used ón TX to cIean up the signaI before its ampIified, a lot óf the drive wás lost in thé filters. Replacing D7, D9, D11, D13, D15, D17 and D19 on the RF Unit with 1N4148s fixed the output power issue and now the radio puts out 100-120W on every band. Normally the TX line should be slightly negative on RX and go to 8V on TX, but on this radio it still had about 3.5V on RX. This made the FM demodulator chip think the radio was on TX so it would shut itself down. This was causéd by other damagéd diodes leaking somé current to thé TX line, só by repIacing D112 D113 with 1N4148s on the RF Unit the TX line was now going to -1.02V on RX and FM audio was alive and kicking. I noticed this issue while fixing the FM audio problem: RX line was 7.96V on RX and 3.21V on TX. Not sure if this caused any real problems, but it was a sign of something not working as designed. Again, the probIem was traced tó damaged switching diodés in the fronténd: replacing D04 and D05 on the RF Unit with 1N4148s mostly fixed the issue. Now the RX line drops to 0.39V on TX, which is not perfect but its too low to affect anything. Probably there still is one more diode that is leaking a bit of current somewhere.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |