Fault: R5 on transducer PCB may be burned: it is 51 ohms 1/8 W resistor (1/4 W works too).
Fault: the sonar has lost it's sensitivity in some 5 years of use. For instance it may marginally give a faint bottom echo at 15 m depth even at max. sensitivity (GAIN) setting. The auto-range also soon looses track of bottom when started in shallow and prodeeding out to deeper waters, as it does not switch to deeper range and does no increase gain in gain-auto mode: replace all electrolytic capacitors on the transducer PCB and re-tune three drum core coils.
Rule 1: Don't use metal tools to adjust the coil cores
Rule 2: same as rule 1 and do not attempt to adjust the output transformer, you will crack the core only- it's glued firmly.
The rear connectors are:
DC and temperature sensor: 4 pin connector: 1. +12 V, 2. -12 V, 3 and 4 temperature resitive sensor
Transducer sensor: 8 pin connector: 3. transducer (black), 4. shield, 5. transducer (white)
Log: 3 pin connector: unknown.
Loosen 4 big screws on casing's rear side's blue
The electrolytic capacitors are 100 uF (C19, C21), 22 uF (4 small ones), 4.7 uF (C 25) and about 1500 uF (the big one over the power amplifier +Vcc, installed horizontally), voltage 16 V or more, total 8 pcs. Use hot melt glue or MS polymer to fasten the 100 uF and the big 1500 uF cap's cans, so vibration of the boat will not break the capacitor's leads. I added a 1 uF polystyrene capacitor over the 1500 uF electrolytic to increase HF peak current source and reduce strain from the electrolytic.
Once done, power up the sonar with transducer
attached (a hint the transducer power amplifier output stage may not like
an open or shorted load and may even fail in some sonars).
Tx driver is pre-tuned by listening to the transducer ticking and peaking it initially, then proceeding to the echo strength method of tuning. Oscilloscope did not seem to be effective in this job. Set range to 0-3 m and gain manually to maximum: try to get echo from a close distance from a glass window, pointing the transducer's beam directly and exactly at right angle on the flat glass - the beamwidth is just 10 degrees. Once you find the echo on the LCD screen, slowly pull the transducer away from the glass until the echo is just 1 pixel thick, but stable. Tune TX and both R marked (T1 and T2) coil cores with plastic tool (or glass fiber) (per the picture above) for thickest echo trace - it may widen to 3 to 4 pixels.
Pull away again the transducer from the glass to get the echo line narrow again and re-peak those coils to get echo to maximum thickness on the LCD. The sonar indicates the distance as being 4 times too long since in air speed of sound differs from that of water, so you may need to pull away far enough to change range to 0-6 m to get the echo back within range again.
The PA output coil's tuning core is well secured with some gry goo and I did not try to tune it - something to gain there, maybe not?
The unit should draw at 13 V about 200 mA, the current meter needle pulsing around that value. Those TO-220 cased push-pull power amplifier's darlington-coupled transistors flanges should both! be slightly warm, about +40 C or so, after 15 min of running indoors with enclosure open. The maximum echo distance reached in air bouncing from the window glass was about 1.2 meters.
The FUSO 310 unit is from early 1990's, so need for these tips may be low, but if you are trouble shooting a unit which has any resemblance to this, the tips may be useful. I have no idea why Fuso did not secure those coils' tunable drum core's in production.
Before closing the sonar's enclosure, you might swap in a new dry bag of dry silica gel on the inside of the back cover to keep the sonar dry inside and those few drops of candle wax on the coil's drum cores to prevent de-tuning from vibration.
© I. Yrjölä, 2012