Help repairing Fisher 1265x

[Koala]

I will start at the beginning.

Originally. The unit would only worked properly in disc 2 until it has been on for about an hour. After which it would work fine in both discs

Then it stopped working altogether.

On inspection the FET Q506 had exploded.

Just replaced Q506 and there is 8V on C505 and 4V on C506. Nothing gets hot.

TX Is oscillating properly.

From headphones and speakers there is only a high pitch whistle in disc 1 and 2. No reaction to targets

No whistle in static mode.

Battery test make a faint sound like you just switched the machine off

Left it on for 1/2 hour. U302 TLC27M4 is quite warm. All the other ICs are cold.

Not an easy board to work on. The drilled holes are tiny.

Is there anything else I can check

[dbanner]

u303 kaput? c504 tantalum

[Koala]

Bit of progress. Changed

U302 TLC27M4

. Still got the annoying buzzing.

With the volume at zero and discrimination full I have detection of about an inch.
Turn the volume up and target detection is lost
Turn the discrimination down target detection is lost
It's almost ad if everything is reversed

[Nandor]

Hi
I would check the audio stage, audio oscillator or something near it. Hope it helps and you find the faulty component!

[Andi68]

I would also check all the elkos

[homefire]

Yep, start checking all Electrolytic Caps. Have a Scope ?

[Sean_Goddard]

I used to repair these around 12 years ago, can't remember what causes this but that Q506 is a common problem. If you have a schematic it can't be that hard to work out what signals and voltages should be where.

[Rigol]

Search transistors near Q506 if they are good.

[Koala]

Yes have oscilloscope


Have to look at it again in a few days time.

[Koala]

Search transistors near Q506 if they are good.

I assume the rest of the power supply is okay as I have a good +4 and 8 volts

[Rigol]

From an electronic board that i have the mosfet broke down and the transistor with him.That's why i suggested you this.

[Koala]

From an electronic board that i have the mosfet broke down and the transistor with him.That's why i suggested you this.

Many thanks for the suggestion. Did check near by components for shorts. That's not to say something not out of spec.

[Koala]

Everything's failing on me. Thought I would check how many amps it's drawing.

Meter not working in the mA range. In the 20A range it's showing .140 so looks like it's 140mA but how accurate that is I don't know.


Thought I would see if any mode was different to eliminate parts of the circuit.

Disc 1, disc 2 and static are the same.

When I press the battery test the unit shuts down and draws zero amps.

[dbanner]

If your battery test does not work you might want to take a look at this section here for fault, since this section should at least work for battery test even if other part of circuit is bad. This is audio section, therefore a fault in this section would render entire detector inoperative. Just some Spock logic, or a hunch?

[Koala]

Mmmmm lifted C502 and C407 both have leaked. Both test within 10% though. 502 not so much. 407 a fair bit.

See if I can find some new ones.