Fisher 1265 Revival

[joecoin]

"I believe that that nut is the only thing that stands in the way for a completed revival! "

That's what my wife says about me.


Congratulations. I just finished rehabbing a Fisher 1280-X that someone had butchered. I'm waiting on the replacement seals so I can get it wet.

[KingJL]

.... Cannot check out remaining circuitry until I get some replacement potentiometers.

THE BEAST IS COMING TO LIFE!!!!

To be continued...

Well, the parts arrived at about 1 PM today. Got to work replacing potentiometers. Had to substitute for the Sensitivity control; A tight fit but got it to work. Connected the ribbon header to the header socket; Connected 10.3 voilts to the off/on switch and powered the beast up. I have sound!!

Went through the setup procedures that Qiaozhi wa kind enough to provide. Something not right! I can detect a dime and a quarter, nothing else. All metal/pinpoit mode works OK. Disconnectect ribbon cable and coil, re-inspect board. Just for grins, I give it a good brushing with my soft paintbrush. Take the shell off of the coil cable connector. Lots of rust (remember hurricane Ike)! I clean all the rust and re-assemble coil cable; Re-connect the ribbon cable header and power up. We have working metal detector!! Disc1 and Disc2 functioning properly.

Now to re-assemble all the peices into the housing. I am missing a metric nut for the Mode switch!!! I need to go to Ace hardware tommorow to try and locate a nut that fits.

I believe that that nut is the only thing that stands in the way for a completed revival!

[ivconic]

Thanks KingJL!
I was off for a while due some obligations.
I rechecked everything suspectable and still havent found a problem. I guess problem is in coil or at LM339 finally. Will continue in a day or two.
Best regards!

[KingJL]

The more I think about this and considering my problem that I fixed, I suspect you may have a U101 LM339 problem. Mine was showing a faulty section connected to the audio transformer and was feeding into the oscillator section. Studying this leads me to believe that this type failure might not be that uncommon for the 1265/1266. The LM339 is connected to the step-up side of this audio xformer. Any significant current transient on the low side would generate one heck of a voltage on the step-up side and thus might damage the LM339.

I just got off the phone with a tech at Fisher (I was trying to find out where I could get a replacement sensitivity control). He mentioned that U101 LM339 failure was a somewhat common occurrence.

BTW: Fisher has depleted their stock of 1265/1266 sensitivity controls.

Regards,
J. L. King

[KingJL]

What i dont like is that transformer and oscillator.
To tricky to be handmade by amateur like me. So .. my main point of interest is to replace that part with some capacitive coupling or something else.

OK, if you want to get rid of transformer yet have perfect quadrature signal for demodulator; Use a 4077 XNOR and use two of the sections like in the attaced schematic.

Ok...i am ready to replace whole oscillator part also cause that rare quartz.
I guess cd4060 will do the job just fine there. ...Also power supply gives me some worries. Those fets are rare in local shops, but that is much less problem to be replaced.

OK use 4060 in conjunction with XNOR circuit mentioned above. Also ifyou use a different method for the PS as you desire, you can completely eliminate U101 LM339.

Regards,
J. L. King

[KingJL]

MPSU51...hmmm, once i put there BD180 (as i recall) and it worked..
Not an isssue actually.

No not an issue. The key is to match the hFe to maintain the designed current limit with the same bias current. Also to be careful to get the leads in the right hole. MPSU51 is EBC configuration vs the normal ECB configuration.

I just like the way it is used. So many detectors exhibit the tendency for the audio to overload and introduce instabilities/frequency shifts due to anomallies in the regulated voltages caused by the overloading audio. But the MPSU51 (or sutible substitute) configuration used in the 1265/1266 audio prevents this by providing current limiting & battery voltage droop regulation at the audio stage.

Regards,
J. L. King

[ivconic]

I actually like the audio xformer concept. With thw main current going through the 1 ohm resistor, you get an almost perfect 90 degree phase shift to the transformer. That being fed to the LM339 give a near perfect 90 degree phase shift gate to the demodulators. It will always be the near perfect 90 degree phase shift regardless of frequency!

I have substituted different low power MOSFETS of equivalent current/power rating and had great results in simulation models. I did that for the precise reason you stated, hard to find the part (until I found the right part number).
It is actually quite a robust little PS.

Another thing I like is the way they use a MPSU51 as a current limiter in the audio section. The way they use it prevents the audio from oveloading and creating interference by dragging the battery voltage below what is needed to maintain the positive rail in the PS. These can be hard to find also, but I have about 50 of them!

I have come to have a high regard for the 1265/1265 design. Not flashy, but elegant and fairly simple.

Regards,
J. L. King

No, concept is fine...excellent. But i mean it is hard to wound such small transformer in amateur conditions like my. Ok...i could search for suitable transformers in old radios also...but i need constant source of those if intend to make more devices, not only one.

MPSU51...hmmm, once i put there BD180 (as i recall) and it worked..
Not an isssue actually.

Now i am really going to sleep. It is 3:25 CET here... i am so tired.

Regards!

[KingJL]

What i dont like is that transformer and oscillator.

I actually like the audio xformer concept. With thw main current going through the 1 ohm resistor, you get an almost perfect 90 degree phase shift to the transformer. That being fed to the LM339 give a near perfect 90 degree phase shift gate to the demodulators. It will always be the near perfect 90 degree phase shift regardless of frequency!

Also power supply gives me some worries. Those fets are rare in local shops, but that is much less problem to be replaced.

I have substituted different low power MOSFETS of equivalent current/power rating and had great results in simulation models. I did that for the precise reason you stated, hard to find the part (until I found the right part number).
It is actually quite a robust little PS.

Another thing I like is the way they use a MPSU51 as a current limiter in the audio section. The way they use it prevents the audio from oveloading and creating interference by dragging the battery voltage below what is needed to maintain the positive rail in the PS. These can be hard to find also, but I have about 50 of them!

I have come to have a high regard for the 1265/1265 design. Not flashy, but elegant and fairly simple.

Regards,
J. L. King

[ivconic]

CHEEEERS!
Good night!
See ya !

[KingJL]

I am not sleeping yet...
Ok...i am PASTE-ing all your suggestions to notepad and later will print as sort of reminder, so tomorrow i will go step by step.
It is not imperative me to fix this exact 1265, but to learn more about it. As time goes by, more and more i am reconsidering option to take 1265 schematic as starting "platform" and project some mods over it. Finally i might come to some new and fresh design! Why not?
TGSL is done many times. It has some limitations. Other hand, 1265 concept is more "open" and more versatille in manner to improve it and extend some features. What i dont like is that transformer and oscillator.
To tricky to be handmade by amateur like me. So .. my main point of interest is to replace that part with some capacitive coupling or something else.
Ok...i am ready to replace whole oscillator part also cause that rare quartz.
I guess cd4060 will do the job just fine there.
The rest of 1265 schematic is very adaptable and pretty cute.
Also power supply gives me some worries. Those fets are rare in local shops, but that is much less problem to be replaced.
Again..many thanks for suggestions!

Also recheck that 1 ohm across the audio transformer. The in circuit resitance measured here should be on the order of 0.6-0.7 ohms (audio transformer winding 1.7-1.8ohm in parallel with 1 ohm). If the resitor is bad, it will tend towards a saw waveform and put >24Vpp on the secondary to the LM339.

Enough for now, I am getting a headacche!!
Regards,
J. L. King

[ivconic]

I am not sleeping yet...
Ok...i am PASTE-ing all your suggestions to notepad and later will print as sort of reminder, so tomorrow i will go step by step.
It is not imperative me to fix this exact 1265, but to learn more about it. As time goes by, more and more i am reconsidering option to take 1265 schematic as starting "platform" and project some mods over it. Finally i might come to some new and fresh design! Why not?
TGSL is done many times. It has some limitations. Other hand, 1265 concept is more "open" and more versatille in manner to improve it and extend some features. What i dont like is that transformer and oscillator.
To tricky to be handmade by amateur like me. So .. my main point of interest is to replace that part with some capacitive coupling or something else.
Ok...i am ready to replace whole oscillator part also cause that rare quartz.
I guess cd4060 will do the job just fine there.
The rest of 1265 schematic is very adaptable and pretty cute.
Also power supply gives me some worries. Those fets are rare in local shops, but that is much less problem to be replaced.
Again..many thanks for suggestions!

[KingJL]

Oh no...thank YOU very much for suggesting me possible path to problem source. Now is to late so i will check all tomorrow.
Keep in touch!
Regards!

The more I think about this and considering my problem that I fixed, I suspect you may have a U101 LM339 problem. Mine was showing a faulty section connected to the audio transformer and was feeding into the oscillator section. Studying this leads me to believe that this type failure might not be that uncommon for the 1265/1266. The LM339 is connected to the step-up side of this audio xformer. Any significant current transient on the low side would generate one heck of a voltage on the step-up side and thus might damage the LM339.

Just a thought!!

Regards,
J. L. King

[ivconic]

Oh no...thank YOU very much for suggesting me possible path to problem source. Now is to late so i will check all tomorrow.
Keep in touch!
Regards!

[KingJL]

So... Disc1 and Disc2 pots at this 1265x are the same; 8.9k linear...!?
I guess those were 10K and lost some of preferences during time and various objective working conditions like moisture etc...
But those are identical on measurements ; 8.9k ...??? Strange?

Thanks. That clears up that little discrepancy. It is strange though, both my Disc1 and Disc2 measure 5K and both show logrithmic taper. I will replace with 10K linear.
Thanks again.

Reffering my problem; i havent solved it yet. 2N5087 are put on places and still TX amp is bairly over 2vpp!? 1ohm resistor in serie with coil is ok. Everything else is ok..??? Could it be coil? I dont have right now another to check...damn!
What bugs me mostly is shape of signal - saw??? The heck!
Cheers!

In some of my modeling I have encountered that saw shape. It could be coil. It is showing integration of square wave. What was the frequency of the saw waveform? I just went over to LTspice and fed a 1265 Tx circuit with 6.5KHz squarewave. Got a 2Vpp 6.5KHz saw waveform! 8KHz gives extremely sharp saw waveform of 1.5Vpp. I think your TX is wrong frequency or coil is for a 2KHz 12xx.

Dissconnect coil and check the feed to the 4025 (actually check LM339 U101 pin 2. It should be ~77KHz square wave. If it is not, check out U101 (and be sure that you are not getting feedthrough of another section of U101; I was getting feedthrough from pin2 to pin 1 and vise-versa). Also check that the output of U102 (4025) is divide by 16 (4.8KHz).

P.S.
Sorry i am late. Took me more...had some troubles..

I thought you did extremely well. Especially since you went out of your way to do this for me.

Thanks again,
J. L. King

[ivconic]

So... Disc1 and Disc2 pots at this 1265x are the same; 8.9k linear...!?
I guess those were 10K and lost some of preferences during time and various objective working conditions like moisture etc...
But those are identical on measurements ; 8.9k ...??? Strange?

Oh yes; i had to unsolder wires cose when wired up those gave values 3.8K each... end to end.

Reffering my problem; i havent solved it yet. 2N5087 are put on places and still TX amp is bairly over 2vpp!? 1ohm resistor in serie with coil is ok. Everything else is ok..??? Could it be coil? I dont have right now another to check...damn!
What bugs me mostly is shape of signal - saw??? The heck!
Cheers!


P.S.
Sorry i am late. Took me more...had some troubles..