Fisher FM-200-B microtune question

[Patrice B]

Hello,

Stumbled on a thread here I think and also on the FM-200-B manual on the AK database: microtune circuit is supposed to engage when touching the tuning knob and the sensitivity should be adjusted with the pot at the rear.

Problem is that on mine, it doesn't work "here" (metropolitan area, floor with a rug) nor with strong signal (home made Yagi on rooftop).

It works if:

1- antenna is disconnected (not good as the signal is poor);

2- if my feets are on a concrete floor, like at my little cottage, 60 miles from here... even if a Yagi is connected there also ????

Someone has any hints?

Thanks.

Patrice

 

[dcgillespie]

I'd ask if it used to work under the exact same conditions at some time in the past.

Certainly a static charge build up in your body from a rug floor could upset the circuit. I suppose that strong RFI could do the same thing if you are in such an area. Assuming all is well with the Microtune circuit itself, then I would also make sure that any AC line bypass cap that is installed in the unit is in good shape as well. Having the chassis reference a good ground source could be a key to proper circuit operation.

Dave

 

[Patrice B]

Go figure, it works now... somewhat: it's not so responsive (one time it deactivates right after touch and other times taking a long long time!). Sometimes, the AFC circuit doesn't even come back after???

Maybe it's time to clean the ECC88 socket again (microtune amp) and verify the tube.

I've played a bit with the pot also, maybe it's in need of cleaning too.

Thanks Dave, good explanation as always. I've heard that the hand induce some hum with the tuning knob, triggering the circuit (or something like that). Is that correct?

Patrice

 

[dcgillespie]

In essence, that is correct. The knob is like an open input you touch, which is amplified, rectified, and used to drive a relay control tube that ultimately activates/deactivates the AFC circuits of the tuner. But the input from the touch control is a pentode with only grid leak bias applied to it, making it very sensitive to any available influence.

If RFI is a problem in your area, you might consider connecting a 10 pF cap at the input grid of the microtune circuit to ground. This should make the circuit insensitive to such signals, while not affecting the 60 Hz signal your body basically injects.

It is not all that uncommon for such circuits to be flakey at times.....

Dave

 

[AlTinkster92]

This is a great thread, the FM200B is one of the best tuners ever made IMHO...very similar to the FM1000 but just one less IF stage I think? I can remember back in the 60"s touching the tuning knob on my Dad's 1000 and one of the jewel lights would go off or on. I don't believe my 200B does that now, hmmm... Patrick you have me thinking and thanks Al

 

[Patrice B]

In essence, that is correct. The knob is like an open input you touch, which is amplified, rectified, and used to drive a relay control tube that ultimately activates/deactivates the AFC circuits of the tuner. But the input from the touch control is a pentode with only grid leak bias applied to it, making it very sensitive to any available influence.

If RFI is a problem in your area, you might consider connecting a 10 pF cap at the input grid of the microtune circuit to ground. This should make the circuit insensitive to such signals, while not affecting the 60 Hz signal your body basically injects.

It is not all that uncommon for such circuits to be flakey at times.....

Dave

About RFI: the Fisher is right near to my Wifi router, maybe it's the culprit!

As always, thanks for your clear explanations and the possible solution!

Patrice

 

[Patrice B]

Rotating the knob at rear made the things more responsive: the pot isn't clean. I've never thought that it would need a bath of Deoxit! I'll do it tomorrow...

 

[dcgillespie]

That, and there's a possibility that the general digital noise from your router (not just the RF signal it produces) could be giving it fits as well. Then there's any electronic florescent ballasts, CFBs, cell phones, computers, cordless phones, and a bazillion other digital noise makers in the area that it might be fighting as well.

A portable AM radio makes for a good digital noise sniffer. Just tune it between stations mid band, and advance the volume. Now go around your house and bring it near all your electronic devices to see which are the worst offenders. As mentioned, don't forget any permanent light fixtures, CFB, wall warts, etc. You'd be surprised at the amount of digital garbage floating around at any time -- and we haven't even begun to address all the crap on the AC line itself now. The battery charger for my cordless drill gives off enough noise for my radio to pick it up throughout the entire house!

The Microtune feature of your Fisher was designed in a day when none of the digital interference existed, and the devices mentioned could only be dreamed about. Ah, for the good old days, when the only real interference came from Mother Nature. I can dream too!!

Dave

 

[Patrice B]

This is a great thread, the FM200B is one of the best tuners ever made IMHO...very similar to the FM1000 but just one less IF stage I think? I can remember back in the 60"s touching the tuning knob on my Dad's 1000 and one of the jewel lights would go off or on. I don't believe my 200B does that now, hmmm... Patrick you have me thinking and thanks Al

In "microtune" AFC position, it should do it but as you had seen in this thread, it's not always working well 50 years later...

 

[AlTinkster92]

Thanks Patrick, I believe it is working ok now, played with it last night and light worked fine when touching it.. Off and on) Good luck with yours!

 

[notdigital]

Having just sorted out the Microtune feature in my own unit, I thought I'd revive this thread for future consideration.

Yes, simply touching the tuning knob deactivates the Microtune circuit when you are tuning in a station. Once the knob is released, the circuit is reactivated as indicated by the front red panel light. Or so it is designed to do. Mine didn't do that no matter how much I messed with the Microtune pot. I took the following steps to address the issue:

1) Clean the contact pin of the tuning wheel shaft and the brass "spring" behind it so that they make firm contact. I retensioned that "spring" by bending it toward the pin ever so slightly. You can do so by removing the two spring clips that fix the tuning shaft in place and pulling the shaft slightly away from the spring. You will widen the gap giving you room to clean both surfaces and bend the spring forward. Check for solid continuity.

http://www.audiokarma.org/forums/attachment.php?attachmentid=566409&d=1420904737

2) Clean the sockets and pins of V-12;

3) Clean the Microtune pot in the rear of the unit. Since there are only 2 wires and a bare grounding wire connected to it, I decided to tear it down completely instead of wasting more cleaner on it. I disconnected the grounding wire only and disassembled it with the two wires still connected. As with most pots of this vintage, it was pretty dirty;

4) Fiddle with the Microtune pot as per the operating instructions until desired sensitivity is reached.

I can't be certain which of the above steps solved the problem but suffice it to say, that slick little feature now works :thmbsp:

 

[actuator]

Re: Fisher Microtune

I have a working FM-200 and the red AFC light only goes out when touching the tuning knob if the unit is NOT grounded. For example, if RCA cable is connected to preamp - red light off doesn't work. If the chassis is grounded (and no RCA cable) - red light off doesn't work.

Not a huge deal since I typically just leave AFC off, but kind of strange. Thoughts?

 

[Patrice B]

Re: Fisher Microtune

I have a working FM-200 and the red AFC light only goes out when touching the tuning knob if the unit is NOT grounded. For example, if RCA cable is connected to preamp - red light off doesn't work. If the chassis is grounded (and no RCA cable) - red light off doesn't work.

Not a huge deal since I typically just leave AFC off, but kind of strange. Thoughts?

Hmmm, mine works connected with RCA cables to my Fisher X-100-2. I guess your preamp is a newer one, not the same era as the tuner? Three prongs? When you say chassis grounding, do you mean a separate wire going to the ground round hole in your AC wall receptacle?

If the answer is yes to the two questions, I would guess it’s normal as this circuit works with you being the ground somewhat…

Real experts would probably chime in. @dcgillespie ?

***Oh well, my explanation is far from ok (ground being the hand or whatnot) … and this thread… was mine !

See the post #4:
Post in thread 'Fisher FM-200-B microtune question'
https://audiokarma.org/forums/threads/fisher-fm-200-b-microtune-question.617663/post-8165138

 

[actuator]

Hmmm, mine works connected with RCA cables to my Fisher X-100-2. I guess your preamp is a newer one, not the same era as the tuner? Three prongs? When you say chassis grounding, do you mean a separate wire going to the ground round hole in your AC wall receptacle?

If the answer is yes to the two questions, I would guess it’s normal as this circuit works with you being the ground somewhat…

Yes to your questions (about modern preamp with 3 prongs and trying a ground wire).

That was it. I connected the FM-200 to a Fisher 400-CX-2 (original 2-prong power cords on both units) and the AFC red lamp off/on is working now.

I guess sometimes vintage gear needs to stay in the same era. Thanks for the help.

 

[gadget73]

wonder if it was just leakage to chassis causing the problem. Either that or with everything being grounded there wasn't enough of a hum field to register.