fisher: Fisher 100 -- loss of bass and treble controls

good morning all.

i just received a fisher 100 microreceiver (small tabletop radio) with two exposed transistors mounted in the back, where they're accessible without removing the case -- an amperex 2N4077 and a 2N4078 "Germanium" transistors. they have twin prongs that plug into the back of the unit). i've lost my bass and treble controls. i don't have a schematic. i'm hoping that replacing these transistors might solve my tone-control problem. i'm getting plenty of good sound out of the speakers, even with these transistors unplugged. i'm getting no continuity between the prongs of both transistors. i just got this very impressive radio and am trying to determine what has happened to the tone controls. i thought they worked when i received the radio, and i'm troubled that they've gone out so fast. can anyone please help?

thanks very much.

--charles jones

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Dear Charles,

the transistors you have mentioned are complementary germanium power transistors, rated at 1 Amp, 32 Volts, 7.5 Watts. They are most likely associated with the speaker output power stage. If your radio is still running without them, and if the are pluggable without any fixing screws, they might just be spares! The real power output transistors in operation should be mounted with two metal screws, as their case is in itself an electrode. See the picture on the model.

If your tone control has gone, look into -

1.) the tone control rheostat. It happens that a rheostat slider becomes tarnished to the point of non-conduction, or sticky grease interferes with the contact. Try measuring from the center tap to either of the side taps. There should be a resistance varying with the tone setting. This shows the rheostat is working. If it isn´t, try cleaning it with a soft contact cleaning spray that leaves no residue.

2.) one or more of the condensers in the tone control circuit might have shorted and so stop the tone control network from working properly. As this is a transistor set working with comparably low voltages, this fault is unlikely - with tube sets it is not uncommon.

Yours,

Nikolaus

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thanks so much, nikolaus.

the outside-mounted transistors aren't just spares; they hook into some contacts that are ganged with many important-looking little diodes, caps, filters... i don't know what (but i'm trying to learn). so they seem to perform some important function. i'll look into the rheostats. i have several old radios that need good pot cleanings. i'll also try to check the ratings you gave me for the transistors. at the least, i would have thought i should get continuity from one leg to the other, but i don't. thanks again for your kind assistance.

-charles

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Dear Herr Lowe,

i put your lesson into practice and, as i set the second screw through one of the transistors into the case, a huge spark flew and -- silence. apparently, i had killed my new radio. i inspected the insides carefully and could find no evidence of fire or burnt parts, and the soldered-in fuse was intact. when i pulled both exterior-mounted transistors, it seems the sound signal comes through uncontrolled; that is to say, a distorted, uncontrolled sound of a broadcast through the speakers. then, after re-mounting the transistors, nothing. it was only after i discovered that i had mounted them in the wrong spaces that i got the radio back to apparent good health. i switched them back, and all is good.

as to the tone rheostats, my belief now is that they are -- and probably were -- working fine; they are just much more subtle than i am accustomed to. i only pray that herr erb doesn't kick me off the forum for a fool. i am expecting an isolation transformer in the post in hopes i won't kill myself too soon. hope all is well in berlin as we head into winter.

all best,

charles

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Dear Charles

I´m glad you got your radio back to working condition. Here´s a trick that helps me a lot when I´m working on something I haven´t completetely figured out, or that has been messed up: First I take a number of digital photos, especially showing the wiring and part labels (like transistor type numbers). This helps me put everything back together. If I have a question concerning the device, I can post a photo along with my request to radiomuseum.org.

About conductivity between the two prongs of your transistors: There should in no case be low resistance between any two connectors of a transistor - that always indicates a bad transistor. Instead, the two prongs in your case should behave like a single diode (which in fact is part of the internal structure of such a transistor). If you have a diode tester, there should be a forward voltage of about 0.5V in one direction, no conductivity in the other one. This would be the test of the "Base-Emitter diode", which can in many cases even be performed with a transistor in circuit. However, with the device in circuit there might not be infinite resistance in reverse direction, depending on the circuit design. Look up a transistor table or consult radiomuseum to identify the two connectors "base" and "emitter". There is another internal diode between "base" and "collector", which you can test in the same way.

You wouldn´t believe how many people have been fixing everything in their radio except for what´s wrong, messing things all up. I own a very early crystal receiver, actually quite a simple device to repair, and still it appears someone thought they could "improve" the set by changing around some wires. I wish he´d taken photos before!

Yours,

Nikolaus

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Just been reading your exploits with this radio and thought a couple of additional points may help.

The transistors are probably either a TO66 or TO3 type of package where the metal can is the collector. The two prongs are the base and emitter. Sound may come through the speaker when these are removed since only the driver transistors will be supplying the output but the sound will be reduced and possibly distorted. The output ones mounted on the back are supplying the real output power to the speaker and are mounted on the rear panel as a heatsink. They may run warm. Make sure that they are not mounted on a mica washer and have plastic bushes to insulate the can from the metal chassis. The bushes insulate the bolt and terminal used for the collector. Also sometimes a heatsink compound is used to help with the heat transfer. This is a white sticky paste. It is real easy to blow up transistors in amplifier stages and it only takes fractions of seconds to do this. I suspect you may have  a couple away unless you have already solved it.

Regards

Mark

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hi, mark

thanks for stepping in. yes, one of these transistors does have a clear-plastic washer between it and the case. i'll remove it if you think that would serve to keep it cooler. the radio is amazingly powerful for such a small table-top model; it can fill a medium-sized room with sound of a very good quality. i'd almost compare it with my opus 6, which i keep in the same room, in terms of sound quality with classical music. mind you, the opus 6 has not had its 50-year-old capacitors, etc., updated yet, and i must rebuild the 2 electrostatic tweeters, which are dead (because of rotted foam, i suppose). i understand now why these transistors in the fisher 100 are mounted externally, if it is to keep them cool.

all best,

charles

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Hello Charles,

Glad it is now working however DONT remove the washer!! This is to insulate the transistor from the chassis. If you do this, you might end up with a short somewhere or worse a blown transistor.

Regards

Mark

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msg received and understood. thanks, mark

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Turns out, this radio system (which includes tuner, amp, preamp and free-piston hi-fi loudspeaker) uses "Baxendall Feedback Passive Tone Controls," which are much different than the tone controls to which I have become accustomed. For the engineers and mathematicians, here is an explanation of that system:

www.duncanamps.com/technical/tonestack.html

www.schmarder.com/radios/tech/tone.htm

www.lacieg2s.ca/public/w3terra/mil/mil-baxandall.htm

• Baxendall Tone Stack (12 KB)

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