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Dynaco FM-5 indicator lights

NealinNevada

NealinNevada

THE Bozak Museum of NV
Does anyone know the voltage of the "STEREO" and "TUNED" indicator lamps on a Dynaco FM-5? What should the voltage read at pin #28 of the PC-26 circuit board where the twisted pair of blue wires for the indicator lamps connect? Many thanks in advance!
 
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Do you have the manual for the FM-5? It might supply the answer. I have an AF-6 (same or similar FM section as the FM-5) and the AF-6 manual and can check the unit to answer your question. I am not at home now so cannot provide the answer until tomorrow. Are the lamps working in your tuner?
 
2chanman said:
Do you have the manual for the FM-5? It might supply the answer. I have an AF-6 (same or similar FM section as the FM-5) and the AF-6 manual and can check the unit to answer your question. I am not at home now so cannot provide the answer until tomorrow. Are the lamps working in your tuner?
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Yes, I do have the manuals (assembly/operation) but there is nothing concerning the voltages. The lamps are not working on the unit, which means that there is no muting between stations (if tuned indicator lamp is open). I measured +17.7 v dc at pin #28, but something tells me that is a not correct.
 
!7.7 volts may be correct if the lamp filament is open. No doubt it will be much lower with functional lamps in place. It is probably possible to substitute led's for the lamps in conjunction with series resistors to limit current. The resistor values would have to be determined experimentally. Circuit operation may be affected by the change. led's did not exist when the tuner was designed. Substitute incandescent lamps may be difficult to find and obtain as most equipment uses led's instead these days. I am heading home now and will open up my AF-6 and try to get some answers. I will try to post something here tomorrow morning if possible. If you haven't already done so, you might want to verify that the lamps in your FM-5 are actually open. If they're not, there may be some other problem.
 
2chanman said:
!7.7 volts may be correct if the lamp filament is open. No doubt it will be much lower with functional lamps in place. It is probably possible to substitute led's for the lamps in conjunction with series resistors to limit current. The resistor values would have to be determined experimentally. Circuit operation may be affected by the change. led's did not exist when the tuner was designed. Substitute incandescent lamps may be difficult to find and obtain as most equipment uses led's instead these days. I am heading home now and will open up my AF-6 and try to get some answers. I will try to post something here tomorrow morning if possible. If you haven't already done so, you might want to verify that the lamps in your FM-5 are actually open. If they're not, there may be some other problem.
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They are open and it is possible to insert LED's for them. The "Tuned" lamp resistor is tied to terminal 3 on the PC26 board - it should be changed from its original value of 150 ohms to 560 ohms. The "Stereo" lamp resistor is tied to terminal 9 on the PC26 board - it should be changed from its original value of 150 ohms to 680 ohms
 
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Strange how lamp current draw isn't mentioned here, as it's what the muting circuitry is actually looking for. Say the original bulb was 50 ma and the led is 5 ma. It'll light but the the circuitry probably won't even notice it as a load. There's been a few article's on doing the led transplant, but includes a parallel loading resistor.
 
Neal:
Looks like I did a lot of work for nothing. You're way ahead of me. Anyhow; The original lamps were 12 volts and 33 ma is the current they were operated at in both mute and stereo circuits. That is a very low current for a 12 volt lamp, so it may be nearly impossible to find incandescent substitutes. If you substitute a white led (Thru-hole type) that wants 3.2 volts across it, 330 ohms in series with the led should do the trick, if you leave the original 150 ohm resistor in place. If you wish to substitute a different value in place of the 150 ohm without an additional series resistor, I suggest 470 ohms. Make sure you get the led polarity right.
Red, green and white led's have different forward voltages, and led's in each color group also vary. You will probably have to experiment to get the optimum result. I don't think you need any input from me to get this done. As Renzor213 suggests, you may need to include a parallel resistance. 33ma is the operating current you want to match. Please let me know what you end up with via PM.
 
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Wouldn't exactly say you did a lot of work for nothing, as you posted some very important information. I also have an AF-6, but it's been packed away for year's, and all the lamps were still functioning. These are actually very fine performing tuners. I used to have lot's of fun wth mine hooked to a rooftop rotating 10' Yagi, and would pull in all kind's of stuff. Don't do much FM anymore, as there's not much worth listening to around here.
 
Renzor213;
What I meant was that Neal came up with the needed info independently and actually didn't need any help from me. The original lamps in my AF-6 are still intact too. I agree that the AF-6 or FM-5 is an excellent performer, especially compared to the FM-1 or FM-3 which I had many years ago. The AF-6/FM-5 is drift free, even with dynatune off, is far more sensitive and selective, cleaner sounding, reliable, and holds alignment indefinitely.
As for what to listen to: there are about 10 listener sponsored stations around here, including college stations and 3 stations that play mostly classical music. There are lots of garbage stations too, but I ignore them.
 
I found a couple of bulbs (incandescent) from a friend and installed them...Here is the problem I am still experiencing. If the Stereo/Mono switch is set to Stereo or to Filter the stereo indicator light comes on but stays on no matter where you turn the tuning dial to. If the FM-5 is turned on with the switch set to mono, the FM Stereo light does not come on at all. The "Tuned" bulb never comes on. The tuner works perfectly other than these indicator light problems and the fact that Dynatune doesn't work because of the bulb not working....drives me nuts. I could really use some help figuring this out.
 
Neal:
Are the lamps you installed the special low current types that draw 33 ma @ 12vdc?
I have seen the stereo lamp in my AF-6 stay lit between stations occasionally. It should not light at all in the mono mode, so at least that part seems correct. The tuned lamp should light in the mono mode, either switch selected mono or in the stereo mode when tuned to a station that is broadcasting mono(no 19khz stereo pilot) and when the station is broadcasting stereo. It should not light between stations. It may be that your tuner needs the so called "professional alignment" that is in the Dynaco manual. This requires the use of a professional quality signal generator and other gear which very few service facilities have. Such an alignment by a professional would cost more than the tuner is worth and it may not cure the problems. You might want to check the integrity of the front panel switches and treat the switch contacts with contact cleaner.
If the lamps you installed draw heavier current than the originals, they may adversely affect muting and stereo operation. Have you checked how much current the new lamps draw with 12 volts DC applied? Also, have you verified aurally that the tuner is delivering stereo sound when the stereo light is lit and the station tuned is known to be broadcasting stereo? Most stations leave their stereo pilot on all the time even when transmitting monophonic material. I have noticed that most of the listener sponsored stations transmitting NPR programming in my area often are not broadcasting the stereo pilot even though they have stereo broadcast capability.
 
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Alright, now you guy's did it. Put the AF-6 on the work bench and took some real word measurement's. Tuned lamp operate's 10V @ 42ma and Stereo lamp 9.4V @ 40.1ma. Two different meter's show the same thing. Maybe try a lamp/load resistor combo to come up with the same current draw, and see what happen's.
 
Renzor213:
I DID do "real world" measurements and found both lamps in my AF-6 with 12 volts across them while passing 33 milliamps as I stated earlier in a previous post. I deduced the 33ma by measuring the voltage drop across the 150 ohm resistors in series with the lamps. I have no idea why your tuner shows a different result. Perhaps Dyna used different lamps in your tuner, or maybe they are not the original lamps, or perhaps the variations are typical.
I am in the process of selecting resistor values to use with some red and green led's removed from junked gear to put in the "tuned" and "stereo" lamp circuits. I plan to match the 33 ma current level in both circuits. These are narrow leds that will poke through the original light holes behind the front panel.
 
Don't know what to tell you "Chanman", but that's what mine read's and all is well. Voltage reading's were between term's 3 & 28 for tuned lamp and 9 & 28 for stereo. Unsoldered the wire's from term's 3 & 9 and iserted the meter for current reading's. Not sure what the actual lamp rating's are, but that's the way mine's operating.
 
To anyone interested:
I installed a green led in the "tuned" position and a red led in the "stereo" position. I mounted the led's in the blue plastic tubes that originally contained the incandescent lamps, drilling out the face of the tubes so that the led's protruded slightly. Each led was fed by a 220 ohm 1/4 watt resistor from the 17 volt supply. A second 220 ohm 1/4 watt resistor was mounted on the circuit board in place of the original 150 ohm 1/4 watt resistors to complete each circuit. Thus each led had a total of 440 ohms in series with it. This delivered 33ma through each led which is what the original lamps drew. I used 2 resistors because this allowed me to use 1/4 watt resistors which each dissipate just under 1/4 watt.
Circuit operation was unchanged and the led's were just the right equal brightness and looked much nicer than the original lamps. I like the color differentiation indicating
the two functions. I think Dynaco would have done something like this if led's had been available when the tuner was designed. The voltage drop across each led is about 2.2 volts.
 
I just came across a possible source of incandescent bulbs, see http://www.mcmelectronics.com/product/25-505 or http://www.newark.com/mcm/25-505/la...ll+Categories&categoryNameResp=All+Categories. A spec sheet is at the Newark web site. I've ordered some to try, I'll update you.
I made an LED replacement which turns on and off properly, but I'm wondering if the incandescent bulb is part of the "dynatune" auto-tuning circuit? Dynaco advertised that you could tune close to the station by hand and turn on the dynatune switch, and the tuner would automatically fine-tune. I've never tried to figure out how that worked, but could the fact that an incandescent bulb's resistance increases substantially as it heats up have anything to do with how it works? I have a very old HP tube oscillator that uses a cold incandescent bulb to stabilize against temperature drift. Just speculating here, not stating a fact!
 
Capitol C said:
I just came across a possible source of incandescent bulbs, see http://www.mcmelectronics.com/product/25-505 or http://www.newark.com/mcm/25-505/la...ll+Categories&categoryNameResp=All+Categories. A spec sheet is at the Newark web site. I've ordered some to try, I'll update you.
I made an LED replacement which turns on and off properly, but I'm wondering if the incandescent bulb is part of the "dynatune" auto-tuning circuit? Dynaco advertised that you could tune close to the station by hand and turn on the dynatune switch, and the tuner would automatically fine-tune. I've never tried to figure out how that worked, but could the fact that an incandescent bulb's resistance increases substantially as it heats up have anything to do with how it works? I have a very old HP tube oscillator that uses a cold incandescent bulb to stabilize against temperature drift. Just speculating here, not stating a fact!
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Could you either post here or let me know if your light solution works. Thanks. I am in the same situation. Burnt out tuning light which means no mute circuit working.

Ron
 
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