Keeping the Dyna FM-3 Working

From Audio Basics, Volume Nine, Number Eleven, November, 1990

Why (might the uninitiated of you ask) would anybody possibly be interested in a 30 year old vacuum tube FM radio circuit design? Well, folks, for one very good reason - its sound. The Dyna FM-3 tuner, properly working and aligned, is more naturally musical than any tuner that ever came out of Japan at any price, and is only exceeded in pure listening quality by a handful of modern tuners. The best vacuum tube Macintosh tuners are better, as is the Marantz 10B. But for music per dollar, especially if you know how to maintain this little Stewart Hegeman designed wonder, the Dyna FM-3 cannot be equaled. Dynaco's unique achievement was to design the FM-3 as a user assembled kit (from bare PC cards up) and to allow an intelligent kit builder to completely align the unit without test instruments for excellent and repeatable performance. Because of the success of these design goals it is still possible for an owner to keep an old FM-3 inexpensively aligned and musical, with a few catches here and there that we shall tell you about. Lets get started.

You must have the original Dynaco construction and alignment manual for the FM-3 to make sense of what we are saying and to perform any work or alignment on your tuner. If your manual is gone you can order a copy from us for $20.00. There is also a chance that your unit may be the older FM-1 model with the FM3X multiplex module installed later. We do have a beat up, but readable, copy of this older FM-1 manual we can photocopy for you for $20.00. There is also one further iteration - the mono version of the FM-1 with a tiny 10 watt power amplifier built in instead of the multiplex board. We have the manual for that too (another $20.00 photocopy process) but we highly don't recommend its use - it ran so hot it fried itself and all the surrounding tuner circuits and there probably are not any left alive now.

First, although we can make silk purses out of sow's ears, we need a live sow to start with. So let's start by making sure your sow isn't dead. The FM-3 has a few fatal problem spots ­ parts that may need replacement that simply are not available any more and that the tuner cannot live without. Lets get them out of the way first so you won't try to resuscitate a dead pig.

The most obvious fatal injury is the tuning eye tube. If that is defective it is all over, there aren't any more. (6/23/97 Hot news flash - we have been informed that there still are a few EMM-801 Tuning Eye tubes available at $50 each from either Antique Electronic Supply (602-820-5411) or Daily Electronics (800-346-6667). If yours does work, be kind to it, carefully remove it and guard it with your life when tinkering with the circuits in your FM-3. The next two weak spots are the discriminator transformer (T2) and the dual slug Mplx transformer (T73). Both of these parts are gone and both must be good for the tuner to work. If the main tuning capacitor (C1) has warped plates from old age and lots of heat, proper dial tracking alignment from one end of the band to the other will not be possible, if its bearings are noisy, you will get static when you tune. Needless to say this part is made of unobtainium too. Finally you cannot depend on long distance service to keep an FM-3 running. There are many tuning adjustments and some are almost certain to shift during shipping, sending the tuner out of alignment. We only work on units that you can carry in to us and carry home. The alignment simply won't survive the vibrations of long distance shipping.

If your unit is older and has flat topped IF transformers they may be defective because they melt inside with age and the tuning slugs bind and break. Make sure you get the plastic tuning wand for adjusting these transformers too. You can kludge in a replacement for the quad filter capacitor with a stack of four discrete capacitors of the proper value). Anyway, now that you think you know whether or not your FM-3 is salvageable, we will tell you about a few of the problem areas — things to check in cleaning up your unit that makes the difference between music and noise.

The typical FM-3 we see to service is on its last legs. It still plays and tunes, but the output has excess hum, the tuning eye tube won't close tightly even on strong signals and the Mplx section doesn't narrow much on stereo signals. The stereo separation is completely gone and the unit suddenly jumps off station as it warms up or if it is bumped. The sound is muddy and compressed and the highs are almost completely gone. What do we look for and what do we do to restore the unit?

First of all, some of the vacuum tubes are weak links. On older units the 6AT8A tube on the front end board is almost certainly defective. This tube tends to fail much quicker than any of the others. Next, the two 12AX7A tubes probably need replacement. A pair of the Chinese tubes we select for Super Pas preamps are a good choice here ($28/pair + $6 shipping from us). The 6V4 tube should not be replaced with diodes because a solid state rectifier will increase the B+ voltage enough to change the gain on all the IF tubes, thoroughly messing up the alignment and stability and making it impossible to realign without a FM generator. The other tubes tend to last and last. Don't replace them unless you are sure they are defective.

The next weak link is the quad filter capacitor. If your unit has any significant amount of hum audible at the output, the quad filter cap is likely on its last legs. You can buy four 33 µF 450V radial lead electrolytic capacitors and cobble together a discrete replacement.

Another problem area is mechanical. Many units come in with the tuning shaft rubbing against the chassis or the knob rubbing against the plastic tuning eye window. This is caused by the improper mechanical alignment of the front end PC card and the tuning capacitor in the chassis. To cure, you must loosen (but not remove) all the hardware fastening the front end board to the chassis, loosen the screws holding the tuning capacitor cover to the chassis, loosen the hardware holding the tuning capacitor itself in the chassis, and remove (use a solder sucker) the solder connections between the front end (PC-7) board and the chassis, and between the tuning capacitor lugs and the chassis. Now the entire tuning capacitor - front end card should be free to move somewhat in its mounting hardware. Simply grab the tuning knob and pull it gently towards the outside of the chassis (allowing maximum clearance to the plastic tuning eye window). Then tighten enough hardware to hold the assembly in this position, check that you have made enough clearance to stop the rubbing, and then retighten all loosened connections. Sometimes you can eliminate the rubbing of tuning knob by simply moving it out on its shaft a bit further from the faceplate.

Intermittent operation of the tuner is almost always caused by the combination of two problems. The first problem is a loose mechanical connection at the C8 dial tracking variable capacitor. This is a strange little part consisting of a white ceramic barrel with a metal outer foil attached to the PC-7 card and with a screw running through the barrel and fastened to a nut soldered to the PC card. The screw head is accessible from the bottom of the chassis and turning it even slightly changes the dial tracking significantly. The electrical integrity of this part depends upon having a reliable electrical contact between the screw and the nut. As the tuner ages, this contact oxidizes and becomes looser. Then as the tuner warms up or is even slightly bumped, this electrical connection changes, causing the tuning to drift violently. The fix is easy, a little DeoxIt contact cleaner at the junction of the nut and screw. Go back to the Dyna instruction manual and re-read the installation instruction for this part. Note that the spring loaded nut was slightly compressed in the installation process to provide a good tension contact between the screw and the nut. You may have to unsolder the nut and reinstall the assembly again to get that compression fit back to maintain good contact. If all else fails, you can substitute a small fixed value dipped silver mica capacitor (somewhere between 5 and 10 pF at 300V) to stabilize the dial tracking. You will have to play with the exact value to get the stations to tune close to their proper assigned frequencies.

The second (and likely) problem causing intermittent operation and mistuning is bad solder connections on the PC cards. These connections become resistive with heat and age and on kit built units many were never good in the first place. Our advice is to resolder every single PC card connection in the entire tuner and then to use Ronsonol lighter fluid and a toothbrush to dissolve and remove all the excess solder flux. Note that some connections had riveted in eyelets. It is common to find that the solder connection was made between the lead and the eyelet only, but with the connection between the eyelet and the PC card loose and intermittent. Make sure all eyelet connections flow from the lead to the eyelet and on to the PC card foil itself. Bad connections make for bad tuner performance. Use a very high quality solder such as Ersin Multicore SN60 or similar. For proper circuit operation, the tuner must have very low impedance connections between the ground foil on the IF and front end boards and the chassis. This contact must be improved by soldering the PC card ground foil to the chassis bottom at multiple locations around each card. Make sure you solder to the common ground plane only, not to any of the component linking traces. While you are at it, make sure you have very good solder connections from the quad filter capacitor ground lugs to chassis ground and from the tuning capacitor mounting lugs to chassis ground too.

Now lets look at some problem child parts that cause degraded performance. The worst culprits are the two large capacitors located at the Mplx card end of the IF board. These parts, C29 (.47µF) and C31(.22µF) become resistive with age and have poor high frequency response. This causes poor stereo performance because the 38 KHz pilot signal to the multiplex section is attenuated. Replace both of these capacitors with modern film types such as Panasonic EF series film .22 µF/250V and .47 µF/250V capacitors from Digi-Key. While you are at it, replace the two output coupling capacitors on the Mplx (PC-12) board too. Both C82 and C83 (0.1 µF/400V) should be replaced with much larger value capacitors to avoid rolling off low frequencies. We would suggest a Panasonic EF series 1 µF/400V capacitor in each of these two locations. If any of the four multiplex switching diodes are defective the Mplx section will not work. These (D71 through D74 IN541) are special low forward resistance Germanium diodes. Normal silicon diodes will not work in this application. If any are bad, replace all four. Finally, make sure all the small ceramic capacitors on the front end board (PC-7) are positioned straight up on the board. Bending many of these over will affect the alignment.

Once you have the unit put together solidly, make sure it is clean. Use a soft paintbrush and toothbrush to get all the dirt and crud off of the circuit boards on both sides. A dirty tuner runs hot and drifts out of alignment. Now you will need to align your tuner exactly following the procedure in the Dyna manual. The tuner can be aligned without test equipment with the following little caveats that Dyna did not tell you about.

First, there actually was some unadvertised pre-alignment done at the factory. The IF transformers were each pre-aligned to 10.7 megahertz because there was no way for the end user to tune them in to this frequency without test equipment. In the Dyna manual the user is cautioned to not make much change in the alignment of the first IF transformer, but is not told why. The Dyna alignment procedure allows for the alignment of each of the following IF transformers to the first one, but does not guarantee the certain alignment of them all to 10.7 meg. Thus, if the adjustment of the first transformer is changed very much, then further alignment downstream only misaligns all the others to whatever frequency the first is now randomly sitting at. A misaligned IF section will have poor dial tracking, poor sensitivity, poor frequency and phase response, and will be sensitive to external out of band signals (interference). If you suspect your IF section has been adjusted all to hell, then you must take your tuner to a shop that can do a quality FM alignment so they can set the IF transformers back to a 10.7 Meg frequency. You simply cannot do this yourself without expensive equipment.

Second, the alignment procedure includes installing a "gimmick" — a small value capacitor made of a short piece of wire, on the IF board after part of the alignment is complete. Inevitably, the hobbyist forgets to remove this little part before starting the alignment process over again later. It has to be saved for later re-installation. Remember, read all the directions before you start unless you like to do things over again.

That is about all we can tell you. If you have a FM-3 put together solidly out of working parts, and you follow the factory alignment instructions properly you should be rewarded with an open, smooth, and musical FM performance well worth the time and effort expended to get your tuner working at its best.

Frank Van Alstine

Copyright, Audio by Van Alstine, Inc., 1990. No part of AUDIO BASICS may be reprinted or reproduced in any manner without the permission of the publisher.

Good Follow up Information Supplied by another FM-3 owner 6/24/97

Dear Sir,

I read the article on your website and am glad to see that I am not the only one who recognizes the merit of the FM3 as a high performance tuner. I would, however, like to point out a couple of details I have come across that are not mentioned in your article.

On the IF strip are two 10K, 2 watt resistors that are right on the ragged edge of being too small. If memory serves, one is dissipating 1.9 watts and the other is right at 2W. When I rebuilt my FM3 I used 3 watt metal oxide R's and mounted them so they don't touch the board. Each sits about an inch up. I mounted them high because the board was charred where the old ones touched it.

With regard to the hum problem mentioned in the article, mine hummed even after the power supply filter had been updated with discrete caps.It turned out the problem was in the way the output was wired. The signal return was connected to chassis. This was not an error; the manual shows that it should be assembled that way. I lifted the wire from the chassis and ran a 22 AWG wire from the shield connection of the RCA jack back to the ground bus on the MPX board and the hum completely disappeared.

Last but not least, I urge forced air ventilation for all FM3's regardless of how they are installed. On mine the face plate got hot after an hour or so. So did the power transformer. I placed a small muffin fan behind it that is plugged into the outlet on the back. It runs cool as a cucumber now. Since the IF cans and the power transformer can no longer be replaced economically this seems to be the best way to maximize service life. The tubes benefit from ventilation as well.

FYI, I rebuilt mine in '92 using surplus MIL spEC parts. No major mods, just a stock rebuild. It has been in continuous use ever since seeing about 2-3 hours of service a day. I don't recall the last time I touched the tuning knob. Before I rebuilt it, it was necessary to touch up the tuning after about 15 minutes and again about an hour later. No longer. The only time I touch it is on those rare occasions when I change the station.

Thank you for your attention and keep up the good work.

Carlyle Rabe