I just scored a Dynaco FM-3 for $60 to complete my Dynaco system. I’m going to clean and refurbish it. Are there any common things I should look for? I’m debating on changing out all the carbon comp resistors with new metal film ones. I’m going to replace the multi cap with individual caps under the deck. I’ll also do electrolytic to form on any signal path caps. What else should I look at?
Was it a kit, or factory assembled?
You may get unexpected problems changing the carbon comp resistors in the RF circuits to metal film.
You may get unexpected problems changing the carbon comp resistors in the RF circuits to metal film.
It’s a factory built not a Dynakit. was going to measure all the resistors to see how far off they are before changing them out. I’ve had great results doing that on a PAS-3 and ST-70.
Very lucky, the factory built tuners are usually much better than most of the kit versions.
Dyna hired a team of women to do the assembly for FM-3A tuners (and other units),
and they did a great job.
However, the RF circuitry is very different from the audio part. A word to the wise.
Dyna hired a team of women to do the assembly for FM-3A tuners (and other units),
and they did a great job.
However, the RF circuitry is very different from the audio part. A word to the wise.
Don't use metal films in the RF/IF sections, look for fresh Allen Bradley carbon comps. Many will have drifted significantly unless its a low hour and properly stored unit.
Make sure it is well ventilated, they run hot and will drift. Don't mess with any of the cog/npo caps in the tuner front end unless you never want it to work correctly again.
These are very nice tuners, and capable of quite good sound. I had one for a number of years, regretted getting rid of it.
Make sure it is well ventilated, they run hot and will drift. Don't mess with any of the cog/npo caps in the tuner front end unless you never want it to work correctly again.
These are very nice tuners, and capable of quite good sound. I had one for a number of years, regretted getting rid of it.
Yes, if you use it for long periods of time, a slow, quiet fan will keep it nice and cool.
And there''s even a rear panel AC outlet for a 120VAC fan.
And there''s even a rear panel AC outlet for a 120VAC fan.
Good ideas, I don’t want to have to realign the tuner! I have access to AB carbon comp resistors but I have to drive to Apex Surplus and hunt for them. It’s a worthwhile venture because I always find other stuff I need there. I also have some values on hand. Thesr things are not that common these day. I’ve also got a nice vintage Scott 350 tuner and a McIntosh MX110 to compare it to. It becomes an obsession!
Take your DVM with you, and measure before buying (if possible). The CCs can absorb moisture in storage.
Alignment is not difficult, but is not necessary if the tuner works well.
It probably has never been realigned since assembly, and very well may not need to be.
They also had excellent tubes supplied from the factory, so don't replace any unless absolutely necessary.
Be sure to compare with the same antenna.
Alignment is not difficult, but is not necessary if the tuner works well.
It probably has never been realigned since assembly, and very well may not need to be.
They also had excellent tubes supplied from the factory, so don't replace any unless absolutely necessary.
Be sure to compare with the same antenna.
What about carbon film? Better temp stability and accuracy than carbon comp. Not as problematic as metal film in RF apps?
About the same problems as metal film. The CC are bulk material and work well at RF.
New CC will not have much moisture and should measure ok. The tuner's heat will keep them dried out.
If you buy surplus CC, they could be off their values due to moisture absorption, but may come back
with use in the hot tuner environment.
New CC will not have much moisture and should measure ok. The tuner's heat will keep them dried out.
If you buy surplus CC, they could be off their values due to moisture absorption, but may come back
with use in the hot tuner environment.
There are some typical problems, but they are only in the kit-built version, which you don't have.That's good.
There is an inherent design fault that I can later on tell you how to fix, if you have hum problems.
There are two things I would do first.
1) Replace the volume control with a two resistor L-pad per channel, with attenuation to match your desired output level.
Keep the same 250k sum for the two resistors. A series 200k and a shunt 50k is fine. Lose the mono switch if you want.
2) Change C29 to around 5.6uF @ 100V film cap. This is a quite dramatic change for the better in the bass,
which is the FM-3's weak point for audio quality. There are two coincident poles, and this moves one of them lower.
Now compare it to the other tuners. If you are still not happy, replace the three PECs with discrete parts, kits are available.
If you still have objectionable hum, reconnect the ground end of the final cathode resistor on the IF board (going to the ground plane)
instead back to the main audio ground at the quad capacitor. The filament currents in the plane cause too much noise.
There is an inherent design fault that I can later on tell you how to fix, if you have hum problems.
There are two things I would do first.
1) Replace the volume control with a two resistor L-pad per channel, with attenuation to match your desired output level.
Keep the same 250k sum for the two resistors. A series 200k and a shunt 50k is fine. Lose the mono switch if you want.
2) Change C29 to around 5.6uF @ 100V film cap. This is a quite dramatic change for the better in the bass,
which is the FM-3's weak point for audio quality. There are two coincident poles, and this moves one of them lower.
Now compare it to the other tuners. If you are still not happy, replace the three PECs with discrete parts, kits are available.
If you still have objectionable hum, reconnect the ground end of the final cathode resistor on the IF board (going to the ground plane)
instead back to the main audio ground at the quad capacitor. The filament currents in the plane cause too much noise.
C29 the .47uf off the PEC to a 5.6uf? That’s a huge change! Is that to move the pole much lower in freq? I really don’t need the volume control so i might measure where I set it and replace it with two fixed resistor. I’ll just leave the pot on the faceplate for looks. Thanks for the tips, tuners are not my thing. I figured for $60 I could take a chance plus now I have a complete Dynaco set of tuner, pre, and amp. I’ll use them as an office system.
Yes, the bass is ill defined due to two coincident LF poles. Moving that one pole a decade lower allows the bass
to sound much more defined and tonally accurate. It isn't subtle, so listen before changing the cap and see.
If it's an office system, running the tuner all day, I'd definitely use a slow, quiet fan with the tuner.
Just a little air movement makes a big difference in its internal temperature.
Just bought an FM-3 on eBay. It's a "factory wired" version. It has no tubes, however, I have all the tubes except for the 6V4 rectifier. 6V4s are available on eBay for $20-$30, however, I have tons of 6X4s. The 6X4 seems like a more than capable substitute for the 6V4 if the socket is changed to a seven pin (which I am willing to do). Do any of you foresee a problem with switching to the 6X4? I also have a few good 6CA4s, which are plug-compatible. However, heater current is 1.0 amp on the 6CA4, compared to 600ma on the 6V6. There is also the simple alternative of using silicon diodes with a dropping resistor. I only spent $40 on the tuner, and I am looking for the cheapest effective solution. Any thoughts?
Haven't ever had to do that, however this tuner is worthy of being kept in its original condition.
It isn't just a cheap tuner to get by with. Would not use silicon diodes either though.
But test the tuner for functionality before going much further.
It isn't just a cheap tuner to get by with. Would not use silicon diodes either though.
But test the tuner for functionality before going much further.
Note all the fear of drift AB carbon comp resistors in posts 5 & 8 are not my experience. About 1961 AB & sprague started using a paint that was a barrier to moisture. I have hundreds of AB post 1961 resistors in dynaco preamp & amp, hammond organs, none have drifted. Pre 1961 AB & sprague were just as bad as anybody else. European owners of hammonds built in holland have reported carboncomp resistor drift. Particularly near the sea. The moisture barrier paint used on rcr07g# resistors by AB & sprague may have been a military controlled process not alowed to be exported to Europe. Resistors that stayed put in value at sea would have been a huge advantage to the US Navy.
Whatever, measure your resistors with a DVM. You may be pleasantly surprised.
For vacuum tubes I mostly use tubesandmore.com of AZ, also known as antique radio.
Whatever, measure your resistors with a DVM. You may be pleasantly surprised.
For vacuum tubes I mostly use tubesandmore.com of AZ, also known as antique radio.
Is there a difference in their appearance?
If so, can you post a photo of the two kinds side by side for a comparison?
This is a great thing to know for vintage audio gear, esp tuners.
Still, I prefer metal film for the audio circuits, and even for supply dropping resistors
(not subtle), except for maybe grid stoppers.
If so, can you post a photo of the two kinds side by side for a comparison?
This is a great thing to know for vintage audio gear, esp tuners.
Still, I prefer metal film for the audio circuits, and even for supply dropping resistors
(not subtle), except for maybe grid stoppers.
No. the post 1961 AB sprague resistors looked just like the pre 1961 ones. Also the European CC resistors looked just like the American ones. Difference was only moisture resistance. I've seen no write-up of this, but my 1966 A100 and 1968-69 H18x hammond organs built in USA the resistors have no drift problem. Hammond A's, B's, C's, D's, from the earlier dates had notorious moisture drift in the preamp resistors. An organforum poster with a 1970? Hammond X66 built in holland, that lived on a channel island, had to replace nearly every resistor for drift. As the AB & sprague 1/4 watt CC resistors looked just like the RCR07g# Ford aerospace was buying for my projects in 1976-79 I presume the civilian AB & sprague resistors were made on the same lines as the military products and received an unadvertised benefit due to the paint upgrade.
Metal Film & carbom film have a spiral construction, which may lead to unfortunate results substituting for carbon comp in RF circuits. Spirals have more self inductance. Metal and carbon film are fine for audio circuits; I use only metal film or wirewound for audio.
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