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FM-3 voltage checks and alignment. Anything to watch for?

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Posted on July 31, 2009 at 18:48:27
tvr2500m
Audiophile

Posts: 1380
Location: Boston
Joined: February 2, 2003
 Just a little bit ago I purchased an FM-3. Factory wired. Original tubes, all good except the RCA 6AT8A (indicates very gassy on my TV-7D/U!?).

I only just powered it up for the first time this evening. Seems to be "working" okay. Are there any checks I should do before running through an alignment procedure? Voltage checks would be wise to do before an alignment, I'm guessing. Are there any subtleties, nuances, "secrets", "special", or undocumented things to note in doing an alignment? How about the plastic alignment tool? Any recommendations for this? I'd worked with my father's FM-1 with the multiplex adapter, but that was about 25 years ago.

The Dynaco FM-3 manual specifies to use a VTVM for voltage checks (plus, some measures using a 100K resistor in series with the probe). I don't have a VTVM, just a modern DMM. Any compensation I need to make in doing these measurements with a DMM rather than a VTVM?

Just took a look at at Bill T.'s FM-3 restoration on Audio Circle. Thanks, Bill! Yeah, helpful. Great stuff. Has it been completed? I was looking for the alignment and then performance and sonics reports.

- SJ

RE: FM-3 voltage checks and alignment. Anything to watch for?, posted on October 9, 2009 at 22:41:26
GP49
Audiophile

Posts: 112
Location: No Cal
Joined: April 21, 2009
 "...The Dynaco FM-3 manual specifies to use a VTVM for voltage checks (plus, some measures using a 100K resistor in series with the probe). I don't have a VTVM, just a modern DMM. Any compensation I need to make in doing these measurements with a DMM rather than a VTVM?..."

A modern DMM will do just fine.
GP49
RE: FM-3 voltage checks and alignment. Anything to watch for?, posted on October 9, 2009 at 18:03:01
MM
 What pin is the primary on T1? Where do I attach the voltmeter to check the peaking voltage during adjustment and what king of voltages am I looking for? Also, Do I remove the gimmick before I start anything?

RE: FM-3 voltage checks and alignment. Anything to watch for?, posted on August 2, 2009 at 00:05:41
tvr2500m
Audiophile

Posts: 1380
Location: Boston
Joined: February 2, 2003
 Thanks. Not having a quality FM/MPX generator or oscilloscope, I performed a "kitchen table" alignment. I also didn't have handy a 1.2 megohm resistor for the stereo decoder alignment - that that Bill points out is so important - so I wasn't able to complete this.

BUT, what a difference doing everything else made. The tuner works and sounds great; much better than before the alignment. I finished in time to listen to a live performance broadcast of some Copeland from Tanglewood. This is the kind of thing I wanted this tuner for. There only a couple/few really good broadcast quality stations around here, but they sure can sound really good.

I indexed the starting positions for reference of every tuning slug before I turned it. I didn't have to change any of them very much to get to the loudest noise levels.

I removed the gimmick wire/capacitor before doing the discriminator alignment, did the alignment with a very high impedance DMM, then reinstalled the gimmick.

One thing looked/felt a little odd. I had to turn out the C8 screw an amount that felt like quite a bit to get the dial tracking right.

I did install a very healthy RCA 6AT8A, at least very healthy as reported by my TV-7D/U. I also bought a few NOS ones to have on hand.

I haven't changed any parts or done any modifications. I wanted to establish that I have a working, stock, serviceable FM-3, calibrate/benchmark myself to this, then decide what I'd like to do next. Looks like some resistors and caps need to/should be changed. Maybe an SDS power supply board? New PECs?

Those two resistors, the two coupling caps on the MPX board. Van Alstine mentions some others. Bill, I see from your Audio Circle rebuild you've changed some of the ceramics to mica and film, and some other resistors. Most folks seem to leave most of the ceramic discs in place. What's up with this? Ceramics aren't noted for their good sonics. Not so many in the audio signal path? Are they especially well suited for the job their doing in the positions they're in? ??

Otherwise, yes, not difficult to do a "kitchen table" alignment.

Thanks again for the responses. I appreciate them.

- SJ

Re: Capacitor changes., posted on August 2, 2009 at 07:05:52
Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004
 Yes, I changed the ceramic disc coupling capacitors to something a bit more "audio-friendly". You're also right about changing out the PEC modules. The ceramics work "ok" for RF purposes, but around the discriminator I decided to attempt to get things a bit more closely matched by using the micas, along with carefully matching the resistors associated with them.

Sorry about the holdup on the alignment and final assembly. The main reason for the delay has to do with a lot of rain! (I also have a new project that is about to begin, but is also "on-hold" until I can snap some pictures.)

All will be made clear VERY soon.

Bill

Just a *couple* of things., posted on August 1, 2009 at 14:00:36
Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004
 Replace the two 10K 2 Watt resistors on the IF board with at LEAST 3 Watt parts. Mount them above the board to allow air to circulate all around them for cooling. Increase the Output coupling capacitors from 0.1 to 1.0 uF.

Make SURE your 6AT8A is as good a tube as you can find. This particular tube takes a REAL beating in the FM-3.

While Dynaco doesn't mention it, you should peak the bottom of IF transformer T1 for 10.7 MHz before doing the rest of the IF alignment peaking. This will require a signal generator set to 10.7 MHz and a frequency counter to verify the frequency. Pull the 6AT8A and inject the signal into the Primary of T1. You can use the tuning eye tube to observe the peak. If the EMM801 closes completely, reduce the signal level from the generator.

Other than that, you can use the EMM801 to align the tuner or you can use a Voltmeter with a high Input Impedance to do the job. Either method will work just fine.

Contrary to what GP49 has stated, doing an alignment with a bunch of test equipment (other than to peak the Primary of T1) will NOT produce any better results than using the method outlined in the manual.

Also, make SURE you install the "gimmick" capacitor - as outlined in the manual, after you complete your alignment and "button up the tuner."

Alignment of the Stereo decoder board is quite critical. Small changes in the slug positions on T73 will produce BIG changes in the Stereo quality. Follow the manual and "tweak" carefully.

Other than that, it's a piece of cake.

Bill

RE: Just a *couple* of things., posted on August 1, 2009 at 15:03:45
GP49
Audiophile

Posts: 112
Location: No Cal
Joined: April 21, 2009
 Bill wrote:

"Contrary to what GP49 has stated, doing an alignment with a bunch of test equipment (other than to peak the Primary of T1) will NOT produce any better results than using the method outlined in the manual."

I've found myself able to align and peak the tuner much faster with proper test gear. I THINK more accurately, too. Also I've been able to spot some pitfalls...the first IF not being precisely at 10.7 MHz is a major one...from what the detector curve looks like, thus saving a lot of time. I'm not sure I'd have been able to spot that, otherwise.

This is not to say I haven't done my share of "kitchen table alignments" on the FM-3 and FM-1.

"Also, make SURE you install the "gimmick" capacitor - as outlined in the manual, after you complete your alignment and "button up the tuner.""

Probably a third of the kit-build FM-3 and FM-1 I have had to align, still had the "gimmick" in there; the kit builder had never removed it.
GP49
RE: FM-3 voltage checks and alignment. Anything to watch for?, posted on August 1, 2009 at 00:20:22
GP49
Audiophile

Posts: 112
Location: No Cal
Joined: April 21, 2009
 For aligning a tuner, an analog meter is easier to use than a digital because you are peaking circuits and the analog meter allows you to better follow the trend of your adjustment, up or down. Some digital meters have a bar scale in the display, and that helps. When aligning the detector stage, there is one step where you tune the detector to zero volts. That is easiest to do with an analog VTVM or TVM, adjusted to offset the zero position of the needle to center scale. However you certainly CAN use a digital meter...it is just not as easy.

My personal preference is to use an oscilloscope.

You definitely need a good plastic or hardwood hex alignment tool. A steel hex tool will detune the very coil you are trying to adjust, making all such efforts useless; and may crack the powdered iron core, making the attempt to align the stage fatal. DO NOT BREAK THE CORES, or you will be looking for new IF and/or detector transformers.

The no-instruments alignment procedure in the assembly manual will get the tuner working but a proper bench alignment using a quality FM/MPX generator as a source and an oscilloscope as a readout is far easier and will yield better results.

Another page with good advice for perking up an FM-3:

http://www.avahifi.com/root/audio_basics/1990-11_fm3_keep_working.htm

And you don't have to be told, don't break the EMM801 tuning eye tube. They are not totally unobtainable: Tubeworld says they have them, or will be getting more. But they will charge you $150 for one.

GP49

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