[gtalmo]

Problem:
  I can't get the PC-12 multiplex board to align and output in stereo on my “refreshed” Dynaco FM3. 

Background:
  My brother gifted me a Dynaco FM3 a few years back and I finally took to giving it some love over the long Minnesota winter. I replaced all of the electrolytic caps, the big power can-capacitor, the PECs, power resistors, and all of the tubes (keeping the originals) including V8. Everything went well, following the Dynaco instructions and Bill Thomas’ "Double Dose" guidance and back issues of “Audio Basics”. Currently I can align the IF and RF stages of the FM3 just fine and install "the gimmick". The tuner is stable, sensitive, and musical. But the PC-12 won’t align so it’s all mono. : (

Troubleshooting:
  - everything up to T73 seems good
  - v8 seems to work properly, the bottom closing on strong stations, and the top STEREOCATOR closing on known stereo broadcasts yet no discernible stereo output.
  - replaced all of the electrolytic caps on PC-12
  - tested/confirmed the stereo/mono switch is not shorted
  - T71 and T72 respond and align in step 31
  - I can get through step 32 of the FM3 alignment instructions just fine
  - When hitting step 34 the STEREOCATOR indicator opens fully and will not close or respond at all to further adjustments. The “signal”indicator of v8 works properly throughout
  - up to step 33 the STEREOCATOR operates fine, closing on known stereo broadcasts, but after step 34 it does not operate
  - swapping out v71 for valve "A" (the original valve) causes the STEREOCATOR to operate properly up to step 32 and then not light on step 34
  - swapping out v71 for valve "B" (a "new" valve) causes the STEREOCATOR to not function at all -- as if the valve were removed
  - swapping out v71 for valve "C" (a "new" valve) causes the STEREOCATOR to operate properly up to step 32 and then not light on step 34
  - removing v71 entirely causes the the STEREOCATOR to not function at all (as expected)
  - swapped V7 -- no change
  - swapped V72 -- no change
  - tested/confirmed the diodes on the PC-12 board with a VOM, they check out
  - finally, measuring between the two red wires on the volume control, with FM3 off and the volume control pushed in (e.g. in stereo mode) I measure 0 ohms resistance and 0 farads of capacitance across the two red wires. That seems incorrect and an indication the T73 is bad since those wires solder directly to lugs on T73.

Diagnosis: bad T73 transformer?

Questions:
  - am I troubleshooting this correctly, is it likely the T73?
  - how can I confirm (or disprove) my diagnosis before I buy a replacement T73? Is there a way to test the T73 transformer when it's installed?

Thank you for any help!

 - greg

[BuddyDave]

I made a detailed reply last night.  Don't see it yet.  Apparently the moderator needs to approve it first.

[adminRH]

BuddyDave,
You have no other posts in this thread awaiting moderation.

[BuddyDave]

You have done quite a lot to find the cause, and that right there is commendable.  I had a pretty much identical experience--you get to about the last step in the alignment, and then nothing--the EMM801 does not respond.  Not a good feeling.  Unfortunately, my solution was a shotgun approach, and I never did know what I did that got stereo back.  Replacing T73 did not do it (although I can give you one if you want it).  I then replaced nearly all the caps on PC-12 (except C71, C72, C80, C81, C84, as I recall) and then several resistors, as one was pretty far out of tolerance.

I also checked all the tube and multi-section filter capacitor lugs voltage test points in the manual (inside back cover), as that can uncover some problems.  My tube voltages had several issues that gradually got to normal after replacing some caps and resistors on PC-12.  High voltages there, so watch out.

From my reading on this, apparently T71-T73 can be touchy.  The manual explains for most of the ones on the FM-3 where to have the starting point on the slugs.  In addition, not that it has much to do with your problem, but V2 (6AT8A) is one flaky tube, and about half of the ones I have tested were sub-standard.  V1 (6AQ8) should be checked as a general measure, too.  Confirm that V71 and V72 are good.

Another option is just replacing PC-12 if you can find a clean one, ideally a factory-soldered board.  They come up now and then.  My parts unit was done by a novice kit builder and is a mess, so I just removed T73.  That's another thing--having a parts unit is not a bad idea.

In terms of testing and tuning transformers (IF coils), you can do it with a scope, it seems.  It works pretty well on a removed T73 (or other).  In circuit, I don't know.  There is a Youtube video of somebody tuning a radio coil out of circuit, and I used his method.  It didn't solve any problem, but I learned something.  Sorry, no reference to that video.

By the way, what PEC boards did you use?  I built mine from plans from Sheldon (forgot his last name).  I have not yet installed them.  There is not much room there on the board, and this design would have long exposed bared leads and sit high off the board, which makes me uncomfortable.  If you want the T73, let me know.  My gut says it is not the problem unless the slugs are really loose or frozen, etc.  No fun removing.

[BuddyDave]

T73 could be the issue, but check everything else first.  I'll check those points on the volume control and let you know what I find.

[gtalmo]

BuddyDave, thank you for the reply!

I’m curious to discover your measurement of the 2 red wires on the volume control. Depending on that result I may take you up on your T73. I’ll DM you if so.

I built my own PECs with breadboards based on Sheldon Stokes’ PCB designs. They’re rather big but face away from each other and fit tightly to the PC-12. I’ll send a picture later.

I don’t have a tube tester, but based on performance and behavior I think all of the new/current set are good. I will go through the tube output measurement/reference again.

Thanks to member arindamcc I have the Gerber files for the FM3 so I can have a new PC 12 printed, if needed.

I just keep staring at the schematic with my limited knowledge of electronics and it looks like the T73.  TP88 is one cap away from lug #3 on T73 and when soldering V8 (e.g. TP75) to TP88 in step #33 the STEREOCATOR stops working.

Thanks again!

- g

[dB Cooper]

I suggest you contact Frank van Alstine and first get a copy of his old Audio Basics newsletter issue "Keeping the Dyna FM-3 Working" (downloadable from his website) and/or call him. He's very knowledgeable about this unit from years of supporting them as a dealer. BTW almost all the tubes except the tuning eye are available at reasonable cost. I had JAN mil-spec tubes in mine. I would email him with your troubleshooting list so he has some time to look it over and follow up with a call. Sounds like you.re onto something though. Seems to me there were some other potential causes for degraded stereo mentioned in his article. They could be contributing if not deciding factors.

[BuddyDave]

gtalmo:  Measuring resistance at lugs 4 and 5 on the stereo/mono switch opposite the volume knob per the detailed drawing pullout, I get 116 ohms.  These are the twisted red wires that on the other end connect to lugs 2 and 3 on T-73.  Power off, stereo switch in.  Capacitance is probably not a useful measurement here, but in either case I measured very low, insignificant. If you measured somewhere else, let me know.

[BuddyDave]

To clarify:  I measured across lugs 4 and 5 on the stereo/mono section of the volume control assembly, not up at the volume control itself.

[gtalmo]

BuddyDave: my previous post was poorly worded. My FM3 reads 117 ohms across lugs 4 and 5 on the stereo/mono switch like yours. I might take you up on that T73.

Sounds like my diagnosis is wrong.

dB Cooper: I’ve been through the back issues of Audio Basics and reached out to Frank, who graciously told me they don’t work on old FM gear any longer, lacking operable test equipment. Frank is local, I was hoping to bring it to him for in person repair.

[dB Cooper]

BuddyDave: my previous post was poorly worded. My FM3 reads 117 ohms across lugs 4 and 5 on the stereo/mono switch like yours. I might take you up on that T73.

Sounds like my diagnosis is wrong.

dB Cooper: I’ve been through the back issues of Audio Basics and reached out to Frank, who graciously told me they don’t work on old FM gear any longer, lacking operable test equipment. Frank is local, I was hoping to bring it to him for in person repair.

IIRC There are some caps that become resistive with age and attenuate the stereo pilot signal causing degraded (or no) stereo performance. Also IIRC they are the big film caps at the righthand end of the IF strip. They are mentioned in the FM3 article. Thought it was worth a shot.


Not too easy keeping a vintage tuner running anymore if something goes wrong. This goes double of old hot-running tube tuners. At least the SS units tend to run cool. Too bad. My AVA FM5-based tuner was the best tuner I ever heard, including the Magnum Dynalab I bough for ~3x the price.


RIP FM.

[BuddyDave]

A few more ideas.  Make sure V71 (6BL8) is good.  That is key.  Looks like you have this one covered.

The ballpark T-73 slug settings:  Bottom Slug:  2 turns clockwise from bottom stop.  Top Slug:  6-8 turns clockwise from top.  Would not hurt to check.

A replacement T-73 might do it. 

Otherwise, short of a new PCB, which you would have to re-populate entirely, you'll probably have to start testing and replacing components.  I would start with the resistors.

First, though, I'd check voltages on those two tubes (V71 and V72), but carefully.  Should not be off by more than, say, 20% maximum from table on manual inside back cover.  Identifying the pins can be tedious.