FM-3 enhanced
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| Posted on August 8, 2012 at 18:33:55 | |
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Posts: 17
Location: Brentwood,Ca 94513 Joined: February 17, 2009 |
This unmodified FM-3 was ebay purchased in June 2001 for $128 with 4 marginal tubes: 6AT8A,6AQ8,6BL8,12AX7. I replaced 4x tubes and installed "sonic upgrade kit" in 2002(2x PEC 555001 modules,C29,C31,R11,R13). The tubes improved sonics and the "kit" also seemed to help. I was too unskilled in 2002 to attempt alignment---probably this unit was never aligned until last week! After much study of FM-3 on this excellent forum and others I timidly rebuilt this FM-3 in 2012. I was afraid because I dont understand RF,IF and my Fluke 83 meter does not measure high-freq. The traces and parts seemed more fragile and awkward than my PAS3X & ST70 rebuilds(posted 12/20/2009 and 01/05/2012). I would like to publicly thank Bill Thomas(rip),James Lin MD,GP49,baddog,John Buddha Camille,FM-3 Zone,Sheldon Stokes. Please also note my personal&predjudiced Dynaco attitudes below. REBUILD NOTES PC-7: C12 is totsig and not NP0 so I upgraded to silver-mica. At 10.1MHZ I felt a coil-like polystyrene might be too inductive. I fantasized about totsig C4 upgrade to silver-mica but it is NP0 so I let it stay. My predjudice against totsig mica caps is less than my thermal instability fears. I also verify-measured(in-circuit) R2 & R3 because Dynaco specifies 5%. PC-8: R11 & R13 upgraded to 3W and intalled 1/4" high for cooling. PEC 555004 is totsig so I used the board from FM-3 Zone(with the upgrade of its 510pF to polystyrene). R25,R26,C24,C25 are totsig so they were repaced with 1% matching and silver-mica--this is post-discriminator so silver-mica upgrade over NP0 seems ok here. Volume switch can be pushed aside for this by unsoldering its gnd wire. C26 & C31 are totsig so K40Y-9 were my new choices. C29 is totsig so SBE 716P premium polypropylene was used. C30 is totsig so TRW polystyrene was used. C15,C17,C20 are totsig but NP0 so I did not replace them. C27 seemed too awkward to replace. PC-12: R81,R82,R83,R84(totsig) need 1% matching and cant be measured in circuit so I used Caddock MK132 and combined my choices until i had 4x resistors within .2% C82&C83 totsig caps were upgraded to 1.0uF Auricaps C79 upgraded to Auricap C81 became polystyrene C74,C76,C71,C75,C80 became silver-mica the 2x PEC555001 are totsig so this David Berning redesign(Audio Amateur 02/1988)needed the 100Pf cap upgraded to polystyrene. C73,C78,C77 are John E Fast types of unusual values so they were not replaced. power-supply: I used the SDS capboard with 4x 47uF caps at 450V(Digikey 493-4444-ND) with 2X polypropylene bypass caps across last 2 former quad-cap sections. The SDS board DOES have the wider-separated traces that are needed for cap-value experimenters like me. It is not hard to reorient the mounting positions of the larger/upgrade 47uF caps on SDS board. PSU upgrade showed audible upgrade over my previous 40 year old caps. star-grounding: I ran one continuous solid-core copper wire to all the tube grounds on PC-7,PC-8,PC-12 and then to "central-ground" where all the new ground-wires also met from power-trans,tube-heaters,psu,output-RCA's. alignment: Foolishly I procrastinated this much too long! T4----T1 adjustments top did narrow the tuning beam. However T5 adj made a HUGE improvement! Misc: -sanded & tested 8 tube shield grounds -reflow solder joints: PC-7 & input-antenna screws,6V4,ground-planes of PC-7&PC-8 to chassis -4 new softer feet from Dynaparts -add sheet-metal emi "fence" to shield PC-7,PC-8,psu -add 5x holes 3/8" dia to bottom plate for better cooling -2x gold plated RCAs -panel-mount Steren F-connector jack for future coax antenna lead -Woods 16/2 polarized power-cord and rewire so that hot 120V wire goes to fuse then power-switch then to power-transformer. Also remove the undesirable 120V outlet thus creating another needed vent. -new toggle power switch(ACE 33491) (15/32" hole) & wire-tie copper-foil around power-switch wires underneath IF PC-8 area. My Dynaco predjudices: I identify as an enhancer rather than a modifier or restorer. I try to keep original design so that Dynaco identity is maintained however this is NOT a museum piece and changes ARE needed. I do always improve power-supply All caps in total signal path of FM-3(aka "totsig") should be upgraded with serious consideration of heat,emi,stability/drift. Also re-consider that ground is the under-appreciated "other side" of signal-path and power-supply. When Dynaco specifies 5% matching I try for <1% Unit now performs quietly and sounds great!
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