Erm.. Why!? putz around? Update My Dynaco
has 'kits' for rehabbing Dynaco ST120's (and others).
For surprisingly Few $$ Likely not more $ than your BOM for a diy patch job.
Plus they come as discrete sections ie; Power supply Audio boards etc.
So only need to replace what you want to.
Although the Complete Update install is leagues better than Any Dyna 'rehab' of the dark past. IMO do it all... or shortchange your results.
has 'kits' for rehabbing Dynaco ST120's (and others).
For surprisingly Few $$ Likely not more $ than your BOM for a diy patch job.
Plus they come as discrete sections ie; Power supply Audio boards etc.
So only need to replace what you want to.
Although the Complete Update install is leagues better than Any Dyna 'rehab' of the dark past. IMO do it all... or shortchange your results.
I looked at updatemydynaco
His output boards appear to be lm3886 boards. Those have nice detail and solve the gritty cold crossover distortion of the original PC-15's. However, you'll never get 70 w for 5 seconds out of a lm3886. I think 10000 uf output caps are silly for 8 ohm systems. His finned heatsinks are nice, a nice upgrade on the 3/8" aluminum bars he was using when he was developing the kits. Note the LM3886 board for $148 the pair don't come with the heatsinks. And my unit doesn't thump turning on or off, possibly because the 3300 uf main cap is small. I've got the djoffe bias control on my original PC-15 which with 4 mhz output transistors drive distortion down to sub .1%. That bias control board is 7 $.08 transistors and a half dozen $.012 resistors, on 5 sq in of a $18 Nema-C 1/16" 12"x24" board. New MJ15015 got down to $1.25 each although since mine were stolen 2020 I had to replace them with $10.32 MJ21194 this month.
The other PC-15 was butchered by TO-220 replacements for TO5 cans, so it is now an Apex AX6 board. One $.08 TO92 transistor, 3 $1.50 TO220 transistors, two $1.15 MJ15015, two $1.20 ceramic caps, 5 $.50 electrolytic caps, 7 $.012 .6 W resistors, 4 $.30 3 watt resistors, 2 $2 5 watt resistors, add $3 for a new 3300 speaker cap if you want a new one. 15 sq in of the $18 NemaC board. Three TO-220 heatsinks, free because sawed out of window frames. 10' $.33/ft 30 ga wire, 2' $.70/ft 20 ga wire. Maybe <$30?
His output boards appear to be lm3886 boards. Those have nice detail and solve the gritty cold crossover distortion of the original PC-15's. However, you'll never get 70 w for 5 seconds out of a lm3886. I think 10000 uf output caps are silly for 8 ohm systems. His finned heatsinks are nice, a nice upgrade on the 3/8" aluminum bars he was using when he was developing the kits. Note the LM3886 board for $148 the pair don't come with the heatsinks. And my unit doesn't thump turning on or off, possibly because the 3300 uf main cap is small. I've got the djoffe bias control on my original PC-15 which with 4 mhz output transistors drive distortion down to sub .1%. That bias control board is 7 $.08 transistors and a half dozen $.012 resistors, on 5 sq in of a $18 Nema-C 1/16" 12"x24" board. New MJ15015 got down to $1.25 each although since mine were stolen 2020 I had to replace them with $10.32 MJ21194 this month.
The other PC-15 was butchered by TO-220 replacements for TO5 cans, so it is now an Apex AX6 board. One $.08 TO92 transistor, 3 $1.50 TO220 transistors, two $1.15 MJ15015, two $1.20 ceramic caps, 5 $.50 electrolytic caps, 7 $.012 .6 W resistors, 4 $.30 3 watt resistors, 2 $2 5 watt resistors, add $3 for a new 3300 speaker cap if you want a new one. 15 sq in of the $18 NemaC board. Three TO-220 heatsinks, free because sawed out of window frames. 10' $.33/ft 30 ga wire, 2' $.70/ft 20 ga wire. Maybe <$30?
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As I see it, after these upgrades, you dont have a Dynaco anymore, but a cheap chipamp instead.🙄 Why not cloned replacement boards and improved components to keep/save the soul of the original?
The original sounded bad. See Greg Dunn for the 1966 stereo review article. Too much crossover distortion cold & soft, dull highs due to slow output transistors. People kept buying tube amps. I did in 1970. The TIP mod helped the highs (TIP3055 instead of 40636 O.T.) but it took the djoffe bias control board add on to push the idle bias current up to 20 ma. Dynaco Stereo 120...can be beautiful - diyAudio
The original also frequently burned output transistors. Mine was bought that way. 20 w/ch for 3 hours did it again. Shorted OT's set resistors on fire on my repaired unit at a choir rehearsal. More heatsink Scotty, or on my unit, 2 fans.
LM3886 amp sounds okay, but 2 TO3 output per channel properly driven allow the 420 VA power transformer to give the speaker more dynamic range on classical music.
The original also frequently burned output transistors. Mine was bought that way. 20 w/ch for 3 hours did it again. Shorted OT's set resistors on fire on my repaired unit at a choir rehearsal. More heatsink Scotty, or on my unit, 2 fans.
LM3886 amp sounds okay, but 2 TO3 output per channel properly driven allow the 420 VA power transformer to give the speaker more dynamic range on classical music.
I had one in the midt seventies with Tip41/42 drivers and 2n3773 outputs and I remember it as warm sounding and with a lot of punch in the low octaves. As far as I know with no other mods.
Mine was a Stereo 120A. I dont know if that makes a difference, but I never had any problems except the wofermovement at turn on
In total agreement on the punchy bass, which I call dynamic range. 1975 2n3773 were fast enough OT's; mine was a 1970 unit. Dynaco quoted "Power Output Rating: 60 watts average continuous power per channel into 8 ohms at any frequency between 25 Hz and 15 KHz at less than 0.5% total harmonic distortion. Distortion reduces at lower power levels. This is in accordance with F.T.C rating requirements, including preconditioning."
The FTC preconditioning warm up solved the cold crossover distortion. Tricky of dynaco to quote the HD after warmup. Cold crossover distortion was obvious on my unit which usually operates at 1/8 watt unless the orchestra is playing ff (loud). Playing loudly enough long enough warmed up the unit to reduce crossover distortion, but there was that first hour. The 7 transistor djoffe idle bias control board solves the crossover distortion at all temperatures. The messy point to point build of this board, and my point to point hand build AX6 board, kept the burglar in 2020 from stealing my ST120. He took all my pro-grade Peavey gear.
One advantage of the ST120 over all other SS amps is that the 100 kohm input impedance is compatible with the dynaco PAS3 preamp. The 12AX7 output of the preamp only has only 250 kohm drive current. So 10 kohm input solid state amps are not compatible with my PAS2 preamp. I had to tweak my AX6 board to increase input impedance, which the 300 gain MPS8099 input transistor allowed easily.
The FTC preconditioning warm up solved the cold crossover distortion. Tricky of dynaco to quote the HD after warmup. Cold crossover distortion was obvious on my unit which usually operates at 1/8 watt unless the orchestra is playing ff (loud). Playing loudly enough long enough warmed up the unit to reduce crossover distortion, but there was that first hour. The 7 transistor djoffe idle bias control board solves the crossover distortion at all temperatures. The messy point to point build of this board, and my point to point hand build AX6 board, kept the burglar in 2020 from stealing my ST120. He took all my pro-grade Peavey gear.
One advantage of the ST120 over all other SS amps is that the 100 kohm input impedance is compatible with the dynaco PAS3 preamp. The 12AX7 output of the preamp only has only 250 kohm drive current. So 10 kohm input solid state amps are not compatible with my PAS2 preamp. I had to tweak my AX6 board to increase input impedance, which the 300 gain MPS8099 input transistor allowed easily.
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My Dynaco 120´s fuse blew up when switching it on. After some testing, it appeared that 2 of the first (bridge) diodes were destroyed.
After replacing all 4 of them, things started to work again, but after maybe 4 minutes, a hum appears and both end transistors of one channel gets very hot. Any suggestion of what I should check?
Thanks, robert
After replacing all 4 of them, things started to work again, but after maybe 4 minutes, a hum appears and both end transistors of one channel gets very hot. Any suggestion of what I should check?
Thanks, robert
Sounds as if idle bias current of output transistors is out of control. On PC14 of the side that gets hot, with AC power disconnected and C11 & C12 below 1 v, (power supply caps) check D1 D2 D3 with diods scale of DVM. should be 550-700 forwards, 9999 or ---- backwards. Since the kits are built by amateurs, could instead be a bad solder joint in the bias voltage stack between emitters of Q3 and Q4.