Marvel Posted December 28, 2004 Share Posted December 28, 2004 This is one of those philosophical things. How much can you replace before it isn't what the name says? I have had an ST-70, since the early '70s, but it really needed some work. I have replaced the power supply and the driver board. I don't really consider this a Dynaco anymore, since all the main circuitry is different. But, the power transformer was running really hot (thanks to Mark for helping me through some of the diagnostics). Secondaries were running very high. Putting it on the variac, I cranked down the ouputs to where they were the correct values (360-360, etc.). Then I measured the primary and it was sitting right on 110 volts. Hmmmm, it's supposed to be a 117V tranny, but who knows, this was one of the later versions where the transformers were made in Japan. I ordered a new power tranny from Ned, and it arrived today. His ads say it has a 120 v primary, which would fit where I live just fine. Supposed to run cooler at full load as well. This thing is HUGE! The laminations are stacked 2 3/8" compared to the original's 1 3/8". The other physical dimensions are the same so that you can drop it into place. (carefully). I'll get some pictures of the two together. I won't have time to swap it out until later in the week. While I was at it, I went ahead and got the replacement choke. So it won't be a Dynaco at all when I am done, other than the chassis and the output transformers. I guess that a new chassis may not be too far behind. Whew! The ST-70 chassis doesn't give you a lot of room to work in. Marvel Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 29, 2004 Share Posted December 29, 2004 The Dynaco ST-70 is a old vintage design. So it ain't a original Dynaco anymore. But I bet it's starting to sound better than it ever did. It's a "Marvelco ST-70" now. Quote Link to comment Share on other sites More sharing options...
edwinr Posted December 29, 2004 Share Posted December 29, 2004 ---------------- On 12/28/2004 7:46:47 PM Marvel wrote: So it won't be a Dynaco at all when I am done, other than the chassis and the output transformers. ---------------- But, Marvel, you'll still love it just the same. Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 29, 2004 Author Share Posted December 29, 2004 I didn't care that it wasn't going to be a "Dynaco". It does sound great, or did until one of the old EL34s bit the dust this week. A new set is on order, and gives me time to swap out the transformer and choke. Some people get all bent out of shape with having it still look like an original. Doesn't really matter to me, since the circuit is no longer a Hafler design. Such a sweet sound. The EL34s are plenty of power for my listening levels, and not overkill. I am tempted to build one from scratch the next go 'round. I almost did that instead of fixing this first, but knew it would be good to follow something I basically understood first. Maybe I'll rip apart the little Knight amp I have... Marvel Quote Link to comment Share on other sites More sharing options...
elcapitan83 Posted December 29, 2004 Share Posted December 29, 2004 ! you have a knight! which model? i did a recap on a knight kn-740, stereo integrated amp with 4 6l6gcs, push-pull cathode biased, with 5 12ax7s in the preamp. that little amp sings! granted, i may have increased the capacitance too much (doubled it), i get arcing randomly in the 5u4gb. how do i get rid of arcing? my power tranny also runs hot, line voltage here is 125v! i am looking to get a variac, i suppose Quote Link to comment Share on other sites More sharing options...
D-MAN Posted December 29, 2004 Share Posted December 29, 2004 Reminds me of the old adage, which I think is true: Mutts make the best pets. DM Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 29, 2004 Author Share Posted December 29, 2004 It's a mutt alright. And a beautiful one at that. elcapitan, I can't recall off the top of my head. This is a little one, with EL84 outputs, and an EZ81 rectifier. The new tubes I bought from NOSValves have such skinny pins that they are loose in the sockets. I have tried working on the sockets, but they probably just need to be replaced. When all is working, it has a nice sound, although not much power. I haven't been able to find a schematic for it anywhere, and one of the caps is a small ceramic with four leads on it. I think it's on the tone controls somewhere, but it has been a while since I have had it out. It was a freebie, so I am not too concerned. The transformers are small on this guy, but it probably only does about 12-14 watts a side. Marvel Quote Link to comment Share on other sites More sharing options...
D-MAN Posted December 29, 2004 Share Posted December 29, 2004 ---------------- On 12/29/2004 3:35:05 PM Marvel wrote: It's a mutt alright. And a beautiful one at that. elcapitan, I can't recall off the top of my head. This is a little one, with EL84 outputs, and an EZ81 rectifier. The new tubes I bought from NOSValves have such skinny pins that they are loose in the sockets. I have tried working on the sockets, but they probably just need to be replaced. When all is working, it has a nice sound, although not much power. I haven't been able to find a schematic for it anywhere, and one of the caps is a small ceramic with four leads on it. I think it's on the tone controls somewhere, but it has been a while since I have had it out. It was a freebie, so I am not too concerned. The transformers are small on this guy, but it probably only does about 12-14 watts a side. Marvel ---------------- It sounds like it's fun. On the socket thing, I would try to gently "bend out" the pins on the tubes rather than mess with the sockets. But that's your call as we can't see it, of course. So how about some pics? DM Quote Link to comment Share on other sites More sharing options...
Colin Posted December 29, 2004 Share Posted December 29, 2004 mutts have great personalities! pics? Quote Link to comment Share on other sites More sharing options...
D-MAN Posted December 29, 2004 Share Posted December 29, 2004 I too prefer mutts, but perhaps I should have called this a "Hybrid" rather than a mutt! Sorry, the connotation of "mutt" is a happy one for me. No offense intended. DM Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 29, 2004 Author Share Posted December 29, 2004 This is a couple shots of the Knight 928 mutt. He's not barking a lot right now. The multi part ceramic caps are seen mounted on the bass tone pot. I'll have to get some pics of the ST-70. I have some before I started doing any of the work, but they aren't the best. Quote Link to comment Share on other sites More sharing options...
3dzapper Posted December 29, 2004 Share Posted December 29, 2004 Marvel, Are you talking about those flat orange things? If so they are resistor/capacitor networks. A lot of amps use them for RIAA equalization. If not, nevermind. Rick Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 29, 2004 Share Posted December 29, 2004 Those are those PEC's. "Printed electronic circuit". I hate them. If they are the Centralab branded PEC, you can figure out the values from this website. http://www.peacockcollectibles.com/radios2/radios3/pec.htm I had found it, but the PEC I wanted to know values for was a RCA brand for volume control loudness compensation or something like that. The RCA numbers do not match to Centralab, bummer.... That little Knight amp would probably sound pretty sweet. Trace out the circuit and draw a schematic. That Knight 6BQ5 PP circuit is probably point to point wired and simple at that. I bet the 6CA4 PS doesn't even have a choke. CRC or something... If I can do it, you can do it Marvel. I'm just a dumb 40 year old. Starting from the right and going to the left, is kind of a rule of thumb. I'd rather chase that Knight amp out, than a Lowery 6550 PP mono organ amplifier. With a 6V6 screen regulator pass tube and most the passive parts affixed to BOTH sides of a strip board. I almost got it traced out however. I don't like spending money on schematics for something like a vacuum tube amplifier. Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 30, 2004 Author Share Posted December 30, 2004 Thanks Mike. I started to trace it out but just hadn't taken the time to finish it up. I'll get back to it over the winter as soon as the Dynaco is cookin' along well. Yeah, it may be a part of the RIAA circuit, but they are hanging on the bass pot. The phono secion is all the way to the right in the picture looking at the back, so that would make sense. Thanks for the link. I'll get some pics of the Dynaco power trannies in a few days. The new one from Triode is heavy! Marvel Quote Link to comment Share on other sites More sharing options...
fini Posted December 30, 2004 Share Posted December 30, 2004 Marvel, Did you build the ST-70 originally, as a kit? It'll be fun when I finally dig into these Mark IVs I need to rebuild. Thank goodness for this forum (and Clipped & Shorn). Those orange-colored parts look like the kind of Sees candy (without the chocolate) that come two-to-a-paper-cup in those boxes of Nuts & Chews. Mmmm.... As you can see, I will need help staying on course (or "coarse" if Craig helps). Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 30, 2004 Author Share Posted December 30, 2004 fini, I wish I have built the ST-70 originally, but got it from a friend who had built it. I bought the ST-70 and an ST-120 (solid state) for about $100. Sold off the ST-120 in the late '70s. I'm certainly not afraid to work on these as I worked for a while repairing 3 phase motor controllers. It's neat to get things soldered up and have it actually work! I've got an old Heathkit bench supply, that I'm going to use to get a preamp built. Our college was throwing it out. It has filiment and B+ outs on it, and you can set curent limiting on the B+. I will say this -- whatever Craig charges to rebuild these old amps is worth it. It is a pain to do it and he does a very neat job. My Dynaco is a bit messy, but I've got more to do on it. I'm not particularly happy with the Curcio power supply module. I think the SDS labs ones that Triode was selling look a lot better, and have perhaps a bit more thought put into them for the layout. There is someone else on ebay selling PS modules for the Mark IIIs and ST-70s (HK I think), and they look very nicely made. Marvel Quote Link to comment Share on other sites More sharing options...
elcapitan83 Posted December 30, 2004 Share Posted December 30, 2004 hey marvel, check the sams photo facts page for a schem, they might have one on file. if not, i got a scan of a sams photofact schem for my Knight from a guy in British Columbia. I have his email, if you are interested. It cost me $5, and has been invaluable. Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 30, 2004 Author Share Posted December 30, 2004 elcapitan, Please send me his email. I have hunted for months (of and on), but not found anything. I might go for $5. I am more like Mike and don't ever pay for a schematic, but when they have oddball parts in them... Marvel Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 30, 2004 Share Posted December 30, 2004 "I've got an old Heathkit bench supply, that I'm going to use to get a preamp built. Our college was throwing it out. It has filiment and B+ outs on it, and you can set curent limiting on the B+." That bench supply will help you breadboard circuits without a power supply. If you really wanna get that deep. My bench supply is noisy. I need to fix it. Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 30, 2004 Author Share Posted December 30, 2004 The bench supply is a model IP-17, the B+ will do 0-400V @ 100ma, C at 0-100v @ 1ma, 6.3v at 4 amps. Can help out for sure. Quote Link to comment Share on other sites More sharing options...
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