Long story made short- I got this amp for about the price of shipping (essentially free). It did not include 845 tubes, but I had a pair of ShooGoo 845M's (metal plates) laying around. These tubes were a freebie too- jacked up by the previous owner; no doubt they were cooked on too high a voltage, but nothing that would indicate they would not perform. I plugged them in and fired up the amp. It played beautifully for approximately 5 minutes, and then a small bit of smoke was visible near the 845's. They didn't glow red, or seem unusually hot (no more than a standard 845 scenario). I cracked open the bottom cover and can find nothing burnt, or bad smelling. I haven't prodded around to check anything, but no obvious signs of BBQ could be found.
The amp is a "SOUNDTRACK ST-845" as sold on eBay out of Hong Kong. I had a pair of MingDa 845's that, while much better built, had a course grainy treble that would not go away. This soundtrack amp showed a lot of promise in its 5 minutes of glory. Barring the obvious need for a new pair of carbon plate 845 tubes. How do I fix it? What do I do? How could I improve it? I have no schematic for it, so if anyone has one- it would be a great help.
The amp is a "SOUNDTRACK ST-845" as sold on eBay out of Hong Kong. I had a pair of MingDa 845's that, while much better built, had a course grainy treble that would not go away. This soundtrack amp showed a lot of promise in its 5 minutes of glory. Barring the obvious need for a new pair of carbon plate 845 tubes. How do I fix it? What do I do? How could I improve it? I have no schematic for it, so if anyone has one- it would be a great help.
Soundtrack..
I looked in Ebay but nothing as you described. Do you have a schematic for this??? Some of the Chinese stuff comes & goes very quickly.leaving customers without recourse if they have problems. Someone here quite probably has one or has heard of your amp.....just a matter of time really, hang on.
Chinese gear is a good jumping off point to rebuild/modify/improve.
___________________________________________________________Rick...
I looked in Ebay but nothing as you described. Do you have a schematic for this??? Some of the Chinese stuff comes & goes very quickly.leaving customers without recourse if they have problems. Someone here quite probably has one or has heard of your amp.....just a matter of time really, hang on.
Chinese gear is a good jumping off point to rebuild/modify/improve.
___________________________________________________________Rick...
Grab your DMM,and start going over the resistors to make sure they match the value marked on them within a fair degree(10-20%),check any diodes,and perhaps give the caps a resistance check to make sure none are shorted/leaky -you'll likely have to lift one leg of each cap to effectively remove it from the circuit,otherwise you're just reading the circuit resistances,etc. The caps should read open-circuit,or at least several Megohms,very high resistance.
If all of that checks out,pop in some new tubes,and keep an eye on things for a while,just to make sure nothing funny happens.
I've had lots of funny smells and stuff come off of old tubes before,as they heat up the oil from your fingers,dirt,dust,etc. tends to burn off. I'm not sure I've ever seen any actually smoke,but I've never worked with 845's before,I assume these run pretty hot.
OK, will do. I'll report back when finished...might take a while.
Yes, 845's are like the light bulbs on airfield landing strips; they can really light up a dark room and have the potential to get blazing hot. I was actually quite scared of my first one!
845
Sorry didn't notice you didn't have a schematic. Things I did notice however.......perhaps the Chinese are stingy in the chassis department, but that chassis is WAY tight! The TXs' are potted of course so close proximity is sorta OK in that regard but the clearances of the 845s to that central TX is way too close....could the smoke you saw have originated by the paint burning off of that TX??? The corner??
Morgan Jones says, one and a half envelope widths is a good spacing, tube to tube.
This chassis throws out this spacing suggestion.
Just an idea...
__________________________________________________________Rick.....
Sorry didn't notice you didn't have a schematic. Things I did notice however.......perhaps the Chinese are stingy in the chassis department, but that chassis is WAY tight! The TXs' are potted of course so close proximity is sorta OK in that regard but the clearances of the 845s to that central TX is way too close....could the smoke you saw have originated by the paint burning off of that TX??? The corner??
Morgan Jones says, one and a half envelope widths is a good spacing, tube to tube.
This chassis throws out this spacing suggestion.
Just an idea...
__________________________________________________________Rick.....
You should see under the hood! It's a layered bird's nest of corrective leads running back and forth because the layout is so lame.
weird thing is the "central TX" is actually one of the output TX's. The power TX is one of the rear ones. Think I should switch it so both output TX's are on the back?
Maybe- never thought about that....
Cool. I know it's a bit of a pain,but sometimes components go poof,or other odd things happen that aren't readily visible..(FE,I've had resistors go open that looked fine,not charred or anything) So at least we're sure that's all OK.
Maybe just dirt/dust,or like Richard said,paint,burning off.
Maybe just dirt/dust,or like Richard said,paint,burning off.
It's up and running (and sounds fantastic!) despite the odds. The problem was in the factory wiring- they put the power transformer secondaries in the wrong places. It took me forever to figure it out. I had to test the power transformer to see what was up, and the 5V was in the wrong spot. I fixed it and now she sings. These are decent amps, and they have HUGE output transformers.
Only one pic-
This is the only pic (excuse the poor quality!) I took when I had 'em cracked open. This is the power transformer with the leads loose. I had to check the voltages of them, and I ended up swapping the position of the power and one output transformer (they are the same size!). The inside looks rough in this pic- quite the bird's nest. I later twisted all the filament, ac, and signal leads and tidied it up a bit. It looks better. I plan to upgrade the power supply and coupling caps in the near future, so I'll take more pics then.
This is the only pic (excuse the poor quality!) I took when I had 'em cracked open. This is the power transformer with the leads loose. I had to check the voltages of them, and I ended up swapping the position of the power and one output transformer (they are the same size!). The inside looks rough in this pic- quite the bird's nest. I later twisted all the filament, ac, and signal leads and tidied it up a bit. It looks better. I plan to upgrade the power supply and coupling caps in the near future, so I'll take more pics then.
An externally hosted image should be here but it was not working when we last tested it.
Wow, when you said they were cramped I did not quite appreciate just how cramped they actually are..
Quiet fans to remove heat generated by the output tubes somewhere near the amps seem like a wise investment.
Well, the heat of 845's will be a welcome benefit this winter
...and I don't expect to have these by next spring. This is my last foray into 845's (two prior). I have developed a love/hate relationship with these tubes. I have no doubt that a properly built power supply would yield a much better performing amp, but I just can't stand the abnormally inflated soundstage (vocalists sound like they have 4' wide mouths), and the predisposition for sibilance. Plus, they are way overpowered for my application (Tannoy dual concentrics at low/moderate volumes). I can't get 845's to behave properly. I'm gonna try the RH84 before going back to good old 2a3's.- Status
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