Donor stereo 845 amp- now what!?

[blackeye*_0]

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.

[Richard Ellis]

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...

[DigitalJunkie]

Originally Posted by blackeye*_0

I have no schematic for it, so if anyone has one- it would be a great help.

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.

[blackeye*_0]

Originally Posted by Richard Ellis

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...

Hi! It's like this one.

[blackeye*_0]

Originally Posted by DigitalJunkie

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!

[picowallspeaker]

Hello
A bad Output Trasformer ? So both , badly constructed ?
Did you get the demo CD

[Richard Ellis]

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.....

[blackeye*_0]

Originally Posted by Richard Ellis

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!

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.

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....

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?

could the smoke you saw have originated by the paint burning off of that TX??? The corner??
Just an idea...
__________________________________________________ ________Rick.....

Maybe- never thought about that....

[blackeye*_0]

Ok, finished checking everything I could. My eyes are ready to pop out- very tedious. Everything seems fine. I wished I would have had some better caps for the power supply while I was at it...

I have a new set of 845's (carbon plates) on the way....

[DigitalJunkie]

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.