Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: bigsbybender on November 24, 2009, 09:57:45 pm
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First
Does anybody have a Schematic for the Trace Elliot AH150-7 Bass AMP? OR any schematic for the AH-xxx range? Google has not been forthcoming with anything but a useless owners manual.
This one has been on the shelf a while since I am pretty reluctant to dig into non-obvious repairs on solid state stuff.
It blows a fuse as soon as it is turned on. So I hooked up the current limiter and tried it, I get a very bright bulb. The rectifier, power supply and Power amp section are on one board, so I disconnected the leads that run to the separate preamp board to check to see if the issue was there...and it wasn't.
The next step, I disconnected the leads from the Transformer from the board to see if the transformer was the trouble, no bright light and each secondary lead gave a plausible voltage for this amp. It acts like a dead short, but I've even lifted the power supply/power amp PC board from ground and the current limiter still hits full brightness. :sad:
Next I checked the Bridge Rectifier deal... It checked OK, Blocking current where it should and passing it where it should. Then I went to the filter caps, they check OK, as they charge and hold...and after that point I don't know where to go....there a too many components and traces lumped together for me to make sense of.
I'm trying to trace it out without a proper schematic or diagram and having no luck.
If anyone could help, it'd be greatly appreciated.
Thanks,
j.
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http://www.el34world.com/Forum/index.php?topic=8085.msg73041#msg73041
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Wow I should have read through this other topic earlier. Thanks PRR, no sense in repeating yourselves.
Applying this to mine..
Looks like 2 of the power transistors, TIP35C and TIP36C are internally shorted.
I removed them, and the light bulb limiter is very dim now....On the right track I presume.
Although I generally agree with building tube amps into dead SS gear.... This amp suits me well as I am playing bass pretty regularly now. It's just as loud as my tube bass amp that I built a few years back, but a fraction of the weight and the tone suits me well too. I certainly would hate to throw it out for a few dollars in parts. (If this were for a customer, I'd have told them that I wouldn't fix it.)
Well off to scrounge for parts....
THanks again to the posters from the Carvin Thread.
j.
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It works! I just had to wait for the parts. The Thanksgiving holiday slowed up my order, but Mouser came through relatively quickly.
I ended up just following the other solid state Bass Amp thread. I just replaced the TIP35C and TIP36C. The TIP31C and TIP32C seemingly read OK by the ohmmeter, but the TIP31C did give a reading that seemed a little odd so I replaced both the 31C and 32C as cheap ($0.80 per) insurance. I went and checked all of the components directly connected to the Transistors...and everything seemed ok, even the resistors were in tolerance...which I was told by a very experienced local electronics repairman that the "Drain" resistors usually die with the Transistor, this wasn't the case. I went into his shop looking for the Transistors but he didn't have all of them on hand. If I get more confident with Solid State, i may have to steal some of his business. :wink:
After soldering in the new Transistors, I turned the amp on using the current limiter....dim bulb. So I played it for a while on the limiter, after about 10 minutes I had enough confidence to plug it in straight to the wall. I played it again for a half hour. After that I totally reassembled the head, played it again and called it repaired.
In case it doesn't last, I bought 2 of each of the transistors...and didn't break the bank.
Thanks to everyone who helped on the other Carvin thread....
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Congratulations!
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In case it doesn't last, I bought 2 of each of the transistors...and didn't break the bank.
AHHHHH well it didn't last..... :rolleyes:
It died again during a gig. Luckily the place had good enough monitors that I plugged into a DI and ran over the PA.
It would seem to me that there must be even smaller transistors downstream that are bad...... right track?
I essentially replaced the four largest, which read open with the DMM. My guess is that the culprit would be small because when I pull the dead power transistors, then fire it up, the current limiter is dim... so a short there wouldn't draw any appreciable current.
I never did have a schem for this. Peavey appears to be handling Trace in the US, so I emailed their customer service dept a few minutes ago. At this point I really don't know what connects to what after a certain point on the PCB.
How plausible would it be that the little 3-legger transistors would take out the big ones?
Thanks in advance for any insights...
j.
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The possibilities are almost endless.
If it was on my bench, I could ad-lib a series of tests and flounder toward a diagnosis.
The fact that it worked for months and then quit is very distressing. It means the failure may not be simple, nor easily found.
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The possibilities are almost endless.
The fact that it worked for months and then quit is very distressing. It means the failure may not be simple, nor easily found.
Agreed. Could almost be anything. Anything from a funny power rail, to parts on the edge, DC coupled parts interacting strangely.... anything really.
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A bit late in the day, but.. The reason the AH150 cooks its output transistors is instability and this is caused by a really cheap and nasty earth connection via one of the PCB mounting posts. If a shakeproof washer isn't used on both top and bottom of the board to ensure a solid connection, it can easily work loose, cause the board to go unstable and fry the output transistors and the drivers. The 'fix' is to solder a solder tag with the right diameter to the connection pad that the mounting post sits on and run a seperate bonding wire to the earth chassis connecting point just above the mains switch.
Then bolt the whole lot down tight onto the mounting posts by shakeproof washers and solid screws. If it has blown its self,
it's recommended that you replace all the TIP drivers and make sure they all come from the same supplier. Also replace
the quiescent current transistor that sits on the heatsink beneath heatsink compound. Make sure that it is |COMPLETELY covered in heatsink compound, otherwise the fan may cool it down and cause the amp to run very hot. Finally, replace the
TIP35/36s with 2SC4468 and 2SA1695. These are the power transistors used in the 300 watt version and are much more reliable.
Note that all these fixes/upgrades applies only to the Bipolar Bear bipolar power amp. The later MOSFET version has its own issues.
Hope this helps
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Cool Thanks!
I ended up rebuilding the board back in June and have been gigging the amp pretty steadily since then. I just took it back to original spec, and replaced the filter caps too while I was at it.
I'll certainly do the mods for a better ground to the PCB... anything to make this amp more reliable! I really do like the sound of this amp.
j.