Tube Amps / Music Electronics
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|8/28/2004 10:27 AM|
||Carlsbro GLX 80 (Special Edition) schem, please!|
I need a schem for the Carlsbro GLX 80 guitar amplifier. I tried to contact Carlsbro with no success (on vacation?) and I really need it as soon as possible.
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|8/28/2004 5:23 PM|
In this amp there are 3 boards. The power amp / power supply board, then two more, one is the preamp, and at the right side looking from the front of the amp there is another board for the reverb, the two channel master pots and the pots for mixing the effex.
But this board receives the power from the power supply/power amp board via a conector, then via another ribbon it feeds the preamp board. The problem is that in the power supply board there are the usual two big resistors followed by the zeners (36volt zeners in this amp)for the + and - rails, that go hot to the point of melting the solder and ruining the board around these components, and voltage readings are +9 and -9 volts. If I disconect the ribbon that goes from the ps board to the second board, the readings are the spected +36 and -36, no hot, so there's a short in the second board (I can disconnect the preamp board, the third one that receives the power from the suspicious second board, and the problem is still there). I have no time to do nothing at all because it only takes a two or three seconds to beging to smoke from the mentioned point in the ps board so I can't check things properly.
To make things worse I dont have a schem, but I have a question; right in the front end of this board, where the power ribbon is connected, there are a couple of 18 volt voltage regulators (MC7818CT and MC7918CT): can these voltage regulators get shorted?
Thanks for any help or suggestions.
|8/28/2004 9:37 PM|
Hi JC. Have you checked the power transistors yet? It sounds like they are burnt up and shorted. The only safe way to run it in this state is with a ajustable voltage like a variac. Then you slowly turn the voltage up a little at a time and never go over 1.5 amps, hold it there for a few secounds and turn it off. Then feel around on the board and see what is hot and it maybe your short. Hope this helps some. Lee
|8/29/2004 6:56 AM|
Lee but the power amp and power supply share the same board, and the main problem doesn't seem to be there. If I disconect the other boards the heat problem seems to dissapear... Thanks for the variac tip I may use it as a last resource. The power transistors may well be bad too, but there must be a big short somewhere else I believe.
|8/29/2004 9:34 AM|
But the power transistors are not or should,nt be on the PS board. Check them and see if one or both are not shorted.
|8/29/2004 6:38 PM|
The power transistors are in fact mounted on the same board as the power supply (filter capacitors, bridge rectifier etc), and powering this board alone doesn't show any problem.
I have checked them anyway, and they are good.
It surprised me that they are both npn type. Aren't they suposed to be one npn and the other pnp? I guess that they work in parallel in this amp...
|8/29/2004 5:53 PM|
||Re: Carlsbro GLX 80 (Special Edition) schem, please!|
I think if you can disconnect a board and the rails come back up, that board must be loading down the rails. I would be using the variac myself though, looking for excess draw.
But I would indeed explore the power xstrs first, like the above suggestion.
On the suspect board, are there additional zeners or three leg regulators? Follow the rail traces, are there any local filter caps to short.
The fact that both rails load the same leads me to suspect an op amp or something is shorted. Visually inspect - closely - all the chips on the bad board. If any look cracked or discolored from heat, rremove them and see if it restores the rails. If there are any bypass caps at each chip or just across the rails, small things, look them over. Any with a small black divot missing are bad. Normally I would ask you to touch each chip to see if any get hot, but they may not heat up in your few seconds. Worth a try though.
If you have a short across the rails, the schemo may not help much.
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