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Old 25th November 2005, 06:38 PM   #1
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Default Laney PA amp help!

Hey again guy's, been workign on my laney PA amp and have a problem...

Basically there was a resistor burnt up inside it when i got it, it feeds the preamp tubes. the resistor is next to another resistor of a diffrent value which also feeds power to diffrent pins on the preamp tubes.

I got in touch with someone that has the same amp and he had a look inside his and came back that there should be one resistor that is either 47K or 4K7 (he couldnt make out if one band was orange or red) and the other should be 22K or 2K2.

The one that i have in my amp that isnt burned out is 47K so i guess that the other one should be a 22K and i replace it with a 22K 2W resistor which then burns out, replace it with another 22K 2W resistor this time connecting my multimeter in series with it and find out that with no valves installed the amp draws 300mA (too much for a 22K resistor me thinks)

I cant find any faults in the circuit so im guessing that the value of the resistor is wrong?? or am i missing something?? i can try and draw a schematic if it helps and if someone can direct me to a site where i can get a free schematic cad type package from..

many thanks,
Owen
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Old 25th November 2005, 07:15 PM   #2
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ok, done a quick schematic in paint, hope it makes sense, its for ONE preamp valve, there are 6 in total. the valve is an ECC83 and this piccie is from the solder side of the socket not the valve side. i have marked the resistors with their colour codes and the resistor that i think should be a 22k i marked 22k??
sorry for the rubbish quality.
Thanks,
Owen
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Old 26th November 2005, 10:16 AM   #3
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bump!
hope the schematic is right, anyone got any ideas if 22K is right for that resistor? does it look right? :S
Owen
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Old 26th November 2005, 12:54 PM   #4
Giaime is offline Giaime  Italy
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You probably have some shorted cap to ground, that's the cause of high current drawing from the B+.

Carefully check ALL the caps and ALL the resistors for shorted ones, drifted resistors, bad solder joints etc etc.

Also check your preamp tubes (but I haven't seen a shorted preamp tube yet).

Hope this helps!
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Old 26th November 2005, 01:14 PM   #5
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What model is the amp?.

I've got 244 Laney PDF manuals sat on my hard drive!.
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Old 26th November 2005, 07:01 PM   #6
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Hey,
The model is LA100PA Super.... it's a 100Watt 6Channell PA head.

The 300 mA current was without ANY valves installed ( removed them to check incase one of the valves had shorted). How do i test the capacitor? there is one that goes from the resistor to ground that i think if any are faulty it is probably that one, would it be good enough to just dissconect it and measure the current again?

There are also quite a lot of resistors that COULD be faulty but they all LOOK good (havent measured them yet) but i will measure them to see if anythign is going wrong on any of them.

Reason i didnt check the capacitor is that for the few short moments the amp was living it sounded totally awsome! so humm's or hiss's atall.. and then the volume just faded and there was a smell of burning and i turned it off and had a look and the resistor had burnt out again.

Have resoldered all the solder joints already as there were a couple that looked bad but that didnt seem to help.

Will see what the values of the capacitors are and replace all the power supply capacitors i guess as they are probably due to be renewed on such an old amp.

Someone has been in the amp and done repair work in the past before i got it aswell as one of the power tube sockets have been replaced and the power socket has been replaced with a piece of cheap electrical wire with a plug on the end ( which will be returned to standard when i get home again)
Thanks for your help,
Owen
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Old 26th November 2005, 07:52 PM   #7
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I haven't got a PDF of that one, but I do have a JPG, and I've attached it. It sounds like one of the decoupling capacitors is S/C (or at least VERY leaky), you should be able to find it with your meter on the ohms range. In the JPG the suspect ones are shown as the two 16uF either side of the 10K.

Oops - JPG too large, try it at My Website
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Old 27th November 2005, 02:33 PM   #8
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that schematic looks slightly diffrent to my amp but i know the two capacitors you mean, they are on the power supply section of the circuit but on mine the 10K is a 47K and it is connected to the other side of the 22K resistor (which is the one that keeps burning out and the caspacitors in question go from the 22k to ground and the 47k to ground. I will check it when i get home anyway and check and see if the capacitors are shorted. if they appear shorted on my meter would it be acceptable to dissconect them to remeasure the current draw or would it damage other components if i did this?
many thanks,
Owen
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Old 27th November 2005, 07:00 PM   #9
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Yes, you can disconnect them to measure the current, valves are EXTREMELY robust devices (at least electrically!), so it won't do any harm - most likely effect is excessive mains hum, with a possibility of instability?.
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Old 27th November 2005, 09:37 PM   #10
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ok, i tested the two capacitors (15 uF 400V in my amp) and tested them with my meter and sure enough the one conected to the resistor that kept burning out was short circuited... COMPLETELY!..
Removed the capacitor and the amp works well.. slightly unstable at low frequencies and a little mains hiss but the resistor isnt burning out anymore.. am going to look on ebay for some replacement capacitors and replace the two 15uF capacitors and see what it is like then and mabey get some new power tubes for it as it only has 2 in there at the moment as one was broken when i got the amp (had a crack in it )
thanks for all your help,
Owen
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