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Old 28th July 2012, 07:03 PM   #1
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Question Lenco a50 what's wrong?

Hi Guys,

I have a situation here. I got an Lenco A50. I also added the missing capacitors which were removed by someone i know. He had his reasons to remove them. Anyway questions would be:
Schematics: Lenco A50
1. Amp gets hot kind of fast. what could be wrong with it?
2. How good is this amp?
3. Is there any mods for it for more stability, less heat?
4. Is it a wise idea to replace output stage transistors from KT819GM/KT818GM to these MJ15024/MJ15025 ?

-Oz

Last edited by ozracing; 30th July 2012 at 10:40 PM. Reason: more info
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Old 28th July 2012, 08:37 PM   #2
tvi is offline tvi  Australia
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Just Uploaded the best schematic I could find here
Lenco_A50.pdf

Hope it helps

rgds
James
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Old 28th July 2012, 08:51 PM   #3
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Looks like the common blameless topology, will be really easy to get going.
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Old 29th July 2012, 09:53 AM   #4
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Quote:
Originally Posted by tvi View Post
Just Uploaded the best schematic I could find here
Lenco_A50.pdf

Hope it helps

rgds
James
Hi tvi,
yea i got this schematic, and sorry but i need better quality because cant read out a lot of values and part numbers just too blurry to me. But thanks for helping

-Oz
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Old 30th July 2012, 05:32 PM   #5
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Amp is fixed but there is jitter in right channel and amp heats up pretty quickly.
what could be causing that jitter?
Is this amp class A that it heats up so quick?

Any tips & tricks for more power and stability?

Last edited by ozracing; 30th July 2012 at 05:38 PM. Reason: forgot something
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Old 30th July 2012, 07:43 PM   #6
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Without knowing what you fixed or replaced, it would be just guessing what the problem may be. Could you describe the parts fitted and their location in the circuit so we have some clues?

It would be almost certain that if you replaced output stage transistors, the bias current would need to be reset to the specification (perhaps you can find it on the schematic). If this is high, any biased amp. will run hot, perhaps even to the point of thermal runaway. There will likely be an adjustment trimpot for this, situated normally in the VAS stage and sensing the main heatsink for thermal control.

Appropriate AB bias currents are known fairly well for common amplifier topologies but these are by no means standard as manufacturers have always had different ideas what current is best technically, subjectively or most stable in their designs.

Jitter? - Do you mean a harsh distortion or sibilance?
Tips and tricks? - Apart from general refurbishment like replacing all electrolytics, I think you will need to follow some threads about similar commercial products and draw your own conclusions. There will not be many here with experience of modifying this old design. Increasing power involves the whole amplifier; power supply, transformer, voltages, component ratings etc. Effectively, it would need redesigning and rebuilding. A new amplifier would likely be cheaper.
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Old 30th July 2012, 07:58 PM   #7
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Ok got that.
Jitter i mean sometimes when it plays music and stuff right channel becomes distorted. Not much but still it is annoying.

I added missing capacitors. They needed to be there because they was clipped/removed by someone else i know, reason was that that someone needed some caps but was too lazy to go to the parts shop.

Anyway just measured both channels. Left is biased at -0.07 and right is at -0.09 both values are constantly rising towards positive voltage and adjusting both trimpot does absolutely nothing.

Any clues what could be wrong?

Also i had an idea of replacing output stage transistors to MJ15024/MJ15025 as of current kt819gm/kt818gm. Is this idea is bad?
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Old 30th July 2012, 08:14 PM   #8
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I could easily have fixed it if shipping was cheaper.
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Old 30th July 2012, 08:18 PM   #9
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Quote:
Originally Posted by Tekko View Post
I could easily have fixed it if shipping was cheaper.
im sure you would have but i want this to experience myself electronics and stuff is gold to me so....

anyway i think you got the point
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Old 30th July 2012, 08:20 PM   #10
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Measure the voltage in millivolts across the emitter resistors of the output transistors, then use ohms law to calculate the current, should be around 50mA.
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