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
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
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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? 🙂
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? 🙂
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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.
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.
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?
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?
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 🙂
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.
i think i already did this as per values stated above. Or am i mixing something ???
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The pdf seems to be gone from the server, but I suspect the output zobel network may be faulty. It is common for the series resistor to ground (10R?) to fail open and then the amp oscillates
Interesting ill check all the resistors going to the ground.
I also edited first post to include schematics 🙂
'Mighty hard to read that schematic even with your edit! I can make out the bootstrapped capacitor VAS and a VAS buffer transistor but no conventional adjustment for Vbias. Does anyone else have a better view of this? Experience tells me that there will be all sorts of distortion if this bootstrap electrolytic cap. is faulty. Likewise the main electroytics.
Note that the Zobel (series RC circuit) to ground in each channel is at the far right side of the amplifier board sections of the schematic, fitted from the output track to ground.
Note that the Zobel (series RC circuit) to ground in each channel is at the far right side of the amplifier board sections of the schematic, fitted from the output track to ground.
As far i know this is the best quality schematics on the net but i'll try to rework the schematic from my amp piece by piece that will be a challenge but well i really like to get this thing banging some nice old music 🙂
So still the main idea to this point is to replace all electrolytics ?
P.s. I'm a little learner here so what is VAS cap and VAS buffer transistor?
i believe adjustments for Vbias are being done with potentiometers ( there is 2 of them in total) so one is for left channel other is for the right?
VAS?
Lacking the labels of the schematic, the VAS (Voltage Amplifier Stage) is actually the third transistor from the left side of the amplifier boards. It is said to be "bootstrapped" by the addition of a capacitor to the output node. The first 2 transistors are the input stage, commonly called an LTP or Long Tail Pair. This describes the differential pair which compares input signal with feedback to provide a corrected signal driving the rest of the amplifier.
There is a series of resistors connecting the VAS transistor collector to the bottom rail, shown between the amp. channels. Between the resistors, an electrolytic cap is connected also to the output rail. This is the bootstrap cap, used in many older and simple transistor designs. It gives a slighter higher distortion than newer designs using a Current Source, for example, but the particular character of the distortion can give the amplifier a pleasing sound.
I should correct what I said in the previous post, as the 4th transistor is a phase splitter, providing 180 degree phase shift in the signal to the output drivers. However, this doesn't help you much as with all amps over 20 years old, the main task is to replace any deteriorated electrolytic capacitors. If you can't test them confidently, the simple but expensive alternative is just to replace them all. Modern caps are a lot smaller, so using a higher voltage rating or even 50% more capacitance won't cause problems. Try to at least use good quality genuine parts from recognized manufacturers. 😉
P.S. Can you point to the trimpot(s)?
Lacking the labels of the schematic, the VAS (Voltage Amplifier Stage) is actually the third transistor from the left side of the amplifier boards. It is said to be "bootstrapped" by the addition of a capacitor to the output node. The first 2 transistors are the input stage, commonly called an LTP or Long Tail Pair. This describes the differential pair which compares input signal with feedback to provide a corrected signal driving the rest of the amplifier.
There is a series of resistors connecting the VAS transistor collector to the bottom rail, shown between the amp. channels. Between the resistors, an electrolytic cap is connected also to the output rail. This is the bootstrap cap, used in many older and simple transistor designs. It gives a slighter higher distortion than newer designs using a Current Source, for example, but the particular character of the distortion can give the amplifier a pleasing sound.
I should correct what I said in the previous post, as the 4th transistor is a phase splitter, providing 180 degree phase shift in the signal to the output drivers. However, this doesn't help you much as with all amps over 20 years old, the main task is to replace any deteriorated electrolytic capacitors. If you can't test them confidently, the simple but expensive alternative is just to replace them all. Modern caps are a lot smaller, so using a higher voltage rating or even 50% more capacitance won't cause problems. Try to at least use good quality genuine parts from recognized manufacturers. 😉
P.S. Can you point to the trimpot(s)?
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Ok thanks for the info. But i didn't get only one thing. Genuine capacitors
what are recognized manufacturers for them? Actually this is the first time i ever heard such a thing 🙂trimpots:
vr501 and vr502
There are thousands of on-line traders selling fake products of all kinds. Expensive, high quality electronic components are prime targets for these guys, as they sell to individuals like us who probably can't test or know they have inferior parts. There are many threads here discussing fake transistors but there are plenty of poor quality and fake capacitors too.Genuine capacitors
Genuine means the manufacturer marked on the label is the real one and the product quality is declared in a published specification. A recognized manufacturer is a big name company like Panasonic, Elna, Sprague, Nichicon etc. who is known globally to manufacture quality capacitors - just like any other manufactured goods.
The advice is simply to avoid cheap, unbranded parts and parts that are just too cheap to be genuine. If you don't know what good brands are, go to the big marketing company websites like Mouser, Farnell, Digi-key, RS components etc. and read their supplier and parts lists.
Hello ozracing,
I used to work in the Service Departement for Lenco Audio AG in Switzerland. That was from 1978 until 1983. The A-50 was one of the units I was responsible to repair. I recommend you to check all the semiconductors - including the diodes - and the resistors around them and don't forget the trimmers ! The channel that is working normal should be used as a reference. Use an old fashioned Ohm Meter for that, especially if one of the semiconductors has a leakage !! I also can send you a scanned Service Manual with an extremely good resolution (600 dpi) if you wish.....
Max
I used to work in the Service Departement for Lenco Audio AG in Switzerland. That was from 1978 until 1983. The A-50 was one of the units I was responsible to repair. I recommend you to check all the semiconductors - including the diodes - and the resistors around them and don't forget the trimmers ! The channel that is working normal should be used as a reference. Use an old fashioned Ohm Meter for that, especially if one of the semiconductors has a leakage !! I also can send you a scanned Service Manual with an extremely good resolution (600 dpi) if you wish.....
Max
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