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Kasino U100-P repair - HELP!

Started by LJN, December 09, 2014, 07:02:40 PM

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J M Fahey

Agree.
But no problem, it's quite repaireable.

Just a little homework:

1) check that the power amp schematic actually matches what DrGonz posted.

2) sometimes the electrical schematic is the same, but parts designations, as silkscreened on the PCB do not match (say schematic Q103 is screened Q47, etc.) ; in that case relabel parts so they match your PCB , to avoid confusion.

Although it's highly probable they will match.

LJN

Thanks ,Juan. But there's two problems.  I don't have a schematic for this amp, and I can't download a PDF file. I really appreciate all your help.
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL

J M Fahey

Quote from: LJN on December 13, 2014, 06:48:30 PM
Thanks ,Juan. But there's two problems.  I don't have a schematic for this amp, and I can't download a PDF file. I really appreciate all your help.

Then you have NO problems (lucky guy !!  :lmao:  )  because here it is:

just print it as large as possible , grab good light and a pencil to check how well it matches your actual PCB  and if necessary rename parts in schematic with PCB labels.


LJN

Thank you very much, Juan. I just downloaded it. Not sure how closely it matches mine, but it does have all the same jacks,  so I think that may be a good sign. Much appreciated.  :dbtu:
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL

J M Fahey

The cherry on top of the cake: the official Kustom amp testing procedure.

Notice the signal generator attenuation to easily get instrument level signals, the series 100k at the input to better simulate a real world pickup (driving guitar amps from very low impedance generators, typically 600 ohms or less often leads to inaccurate results) , .... and he also implicitly acknowledges that he cheats a little with rated power.

"100W" amps are around 65W RMS ; maybe 70W on a good day, to reach 100W he has toallow quite a lot of clipping.

Oh well, typically "100W Twins" were around 85W RMS at clipping start and so on so way back then that was the name of the game.


LJN

Thanks again, Juan. I don't have any testing equipment, or any idea how to use any.
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL

Roly

Quote from: LJNI don't have any testing equipment, or any idea how to use any.

Which are problems for somebody intending to work on amplifiers.  But they can be overcome.

Without a multimeter you are flying blind.  It doesn't have to be expensive, quite reasonable (yellow) Digital MultiMeters, DMM's, can be had for under $10, but you've really gotta have something.

The only other thing you really need in the way of instruments (c/f tools) is a signal source.  You can use your guitar, but you'll find it a lot easier to be able to play test tracks from your MP3 player, radio, laptop, or whatever.  You can just use music, but you can also download test sound files, or record your own using freeware like Audacity.

There are also freeware "instruments" that will turn your (old/spare) 'puter or laptop sound card into a test oscillator, an AC millivoltmeter, an oscilloscope, or a spectrum analyser.  These are very limited in their capacity and application, but they can still be useful if you have no other such instruments.

An instrument you can busk which is the "poor mans CRO" is a signal tracer, and modding an old unwanted amplified computer speaker is a great starting point.

One of the things you build up over time is a rack full of test leads, leads with different ends, every type of connector you can think of to the connectors in common use in your workshop, 1/4-inch, minijacks, RCA, insulated croc clips, etc, etc, made up as needed and archived on the rack.

You can learn to use whatever instruments you can acquire or build by presenting repair problems to a forum like this and be instructed on how to apply what you've got to get the results you need.  {a lot of the time when you're a tech it's about improvising with whatever is to hand to do the diagnostics and repair.}
If you say theory and practice don't agree you haven't applied enough theory.

LJN

thanks,Roly. I actually do have one of those cheap yellow multimeters. It's really all I have. I don't really see it as a piece of testing equipment.
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL

Roly

Quote from: LJNI actually do have one of those cheap yellow multimeters. It's really all I have. I don't really see it as a piece of testing equipment.

Aww, that's a bit cruel, it beats having to stick your tongue on it.   :lmao:

For the moment we are only concerned with restoring the DC conditions, so even the most basic multimeter will serve.  We will tell you what measurements to make.


I think we need to bring a bit of formal order to this faultfinding.  Questions pending, your homework;

Quote from: J M FaheyLook for PCB labels and post a couple pictures

In particular, is the power amp board marked "PC900"?

Quote from: J M Fahey1) check that the power amp schematic actually matches what DrGonz posted.

To confirm that we are all on the same circuit page.

Quote from: DrGonz78Are you running this amp on a light bulb limiter when you are testing it?

This is a "must" - see Limiting Lamp.

Also disconnect the loudspeaker and leave it disconnected for the time being.

This circuit includes the locations of the transistor on the PCB (on p2);

https://docs.google.com/a/olenik.com/file/d/0B1FxxpOmGO8vZXZWQV9rOTBpNm8/preview?pli=1&rm=demo

I don't think this is a germanium-based amplifier but silicon-based, which will hopefully make repair a bit easier.

The mystery diode 1N3754 is a silicon diode and marked on the circuit as CR902.  Can you please confirm (or otherwise) that the board overprint is this number?

In any case, install a temporary wire link where this diode was fitted.

Power up the amp via your limiting lamp (say around 40W or less).  Does the lamp settle to a dim light, or seem to be full brightness?  (dim is good, bright suggests there is a short somewhere)

Measure the three following voltages;

The main +ve supply.  Should be a red wire coming from the power supply.  It may be as high as +40V, but could be anything if there is a fault.

Ditto for the main -ve supply, green wire.  May be up to -40V, but as above.

The voltage on the half-rail, that is the output to the speaker, blue.  This should be about half way between the +ve and -ve supply, that is very close to ground, only tens of millivolts, but again could be anything from full +ve to full -ve under fault conditions.

Post results.

These measurements should give us a much better idea of what we are dealing with.  And don't fret pet, most of us have been servicing amps without the benefit of a circuit since Noah called to have the stereo on the Ark fixed, and we've walked people through to happy conclusions many times on this site.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

LJN

I appreciate all the help. It's been a while since I looked inside the amp, but none of the parts are labeled on the board. I do remember seeing the PC900 Mark, though. I tried several diodes, but each one caused the amp to sound distorted and the board got very hot.
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL

Roly

Quote from: LJNI tried several diodes, but each one caused the amp to sound distorted and the board got very hot.


Uh huh.

Now when you run out of hair to pull out, try it my way;


Quote from: Rolyinstall a temporary wire link where this diode was fitted.

Power up the amp via your limiting lamp (say around 40W or less).  Does the lamp settle to a dim light, or seem to be full brightness?  (dim is good, bright suggests there is a short somewhere)

Measure the three following voltages;

The main +ve supply.  Should be a red wire coming from the power supply.  It may be as high as +40V, but could be anything if there is a fault.

Ditto for the main -ve supply, green wire.  May be up to -40V, but as above.

The voltage on the half-rail, that is the output to the speaker, blue.  This should be about half way between the +ve and -ve supply, that is very close to ground, only tens of millivolts, but again could be anything from full +ve to full -ve under fault conditions.

Post results.


We can provide you with hundreds of years of experience, but it is up to you to apply it so we can make progress.   8|
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

Did we ever determine the board number for the circuit we are looking at?

LJN

Not yet. I'll get to it as soon as I can. The amp is in an awkward place right now and I'm fighting a cold. I'll get to it soon.
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL

LJN

Hi, all. I just wanted to report that the amp has been working for over 20 days now. It turns out there were a few solder joints in the power section that had come loose. Old age, I guess. Thank you all very much for your help. I' m now working on a tremolo pedal to use with it. :dbtu:
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL

LJN

I do have a question regarding the monitor Jack on the rear panel. What's the purpose? By that,  I mean, what type of monitor will work with it? Thanks,again.
If it sounds good, USE IT!

Epiphone Les Paul, Kasino U100- P, Sears 125-XL