Quote from: Vitrolin on September 22, 2016, 04:49:31 PM
Hi could you take those measurements again with reference to GND at U301 pin4,

pin 6 should be around 15VDC (form D315)
pin 5 needs a scope to be measured see enzos and faheys posts
pin eight should be quite high +300VDC, be carefull it can kill you.!!
the optocoupler on pin 2 is a galvanically isolated feedbak from secondary side, this should be low to operate correctly.

Hey Vitrolin thanks for Ref point,I'm got confused as to where I should put my black DMM lead at. Done what you suggested.

U301 with pin 4 as ref.
Pin 6 has some Voltage fluctuation between 7 to 15Vdc
Pin 8 has from 229 to 300Vdc(yes it is dangerous)

I don't have an isolating transformer..I have a dumb question. If I snap off or dienggage the ground/earth leading out from this amp.Will it be safe for my scope to measure pin 5?

Thank you Vitrolin.
Thank you Enzo.You had saved two lives here.Me and my cheap Velleman scope.Almost put my scope and held the board live

To be honest this SMPS thing is very new to me.I'm not much of a theory guy.Just trying to fix stuff,replace what's broken,give it back or earn a few dollars and move on to the next repairs if there's any.EE knowledge doesn't stay long in my brain.Haha

Ok here's what I gather.When turned on I took some voltage reading(in red).Hope we could pin narrow down the fault.So far what I could understand about this circuit are both primary and secondary are isolated.The optocoupler has somesort of virtual connection there.Ignore my opinion if they're irrelevant to the subject.

This is the best I could blow up the schemes,cut it and do some photo font.I'm not good with IT stuff either

Hello friends,got this sent for repairs.No power.Check there's no secondary coming out from that tiny transformer.Never bought such component before nor have any idea about its type and data.I don't know what those numbers stamp on it means.

Would appreciate any info about it. Thanks.

OK Mr Phatt,will do that.Will be back.

Email with a pic of the bench too?Come to think of it,that should make them know who genuinely in need for SM. I certainly will try this method.

Thanks for the tip Mr Nosaj.

Hello all,its me again.My apology

I'm troubleshooting this unit.Its distorted both channel.The output is low too.This is a second unit.I failed the first one last year.I gave it back due to my lack of time a.k.a I'm slow.

The problem come and goes.Feeding to return seems ok and I think it the preamp but I don't know where.I feed a 440hz sine wave for tracing on a on and off basis as the reading on my scope is ok(I think).Plugged in the guitar to play and trace at the same time is hard to do but that's when the problem can be heard.Is there a common spot at the PCB to focus on about this symptom.?I as much like to solve this with the help of you guys.My Marshall success repair is low.Trying to get back doing it good again.My amps encounter has few and much fewer when I started to do more guitar repairs.Those pays the bills quicker than amps.Hope most of you with such experience could give me pointers.


Thank you

Hello all,long time no post.

I have this Laney sent in after the authorized service center could not fix it.The owner took it back and came to me.I'm not an expert but would like to do my best.Hard to say no to circuit porn.

To begin with I opened the amp found a diode was put back wrong near the power amp tube circuit,I'm unsure what else was done to it.However the inside doesn't match the LC30 service manual I have.They have redesign it all.I'm not good with reading it without SM,I'm unsure whether I'm asking for right version as well.I think its version 3.Anyone here has the service manual to share.It would make a good reference on the bench.Would like to be alive after that too.hehe

Thank you sir.

Thank you for sharing.This will come in handy one day.Kudos!

Hi,my client wants me to put a fuzz in his bass.the bass has an active onboard preamp.
Where would be best to put it.before or after the preamp?parallel?

Thanks guys.

Hello,I came across a Line 6 spider III 3012 that has power power problem,upon checking and comparing with spider II schems,I found some similarity between the two for voltages checking.

upon looking at it I didn't saw op amp near the EQ controls,then I found that they use codec.I am no codec expert but I don't like this,my work came to a stop.There's no such codec in my country.

Who else is using this codec IC?
I don't believe codec is equal to the hard working op amp,trannies etc.
Its a good way to kill us small time tech.

what do you think fellas?

Great.

I know its a long time ago but anyway you're very welcome.
Also thanks to enzo.

good info about the VA thing,now I know what it is.

Thanks a lot.

Hello there,

I too have came across such problem,this is what i suspect,dirty contact at the "send" out plug.

a)easy remedy is take a short 1/4 inch pedal interconnect,plug it in between the "send" to the "return" plug.

b)or open up the amp and you might as well replace the female "send" and also "return" plug.(I recommend)

I hope this helps.bye

keep us posted.

I guess for beginner like me i asked to much hehe.

Thanks teemuk and i also do agree with jack1962.

although i'd skip reading about that sziklai and that quasi thing,i do now understand why most under 20w guitar amps comes with those TDA IC,the design itself is done by the parts manufacturer.so less R&Ding for the guitar amp maker.i guess initial production time,skill labour and cost are very important.

Power amp ic does make replacement easy.

I also have to agree on your last paragraph.

I used to worked at Sharp service house and most of their hi fi uses those STKxxxx ic and it is expensive to replace.I accidently burned a few and its not good when the boss knew about it.On my work records it would went.hehe

Later i worked at a company servicing marantz,sansui and creek home audio stuff.most of them uses those 2sa,2sb,2sc,2sd and other 3 legged trannys,none wre using STK or similar IC's.Replacing them was rare case,even my NAD lasted for 14 years if it wasn't for the faulty transformer.

now i'm in the musical stuff repairs and i'm seeing both IC and Tranny fitted equip. coming in for repairs.Thats why question suddenly pop in mind.

Thanks for the elaborate answers,this is really a great forum.