# Samick SM60rc guitar amp blows fuses



## polar60 (Dec 3, 2021)

So, my friend's guitar amp blows the fuse as soon as you power it up. I checked the filter caps, and the FETs, they were ok,
when I checked the Bridge rectifier, it is blown on one side, I pulled it off the circuit board to confirm it was blown.
it's a KBPC303, I can't seem to find this rectifier or spec sheet anywhere.
Would someone know where I can find the KBPC or an equivalent replacement? The amp is a 60Watt

Any help or leads greatly appreciated.
thanks


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## Mark Brown (Jan 4, 2022)

Is the schematic magically inside the amp?? Can you upload it?


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## polar60 (Dec 3, 2021)

You mean like an old kitchen stove? no this doesn't have the schematics inside and I can't find them anywhere either


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## CathodeRay (Jan 12, 2018)

Huh, I get search results on that, it's a 3A rectifier. Try duckduckgo.


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## polar60 (Dec 3, 2021)

Yeah, I see but is the 303G the same as the 303? and is there an equivalent?


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## CathodeRay (Jan 12, 2018)

If it was me, I'd go with any higher amp rated full wave like 6A and not worry about the details or the part number. There's nothing particularly special needed for 60hz operation. And the original part you're after blew out, so...


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## polar60 (Dec 3, 2021)

I was thinking the same thing, just that I wanted to see the specs to see if I can find a replacement locally to be able to fix it right away. 
thanks


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## Mark Brown (Jan 4, 2022)

Since we are being honest, I just wanted to see the schematic out of curiosity


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## Latole (Aug 11, 2020)

With a 3 A fuse, any 1 A ( or more ) solid state rectifier / bridge will work .
Easy to find at any good electronic store






Diode, Bridge Rectifier 10A 600V PCB


Diode, Bridge Rectifier 10A 600V PCB




abra-electronics.com


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## Latole (Aug 11, 2020)

Some more ;






Search results - page 2







abra-electronics.com






Single ; 





1N4007 General Purpose Rectifier 1A 1000V


The 1N4007 is a general purpose rectifier or also called a diode in a DO-41 package.




abra-electronics.com


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## CathodeRay (Jan 12, 2018)

Latole said:


> With a 3 A fuse, any 1 A ( or more ) solid state rectifier / bridge will work .
> Easy to find at any good electronic store


I think you got the fuse and the rectifier mixed up, this is not the case.


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## Paul Running (Apr 12, 2020)

I would advise that you do not power it up again until you know why the bridge rectifier failed. I believe that there are 2 supplies in that amp...there should be 2 bridge rectifiers.


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## Latole (Aug 11, 2020)

CathodeRay said:


> I think you got the fuse and the rectifier mixed up, this is not the case.


No .


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## polar60 (Dec 3, 2021)

Correct Paul, there is another bridge rectifier, it tests good.
Btw, Paul if you're able to get better resolution on the schematic, can you give me the numbers on those FETs? I wasn't able to .

thanks


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## polar60 (Dec 3, 2021)

Well, I spent another 1.5hrs troubleshooting, I checked all filtering caps, FETs(again) and JFETs(again) although 1 diode gave me resistance(ohm) even with my component tester/voltmeter on diode setting, when I pulled the legs off and it tested good. all other diodes tested good. I did test some transistors around that diode and were ok, but not all as some are in clusters. I can't seem to find any other faulty components or burn/heat marks other than the bridge rectifier. Any other ideas or should I just go with the faulty bridge rectifier?


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## Latole (Aug 11, 2020)

Replace the faulty bridge.


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## polar60 (Dec 3, 2021)

Well, still no go!! I replaced the faulty bridge but the fuse blew again. tested the FETS..all good, tested rectifiers(2) ..all good. I isolated the transformer and I get ac voltages from the taps. the filter caps test ok.All diodes test good, Something is shorted on this board, Only thing left is to remove and test all JFets, this is when I wish I had the schematic.


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## Latole (Aug 11, 2020)

Paul Running have the schematic.


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## Paul Running (Apr 12, 2020)

Sorry, I am unable to locate a clear view schematic. If the fuse blows immediately, it is most likely a hard-short. Try to isolate the power supply from the load. Test the power supply and make sure the voltage is correct and no current draw; then I would do some continuity checks on the load-side...points to ground.


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## Latole (Aug 11, 2020)

Paul Running said:


> Sorry, I am unable to locate a clear view schematic.



The schematic you show answer #12 is not a schematic you have in PDF ?


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## polar60 (Dec 3, 2021)

The transformer on its own is ok, I have to see if I can separate the DC rectifiers from the rest of the board somehow to check the DC output voltages. *Paul are you able to blow up the schematic and increase its resolution? continuity I'll see if I can check continuity from load to gnd , the way I see it now, its either a transistor or a cap.


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## greco (Jul 15, 2007)

polar60 said:


> *Paul are you able to blow up the schematic and increase its resolution?


Using @Paul Running should notify Paul re: post #21.

Good luck with the troubleshooting/repair.


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## Paul Running (Apr 12, 2020)

I am unable to improve on the clarity of the image...not much for public view on the net, I've only located 2 models with descent schematics...Samick support may help.
I'm not well informed of Samick designs, their amps could be made by another source.


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## polar60 (Dec 3, 2021)

@Paul Running , ok thanks. I will try to lift some resistors legs right after the bridge rectifiers to see if I have proper DC
voltages. this board has so many wires soldered to it that makes it a little difficult to flip over. I will try to check continuity load points to gnd after the rectifiers, to see if I find shorts. If anyone can think of anything else, pls let me know


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## polar60 (Dec 3, 2021)

Well google was my friend today, I was able to find the schematic online. I will print it out on 11x17 tomorrow so I can make notes and continue troubleshooting..


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## Paul Running (Apr 12, 2020)

So which bridge did you have to replace?


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## polar60 (Dec 3, 2021)

@Paul Running , I replace D115 (the ~ to + diode was blown on it) , D114 tested good. But after I replaced D115 and fired it up, it did not blow again. only the fuse blew, then I ran out of fuses and started to check continuity across the board.


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## Paul Running (Apr 12, 2020)

polar60 said:


> then I ran out of fuses


You might want to rig a current limit circuit at the mains. When the bridges were removed, how did the AC voltages measure for the secondary windings of the power transformer?


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## polar60 (Dec 3, 2021)

Ac voltages and measurents from the transformer secondary winding are as follows to D115 rectifier (1)top winding=20.06mH, (2)bottom winding=24.76mH, second windings(3) to D114 = 29.79mH
Voltage (1)= 34.55Vac, (2)=34.51Vac, (3)=39.15. I've fired it up at least 6x today with a 0.5A fuse and it never blew up. At this point, the wires from the transformer to D115 and D114 are disconnected, so maybe the issue is on the following stage. Oh and I don't have a variac at my disposal.


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## Paul Running (Apr 12, 2020)

You are correct...the short will be after the rectifier stage. Have you verified the resistance to ground at points: HV+, HV-, LV+ and LV-? If not discharge the filter caps and try it.


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## Paul Running (Apr 12, 2020)

polar60 said:


> Oh and I don't have a variac at my disposal.


You could connect a current limit device such as an incandescent light bulb in series with the mains supply.


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## polar60 (Dec 3, 2021)

Ok, I can measure those points, from which Gnd reference? the center tap? I guess I'll have to build me that current limiting device eventually


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## polar60 (Dec 3, 2021)

So I discharged the caps, from HV+ to CT Gnd..Mohm resistance, same as HV-, on continuity setting, no shorts
same Mohm resistance for LV+ / LV- to CT gnd, continuity setting, no shorts
on Ohm setting the caps charge up


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## Paul Running (Apr 12, 2020)

What are the actual resistance readings?...it may not be a short however, a very low reading could cause an over-current situation; 35VAC would produce a DC voltage of about 50VDC no load. If the resistance measured 100Ω, the current would be 500mA.


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## polar60 (Dec 3, 2021)

Alright I finally got those values, HV+ to Gnd = 10.3k, HV- to Gnd 10.2k
LV+ to Gnd 5Mohms, LV- to Gnd 82K
I took them a few times, LV+ first read 550k, other times went to 5M

Oops! I just realized I took the wrong Gnd as reference when I measured LV+ & LV-, I will re-measure shortly..

So from Gnd perpective of second stage to LV+=325k, Gnd to LV- =55k


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## Paul Running (Apr 12, 2020)

Okay, no low static DCR readings; the issue occurs when the circuits become active. The majority of major failures occur in the output section (top RH section of the schematic). I would check the DCR of the output to ground, with and without the speaker load...it would be wise to disconnect the speaker or any other load from the amp...the speaker could be causing an overload condition. If the DCR measurements of the output measure serviceable then further isolation will be required. To perform power-on tests, I would recommend current limiting the mains to about 1A (you already know that it will draw more than 2A because the fuse blows), and monitor the current.


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## polar60 (Dec 3, 2021)

I suppose this is where that current limiter comes in handy, I'll have to see if I have parts to build one today. Anyhow, when the fuse(s) blew, I never had the speaker connected, but initially I did check all 1/4" jacks for shorts and the only one that had continuity was the headphone jack, I'll have to disconnect it to check it again or its associated circuit.


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## polar60 (Dec 3, 2021)

the headphone jack is ok, I'll need to continue checking more components..


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## polar60 (Dec 3, 2021)

@Paul Running , still no luck The DCR from output to Gnd is 700K, I've checked all discrete components tied directly to gnd(chassis) on the top side (middle) as well as right section. R148 rang short but lifted one leg then read 10 Ohms. I've also checked all diodes, caps and gnds on the input jack board and nothing. Other than the Dim bulb current limiter which I'm getting ready to build, there is probably the option of injecting voltage into the circuit to see if some area is heating up. what do you think?


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## Thunderboy1975 (Sep 12, 2013)

Now you have to search out shitty solder points maybe? 
Whats the black mark on the board in the upper left of the pic of the board? 
Was there a curcuit board above that area?


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## polar60 (Dec 3, 2021)

Yeah, I've looked at most solder points already and I reflow them if I see any questionable ones, the mark you see on the board is a black marker smudge, there are two on the board.
The input jack board is towards the front of the chassis, it does not cover that area.


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## Thunderboy1975 (Sep 12, 2013)

Had a job at Seimens soldering caps and resisters onto boards for sensor and sonar devices. Some came back and i had to reflow. Even under the magnification the solder points looked perfect, but some dude would bring them back to me holding it like a dead mouse shaking his head.


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## Paul Running (Apr 12, 2020)

polar60 said:


> there is probably the option of injecting voltage into the circuit to see if some area is heating up. what do you think?


I would remove the +HV and -HV power to the power amp then, power-up the remaining circuits to verify that the high-current issue is indeed caused by the power amp stage. If that test is good then you could concentrate on the power amp with current limiting...keeping an eye on component temperatures.


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## polar60 (Dec 3, 2021)

@Paul Running, I got all the parts to build the dim bulb current limiter and start building it tomorrow to continue troubleshooting. BUT today, I stumbled onto the problem and I'm 100% sure its the problem as all my testing has found nothing else so far. It looks like someone else has been in the amp before me, at the bottom of the cabinet, I found 4 FETs with unknown numbers..?, then when I looked at the schematic, Q109, should be 2SA1015 PNP but found 2SD600 NPN soldered, Q110 should be 2SC2235 NPN but found 2SB631 PNP soldered, I'm not finished.... Q112 should be 2SC3854 NPN but found 2SB817 PNP soldered & Q113 should be 2SA1490 PNP but found 2SD1047 NPN soldered, looks like someone tried a whole bunch of FETs without looking at the specs. it never dawned on me to look at the numbers. I learned a good lesson, DAMN!!! I can probably swap these around in their proper places but I think I will order the original parts and put this thing back together the way it was intended to be.


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## Mark Brown (Jan 4, 2022)

That is crazy!

It is funny how we can overlook such obvious things simply because we do not suspect them. I know I would have missed it until, like yourself, all other possible options were exhausted.

Hopefully now you can make some progress.


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## polar60 (Dec 3, 2021)

Its unbelievable how someone can mix up these parts and cause all this s*ht. Anyhow, I've build my current limiter and ordered my parts, will report back when all is installed and tested. thanks for the help guys!


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