Kustom double barrel guitar amp help

[duke37]

I see two diodes next to J8. There will be two more, possibly 1N400x.
Disconnect both red leads
Measure resistance across each diode in both directions You will get some conduction in the forward direction a nd matbe some conduction in the reverse direction depending on the rest of the circuit.
If the diode is shorted it will show zero resistance in both directions. To confirm, disconnect one end and test again.

As Ed said you could go for 1N5007, they are cheap enough to replace all four if one has failed.

 

[banjomaniac]

I measured all the diodes and I get readings in all of them in both directions. I pulled 1 diode and then only got the reading in 1 direction as it should be. Here is another close up and I soldered the caps back on too. If I want to test the bridge rectifiers, should I pull those or not? I should have time to build the lamp current limiter today so I'll give that a try, I might be chasing my tail if the transformer is bad so I'll do that first, I just got some more fuses yesterday.

 

[73's de Edd]

Sir banjomaniac . . . . .
( Can you reeeeeeeeely - reeeeeeeeely play the banjo . . . .like a maniac .. . . . . double mark time . . .Dueling Banjos . . . a . . . la . . .Deliverance ?)

Vell . ..I'm bin tellink' you vats . . . . . what, with you now incrementally showing us ONLY just a LITTLE bit more of the puzzle . . . .
We can see that the suspect four separate discrete diodes that seem to be used in constructing your units configuration of a FULL WAVE RECTIFIER function block . . .appears to be the clustered together D15-16-17 and 18 units.
And in comparing their case sizes to the single D10 ( 1N4007 type) diode that is located just above them, these units seem to be one power step up , but less than the mentioned 1N5007 series of diodes.
Now since you have already heard the . . . . . transformer HUMMMMMMMMMMM condition that blew the fuse.
If they are not already, you might now have the transformer primary winding BLACK and WHITE wires connected back just as they always were and then go to the J7 and J9 RED connections and pull the T-former wires . . . .(a la the dukester 37) . . . off from their terminals .
Then have a good fuse installed and plug in the AC power and turn on the units power switch and then expect the then resultant power transformer noise to have fully receded all the way down to the level of a mouse peein' on a cotton ball.
Run awhile . . .no T-former core heating up ? . . . . then cast aside your worries about that transformer.
If so, then the plugged in Pee Cee board is containing your dee-funct power robbing component (s).

ALSO . . . .with that transformer secondary now being totally out of circuit by virtue of the flying free J7&9 leads, would be a good time to re-check those 4 diodes again.
Go into low ohms setting with your meterings function and range and then short the two meter probes together.
Got that resultant reading locked in core memory ?
Then you go to each diode and connect probes across each diodes two leads . . . . .expecting that like reading on a truly shorted / bad diode (s).

In seeing the incorporation of a nearby PT2339 I.C. digital echo generator . . .this unit just might be sophisticated enough to be using a dedicated AUDIO power out module . . . . .so see if another photo will capture your . . .expectedly HEATSINKED . . . . area that constitutes the power output circuitry feeding to the speakers.

73's de Edd . . . . .

Remember now . . . if things just happen to get any worse, you're just going to have to ask me to stop helping you.


.



.

 

[banjomaniac]

I still play banjo though not as much, here's an old video from the campground in Stella NC we played at a few years ago

 

[banjomaniac]

Also, I hooked up my current limiter BUT all I have is a 60 watt bulb so not sure if it's working like it should but when I plugged it in, the light comes on bright with nothing connected to it so I hooked up the power cord from the amp with the transformer disconnected from secondary the light glows bright still and the fuse did not blow. Do I need a bigger bulb?

 

[Martaine2005]

but when I plugged it in, the light comes on bright with nothing connected to it

The lamp should not light.
You have obviously wired the limiter wrong.
How did you wire it?

Martin

 

[banjomaniac]

I tried to just use a regular double plug receptacle but now I see it's not in series so I need a receptacle that has each plug isiis from each other. So what I have now the neutral leads are connected together and the hot leads are connected together on that receptacle and it won't work. So basically what I made was an extension cord LOL. I'll fix it and get back to you BUT the fuse didn't blow so I think that tells me the transformer it's probably fine.

 

[Martaine2005]

please be careful, this is mains voltage!.
Put the lamp in series with the hot. If you do this with an extension cord, you can simply plug the amp into it.


Martin

 

[Martaine2005]

I can’t edit my post.

 

[banjomaniac]

I'm going to resume this tonight but I think I see what I did wrong, if you use a regular plug receptacle from Lowes you need to break the tab or connectibg the 2 sides together so the hots are independent of each other and same with the neutral side.

 

[Martaine2005]

I think you may want to reconsider trying this yourself.
You clearly don’t understand mains wiring. Please seek qualified assistance before you damage something or worse.

Martin

 

[73's de Edd]

Sir Banjomaniac . . . . .

Well your banjo playing is all right by me. . . . . even though I did have to watch it WAAAAAAAAY on up in the peanut gallery.

Now, as per your mentioning Lowes and buzzz words of a double outlet and a "breakout tab".
Let me just give you an AMERIKANSKI-cized 120VAC version of Sir Martaines . . . . . 220 / 40 VAC You are a peein' version just above.

The RED X is noting the twisting off of that BRASS screw sides tab, such that the two outlet connections are then separated.
This now leaves the botttom outlet as a test position with the test lamp being inserted in series with your equipment being tested.
THEN, if all is well, plugging into the top outlet would let the FULL unhindered 120 VAC go to the equipment and blow it to smithereens . . . IF you haven't corrected the existing overloading fault.

Now if the equipment initially passes the dim bulb test, tarry there a bit longer in that testing mode, and go to the two unplugged RED secondary leads and just rapidly / in a flash / temporarily touch a lead to its connector to see if a transformer moan then occurs ?
If only one lead causes that hum, you are then closer to ascertaining which set of diodes are being associated with that lead.
Unless it is not them and being the power audio output section at fault.

DIM BUB' WARRING ENDSTRUCTIONS . . . . .









73's de Edd . . . . .

Accupuncture . . . . . . is . . . . . . . a jab well done.
.

 

[banjomaniac]

Thanks for the diagram. I hope to get back to testing tonight. This amp is for our local high school strings department so I thought I could help save them some money and get it going for them I appreciate all the help!

 

[banjomaniac]

I got back on this tonight. The lamp current limiter is working. I guess I need to hook it all back up cause I plugged in the transformer by itself through the limiter and it works and didn’t blow the fuse so I plugged it directly to power and it didn’t blow the fuse. I also rechecked the diodes and they appear to be good. I’ll try to test more tomorrow after work.

banjomaniac

 

[JacobX13]

New member here gents, I have the same amp with the same issues. Any update to this project? Excellent info!

 

[banjomaniac]

I never got it, I can connect the current limiter turn it on and it hums but never figured it out. I have the schematic and email from Kustom suggesting what the likely problem is. Here is his reply...

I would check C48 & C49

Maybe the output device is shorted TDA7296

Or both

I would definitely replace the power supply caps either way

The fuse is a 630 milliamp slow blow

 

[banjomaniac]

I never got it, I can connect the current limiter turn it on and it hums but never figured it out. I have the schematic and email from Justin suggesting what the likely problem is. Here is his reply...

I would check C48 & C49

Maybe the output device is shorted TDA7296

Or both

I would definitely replace the power supply caps either way

The fuse is a 630 milliamp slow blow

 

[JacobX13]

Thanks banjomaniac! that is golden info there.

 

[73's de Edd]

Hey . . .Youse Guys . . . . .

As per . . . . .Maybe the output device is shorted TDA7296

If that TDA7296 unit is bad one would expect the power outputs built within it to be bad.
How about taking metering in hand and set it to ohmmeter function and at its lowest range . . . . unless your unit is autoranging and it will seek it out.
Then you find the Black / negative meter probe and pin prick into IC5 pin 1's solder blob.
Then use the red probe to test pins 8 /same conn / 15 to see if there is either a very low resistance or dead short.
Then move the red probe to test pins 7 /same conn / 3 to see . . . . you guessed it . . . if there is either a very low resistance or dead short.
If that was not found then we might wonder if it is only under powered up conditions that it fails .

So lets look at the boards foil pattern snippet below, note the short RED and BLUE delineation lines.

Get a single edge razor blade or an Xacto knife with a # 11 blade chucked in and BRACE your fist and slope the blade to the right and cut the right BLUE dotted line.
The move up to the RED dotted line and cut it at that same cutting slant.
Now move to the other sides dotted lines and slant the blade slope to the left and do those two cuts. You only need to have a narrow slit opened up to diisconnect power flow.
The use of those non vertical / sloped cuts should then facilitate getting a centered strips end corner foil initially pried up , using the blade tip and then peeling up the whole central strip.
Check for continuity, to be sure that the foil was completely opened.

Then power up to see if any fuse then blows. If not, expect the TDA7296 as internally bad.
If still blowing the fuse . . . . did you initially overlook FWB diodes D14-15-16-17 or main filter caps C48-49 ?
Widening the foil gaps by scraping to bare copper and pretinning both sides and then using a brigdging solder reflow fillet should restore your foil cuts.

FOIL PATHS . . . . . .


Product TDA7296 . . . . .
https://www.mouser.com/ProductDetail/STMicroelectronics/TDA7296/?qs=/ha2pyFadugE/UPLZoVc02JfJIBQ/rENc4uOPrkDCwg=



73's de Edd . . . . .


All TRUE WISDOM . . . . . is usually to be found . . . . being displayed on T-shirts.

 

[banjomaniac]

You guys are too smart, I have very limited knowledge on this stuff BUT I found a TDA7296 and soon as I get it I'll switch it out and see what happens. Slow boat from China.

Banjomaniac