gutting a Johnson std/rep tone 15

[elnonio]

Hi

I've just inherited a Johnson 15 (amp says standard, but looks like rep tone).  Power source is good, led lights up.  Speaker is good, but it has stopped producing any sound.  I'm thinking about rebuilding the electronics  while keeping the pots design.  Ideally it would keep the clean/overdrive functionality, with 3 tone eq and the headphone jack.  The pedal jack I'd like to replace with a line in to plug in an ipod or a cd player for beat tracks.  I'm into bluesy rockish music, that type of tones.

Now, I'm not lazy, but I am at the Navy's postgrad school and time is at a premium (as in, I have little time on top of regular duties).  I'm doing a bit of research, but I'm hoping one of you guys will recognize the perfect solution for the problem at hand.

Many thanks in advance,

A. Contreras

[J M Fahey]

Hi elnonio (¿el ñoño?). I guess your best bet would be to try to get it up and running "original". Start by unsoldering the speaker and testing it; replacing it if dead. If it´s good and still no sound, most probably the power chip is dead, read and post here its code. *IF* you get the schematic (and that´s a big if), post it and maybe somebody can suggest some mods, otherwise it´s a wild gessing game. Good luck.

[elnonio]

I've been looking for the schematics online, no luck.  Not sure it would really help anyway, but I could/should start testing the components individually, and who knows, maybe I'll get lucky and find the culprit.

Don't read too much in the handle (aNtONIO, what a kid I used to babaysit callled me since she couldn't say Antonio)

[elnonio]

The speaker is definitely good.  The two amps are JRC 4558D, one has 1168H and the other says 1046D.  You think those are the likely failure points?

[J M Fahey]

Hi elnonio/Antonio (now I know)
From what you found, I think your "1168H" is a uPC1188H, a very popular 20W amp (just ask the guys at Fender), and the most likely culprit.
Download its datasheet http://pdf1.alldatasheet.com/datasheet-pdf/view/115201/NEC/UPC1188H.html
and check voltages at pins: 3 and 10 (+/- around 20 volts) and pin 1 (close to zero)
Remember to rename the datasheet as uPC1188H.pdf because they rename all to "datasheet.pdf" to avoid automated ripping.
If dead you can (carefully) replace it.
The 1046 part eludes me: 2SC and 2SD 1046 seem to be  impossibly large to fit there.
The "JRC4558" are audio dual op amps.
Good luck with your amp.

[elnonio]

I fear miscommunication has occurred!  There's only two chips, not four.  What I should have said is that one says "4558D JRC 1168H" and the other says "4558D JRC 1046D". 

There is nothing on the board that shows where the failure point could be (no discoloration, let alone any flash/burn), and none of the components looks abnormal either.

Stumped (haha),

Antonio

[J M Fahey]

Well, now it´s easy !!! Your amp produces no sound because they forgot to put the power amplifier in it !!!!!!
(Just kidding)
Now you´ll have to grab your old reliable digital camera and post some clean closeups to solve this mistery.
For now, patience.
By the way, "burnt" SS components usually show no visible signs.
Bye.

[elnonio]

You know how it is.  The most obvious thing is often the one you miss.  How about that 3rd chip, with the large cooling fin attached to it...  Well, I rather assumed the issues would be in the low end, since even the headphone jack was silent.  Anyway, it's a TDA2030.

At any rate, the pictures of the board can be found at http://www.elnonio.net/web/amp.php  I cropped but didn't resize the pictures so as not to loose any definition, but did put thumbnails.  Firefox works great using large pictures anyway (sorry if you use IE...)

I really appreciate your willingness to help out even though it's costing us both more in terms of time than the actual value of getting a new one!

[J M Fahey]

Hi Antonio. Don´t worry, I do it just for fun. I *DO* love Electronics and Music.
Download the TDA2030 datasheet as a reference. In your amp the normal values are almost 0 volts on pin 4 and +/- 15 V on pins 5 and 3 respectively. If anything is wrong, replace it. Be VERY careful when pulling it to avoid damaging the board. Use a good solder puller/sucker , if necessary some solder wick, the chip should slide out easily. If stuck, just cut the offending pin and pull it separately. The PCB "is" the amplifier, anything else can be replaced,so pamper it.
If everything is normal but still no sound, i strongly suspect the headphone jack, which switches off the speaker; it may be stuck in the "off" position or it might have lost its ground conexion, which would silence both speaker and headphones. Check it by running a wire straight from pin 4 (audio out) to the "hot" terminal of your speaker.
Post your results.
By the way, excellent pictures.
Note: be VERY careful with the two red wires and fuse on the extreme right, they carry mains voltage. I find it stupid/criminal to do that, they should be on another , separate board. How much can they save?? 1 cent?

[elnonio]

Make sure to hold the two red wires when plugging it in for max effect.  Got it!   

Will let you know what happens (with the real test that is.  As a kid, I got a taste of 220V while plugging a lamp.  Lesson learned for life, trust me).

Antonio

[elnonio]

Bingo, I think.  I got 11.3v on 5, but hardly anything on 0.8v on 3 with a few millivolts on 4.

Seems finding a new amp of these should be easy enough.

I'll close the thread once I get one in!

Thanks again,

Antonio

[J M Fahey]

Hi Antonio. >>  I got 11.3v on 5, but hardly anything on 0.8v on 3 with a few millivolts on 4.<<
Good: 11,3 (nominally 12 Volts) on pin 5 , it's B+ and comes straight from the power supply positive.
Bad: close to 0 on pin  3, it´s B- and you should have around -12V there. Follow the track towards the negative of one (probably the lower one) of the filter caps, a negative that´s *not* grounded. I´m sure that the TDA2030 shorted and burnt it in the process. Replace it with some black wire, straight from Pin5 to the "-" side of the capacitor.
Meaningless so far: close to 0V is fine on pin 4 but since probably the chip is dead and you have no B-, it means nothing yet.
Start by pulling the chip and placing the black cable suggested; if you get +-12V on pins 5/3 you´re halfway there. Bye.

[elnonio]

I got impatient and didn't want to wait 2 weeks for the TDA2030A to make its way here from Thailand, so I dropped by the local music store.  They didn't have one, but they told me who would.  So I go to this parts store, get a new amp and a new input jack.

The clean channel is back, but the overdrive channel is still gone.  Now, the original jack had 4 prongs, the new one 3, even though they both short the jack when unplugged.  So I thought the 4th prong from the original jack drives the overdrive channel.  A little more soldering and this thing should be back as it should be.  Not!  I'm getting a little sound from the overdrive channel now, but still way less than normal.

I now wonder what else had blown out.

Y como no tienes nada mejor que hacer que ayudarme con este rompecabezas, espero tus consejos! 

[J M Fahey]

Hi elnonio.
I actually read ssguitar during cofee breaks. Which being coffeeholic and self-employed (no boss yelling ¡back to work! in my ears) happens very often.
You didn´t have to order from Thailand !!!!!
Although many of them come from there (my ST ones do), it´s a very common part which can be found anywhere.
I guess they must be scratching their heads over your order in Thailand, maybe they think it´s a typo and send you their regular order: just 1 (one) ..... *million* 
Good to see that your amp is back.
Try to get the same input jack , because modern ones use their auto switching not only to short inputs when unplugged but often killing some other signal point to reduce hiss. So, try to get a 4 pin one just like the original.
Get links to electronic hobbyist supply houses, they have huge online catalogs, pricing, etc. and ship very quickly.
Many of them come to mind, (Mouser is one, Enzo mentioned another very good one , just read posts here).
As far as the distortion circuit goes, not having the schematic makes helping you beyond this point an impossible task.
Anyway, maybe just the switch is dirty.
Respecto al Español, veo que es mejor contestar en Inglés para ser de utilidad a la mayor cantidad posible de amigos. Cuando respondí a un colega Chileno, otro miembro (creo de Miami, quizás de origen Cubano, no sé) tradujo la respuesta al Inglés porque la halló útil.
So, let´s keep using English.
By the way, when you receive your "immigrant" TDA2030, just frame it and hang it in the wall, as a very special one.
Good luck/buena suerte con lo tuyo.

[elnonio]

Oh yes, I keep foreign languages to a minimum.  When Al Gore invented the internet, he also dictated a new world language...

Check this out though.  The local store charged me $15 for one of those puppies, and said they listed at about $25... The two from Thailand were $5.33 shipped!  So if they send me 2 million, I'll be opening my own store!

Thanks again,

Antonio