Kustom Amplifier, need help with troubleshooting

[Dimarino]

Hello,
I´m new to this forum and you could say that im new to electronics as well. So please be gentle!

Basically i need help troubleshooting a KPM 7250 (old model) kustom PA amplifier. Even if you don´t know the exakt model, it´s just very basic electronic stuff.

In the master section it has a pot which says "tape in/aux", which probably affects the mono aux in and the stereo phono input (tape in)

The problem is that the tape in/aux pot doesnt affect the aux at all, which is always on full, and you can only hear a tiny bit of the "tape in" signal when the pot is turned to 0, when i turn it towards 10 the signal disappears.

I have checked for loose cables and any "obvious" problems and everything seemed to be fine. So, my question is then where do I start looking at?

More info:
* Everything else is working as it should, all other inputs and the master section in general.
* I don´t think its either the resistors or capacitors since it should be more or less dead then. Am i right?
* The pot output goes to an IC (4556? maybe) which i believe might be the ugly one in this case.

.. But then since i don´t want to order another IC and change it without being shure that it is problem. How do I check it´s working properly with a basic multimeter?

Thanks alot in beforehand

Regards /Dimarino

[vtech]

KPM7250 is still a valid model number but perhaps yours is slightly different?
There is no "aux/line in" pot from what I can see in the manual
http://www.kustom.com/Content/20082/KPM7250T_Owners_Manual_27DEC04_1.pdf

You are correct, while possible, chances of it being a resistor or capacitor is unlikely. Either way, you may infact have a problem with the IC or even the mentioned pot but without experience and just a meter there isn't a whole lot you can do? Other than maybe checking for continuity on the Pot or possibly rail voltages on the IC which is usually a good indicator of it's health--
More than likely there are more of the same Ic's used elsewhere in the unit...Are you good at soldering or have a soldering iron? Worst case, you can always try swapping it with anothere one.


I always liked the Kustom products & used to play thru a "tuck& roll" guitar amp during my band days.

[Dimarino]

Thnx for the answer,

You´re right, the one in their website doesn´t have that function because it has built in effects. This is the old one i guess.

I have checked the pot and it´s working properly. One thing was that it seemed like 2 of the solderpoints on the pot legs had contact, it was like a small bridge of solder between them. I took a knife and scraped away the solder.
Im thinking maybe thats what made the IC go bad. I don´t know.

I´m a very good friend of my soldering iron so that´s not going to be any problem. My problem is that i really don´t know how to measure or check IC´s and Capacitors to see if they´re working or not.

Is it possible to check if the IC is working with the multi meter?
I´ve checked the voltages in the legs when the whole thing is turned on and playing, but I really don´t know what im looking for. One leg says -15 volts, the rest is 0. I suppose that´s the feeding voltage, because it´s the same on all three IC´s.
But then i´m stuck. Any ideas?

[vtech]

Without having the unit , it is rather hard to say but, as i said earlier, just about the only thing you can do with a meter is to measure for the supply voltages ...did you not say a 45xx was the IC? ---if that is the case, you need
to have BOTH plus AND minuse voltages on pins 4 and 8. --perhaps you have a different IC all together?

Google the datasheet for the IC to see exactly......those series can not work with a single supply!. ---the bit about the solder bridge across the pot can not cause the ic to go bad.

[Dimarino]

Problem solved!!

It was the IC that had gone bad. I swapped them and then the pot started working just as it should.
The IC is a 4558D if anyone is interested.
It would be interesting to know what made the IC go bad, maybe i could fix the source of the problem so that it doesnt occur again, since it´s a customers amp.
I didnt think that IC´s usually go bad, but maybe they do?

Thanks for your support vtech!

[vtech]