Identifying burnt resistors in power supply section of commercial amp using STK4192 II IC.

Thread Starter

Max12345

Joined Aug 27, 2013
63
Greetings to all

I have an amplifier using a STK4192II IC. It seems that a number of manufacturers have used this same IC in their amps.

There are 2 burnt/blown resistors in the power supply section which I want to replace, but obviously their markings are no longer visible.

I measured them out of the circuit and one was open circuit while the other read 5.3 ohms. They seem to be identical resistors (unsure) as they both appear to to follow similar paths to the chip: from power supply capacitor bank to two unmarked caps, to the resistors in question, to the fuses, to the chip via a couple of other component (caps and resistors).

I have attached pics of the circuit board as well as pics of some (identical?) amps.

Does anyone have a similar amp and can tell me what the values of these resistors are please? Or anyone have an idea of what they are?

Thanks
Max
 

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AlbertHall

Joined Jun 4, 2014
8,374
Before replacing the resistors you need to establish the reason that they got smoked otherwise any replacements will go the same way.
If these resistors are in series with the power feeds to the amplifier chip then a value around 4.7Ω is not unreasonable so once the main fault is fixed I would replace them with that value and see whether it works.
 

Thread Starter

Max12345

Joined Aug 27, 2013
63
Thank you for the reply, AlbertHall. I had assumed that the resistors had too low a power rating.
The bodies of the resistors are 9mm and are too big for ¼W bu too small for ½W.
Once I knew what the resistance was, I was going to increase the power rating to 1 or 2W.
Any tips on how to go about looking for the main fault?
Thanks, Max
 

AlbertHall

Joined Jun 4, 2014
8,374
It is unlikely it is a design fault.
You will have to see what they are connected to, probably one of those things is shorted. Prime candidate is probably that big chip but could also be electrolytic capacitors.
 

AlbertHall

Joined Jun 4, 2014
8,374
It is most unlikely they were 2.2k resistors - 23V across 2.2K would dissipate just 0.25W, so the voltage to burn them so thoroughly just isn't there.
Are the burnt resistors connected via 0.1uF (or some value in that region) capacitors to the speaker terminals?
If it is those two then it seems the amplifier was producing a lot of power at high frequency - above audio frequencies. Maybe it was oscillating.
 

Thread Starter

Max12345

Joined Aug 27, 2013
63
I think that they are the resistors across the speakers as i could trace them to the 2k2 resistors and the 47uF caps to pins 5 and 15. Dodgydave, i can't seem to find a schematic for the amp unfortunately...
 

Thread Starter

Max12345

Joined Aug 27, 2013
63
KISS, no radio stations close by, but fairly close to a mountain which transmits TV signals. Why would it oscillate and how do I prevent it from occurring?

SLK001, no neither fuse blew. The amps looks like the pics (the black one) but they all look identical except for the colour. I had to replace the knobs as there weren't any so my knobs are silver.
The name is missing from the front cover, but essentially they are called PA-3038.
 

AlbertHall

Joined Jun 4, 2014
8,374
Does the amplifier still work? If it's those two resistors I would expect it to make some attempt at working. If it doesn't work at all that is probably the easiest fault to find.
 

Thread Starter

Max12345

Joined Aug 27, 2013
63
Does the amplifier still work? If it's those two resistors I would expect it to make some attempt at working. If it doesn't work at all that is probably the easiest fault to find.
Yes it still works, but I don't want to blow the IC by using it often until I replace those resistors.
 

AlbertHall

Joined Jun 4, 2014
8,374
Do you have an oscilloscope that could look for oscillations?
I would replace those resistors (perhaps buy 4 while you're at it) and see what happens. If they smoke or the 'scope sees igh frequency output, then you still have a problem that needs fixing. If the resistors survive then keep your fingers crossed...
 
KISS, no radio stations close by, but fairly close to a mountain which transmits TV signals. Why would it oscillate and how do I prevent it from occurring?
http://audiosystemsgroup.com/Ferrites-Ham.pdf

You could try a ferrite bead on the inputs. Use twisted pair for the speakers. It does not have to be shielded.

What frequency is the TV transmitter? What are the lengths of your cables including the speaker cables?

Problems if they are a multiple of the wavelength including 1/4, 1/2 etc. See: http://www.1728.org/freqwave.htm
Changing the lengths could help.

I'd probably start with twisted pair speaker cables. A variable speed drill can twist wires nicely.
Even making your audio cables out of twisted pair could help.

A typical example: http://www.bellscb.com/products/accessories/RF_Choke_Snap-On_Large_FRCK.htm
 
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Thread Starter

Max12345

Joined Aug 27, 2013
63
Do you have an oscilloscope that could look for oscillations?
I would replace those resistors (perhaps buy 4 while you're at it) and see what happens. If they smoke or the 'scope sees igh frequency output, then you still have a problem that needs fixing. If the resistors survive then keep your fingers crossed...
AlbertHall I have replaced the resistors with 4R7 2 W ones but havent switched on yet. KISS thanx for all the info. I amm going to go thru that at my leisure. The transmiitters transmit on VHF. For now I am going to do the twisted pair speaker wires. They are about 3m long Thanx guys
 
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