Need help with STK4048II audio Amplifier

[venandane]

Hi all,

I need help with this old amplifier I got a while ago. Model number sx 9800, a 350 watt amp based on twin STK4048II chips. This amplifier was part of a larger system and was connected to a sc 9800 AV unit which has gone bad. I want to salvage the amp.

I tried hard to obtain a service manual but to no avail. I don't know what are the correct voltages across the transformer to know if it is bad or not.
I need help in testing the transformer, the IC and whatever else, to bring this old chap back to life.

The amp connects to the av unit with regular red-white RCA wires. The amp also has a kind of "controller wire" which connects to the AV unit. I don't know the purpose of that wire.

Currently the amp lights up, that's all. Please take a look at the photos.
Any help is appreciated. Thanks.

Alvin

[irishlyrucked]

google the numbers on the transformer, and you should be able to find a datasheet. Since the amp lights up, i'd start looking for burn marks on any of the PCBs as that's a pretty good indication of a fried component. Also, feel the heatsink when it's turned on to see if those chips warm up at all, that'll tell you if any power is getting to them. Lastly, check all the fuses.

[venandane]

Thanks for the reply.
No luck with the numbers on the transformer. I tried them before I post. There are three sets of numbers on transformer:
TKP 0279 - 8
P1711AF
SK 261
Anyone with experience on these? There are 5 colored wires on the secondary output, green, orange, yellow, red, and brown. Does anyone know what voltages these carry? And irishlyrucked if the heatsink gets warm, does it mean the chips are good? Because it did get warm, almost hot. It would be nice if the chips are good. One less thing to worry about.

[raindog]

The simplest way to know what voltages the transformer gives is with a voltmeter ( for the stk4048 you shoud have 2x50 volts AC ). Unfortunately the stk chips are not very reliable in my experience ( mostly because they are designed for 8 ohm loads and many people use them with 4 ohm loads ) so , if everything else looks ok ( fuses , pcb, elecrolityc caps etc ) most likely the stk's are toast.

[gain-wire]

Hi Alvin,

you need to do a couple of things. First is measure the voltage on the two big filter capacitors, the two black ones. With your common probe referenced to gound, you shall have (as per raindog's advice) around 63V DC. You should get equal readings, but of opposing polarity. Once you have verified this, you can check to see if there is any DC at the speaker terminals (obviously with no speakers connected). There should be, as you guessed, no DC voltage more than 50mV on the positive terminals(ground to positive terminals, more precisely).

If you have an oscilloscope you can check what kind of ripple there is on the voltage rails, this will give an indication of whether or not the filter caps are doing their job.

At first I thought it's a Pioneer power amplifier, with the 9800 number, but by the looks of it, it definitely is not a Pioneer... What is it?

[venandane]

Many thanks to raindog and gain-wire. I will get onto it. Please check back to see my progress if any. Is there a way to test the chips? I only have a multimeter. The unit is a sharp model...

[venandane]

Ok gain-wire and raindog, here are the latest updates on this amp:
I tested the ac voltages in the secondary, the mains are 126V AC;
green wire 10 V
Yellow 10 V
Orange 52 V
Red 0 V (Is this Ok or not?)
Brown 52 V

Then I had to pull the amp apart to get under the PCB...
I measure the DC voltages on the two large caps: both give the same voltages 68.5/+- DC.

But, unfortunately, I notice two dark spots on the solder on one of the chips. I think this chip is blown. The other one, I notice maybe a dark spot, not sure, but since no channel is playing, I assume this chip is also burnt. Please take a look at the pictures.

At this point I need advice as to the next step. Is it cost effective to replace the 2 chips? Is there a short in the amp that caused them to burn in the first place? What are my options? Thank you.

[raindog]

The voltages seem ok to me ( so probably no short ) the chips may have blown as a result of low impedance speakers or overheating ( they don't have any protection built in internally ). As far as the cost of replacing them it depends on where you live ( here in Romania it would cost me around 100 USD ). It would probably be better to replace them with modules build around more modern ( and better protected ) chips who require the same power supply voltages ; I'm sure the people in this forum can help you with that ( I'll start looking myself later )

[venandane]

much appreciated

[raindog]

after some ,,googling" I can't seem to find any power amp IC that can take +/- 68 volts ( even the high power tda7293 can take +/- 60 volts as ,,absolute maximum" ) so I guess you are stuck at just replacing the stk chips with new ones . I would also recommend replacing the heatsink with a bigger one and use good quality thermal grease ( like the one used for CPU's in a PC - since the STK4048 doesn't have thermal protection built-in and you can't be sure the manufacturer of the amp bothered to put one in ).