Someone help about transformator

Thread Starter

Dzoro

Joined Feb 1, 2019
148
The first post said "no resistance" on the primary. Do you mean zero resistance? Or do you mean no conductivity? And if you are not able to do any more checking how do you expect to be able to wind another transformer and have it work?
My guess is that you have an ohm meter with a single resistance range with perhaps 1500 ohms mid scale, and a low resistance shows tha same as a short circuit.
So at this point you are going to need to do some circuit tracing and see what the two secondary windings feed. If the one set is just for the lights, then you are in luck.

If you can tell us the type number of the output transistors then we can figure out the voltage that is needed for 80 watts output.
One more thing, which is that the numbers in the picture are very hard to see.
SO please read them and post what they are , and THAT may provide us with some useful information.

What were you repairing on the amplifier before you found this problem?
AND, REALLY, if you can provide us with the type numbers of the output stage transistors than quite probabkly we can give some worthwhile assistance.
So i listened to music and sudenly the system just stoped playing with no power on the screen i thinked it is a fuse but when i opend it it wasnt.So i removed the output wires from the transformer and put mains in it and nothing happend i checkd the output voltage and there was 0 so i unpluged it from the mains and check the resisstance on the primary beacose the secondary was good but it was showing over limit on the multimeter so i smeled it and it has that smell when the insulation on the wire is burned.And you said you need the output transistors but there is no beacose everithing is integratet in the chip stk 459 and the transformer has a one winding with central tap that goes to a bridge rectifier with some filtering capacitor and that is powering the amp and the other goes to a different bridge rectifier with filter capacitors and that goes to a conector with two wires obviusly positive and negative and that goes to the equalizer of the amp.its realy old one and theres a lot of wires but the amp itself is realy simple.i reversed engeenerd it and it is realy close like the yakov send the picture and in mine it has some transistors when the conectors are conected
 
Last edited:

Thread Starter

Dzoro

Joined Feb 1, 2019
148
If the power transformer is open on both the primary and secondary from some kind of power surge then the chip is probably toast.
The schematic for the STK459 shows a bi-polar supply.
SG
I hooked a bipolar psu +12 and-12 ehich is 24 total and nothing hapend
 

MisterBill2

Joined Jan 23, 2018
4,817
OK, and if the circuit is what would be a typical application then the amplifier is not an 80 watt amplifier, except for some fantastic claims by marketing people. So a reasonable guess at the transformer is about 40 volts center tapped with a current rating of about 2 amps. The equalizer section probably uses a lower voltage, possibly 12 volts. And since you had mentioned that the system also has two cassette decks, 12 volts would be reasonable for those as well. So probably the other winding is about 12 to 15 volts rated at perhaps 1 amp, although 2 amps is a safer value.
Now for some other questions: Does the system have any lights? They may be useful for discovering the voltage of whatever transformer winding fed them. Also, the voltage rating of the large size filter capacitors will help in knowing the power supply voltage. And still there is a question as to what made the system fail, because transformers seldom fail for no reason at all. Some other part makes the transformer fail, and it is seldom a power surge, unless the power surge makes some other part fail first. So did the system sound OK until it failed, or was there all at once some distortion? Was one of the cassette decks playing at the time? Or the one on top, which may be a record turntable, not a cd player.
 

sghioto

Joined Dec 31, 2017
1,038
I hooked a bipolar psu +12 and-12 ehich is 24 total and nothing hapend
The recommended supply voltage is + and - 21 volts. 12 volts may be too low. There are no specs that I could find that mention the minimum voltage required. Most likely the chip is toast. Replacements are still available. From the specs the amp is rated at 15 watts rms at 8 ohms using a 21 volt bi-polar supply.
SG
 

Thread Starter

Dzoro

Joined Feb 1, 2019
148
OK, and if the circuit is what would be a typical application then the amplifier is not an 80 watt amplifier, except for some fantastic claims by marketing people. So a reasonable guess at the transformer is about 40 volts center tapped with a current rating of about 2 amps. The equalizer section probably uses a lower voltage, possibly 12 volts. And since you had mentioned that the system also has two cassette decks, 12 volts would be reasonable for those as well. So probably the other winding is about 12 to 15 volts rated at perhaps 1 amp, although 2 amps is a safer value.
Now for some other questions: Does the system have any lights? They may be useful for discovering the voltage of whatever transformer winding fed them. Also, the voltage rating of the large size filter capacitors will help in knowing the power supply voltage. And still there is a question as to what made the system fail, because transformers seldom fail for no reason at all. Some other part makes the transformer fail, and it is seldom a power surge, unless the power surge makes some other part fail first. So did the system sound OK until it failed, or was there all at once some distortion? Was one of the cassette decks playing at the time? Or the one on top, which may be a record turntable, not a cd player.
It has some lights on the front they represent like some display and the big filter caps are 35 volts but what is strange is that it has 3 caps and one of them the negative leg is conected directly to the central tap of the transformer (the black wire) and the positive leg of the cap goes to the negative of the bridge rectifier.Is that conected like that beacose of the bipolar supply or anything else.And for the cassete players they weren't playing it was fm radio that plays i am going to picture the whole amp and i will post the pictures and you see what is what
 

Thread Starter

Dzoro

Joined Feb 1, 2019
148
20190301_185511.jpg this is the amp20190301_185741.jpg 20190301_185520.jpg this is the fm radio and the right vertical pcb is the equalizer20190301_185547.jpg 20190301_185626.jpg these are the front and the top that you said is like mixete
 

MaxHeadRoom

Joined Jul 18, 2013
19,665
If the power transformer is open on both the primary and secondary from some kind of power surge then the chip is probably toast.
HIGHLY unlikely that both Pri and Sec are open, in fact it is rare for a transformer to go open at all, most overheat and blow a fuse.
At this point it is not clear as to whether the transformer is actually open!
Max.
 

Audioguru

Joined Dec 20, 2007
11,251
AND, REALLY, if you can provide us with the type numbers of the output stage transistors than quite probabkly we can give some worthwhile assistance.
He said the amplifier uses STK459 hybrid power amplifiers (made by Sanyo). They have a recommended supply of plus and minus 21V and produce 15W at low distortion into 8 ohms. The amplifiers probably produce a maximum of 12W of heat so if the amplifier is stereo then the transformer must be rated at 30V center-tapped, 50VA or 60VA.
 

MisterBill2

Joined Jan 23, 2018
4,817
He said the amplifier uses STK459 hybrid power amplifiers (made by Sanyo). They have a recommended supply of plus and minus 21V and produce 15W at low distortion into 8 ohms. The amplifiers probably produce a maximum of 12W of heat so if the amplifier is stereo then the transformer must be rated at 30V center-tapped, 50VA or 60VA.
The posted picture of the interior shows ONE STK459 only. My thinking is that is all that there is, and that the 80 watts is either line power input, or that IHF "Music power" rating. 25 watts at higher distortion into 4 ohms would also be reasonable to expect. The sudden failure may be due to the opening of an over-temperature cutout device buried in the transformer. Such cutout devices are common in transformers designed for minimum cost, and they are difficult to remove without causing damage to the windings.
 

Thread Starter

Dzoro

Joined Feb 1, 2019
148
The posted picture of the interior shows ONE STK459 only. My thinking is that is all that there is, and that the 80 watts is either line power input, or that IHF "Music power" rating. 25 watts at higher distortion into 4 ohms would also be reasonable to expect. The sudden failure may be due to the opening of an over-temperature cutout device buried in the transformer. Such cutout devices are common in transformers designed for minimum cost, and they are difficult to remove without causing damage to the windings.
So what can i do about that
 

Thread Starter

Dzoro

Joined Feb 1, 2019
148
HIGHLY unlikely that both Pri and Sec are open, in fact it is rare for a transformer to go open at all, most overheat and blow a fuse.
At this point it is not clear as to whether the transformer is actually open!
Max.
So in this case its not its open on the both windings
 

Thread Starter

Dzoro

Joined Feb 1, 2019
148
He said the amplifier uses STK459 hybrid power amplifiers (made by Sanyo). They have a recommended supply of plus and minus 21V and produce 15W at low distortion into 8 ohms. The amplifiers probably produce a maximum of 12W of heat so if the amplifier is stereo then the transformer must be rated at 30V center-tapped, 50VA or 60VA.
It can't be a +-21 volts beacose the filter caps are rated for 35 volts and 21 and 21 are 42 volts that's going to blow them .
 

MisterBill2

Joined Jan 23, 2018
4,817
It can't be a +-21 volts beacose the filter caps are rated for 35 volts and 21 and 21 are 42 volts that's going to blow them .
Each supply, positive and negative, has a separate filter cap, so while the overall voltage is 42 volts, each separate half is only 21 volts. Bi-polar power supplies seldom have a single filter capacitor across the whole supply. Thus a 35 volt rating is quite reasonable, and suggests that the supply voltage may be a bit higher.

And about the transformer, I still question the meter used to check the resistance. Is it an analog meter, or does it have a digital display? If it is an analog meter, try measuring the resistance of a low value resistor, something less than 100 ohms, and see what the meter reading is. I suggest this because some analog meters have only one range, with 500K ohms full scale, so a 10 ohm transformer resistance looks like zero ohms.
As for bypassing the failed internal over-temperature device, that requires dis-assembling the transformer and understanding what one is doing, and a very delicate touch. It is a whole lot like surgery. So yes it can be done, but it usually requires experience.
 

Thread Starter

Dzoro

Joined Feb 1, 2019
148
O
Each supply, positive and negative, has a separate filter cap, so while the overall voltage is 42 volts, each separate half is only 21 volts. Bi-polar power supplies seldom have a single filter capacitor across the whole supply. Thus a 35 volt rating is quite reasonable, and suggests that the supply voltage may be a bit higher.

And about the transformer, I still question the meter used to check the resistance. Is it an analog meter, or does it have a digital display? If it is an analog meter, try measuring the resistance of a low value resistor, something less than 100 ohms, and see what the meter reading is. I suggest this because some analog meters have only one range, with 500K ohms full scale, so a 10 ohm transformer resistance looks like zero ohms.
As for bypassing the failed internal over-temperature device, that requires dis-assembling the transformer and understanding what one is doing, and a very delicate touch. It is a whole lot like surgery. So yes it can be done, but it usually requires experience.
i have the fluke 179 multimeter
 

Audioguru

Joined Dec 20, 2007
11,251
The posted picture of the interior shows ONE STK459 only. My thinking is that is all that there is.
The front of the thing has STEREO written all over it: Stereo Tuner, Stereo Cassette, Stereo Amplifier …. and of course Left Vol and Right Vol.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,251
It can't be a +-21 volts because the filter caps are rated for 35 volts and 21 and 21 are 42 volts that's going to blow them .
Plus and Minus 21V are DC, not AC. Each filter cap gets 21VDC.
If the amplifier had no voltage losses then +/- 21V would make an output of 42V peak-to-peak which is 14.8V RMS. Then the power into 8 ohms would be (14.8V squared)/8= 27W with severe distortion. But the amplifier has voltage losses from its transistors so its low distortion output is about 11V RMS which is 15W into 8 ohms.
 
Top