i know that the center tap winding is for the amp and the other is for the other circuitry and the wires from the centertap are red black red and the other two are blue so when i mesured from red to red it shows 15 volts and from black to red 7.5 each wich is 15 both and from one blue to other blue it was mesuring 5 volts and that is too low.And also i know that diodes need to be checked out of the board and thanks for the safety tip about that i don't want to touch the primary while is pluged in mains.And i am a student with electronics hoby and i work around 3 years and i never came to such a repair like this.One time i even repaired a 3000wats rms car amp but this old thing is the most difficult thing to work that i came in contact with.OK, that was a bit of a long run-on sentence. Lets see if I understand: You say you connected a 40 watt bulb in series with the transformer, and the bulb lit; albeit dimly. Then the primary is not open. If it were open then the bulb would not light. You also state you get 15 volts across the three wire section of the transformer (the part with the center tap). Yes, that's probably below what it should be. From what I've picked up in the conversations you should see 21 volts from center to either side. Turns out that when you multiply 15 volts (AC) by 1.414 you get 21 volts peak. When rectified and filtered you should see 21 volts from center tap to either of the other two legs of the secondary.
Having gone back and examined the photos you've posted, it looks like there's two secondary coils. The two black wires on the right side as viewed in the photograph would be isolated from the other three wires to the left. So - the three wires on the left appear to be what drives the amplifier (as others have already said) The red wire in-between the two black wires is the center tap. From either black wire (on the left) to the red wire you should see 15 VAC (RMS) (Fancy term for the average useful voltage from an AC sine wave). From black to black you should see 30 VAC (RMS) (Root Mean Squared). In short hand terms, multiply the RMS voltage by 1.414 to get the peak voltage, the voltage you'll see when rectified and filtered. None of that matters. What matters is that your transformer appears to be doing something. If you're measuring 15 volts across the two left black wires then something is probably wrong with the transformer. The secondary can be shorted internally, which would explain both the low voltage AND the high draw to the primary which likely blows the fuse. At this point I'd expect (or hope) that nothing else in the circuitry has been harmed.
So your issue is from where to get such a transformer. Those two black wires on the right - - - what voltage did you get from those? Oh, and I hope these voltage readings on the secondaries was not done with the lightbulb in series with the primary. If so - hard wire the transformer directly to mains and read and report all voltages. For future reference it may help if you label each wire with a number or a letter. Let A & B be the primary side; C, D & E the three wires of the secondary with the center tap, D being the tap; and F & G for the last two wires on the right. By arbitrarily assigning designations to the leads of the transformer we can ask you to perform specific checks on specific wires and have you report your findings.
The idea that when you wiggle the wire you get changing readings may be of some concern, or it may be a weak connection. Could be the wire itself touching the probes of the meter. Please don't do that with the primary side while powered - you'll get an unpleasant shock and likely to holler out my name for not warning you. Of course I think you're smart enough not to touch the primary side while powered.
If the transformer turns out to be good then the next thing to check would be the diodes. Each one should be checked outside of the circuitry. Otherwise you could get false readings difficult to interpret, and reporting errors in that condition can mislead us to diagnosing things incorrectly.
Best of luck.
Thanks for the voltage and i know that if the chip is burnt is bad but this on the look is completly fine but how to chek if it is bad i checked if its shorted on the plus and minus input pins and it's notNo specs on the minimum voltage. Recommended is + and - 21 volts. When a chip is burnt it's BAD.
OH! OK, I'm not getting a good look at the colors. Being that you've specified what colors they are - I'll go on that.i know that the center tap winding is for the amp and the other is for the other circuitry and the wires from the centertap are red black red and the other two are blue so when i mesured from red to red it shows 15 volts and from black to red 7.5 each wich is 15 both and from one blue to other blue it was mesuring 5 volts and that is too low.And also i know that diodes need to be checked out of the board and thanks for the safety tip about that i don't want to touch the primary while is pluged in mains.And i am a student with electronics hoby and i work around 3 years and i never came to such a repair like this.One time i even repaired a 3000wats rms car amp but this old thing is the most difficult thing to work that i came in contact with.
The first thing i did is that i checked the rectifiers and those are good and when i hook the centertaped transformer that i have on the outputs of the rectifiers it shows good i put 18vac and i get 22 vdc on the outputHere's a transformer I took out of an old stereo. The primary is dedicated for ONLY 120 VAC. I have others that can be connected for 120 OR for 240 VAC. Notice the different sections of the secondaries. A, B & C form 65 VAC for the amplifier. D, E, F, G & H form another set of voltages available. F could be considered the center tap for D & H or for E & G. I & J are for 5 volt logic circuits. ABC is one set of secondaries, DEFGH form another set of secondaries and IJ form the third secondary. Each group is isolated from the other. You can't get a voltage reading from A to D or any other point except for B & C. So it is with all other connections, each grouping is its own group on a multi-secondary transformer. The transformer you have appears to be similar to my ABC & IJ.
Like I said in my first post - I had a stereo amp go bad because of a shorted rectifier. That MIGHT be all you're dealing with. Especially if you report you're seeing voltages on your secondaries.
View attachment 171387
OH. So your initial report had me thinking we were talking about total failure. Sorry, I guess I got off track somewhere. What are you getting from the two blue wires?The first thing i did is that i checked the rectifiers and those are good and when i hook the centertaped transformer that i have on the outputs of the rectifiers it shows good i put 18vac and i get 22 vdc on the output
i think 5 volts it's too low beacose that is going to a conector and goes to the equalizer fm radio the two motors for the cassete players and the top mixete thing and 5 volts to power all that stuff is very lowOH! OK, I'm not getting a good look at the colors. Being that you've specified what colors they are - I'll go on that.
While I was busy posting my last post you posted clarification on what you're finding (and the colors). 5 volts? I can't tell you that's what it should be, but I'll guess it probably is supposed to be 5 volts across the blue wires. But that's only a guess.
The colors doesnt matter if they are black red black or red black red is tha same thingAfter looking at the pictures again I see that my memory (dyslexic as it is appearing to become) the three left wires are red black red. My bad. I was going from memory thinking I saw the red wire in the middle. As for the blue wires, I just needed to blow up the picture enough to see it was definitely blue. Maybe I need to start wearing hats to keep the sun from baking my potato.
I don't even know at this point beacose it's mesuring low voltage that is too low to power the amp and one primary wire when i putit up is mesuring 40 ohms and when i putit down it's open and mesuring over limitSo here we go again, is the power transformer good or bad?
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