Transformer Newbie Question

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choujijj

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Joined 2018
I have an Akai AM-A3 that powers on, but the main amp relay doesn't click, so there's no sound out of the amp whatsover.

I believe I have isolated the problem to the transformer.

Please see the attached pic:

I know the primary coil has continuity (amp turns on, but tested anyway since this transformer was re-tapped to bypass a thermal fuse I think).

I found this post that indicates the following for this amp and transformer:
If the AC Primary side is open, check the thermal fuse on the red wire
145* for 120v 130* for 220-240V (it is inside the wiring) If the problem
in on the secondary side the org wires go to Fusable resistors R1 and R2.
They supply the 15 volt supply from bridge D1~D4. Yellow-white-yellow
supply brige D7. +-36 volts. Main supply to amp. blue-black-blue
supply ac to logic board FL. if you need PDF I have that also.
Click to expand...

I tested the secondary coil wires:
- Continuity between the orange pair: Shows continuity.
- Continuity between the blue pair: Shows continuity.
However, any of the blue wires also show continuity with the black wire, which I believe is ground. Therefore the transformer is bad?
 

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choujijj

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Joined 2018
scottjoplin said:
The black wire is a centre tap between the blue wires. There's nothing wrong with the transformer
Click to expand...

Thanks so much for clarifying that. I'm trying to figured out why the relay won't kick on after turning on the amp. So I'll need to connect the transformer to test the voltages just to make sure.

So if the blue is supposed to be 36V, it should be 36-18-36 (meaning 36volts across the blue wires and 18v between blue and black with opposite phases?
 
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choujijj

Member
Joined 2018
scottjoplin said:
You're welcome. There will be 36V either side of the centre tap which will be 0V. Does the relay operate at all, does it switch in then out?
Click to expand...

Thanks again. I really appreciate it.

To answer your question, no, it does not click at all. If I power on the amp, the lights come on but no relay click. Since someone prior to me tapped an alternate point for the yellow (ac live) I figured there's probably a thermal fuse that's blown. So I'm assuming the transformer is the issue. Not sure how much of a change the new tap point could cause in the primary coil. It's right near the beginning of the wire/coil, where it "hooks" on to the plastic support before being coiled.

Got it solder back now and am going to test voltages before connecting it to the amp.
 
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choujijj

Member
Joined 2018
scottjoplin said:
I'll wait till you've confirmed if its the transformer or not. Service manual is available here Akai AM-A3 - Manual - Stereo Integrated Amplifier - HiFi Engine
Click to expand...

I have the service manual, but for the life of me I couldn't figure out what voltages it wants where. Was trying to understand the 7 pins that the transformer's 5 wires connects to.

I just tested the transformer with my multimeter.

The Orange pair reads 3v AC
Blue -> Blue reads 64.4v AC
Either Blue wire to Black reads 32v AC.

The 3V is a far cry from 15V.
Not sure if the 32V is enough for 36V requirement.
 
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choujijj

Member
Joined 2018
scottjoplin said:
It looks like the low voltage supply is just for the bar graph display. The DC is greater than the AC after rectification. Are there fuses after the secondary are they ok? Check you have DC after the rectifier
Click to expand...


Ok. I checked the secondary fuses for continuity and they had tested fine. But let me put the transformer back together and check the rectifier. Will be about 30 mins or so.
 
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choujijj

Member
Joined 2018
scottjoplin said:
There is usually a delay and then it closes, then opens if there's a fault
Click to expand...
It doesn't click at all. Haven't heard it once.


So I measured the rectifier. I get different readings if I'm measuring in AC mode and DC mode.

DC reading is 43volts between each pair of legs :=: and 86volts diagonally. :/:

Not sure if I measured right.

Edit: precise numbers are 42.6 and 85.3
Edit2: Rectifier on the left. relay on the right.
 

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choujijj

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scottjoplin said:
Those voltages are fine. The fact the relay isn't operating at all possibly means there isn't a fault with the amp itself. Check whether there is a DC offset on the amp side of the relay contacts.
Click to expand...


Ok. I'm going to try to figure out exactly what you mean. I can see from the board that the output pins from the rectifier go straight to the 2 main capacitors.

I tested the DC Offset at the speaker terminals, with the volume at minimum, settings balanced, cd input and nothing at all connected. My multimeter's settings only goes as low as 2 V DC. I get a reading of .001 to .002 (I believe that 10 to 20mv is good).

Edit: Oops. I don't think testing at the speaker terminals will help, since the amp isn't engaged. XD
 
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choujijj

Member
Joined 2018
scottjoplin said:
Do you have the detailed circuit diagram and the pcb layout? I have them from HiFi engine. Find the relay contacts on that and you'll be able to test the offset there
Click to expand...


I have the service manual. The same one you pointed me to.I just don't have the best understanding of circuitry.

I'm just trying to figure out the relay pins which are labelled on top of the relay itself (shown in the relay pic i uploaded, pins 1 to 6), and which ones I should test against. Is it possible to tell me based on that?

Otherwise, I can inspect the circuit board to try and figure it out.
 
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