Technics SA 626 blowing fuse before transformer - diyAudio
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Old 22nd July 2014, 02:43 PM   #1
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Default Technics SA 626 blowing fuse before transformer

Blows F1 (T4A) fuse. Measured resistance of rectifier diodes in circuit and they seem to be okay. Got a large reading one way...increasing with time and a neg small reading the opposite way decreasing with time. I am thinking of disconnecting the AC connections from the rectifier and confirm that the transformer primary is good. I think I measured some resistance on the primary...small 1 ohm or so but still not open. Should I put a good fuse in it and use a light bulb on the input? Should tell me if it is a soft or hard short?

Picked this off of another thread. Check resistance between the main plug and the chassis, and between both pins of that AC plug.
What results should I look for in the above test?
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Last edited by rfitts46; 22nd July 2014 at 02:47 PM.
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Old 22nd July 2014, 02:47 PM   #2
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Default 110 vac versus 120 vac switch

technics sa 626 schematic power section.png

The two times the unit was powered up with a fuse in it the switch on the back was set 110 vac. I have switched it to 120 vac but I assume this would not cause Blows F1 (T4A) fuse to blow.

Last edited by rfitts46; 22nd July 2014 at 03:11 PM.
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Old 22nd July 2014, 03:56 PM   #3
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The first thing to do is disconnect the rectifier ac input. With the transformer in a no load situation, feed the transformer through a 100Watt incandescent mains lamp. The lamp should go out. If it stays bright, replace the transformer as it has a damaged primary winding that is usually caused by poor/intermittant mains supply. Transformers don't like to be switched on!
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Old 22nd July 2014, 04:47 PM   #4
Enzo is offline Enzo  United States
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Step back and look at the larger picture. Most common fuse blower in an amp is shorted output transistors - how are yours? You already checked for shorted rectifier diodes, but you used resistance. Use the diode function of your meter to test. A diode does not need to be shorted to read super low at its junction.
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Old 23rd July 2014, 01:47 AM   #5
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cut the transformer secondary wires that supply the rectifier. Using a dim light bulb tester powered up the unit and it was ok. The light bulb stayed Dim the whole time and I was able to measure 63 vac on the secondary that supplies the rectifier. a lot of the switch indicators on the front panel work.
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Old 23rd July 2014, 01:52 AM   #6
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This receiver was purchased overseas so on the back there is a switch that can be set for 240/220/120/110. The first two times it was powered up it was set to 110. Since then it has been switched to 120. Is this a possibility for blowing the fuse?

Should I reconnect the secondary lines to the rectifier and bring it up on the DLB tester?
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Old 23rd July 2014, 02:16 AM   #7
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Its a matter of disconnecting each stage until the mains lamp stays on.
Disconnect secondary to rectifier. Test.
Reconnect. Disconnect output from smoothing to pcb. Test.
If you get to this point it is probably output transistors blown.
Replace them but still test using lamp in series with mains.
If the lamp comes on then something in the driver, VAS or LTP stage will be at fault.
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Old 23rd July 2014, 02:59 AM   #8
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Selecting the 110V position over the 120V position is very unlikely to cause damage - if OTOH it is actually 100V (for Japan) it is possible that something got damaged.

Disconnect the power amplifiers from the supply capacitors and check for shorts from each rail to the outputs.

Check the diodes out of circuit if possible as well.
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Old 23rd July 2014, 01:53 PM   #9
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thanks for the support!

Disconnect filter capacitors (smoothing caps)C701 and C702 56V 12000 uF the big PS caps?

To disconnect from supplying the non filtered +48VDC and -48VDC to the amplifier output stage wouldn't the circuitry on the printed circuit board have to be cut?
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Old 23rd July 2014, 01:55 PM   #10
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Kevin, I wasn't thinking of damage as much as blowing the main 4A fuse in front of the transformer.

I want to rule this out as a possible cause of fuse blowing. The only other way is to reconnect secondary and see what happens to the mains lamp (DLB).

Last edited by rfitts46; 23rd July 2014 at 02:11 PM.
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