Technics 1210 Mk2 Internal Fuse keeps blowing

[dunksterp]

Hi there,

I'm new to these forums and am desperately trying to solve this problem.

I have a 1210Mk2 Turntable that the internal 250mA fuse keeps blowing on. It lasts for about half a second the blows once the power is turned on.

Is this a common problem and is there a common fix?

Cheers

Dunk

 

[dangus]

Based on the symptom, I'd check the bridge rectifier D1. Diodes sometimes fail short-circuit.
There's a copy of the service manual here if you need it:
http://www.aboutdj.nl/downloads/diversen/11-technics-sl12xx-mk2-service-manual/download.html

 

[burbeck]

hi Dunk
i would look at the target light wires where they enter the lamp tube the insulation breaks with the movement of the lamp tube up and down, to check this disconnect the wires to the lamp temporarily then put in a new fuse then try again.
this a very common problem

 

[dunksterp]

Based on the symptom, I'd check the bridge rectifier D1. Diodes sometimes fail short-circuit.
There's a copy of the service manual here if you need it:
http://www.aboutdj.nl/downloads/diversen/11-technics-sl12xx-mk2-service-manual/download.html

Do you know what it should be reading? From what I understand from this link...

Highland Electrix TV Repair Advice lo300v Page

With red connected to either ~ and black to positive I get about 727 come up on my multimeter.

With Black connected to either ~ and red to positive it flashes up a high number first 1600+, then settles at 1 on the left hand side of the screen.

With Black connected to either ~ and red to negative I get about 723 come up.

With red connected to either ~ and black to negative it flashes up a high number first 1600+, then settles at 1 on the left hand side of the screen.

I'm not particularly experienced in electronics, but I prefer to have a go, you know learn on the job sort of thing.

Cheers for any help 🙂

Dunk

Edit, if I connect red to negative and black to positive, I get the 1843 come up on the multimeter.

The other way round and I get a high number (around 1000) then it settles to number 1 on the left hand side of the meter.

Cheers again

 

[dunksterp]

hi Dunk
i would look at the target light wires where they enter the lamp tube the insulation breaks with the movement of the lamp tube up and down, to check this disconnect the wires to the lamp temporarily then put in a new fuse then try again.
this a very common problem

I have actually tried disconnecting the cable from the main PCB for the lamp and retested, it just blew the fuse again. Doh!

Cheers though 🙂

 

[sgrossklass]

If the fuse keeps blowing, the usual suspects would be the big filter capacitor (C1) or the rectifier bridge (D1).

(Power supply section, SL-1200M3D)
From your measurements, the rectifier bridge seems OK - diode voltage drop in one direction, no conduction in the other. The electrolytic may or may not be - it's not a dead short, but when these things age, they may not withstand their rated voltage any more and short out when a certain voltage is exceeded. The little 22µ 25 V (C3) after the voltage regulator is even more critical since it's operated closer to its rating. Try measuring across that. There's another electrolytic effectively in parallel to C3, C108 near the motor driver IC (100µ/25V).

You can try unsoldering C1 - the 'table won't work terribly well, but it shouldn't blow fuses any more. If it still does, reinstall C1 in its previous orientation and remove C3 instead. Same procedure for C108. If both C3 and C108 are out and you still measure a short, I'd guess the AN6675 is blown.

Before you do that, however, definitely check the regulator transistors. (A transistor can be treated like two diodes back to back here, see transistor model.) If, for example, Q1 were shorted out collector to emitter, our poor 25V electrolytics would see 30+ volts and be anything but amused. Same for a non-working Q3 (either base, emitter or collector disconnected).