Technics SA-160 repair advise

[JayPare]

Why insane? None of the outputs from this section of the power supply are correct, and there are some burned parts. Any/all of the parts I mentioned could be bad, and replacing some of the parts and leaving other bad parts still in the circuit could result in those new parts frying again. Better to check them all the first time through. As for the 2 10W resistors.... The schematic indicates a 1/4 W resistor, and a 4.7 ohm 1/4W resistor will handle 200 mA or so. Should be sufficient. It does look like they are using that resistor as a safety fuse in the circuit, so replacing with the proper part is a safety related repair. 'I' might put a 1/2W resistor there, but I won't tell anyone else to do so.

Why are they there? Might be all the previous 'tech' had, or the original 4.7 ohm may have failed because something downstream is drawing too much current, and rather than fix downstream a high wattage resistor was used - kind of like wrapping a fuse in aluminum foil because the specified part keeps blowing.

Insane in the good sense! Meaning it blew my mind how it's clear and helpful! I wasn't expecting such help from the forum! You deserve a lot of kudos! (Sorry my native language is French).

 

[JayPare]

Ok I did some more work, here are the results (tested the transistors with a small apparel I bought in the past but never used it before):

Q701: hFE=216, vbE=574mV, IC=6.5mA
Q703: blown
Q704: blown

From the spec sheet, Q701 (2SD1762), IC should be in the 3A range and vbE should be in the 5V range. Can it be my tester?

D705: Reading 518 on diode mode on multimeter (what is that unit?) and Vz = 625mV on my tester.

Didn't find the spec sheet for this one. Schematic shows it as RVDMTZ6R2BTA. For other parts, I find RVD is only added by Technics so it might be MTZ6R2BTA.

R777: 4.9 Ohms
R778: 5.1 Ohms

Those seems in specs.

Q708 (2SB1185): hFE=109, vbE=577mV, IC=6.5mA
Q709 (2SC1685) : hFE=326, vbE=659mV, IC=3.1mA
Q710 (2SB1185): hFE=167, vbE=574mV, IC=6.5mA

All of them are out of specs, so me guess is my tester is not good. I will check how to test them with a multimeter.

Still need to take out D709, D710 and D711 and still need to test R779 and R781.

However, I never found the D781. I followed traces and looked all over the board several times, never seen it. Will look again when I will do the last step I didn't get time to do.

For the reading at the cathode and at the anode, should I use the DBT when doing it?

Thanks again!

 

[Ylli]

Sorry JayPare, something has come up and I am not able to follow through on troubleshooting this right now. Perhaps one of the other techs will be able to pick it up for you.

 

[JayPare]

Sorry JayPare, something has come up and I am not able to follow through on troubleshooting this right now. Perhaps one of the other techs will be able to pick it up for you.

No worry @Ylli, I really appreciate all you help! Wish you the best!

 

[JayPare]

Negative side. Do R777 & R778 appear OK? Better measure them with your ohmmeter - in circuit should be OK here. Then pull and test Q708, Q709, and Q710. Also diodes D709, D710, and D711. While Q710 is out of the circuit, test resistors R779 and R781. Add any bad parts you find to your shopping list.

Did the remaining work. Tested D709, D710 and D711:

D709 : Vz = 746mV
D710 : Vf = 1.31V, C = 6pF
D711 : Vz = 14.4V

D709 (MTZ30B) is supposed to be between 27.7 Vz and 29.13 Vz. Way off the measuring of my tester of 746mV.

D710 (MA29WA) is supposed to be, with an If of 3A, between 1.18 Vf and 1.28 Vf. No so far from the measuring of my tester of 1.31V.

D711 (MTZ15B) is supposed to be between 13.89 and 14.62 Vz. My measuring is right on point with 14.4V.

R779 and R781 are measuring fine at 120 ohms each.

Therefore, I think I will need a replacement only for D709. What do you guys think?

Let's power it up and measure the voltage at the cathodes of D701 & D703 (same point). With the speaker switch in the 8 ohm position, it should be 35 volts or so. Then measure the voltage at the anode of D781. Should be -35 volts or so.

Did the measuring at D701 and D703, got 43V. Never found the speaker switch though, neither the D781. However, at D704 and D702, I'm getting -43V. Is there anything wrong with the transformer?