Sansui 7070 no power, please help

[amason2308]

Hi, long time lurker, first time poster here. I’ve had a Sansui 7070 for about 7 years, with it hooked up to a laptop + projector as part of our family home theatre setup. Though I don’t think I have speakers that match very well (90s Polks) I’ve always loved this receiver. Unfortunately my kids had the laptop volume low and receiver volume cranked all the way up and something has blown inside. I checked the four main fuses (5V on the outside, and the 3V, 7V, and 7V on the inside) and all are fine. When I try to power on I don’t see or hear anything, not even the relay clicking. Would someone with far more knowledge than I direct me where to look next? I’m comfortable with a multimeter and changing components, but I need some help directing me on where to look. Thanks in advance, folks on forums such as this are always so passionate and helpful.

 

[EastPoint]

Test the three fuse resistors. They’re in-between the two fuses and the other fuse in your second picture.

 

[amason2308]

These 3 here…? I added number labels to the pic.

 

[kwayde]

These 3 here…? I added number labels to the pic.View attachment 2981357

Yes

 

[amason2308]

R27 = 1 ohm
R28 = 1 ohm
R29 = 4.7 ohms

 

[Robisme]

Test the switched outlet on the rear of the 7070. It could be a power switch issue.

Rob

 

[amason2308]

With the receiver plugged into the wall, I get power to the unswitched outlet in the back, but not to the switched outlet.

 

[avionic]

I checked the four main fuses (5V on the outside, and the 3V, 7V, and 7V on the inside) and all are fine.

AC or DC ? ( AC ) Pull the fuses and measure there resistance with your multimeter. Looks like they could use a cleaning at least..

 

[amason2308]

Hmmm, I already tested for continuity on all of them and they all produced a beep, but here are the resistances as well…

The exterior back one (5A) = 0.0-0.2 ohms
F02 (7A) = 0.0-0.2 ohms
F03 (7A) = 0.0-0.1 ohms
F04 (3A) = 0.0-0.2 ohms

I also cleaned up the 2 dirty ones with some isopropyl using a cotton swab

 

[amason2308]

Test the switched outlet on the rear of the 7070. It could be a power switch issue.

Rob

By “power switch” issue, do you mean the power switch on the front?

 

[Hyperion]

By “power switch” issue, do you mean the power switch on the front?

Yes.

 

[amason2308]

I also found an old post that suggests R27 and R28 should be reading 4.7 ohms (see attached). I got 1 ohm on both of these. Are these resistors the likely culprit?

Or is the schematic correct and 1 ohm is fine?

 

[amason2308]

Yes.

Any suggestions on troubleshooting this switch? Do I need to remove the faceplate to access?

 

[amason2308]

Is s701 (as shown in the schematic) the power switch? This would explain why the unswitched outlet (CO701) has power but the switched outlet (CO702) does not. Or is S701 something else?

AK Database PDF Gallery - Sansui/Sansui 5500 Schematic

 

[Robisme]

Plug something in the switched outlet and hit the power switch on the front of the receiver. Does the thing plugged into the switched outlet work? Yes, power switch ok. No, power switch bad.

Rob

 

[amason2308]

Yes, with unit plugged in, unswitched works, switched does not. Thank you everyone, it feels good to have narrowed in on the problem!

I have removed the knobs and taken the faceplates off. I have removed the front screws securing the power switch to the frame, but I still can’t remove the switch without first taking off the black plastic button at the front. Is there a good way to do this? I feel like it should pull off (similar to the knobs) but if so it is a very tight fit. I am reluctant to pull too hard for fear of breaking it. Any tips?

 

[DB-7]

You probably could just trace the wires coming of the power switch back to the other end, and with the unit unplugged check for continuity of the switch where the wires end with meter on ohms or the sound signal if your meter has that function.

 

[DougBrewster]

This is a spare I have for my 9090DB. The knob pulled off easily though there are some traces on the plastic shaft that may have been some adhesive...

 

[amason2308]

Thank you DougBrewster! I will just have to be careful.

I did take the bottom baseplate off also, to trace those wires running from the power switch properly. They go to the switched/un switched sections at the back of the unit. No continuity So must be the switch itself.

 

[amason2308]

Ok, once I knew it was supposed to come off I pulled on it properly and it popped right off. Should I be buying a replacement part or trying to troubleshoot this broken switch? I’m also wondering if that other component could be a problem (a capacitor I think based on the schematic)