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Sansui AU-717 Protection Circuit Problem - no Click, No LED

Zekeman

Zekeman

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I have a fully restored a 717 that in a previous thread had and still has an arcing power switch issue. Besides that issue, the amp worked fine with the exception of the power switch impeding current apparently, but it did work fine but now the LED light the power light doesn’t even flicker and it will not come out of protection. I have it disemboweled on my bench. I have no external boards connected to the power supply - I get + and -50 V on the rails off of the power supply yet the relay doesn’t click so it won’t come out of protection and the LED does not illuminate. One issue I’m having with the protection circuit is there are no voltage is posted on schematic IMG_2499.jpegso I’m not sure what voltage the LED is supposed to be getting. I believe my last measurement was ~3 V, but I’m not sure where to start on troubleshooting this puppy.

Any advice at this point would be great greatly appreciated!

Zeke
 
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The protection led anode should be getting roughly the same voltage as the positive rails going to the amp boards. That voltage also coincidentally powers the protection circuit so that should give you a good indicator of where to start looking. Measure at point 32 then compare to the + pin of C601, they should be very close except that the LED voltage will have lots more ripple which will be obvious on a scope but might get averaged out by a dvm
 
Good morning. On that device, as well as on the AU517, it's very easy for the power LED to turn off while you're working on it. (I don't think that's the case here, but check it). The connector on the front panel slides out easily.
In any case, for that problem, I would check the voltages on the protection board, following the path of the electrons ;-) until you find the problem.
 
It's hard to be specific about those voltages, as the voltage is pulled down by the energising of the relay coil when TR603 turns on. Also the output of the rectifiers D601 & D602 isn't smoothed straight away (so it will be 'rough DC'), it passes through R601, 390Ω before it 'sees' the smoothing effect of C601, 47µF, 50V. (NB: It's a good idea to upgrade this capacitor to 100µF, but no higher).

Note also that those rectifiers have been known to fail, sometimes just one might fail, giving very confusing protection performance.

I think I am correct to say that you will see about 44V at the rectifiers, and about 38V on the protection circuit side of R601, dropping to ~27V when the relay is energised. (those voltages are from memory). The actual voltages you will see will vary somewhat (say +/- 2-3V) depending on your local mains voltage. The LED is attached to the PSU protector board via red and white wires coming from pins 32 & 33 (on F-2663) - clearly shown in your picture at the centre of the top edge of the board. (note the LED won't light if it is plugged on to the connector the wrong way round - long leg on the LED goes to the red wire. ;) )
 
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Thanks. The LED is in correctly.

I am going to diagram voltages throughout the protection circuit this weekend to see where things have gone off the rails. I'll post my findings.
 
If it makes your testing easier to move the faceplate away, you can plug just about any other loose LED into the connector to see the blinking light. Just be careful not to let the LED legs stick out the back of the connector and bob around near the chassis, ask me how I learned that lesson :yikes:
 
Zekeman said:
Thanks. The LED is in correctly.

I am going to diagram voltages throughout the protection circuit this weekend to see where things have gone off the rails. I'll post my findings.
Click to expand...
I've got one on my bench right now (chasing down a weird left channel dropout on phono stage only). I'll check some voltages & see what I can give you.
 
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Power LED wont be on if unit is in protect/TR606 is off.
101 testing, verify no excissive Vdc, measure at F-2663 pin 27(L) and pin 28(R).
Measuring voltages at zener ZD601 and then D606 both anode (to GND) then cathode (to GND) may be helpful.
 
mbz said:
Power LED wont be on if unit is in protect/TR606 is off.
Click to expand...

Then on, then off again, repeat until out of protection ;) Seriously, the power LED does blink when in protection, and that behavior is driven by TR606 and TR607 alternating being turned on until TR603 enables the relay, locking the power LED in the on state.
 
Gents,

After after measuring voltages from the right side to the left side of the schematic, I discovered a cold solder joint on the downstream side of R601. Voltage is shown here are before I found the cold solder joint.

IMG_2513.jpegThe amp now comes out of protection (no other circuit boards connected to F-2663) and the LED stops flickering after several seconds.

I should have begun on the left side…!!
 
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Yea so this thing still has some gremlins in it.

Yes, so I just powered it up cold and the relay clicked as normal after four or five seconds but the LED kept flashing for a good 30 seconds or so before it stopped so something else is going on with this power supply.
 
For LED flashing issue:

Change C601 to 100uf/50V
Replace TR606/607 in Multivibrator circuit
Replace C606/607 if you haven't

See if this cures the problem
 
Replaced 606-7 and upped the timing cap to 100uf. Relay engages after 4 sec yet the LED flashes three time more after the relay engagement.
 
So we now have three extra flashes from 30 second flashing from your previous post? Please confirm. The relay timing is controlled by C604 and R609. You can try to bump up C604 to 560uf or 680uf - may give the power supply more time to settle down before engaging the relay and turn off the multivibrator circuit (flashing)
 
Swapped TR606/7 or C606/7, or all? The multivibrator circuit (TR606/7, C606/7, etc.) is there to flash the LED until the relay is activated and doesn't do anything else. You could ignore the 3 extra flashes with no down side except however much the extra blinks might annoy you. If you want the extra blinks gone, you could try to pick some high hFE KSC945's to replace TR606/7, check that the values of the resistors have not drifted high and that the values of C606/7 are also not overly high.

How long does it take for those extra blinks to stop anyway?
 
Have we replaced D607 yet? - and what is the collector voltage of TR603 with the relay energised?
 
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