Sansui G-8700db AC Protection problem

[OleJeepGuy]

After much searching and reading, I didn’t find the answer to my question. So here we go.

I recently brought home a Sansui G8700db. I just finished rebuilding the audio section. But before I plug in the rebuilt F3074 driver amp boards, I wanted to check all the units power supply outputs.

With both F3074s & the Output Power transistors removed from the unit, I connected power cord thru my DBT. I pressed the units power button and here is what happened:

DBT light ramps bright but within a couple seconds, dims to very dim. This is normal behavior.

I noticed the Soft start protection relay (RL601 on F-3072 Protection board) did NOT set. But after a few seconds, RL602, the speaker protector, clicked set and the front panel Protection LED went from flashing green to solid green.

So with the AC power relay not setting, I noticed 2 things:

The input voltage drops from 119vac to around 90vac.

I then measured the Transformer output.
The power transistor collector voltage s/b +/-75vdc, but tops out at +/-55vdc. So, Input ACV drops about 26% (T701 input) and the DCV output drops a similar 26% (T701 output thru rectifier).

All during this time unit is running with no smoke, no smells, and no heat anywhere I can feel.

I’m thinking all of this AC anomaly is due to Power protect relay NOT setting.

And now for my question:
F3072 IC1 (HA-12002) houses the power protect logic.
What power parameters will prevent IC1 from setting RL601?
Does anyone have any tips on how to troubleshoot this logic?

Thanks in advance.

 

[mrk229]

Do you know if the soft start relay pulled in before you started rebuilding the unit?

What wattage is your DBT bulb? Sounds like it may be too low

IC-1 does not control RL601

Voltage to actuate RL601 comes into the protection board at pin 7. Check it there.

There is a zener diode across the coil that could be shorted.

There are two 1.8k two watt resistors that complete the relay circuit to ground. R613 and R614

 

[OleJeepGuy]

Do you know if the soft start relay pulled in before you started rebuilding the unit?

What wattage is your DBT bulb? Sounds like it may be too low

IC-1 does not control RL601

Voltage to actuate RL601 comes into the protection board at pin 7. Check it there.

There is a zener diode across the coil that could be shorted.

There are two 1.8k two watt resistors that complete the relay circuit to ground. R613 and R614

Thx mark! I like the DBT bulb size idea. Didn’t think of that as it went dim. I’ll check it.

When I first bought the unit, I heard a click (didn’t check which relay clicked) but green LED blinked forever. When I pulled the driver boards and output transistors, the green light went solid but no RL601 action.

Funny how the speaker relay engages and the protect light goes green with all the audio driver section removed from the unit?

Thanks for your check list. I’ll check run thru it all after church today.

Blue Skies

 

[OleJeepGuy]

Thx mark! I like the DBT bulb size idea. Didn’t think of that as it went dim. I’ll check it.

When I first bought the unit, I heard a click (didn’t check which relay clicked) but green LED blinked forever. When I pulled the driver boards and output transistors, the green light went solid but no RL601 action.

Funny how the speaker relay engages and the protect light goes green with all the audio driver section removed from the unit?

Thanks for your check list. I’ll check run thru it all after church today.

Blue Skies

mrk what wattage DBT build do techs use for these high (100+W/ch) powered units?

 

[mrk229]

mrk what wattage DBT build do techs use for these high (100+W/ch) powered units?

I typically use a 116 watt (odd size, it's for traffic lights) bulb for most anything except very low powered units.

Units with a built in soft start can be very finicky about incoming voltages. With both the DBT and the softstart resistor in line you have a double voltage drop.

I expect based on some of your measurements that the combination of the voltage drop from the DBT and the soft start resistor is just dropping too much voltage. You calculated 26%.

Those soft start relays can be very picky about voltage. The voltage at pin 7 will tell us a lot.

 

[OleJeepGuy]

Hello again mrk. You are right on the “mark”! I ordered some 100 & 200w bulbs online and whala, question answered. I was using a 65w DBT bulb. That with the soft start resistor dropped the input voltage (as measured on the DBT) to 93v. This prevented the power relay from engaging.

With the 100W & 200W bulbs the input voltage dropped to 104vac & 114vac respectfully. And power relay engaged with both. Yeah!!!

This was the first power protection problem I’ve experienced. A great learning experience for me.

I appreciate your great help in walking thru this issue with me. Have a great day! Hank

 

[mrk229]

Hello again mrk. You are right on the “mark”! I ordered some 100 & 200w bulbs online and whala, question answered. I was using a 65w DBT bulb. That with the soft start resistor dropped the input voltage (as measured on the DBT) to 93v. This prevented the power relay from engaging.

With the 100W & 200W bulbs the input voltage dropped to 104vac & 114vac respectfully. And power relay engaged with both. Yeah!!!

This was the first power protection problem I’ve experienced. A great learning experience for me.

I appreciate your great help in walking thru this issue with me. Have a great day! Hank

Awesome. Glad it worked out.