Toshiba SA 7100 protection mode

[ronjon489]

Hello All, first post so I hope it is in the right place. The receiver is in protection mode as subject indicates, however I'm focused on Q803 and Q804. The collector voltage on Q803 should be 28V. I'm getting only 2.2V. The voltage from Q804 Base to R812 is 24v but I get only the 2.2v from the other side of R812. I have no idea where and why I'm loosing 21.8 v. can some one please take a lo

 

[planet10]

dave

 

[rsavas]

The base of Q804 is pulled down, turning it on in protection mode.
One or more of q801,802,803 are on, you need to determine which ones are doing it and why.

 

[rayma]

Yes, that IS the protection circuit, and such operation indicates a problem elsewhere that is causing
excessive amplifier output DC voltage offset.

 

[ronjon489]

Ok thanks guys, I’m guessing I should start by checking L601/602 and the output transistors?

 

[rayma]

First measure the amplifier's output DC voltage. This will likely be nonzero.

 

[ronjon489]

Hello Rayma, found some of the 10 ohm resistors out of spec. Yes the output Dc voltage is zero. Order resistors.

 

[rayma]

It is indeed possible for the protection circuit itself to be defective, instead of the amplifier it protects. Good luck.

 

[ronjon489]

Hey intended to say DC was not zero.

 

[rayma]

Then what is the DC output voltage at the coil to ground (before the relay)?

 

[ronjon489]

Hello, I finally got back to the Toshiba SA 7100 and I’m getting sound but getting only 40volts both rails. When I adjust the idle current up the bulb brightens. Any idea of where to start troubleshooting?

 

[ronjon489]

Hello all, so I finally got is playing however I getting only 42v on the supply rails. In addition the two 60watt builds in the DBT look like there is still a problem. Not sure where to troubleshoot. Any help will be much appreciated

 

[rsavas]

Usually one uses the dim lamp as a quick go/no-go test. Usually at initial power up, the lamp is bright, as the power supply ecaps charge up, the lamp dims. If the load draws a normal current, output stages biased, the lamp will show dim and limit the supply so normal voltages will be reduced
try this
Turn the power amp output stage bias current to minimum, when power is off power back up the lamp glow will be very dim, the 42V you measured will rise a bit.
If these tests pass you can power up on regular supply

 

[ronjon489]

Hey thanks Rsavas, I bypassed the DBT and nothing bad happened 😢. However I don’t think the protection circuit is working properly as there is no pause before the relay opens. I’m looking at the schematic to see if I can determine where the problem is on this Toshiba SA7100.

 

[ronjon489]

Well l let the Toshiba run for a longer period without in being hooked to the MBT. The output transistors on one channel really hot. I trifled to set the idling current to the recommended 8mv to 2nd but could only get it to 60mv. Also noticed a hum coming from the transformer. Sound like something still isn’t quit right? Any ideas would be welcomed