SX-727 with failing left and right Channel at high outside temperatures

[pa_200]

Today I had my SX-727 outside on the patio and adjusting the Bias und DC Offset after some work on the Head Amp Unit W21-002 and the Protection Board AWM-027. After the adjustment I had a successful DBT-test.

We had some high temperatures above 86 °F and the open unit was sitting in the bright sunlight for 2-3 hours.

Later, but still hot when starting a listening test with tuner and speakers connected, I had intermittent failures on one channel, either on the right and sometimes on the left sides in Stereo mode. Quite irregular.

Switching to Mono didn’t help as well, the failing channel stayed for some time.
My first idea was the Head Amp Unit W21-002, but that wouldn’t explain the missing channel in Mono.

After the outside temperature dropped below 80 °F, both channel work fine.

I hope it’s not a stupid question, but what part could be so heat sensitive?
Has anyone had already a similar experience?

 

[pa_200]

Looks like I found the culprit.

Checked both channel from the Head Amp W21-002 via AF Unit AWK-010 – Power Amp AWH-011 and Protection Unit AWM-027A to the Protection Relay.

Found a higher resistance between the closed contacts due to some dirt on the contacts. After carful cleaning the contacts with alcohol on a piece of cotton fabric, the SX-727 runs now for several hour, even at Temperatures above 100 °F without any drop out.

Will have a close look on it and if that happens again, I drop in a new relay.

 

[larryderouin]

Keep it under an umbrella or awning outside.

 

[leich mcous]

Looks like I found the culprit.

Checked both channel from the Head Amp W21-002 via AF Unit AWK-010 – Power Amp AWH-011 and Protection Unit AWM-027A to the Protection Relay.

Found a higher resistance between the closed contacts due to some dirt on the contacts. After carful cleaning the contacts with alcohol on a piece of cotton fabric, the SX-727 runs now for several hour, even at Temperatures above 100 °F without any drop out.

Will have a close look on it and if that happens again, I drop in a new relay.

hi i am wondering which contacts you cleaned on the relay ? and how -thanks for intel just have my brothers 727 to fix and am continuing the rersearch mike

 

[larryderouin]

Clean both the top and the bottom contacts along with the flipper (solenoid arm) contacts. DeOxit on a strip of paper and pull it thru the closed (manually manipulate the solenoid arm) contacts. One pull, one piece of paper in one direction only. Do this until no discoloration is seen on paper. Then do the other contacts. This can usually last 1-2 years. Inspect the contacts for pitting. If pitted, REPLACE! Better now than later. Replacement Relay # should be in this thread around post 40 IIRC. https://audiokarma.org/forums/index...-1010-939-838-et-al-relay-replacement.150439/

 

[pa_200]

If you open the relay and want to clean the contacts, take special care with that tiny little spring on top. It likes to jump of and disappear in your home for ever.

Cleaning will help in the first place, but next time I have that issue, I’ll replace the relay with a new one. For the replacement have a look here.

https://audiokarma.org/forums/index...-relay-replacement.150439/page-2#post-6759577

And one more advice, add a flyback diode to the protection board as well. See last picture bottom left.
Quote: avionic and markthefixer said,
cathode (white ring marking/banded end) of the 1n4004 diode is to be connected to awm-027 pin 15. Pin 14 is to be connected to the other (UN-banded) end of the 1n4004 diode.