Magnavox MX931P Recieiver from 1995

[itek]

Hi guys! I got in a Magnavox MX931P RO01 receiver here that has some channel issues. All inputs work, headphones out works fine, RIGHT channel on system A and B puts out very faint sound, LEFT channel on system A and B puts out NO sound. I have cleaned everything up as it was pretty dusty in there and I am still in the process of getting the boards out, will do more checking a post here when I do. Does this sound like a capacitor issue or something else? Thanks

[itek]

Re: Magnavox MX931P Recieiver from 1995

OK so I got the boards out, there is nothing on the bottom,, all caps ESR test OK except this little SMG 6800uf 25v which shows 0.04 ohms ESR which seems high? maybe not... I'm not sure where to go next, all speakers line solder points are good, the main unit is STK4192II if that helps, Thanks!

[retiredcaps]

Re: Magnavox MX931P Recieiver from 1995

LEFT channel on system A and B puts out NO sound.

I'm no receiver expert, but I have watched videos by

http://www.youtube.com/user/mdzacharias

and he is in the stereo repair business. From his videos, I'm guessing you have a blown output transistor?

Anyway, check out his channel for ideas.

[itek]

Re: Magnavox MX931P Recieiver from 1995

I am having a hard time finding these transistors, the only thing on the heatsink is the STK4192II IC......
PS Thanks for that video channel it is a great resource!

[retiredcaps]

Re: Magnavox MX931P Recieiver from 1995

I am having a hard time finding these transistors, the only thing on the heatsink is the STK4192II IC......

ebay has some ...

http://shop.ebay.com/i.html?_from=R4...All-Categories

[itek]

Re: Magnavox MX931P Recieiver from 1995

OK there are 2 GBU4D bridge rectifiers inline with the outputs that are both blown (i have continuity both ways between ~/+, ~/- and ~/~), would this cause my issue or should I give up on this thing? Thanks

[itek]

Re: Magnavox MX931P Recieiver from 1995

we posted at the same time! I don't know how to test that IC, need to do more reading.. gosh I have learned so much in the last month

[Tube_Dude]

Re: Magnavox MX931P Recieiver from 1995

OK there are 2 GBU4D bridge rectifiers inline with the outputs that are both blown (i have continuity both ways between ~/+, ~/- and ~/~), would this cause my issue or should I give up on this thing? Thanks

Have you measured the bridge rectifiers out of circuit? If not, that continuity come from the low resistance of the transformer secundary. If you had the rect. diodes shorted, your filter caps have gone BOOM long time ago...

[itek]

Re: Magnavox MX931P Recieiver from 1995

You are so right I tested them in circuit..... gosh I think i'm gonna give up on this thing, thanks guys!

[retiredcaps]

Re: Magnavox MX931P Recieiver from 1995

gosh I think i'm gonna give up on this thing, thanks guys!

I believe Magnavox is not mentioned in the same breath as Marantz, Arcam, NAD, etc in the audiophile circles so giving up on this one may be a good decision.

[Krankshaft]

Re: Magnavox MX931P Recieiver from 1995

I see this receiver is a Magnapox product . A great majority of amp issues can be traced to fault in the power supply, bad caps, or bad output transistors. Based on what you've done already I'd check the output ICs.

The Sanyo STK series of hybrid ICs are notorious for dying. Just ask guys who used to fix convergence issues in the CRT projection TV repair business.

To check them I'd power the unit up and hook up some audio to the input (like a steady sine wave tone from frequency generator on my PC) it's name escapes me at the moment but it's free. Then I'd use a scope and probe on the input of the STK IC if the signal is there I'd then probe on the output and expect to see a waveform with a higher amplitude.

Without a scope troubleshooting gets dodgy maybe build an LM386 audio amp on a breadboard and hook headphones to the output of the LM386 to check for the input signal.

[itek]

Re: Magnavox MX931P Recieiver from 1995

wow thanks Krankshaft for the insight! Looks like I better buy a scope, any recommendations?

[itek]

Re: Magnavox MX931P Recieiver from 1995

OK I got my Tektronix 465 and a couple probes today!!! Gonna check it out and post results soon

[Krankshaft]

Re: Magnavox MX931P Recieiver from 1995

A very nice scope. They weigh a ton but they're built like tanks and as time has shown they just don't die.

Proprietary ICs are kept to a minimum too unlike the Tektronix scopes from the 80s which are packed with large scale ICs that can't be found anymore.

Since the amp has isolation via it's transformer you can probe safely. But if you are ever working with unisolated devices connected directly to line voltage you'll need to plug the device under test into an isolation transformer to protect the scope.

[itek]

Re: Magnavox MX931P Recieiver from 1995

Wow OK good to know! I'm still getting this thing figured out, not sure what the best settings for this scenario would be.. In the picture is the output side of the board, the only line that shows an effect from turning the volume knob I lined in red, the rest show the same result as the input side of the IC,, maybe I need to adjust the scope some more? Thanks again for your help

[TheLaw]

Re: Magnavox MX931P Recieiver from 1995

Just wondering. Off topic. What's the wattage on this thing?

[itek]

Re: Magnavox MX931P Recieiver from 1995

well the STK4192II IC is rated 50w/chn @ 8 ohms minimum i believe