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 Post subject: RCA New Vista MGW78W One Channel Distortion
PostPosted: Aug Tue 31, 2021 5:03 am 
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Hello there group,

This is my first post here to this forum and I'm hoping that I could receive some help in my current project and hopefully learn something new along the way as I'm still a novice in electronics repair.

I recently acquired this RCA New Vista record player console Model#: MGW78W.

Before even plugging it in I noticed a few of the plastic cased ecaps in amp were cracked and broken, so I replaced all ecaps in amp. Noticed that one channel was low and distorted or static. Thought it may be an output transistor but all four checked out with VM. Did a clip lead test to switch inputs to see if it was isolated to one output channel. The issue moved as I swapped clip leads so issue must be in preamp.

I then replaced all ecaps in preamp and unfortunately same issue.

Up to this point I've been using the FM stereo option which shows that the right channel has low volume and static. To test a different function, I switched to the phonograph and the issue switched channels.

I cleaned all controls and pots with contact cleaner and worked them all throughly.

From anyone's experience, does this sound like the issue is within the function selection switch or transitor within preamp? Could the switch change left to right depending on function but there be an issue component on a board that obviously is isolated to one channel?

I've been using sams photofact set 820 folder 6 for a guide.
Tuner is Model#: RC-1218H
Amp is Model#: RS-209B

I have attached photos for better visual reference.

If anyone is willing to help me it is most appreciated as I'd love to hear this units full potential!

Thank you all


Attachments:
File comment: Amp
20210727_162254.jpg
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File comment: Tuner top
20210727_201219.jpg
20210727_201219.jpg [ 836.12 KiB | Viewed 553 times ]
File comment: Tuner bottom
20210727_201231.jpg
20210727_201231.jpg [ 927.45 KiB | Viewed 553 times ]
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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Wed 01, 2021 2:25 am 
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Joined: Dec Mon 23, 2019 5:52 pm
Posts: 618
Low volume is usually a coupling or by-pass cap. Static is common with those germanium transistors. Do you hear static with no FM or phono signal? There are 4 transistors in each channel of the pre-amp section. Using Sams numbers, if the noise is not affected by the volume control, X21 and X22 in the right channel would be suspect. If static is only heard with the volume turned up, but is not changed by turning the balance or tone controls, then try X20. Finally, if the noise does change with the adjustments of the balance and tone controls, suspect X19.


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RCA MG stereo.jpg
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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Wed 01, 2021 2:33 am 
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Joined: Dec Mon 23, 2019 5:52 pm
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I forgot to ask how AM sounds. The FM multiplex board has 2 transistors and 2 caps that could cause low volume only on the FM stereo setting . Low volume and static on the other channel in the phono position could be the cartridge.


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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Wed 01, 2021 8:09 pm 
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Joined: Aug Tue 31, 2021 3:39 am
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Thank you so much for your response. I'm thankful that someone can decipher the schematic down to the level of detail you have provided, I am not that experienced yet so thank you!

I need to look into getting a transistor tester potentially.

So after going back over the unit I did a little better job of documenting each function selection and jotting down my observations. I have listed them below:

FM Stereo:
- static only from right channel
- worse with treble adjustment but really I think only makes it more noticeable
- lower volume observed from right channel with each balance set

FM:
- static from both channels
- even volume, but lower than than observed from left channel during FM stereo

AM:
Same as FM

Phono:
- added new cartridge and made sure tonearm lead wires were properly soldered and inserted.
- static in right channel, even when phono completely disconnected
- not as loud as I think it should be when at full volume, comparing to left channel during FM Stereo


Side Notes:
- static affected by volume, silent with volume at zero
- static goes away when balanced to left channel only

I hope this helps to pinpoint potential issues and again thank you for all advice!


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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Thu 02, 2021 1:52 am 
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Joined: Dec Mon 23, 2019 5:52 pm
Posts: 618
Since the balance control quiets the noise I would suspect transistor X19 located behind the balance control. The original number is 2N2613 and it crosses to the NTE sub part number NTE102A. The noise these transistors make is sometimes described as "bacon frying". Touching a hot soldering iron tip to the case of the transistor for a second will usually make the noise worse, while cooling it with freeze spray or canned air will quiet it, just to prove the fault.


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RCA MG stereo 002.jpg
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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Thu 02, 2021 2:13 am 
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Joined: Aug Tue 31, 2021 3:39 am
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Thank you again for your help.

I will give the heat/cold test a try and maybe I can pull the transistor to test with multimeter.

Any recommendations for best source for this part? Ebay had a few NOS, but they are $$$.


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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Thu 02, 2021 2:27 am 
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I tried cold test with can of contact cleaner. Might not be as effective since I did not notice any change. I spray each of the previously mentioned transistors for right channel.

I did notice that when I tapped the top of X19 loud popping was observed from right speaker. I tired this on that others and only very quiet popping if anything. X15 did not make any popping. Would this indicate bad transistor by chance?


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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Thu 02, 2021 2:27 am 
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Joined: Dec Mon 23, 2019 5:52 pm
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Testing with a meter may not tell you much. Most all germanium transistors check strange. I don't know of any sources other than Googleing the part numbers.


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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Fri 03, 2021 4:31 am 
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mickman23 wrote:
I did notice that when I tapped the top of X19 loud popping was observed from right speaker. Would this indicate bad transistor by chance?


Try reflowing the solder joints and look for cracks in traces.

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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Wed 08, 2021 6:22 am 
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Joined: Aug Tue 31, 2021 3:39 am
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No cracks in traces and tried to reflow solder. No change.

I did receive replacement transistor for X19 and that did the trick. No more static and volume has definitely increased on right channel, not as much as displayed by left channel but I may just be too picky. With balance adjusted slightly to the right you can't tell the difference.

Thank you all for the help! It is greatly appreciated!


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 Post subject: Re: RCA New Vista MGW78W One Channel Distortion
PostPosted: Sep Thu 09, 2021 2:06 am 
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Joined: Dec Mon 23, 2019 5:52 pm
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Glad you got it resolved. Thanks for posting the solution.


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