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Pls help me fix Fender M-80 SS guitar amp

Started by aaron, June 07, 2017, 02:43:23 PM

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aaron

Hello Experts!

So I love to tinker, and my M-80, vintage 1980, has undergone the surgeon's scalpel (that would be me!)

BTW, two channels, clean (w/reverb) and overdrive.

It has two issues:

1. The reverb makes a sizzling sound (fizz?--imagine the sound of frying eggs)....The higher the reverb know is dialed, the louder the sizzle.Any idea which component would cause that?


Now, when the reverb is dialed to zero, the clean channel works fine, sounds fine, etc.

The board around the reverb knob definitely looks funky, the solder connections look dull, there's brown goo, etc., so I guess it's time to sort out which part is bad and replace it.  (BTW, the reverb tank tests fine, and the reverb WORKS).https://drive.google.com/open?id=0B7FTDBmMiEfcb0NHRzNHRmk4eW8




2.  The distortion channel has a "hum/wierdness/noise" that increases when you dial up the gain knob.  Other knobs (contour, presence, volume) in that channel don't affect the problem.

Unlike the reverb part of the board, everything looks fine via visual inspection, i.e., near the gain knob part of the board.

If there is no input to the amp, i.e., I ground/mute the strings with my hand to keep the guitar silent, the amp is silent too, no hum/wierdness/noise, diming the gain knob doesn't do anything.

3.  The large "cement block" resistors and two diodes in the power section (I think) are clearly overheating and the solder connections on the board are clearly dull and blackened.  There's a bit of brown goo there too.  The two large caps look OK, and the solder connections look fine, but there is a bit of brown goo...I'm not sure if this is a leakage from the neighboring overheating or not.  The parts that appear to be overheating are: CR25, CR26, R94, R95, R42, R43, C24, C25.   C48 and C49 also look abnormal on the underside of the board.

https://drive.google.com/open?id=0B7FTDBmMiEfcbDBqQlA2V3FtWVE


https://drive.google.com/open?id=0B7FTDBmMiEfcSHJrUTJIMlI4UHc





The amp is clean inside, the brown goo is not from spilled liquid.  The electrolytic caps look good.


One point of confusion I have is if what I think are the diodes/resistors that are overheating, is the problem, wouldn't that affect the clean side too? 

So I need help with what's causing the overdrive channel to make it's wierd noise.


Can you visually inspect a "glass" diode?  CR1 doesn't look like it's neighbors on the inside.

I've attached a M-80 schematic and pics that I hope will help you help me!

P.S., I don't have an oscilliscope

Thanks a million!! <3)


DrGonz78

#1
Quote from: aaron on June 07, 2017, 02:43:23 PM1. The reverb makes a sizzling sound (fizz?--imagine the sound of frying eggs)....The higher the reverb know is dialed, the louder the sizzle.Any idea which component would cause that?

Now when the reverb is dialed to zero, the clean channel works fine, sounds fine, etc.

The board around the reverb knob definitely looks funky, the solder connections look dull, there's brown goo, etc., so I guess it's time to sort out which part is bad and replace it.  (BTW, the reverb tank tests fine, and the reverb WORKS). NOTE: This was hidden inside message but I don't think other's will see that he added this image... https://drive.google.com/open?id=0B7FTDBmMiEfcV0ZiT3Z3V2JzUGM
Is this something that has recently developed or has it been this way since you have owned the amp? The brown goo you mentioned sounds like it is left over flux residue which is commonly found from solder joints. Best to clean it up and re-solder these joints while you have it out. Looking at the picture attached it appears that dull solder joints could be cold soldered connections. Remember it does not always have to a bad component that needs replacing and could be just a bad connection. Also, that picture of the board shows a trace clearly lifted up probably from someone soldering it too many times and/or with too much heat applied. Inspect all the solder joints looking for not only dull looking cold connections but also any cracked joints too.

Quote from: aaron on June 07, 2017, 02:43:23 PM3.  The large "cement block" resistors and two diodes in the power section (I think) are clearly overheating and the solder connections on the board are clearly dull and blackened.  There's a bit of brown goo there too.  The two large caps look OK, and the solder connections look fine, but there is a bit of brown goo...I'm not sure if this is a leakage from the neighboring overheating or not.  The parts that appear to be overheating are: CR25, CR26, R94, R95, R42, R43, C24, C25.   C48 and C49 also look abnormal on the underside of the board. NOTE: Another picture was hidden here in the text that could not be viewed....https://drive.google.com/open?id=0B7FTDBmMiEfcbDBqQlA2V3FtWVE
Those are cement wire wound resistors and they drop voltages, most times to the preamp circuits. They do get hot and they do overheat the circuit boards over time creating a browning of the board itself. In fact, those diodes in the picture it looks like they heated up leftover flux residue on the top side of the board and it has turned dark brown. Clean all that brown flux residue up while in there with a alcohol and a q-tip. All very normal and you should focus your energy right now to your very first question about dull looking solder joints and the possibly damaged circuit board traces.

COMMENT: Please attach your google drive images as url hyperlinks rather clicking image tag or we won't be able to see them on the post. Thanks. :dbtu:
"A person who never made a mistake never tried anything new." -Albert Einstein

aaron

thanks for the advice!

I haven't used the amp in a long time, and unfortunately I can't remember how the problem developed.

The reason I mentioned the brown goo is b/c I didn't think it was rosin from the solder.....b/c the rest of the board is so clean.

But I will do as you say, and will resolder anything that looks bad.

Do you have any ideas about what could be causing the overdrive channel noise, other than bad solder joints?

aaron

thanks for letting me know how to fix the pics......I was wondering why I couldn't see them :tu:

aaron

I still can't embed images from google drive....
clicked on "insert image".....nope
clicked on "Insert Hyperlink".....nope
:-\

phatt

The good DrGonz already mentioned the brown goo,, Na that is normal and nothin to worry about as it's not conductive,,, if it bothers you you can gently scrape it off.
A lot of fender circuits from that era do run those parts hot and if it has run this long then I'd leave it alone and only fix it when it does eventually fail.
R94 and R95 drop a lot of voltage and will run hot.
When they run too hot they change colour and often darken the PCB and de-laminate the copper tracks,,, then you worry. ;)
The main Electro Caps (C48 and C49) are a common cold or cracked solder problem and often need to be touched up.

Regarding the *wierd noise* well most of those Amps do make a fair bit of excess hizz and buzz when the OD/ distortion channel is used especially if you crank the gain up high,, again normal.  8)
Worth a note;
**These amps are loud and intended for LOUD Venues**
A common mistake is using these amps at bedroom level and in that situation the Fizz is quite obvious BUT at Gig levels you don't hear it.
So crank the master Volume and if the fizz/hum/buzz is louder than your guitar then yes the circuit maybe faulty.

Having repaired a few of the fender amps from this era the players often NEVER use the Distortion Channel as it's not that good xP,, more often than not they just leave it on the clean channel and use pedals for all the OD/Dist sounds as pedals give you many more options. :dbtu:
Phil.

DrGonz78

#6
Quote from: aaron on June 08, 2017, 06:06:46 AM
I still can't embed images from google drive....
clicked on "insert image".....nope
clicked on "Insert Hyperlink".....nope
:-\

https://drive.google.com/open?id=0B7FTDBmMiEfcV0ZiT3Z3V2JzUGM
All I had to do was to copy paste link between url tag code. [url][/url] Sounds like you tried but perhaps it is a result of being a new member here. We had problems with people joining the forum only to post a spam url link. I am not 100% certain but as a result Joecool (webmaster) could have set it up so new members would have to post 5-10 posts before allowing members to post links on the page?

Edit: oh never mind I just noticed it is now working in the original post and I see the links, nice!
"A person who never made a mistake never tried anything new." -Albert Einstein

aaron

Well, I'm still not seeing any pics embedded.

:grr

aaron

Quote from: phatt on June 08, 2017, 09:09:14 AM
.

Regarding the *wierd noise* well most of those Amps do make a fair bit of excess hizz and buzz when the OD/ distortion channel is used especially if you crank the gain up high,, again normal.  8)
Worth a note;
**These amps are loud and intended for LOUD Venues**
A common mistake is using these amps at bedroom level and in that situation the Fizz is quite obvious BUT at Gig levels you don't hear it.
So crank the master Volume and if the fizz/hum/buzz is louder than your guitar then yes the circuit maybe faulty.

Having repaired a few of the fender amps from this era the players often NEVER use the Distortion Channel as it's not that good xP,, more often than not they just leave it on the clean channel and use pedals for all the OD/Dist sounds as pedals give you many more options. :dbtu:
Phil.

The noise/hiss/whatever is way beyond normal, so it's the circuit, not the amp's characteristics.

I don't see how not using half the amp's channels increases my options.

I was hoping for electronic advice....

In any case, I've resoldered all the usual suspects.  I'll report on the results as soon as I put it back together.

DrGonz78

Quote from: aaron on June 08, 2017, 04:53:20 PM
I don't see how not using half the amp's channels increases my options.
I was hoping for electronic advice....

Our advice is given based on what questions have been asked. It will take a bit patience with us to remotely help you as we are not there to observe the amp directly. At this point I only mentioned touching up solder joints since you had taken the amp apart already and observed some solders that looked questionable. From the start we would not have advised to start by removing the circuit board. Once the amp is put back together then the electronic advice will get more in depth and the community here will help as much as we can. Best regards.
"A person who never made a mistake never tried anything new." -Albert Einstein


Clyde

#11




Don't know much about Google Drive...but clicking on the link brought up the image & rt click/copy image address & pasting between tags as Image Insert got me this.   Seems like there should be an easier way but at least the image posted.

DrGonz78

#12
Clyde I can't see a link or anything it is just white space...??

This is a test of the google drive posting. Attempt with img tag... Plus I set google drive image properties so it is public too.(directly below this img tag was used to post image file link)


Now Hyper-link test with same file....
https://drive.google.com/file/d/0B8JIpzle0qeRRVdTeHVFUVVWeHc/view?usp=sharing

Also below you can click attachments of saved images from your computer below. So why not just do it that way from the start instead of hosting the image on google drive or some other website. Then if you delete the image from google drive it won't disappear from the site here.

Previewing it ahead of clicking post show the img tag is not showing on my post.
Edit: however later they might show up as they did in the original first post of this thread but like a day later and as a hyper link. But that must have the OP going back to change the tags from IMG to URL.
"A person who never made a mistake never tried anything new." -Albert Einstein

Clyde

#13
Interesting.  On my previous post the image showed in preview & after posting & I thought all was good.  Clear cache or try from another browser & blank goes the image.  Sorry about the dead end. 


Enzo

When you can see it but no one else can, it usually means you took the image from your memory, so when you look back at the page it simply recalls the image from memory rather than downloading it.

You have two sources of images, your own computer, or some internet site.  You can click on attachments below here, and click the Choose File button.  The box opens up so I can search my files for an image, which I then select, thus Fender transformer below:


If you find an image on some web site, just copy and paste the URL from the top of the page.  Like this:
https://upload.wikimedia.org/wikipedia/en/5/57/Mr_Natural.jpg

Or I can paste that link in, then highlight it with my cursor, and then click Insert Image from top left to insert the image directly: