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 (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=0B7FTDBmMiEfcbDBqQlA2V3FtWVE)
https://drive.google.com/open?id=0B7FTDBmMiEfcSHJrUTJIMlI4UHc (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)
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 (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 (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:
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?
thanks for letting me know how to fix the pics......I was wondering why I couldn't see them :tu:
I still can't embed images from google drive....
clicked on "insert image".....nope
clicked on "Insert Hyperlink".....nope
:-\
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.
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=0B7FTDBmMiEfcV0ZiT3Z3V2JzUGMAll 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!
Well, I'm still not seeing any pics embedded.
:grr
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.
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.
thanks
(https://lh4.googleusercontent.com/vNfZe_Iebjh5xrRo98pPX0FXEkTJFXOaq-mUg1n9-TR7wxXVn-Tk3atJw-zgAYIcqi6lpFsJ9X7unaY=w1680-h911-rw)
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.
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)
(https://drive.google.com/file/d/0B8JIpzle0qeRRVdTeHVFUVVWeHc/view?usp=sharing)
Now Hyper-link test with same file....
https://drive.google.com/file/d/0B8JIpzle0qeRRVdTeHVFUVVWeHc/view?usp=sharing (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.
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.
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:
(https://upload.wikimedia.org/wikipedia/en/5/57/Mr_Natural.jpg)
off all the posts, including my own, I can only see the image two images of "mr. keep on trucking" from R. Cobb, and the Fender power transformer image below that.
Every other post shows only links.
OK, then my examples worked. I posted three examples. The first one was indeed just a link, but anyone can click the link and open up the image. The next two were images, one from a URL and one uploaded from my computer. So three ways to get an image to the rest of us.
Google Drive seems to behave differently than other image hosting sites I've used.
Experimenting around & using the following:
http://drive.google.com/uc?export=view&id=0B7FTDBmMiEfcSHJrUTJIMlI4UHc
sticking that between <img> tags gets this:
(http://drive.google.com/uc?export=view&id=0B7FTDBmMiEfcSHJrUTJIMlI4UHc)
Quote from: aaron on June 07, 2017, 02:43:23 PM
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.
You may have a bad ground return from the reverb tank,, try cleaning the RCA sockets or even use another RCA lead as they are a common fail point.
Quote from: aaron on June 07, 2017, 02:43:23 PM
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.
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.
That could be a guitar ground issue and not the amplifier, check that the guitar bridge is properly grounded. maybe try another guitar to check.
Phil.
OK, after resoldering (applying the soldering iron) to the bad looking points, the amp sounded great on the overdrive channel, and the clean channel too! :dbtu: I didn't get a chance to try the reverb yet...
but what happened was that the ceramic resistors I mentioned were so freaking hot, that it was creating a strong smell. I didn't want the circuit board to go bad again b/c the resistors heating and reheating the solder, and have everything go back to the original condition again.
There's no way that those resistors should be that hot. I was wondering what to do, and I decided to check out the voltage vs. what it's called for on the schematic. I don't know why, but the AC voltage was less than half on one side, and like a fourth on the other side of what it's supposed to be
So now, I'm working on isolating the transformer to see where the breakdown in voltage is happening.
I really need to figure out why those resistors are overheating....
I'll keep you posted
Quote from: aaron on June 14, 2017, 02:48:28 PMThere's no way that those resistors should be that hot. I was wondering what to do, and I decided to check out the voltage vs. what it's called for on the schematic. I don't know why, but the AC voltage was less than half on one side, and like a fourth on the other side of what it's supposed to be
Please explain exactly what measurement you are talking about and relate that exactly to the schematic so we have a chance to understand better. Be detailed oriented in you explanation. Example-----> "I put black probe on chassis ground (earth) and the red probes was connected _________" Fill in the blank....
Just got to be sure we are approaching this the correct way. Check major DC voltage check points on the amp, main power rails and low voltage supplies.
Yep, you're right
when you're thinking in your head, it makes sense b/c you know what you're talking about.
Unfortunately, nobody else does.
I'll get back to this tomorrow.
Without re-reading the whole thread, many amps have zener low voltage supplies, and there will be a dropping resistor for each one. Those do get very hot normally.
Quote from: aaron on June 14, 2017, 02:48:28 PM
There's no way that those resistors should be that hot.
You'd be surprised. :o
Those are 5watt resistors because they get very hot. (R94 & R95 right?)
The voltages listed on the schematic are DC. One end of each 40V, the other end 16V. (negative voltages for R95)
So one meter probe at each end should give 24V
across each resistor.
Power formula to calculate watts is voltage squared divided by resistance. So (24 x 24) divided by 330. Approx. 1.75 watts. That is lots of heat.
So if there is about 24V across each of those resistors, they are running normal amount of heat. If the voltage is higher than that, then yes you have a problem.
The VDC across R94 and R95 is 22.7-22.8.
I cannot get a DC voltage reading when I put one DVM probe on the chassis and the other probe on either side of the resistor. The meter runs between 40 to 373mVDC with lots of fluctuation.
Chassis to R94 either side = 8.3 to 8.5 VAC
Chassis to R95 either side = same thing 8VAC.
Chassis to either side of CR25 or CR26 = 8 VAC.
Chassis to CR23 35 VAC
Chassis to CR24 18.5 VAC
Chassis to CR22 8.3 VAC
Chassis to CR21 8.3 VAC
I'm doing something wrong, but I'll be damned if I know what it is. I'm going to take a break from this, I'm too busy and it's driving me too crazy.
thanks to all
there are NO AC Volts there only DC Volts,, set your meter to DCV NOT ACV. 8|
Read *g1* notes again,, NO lead to Ground/Chassis your probes go either side of R94. You are then measuring the voltage drop Across that resistor which can then (with g1's maths above) you can then work out the wattage dissipated in that resistor) Do same for R95. :tu:
Phil.
Hi Phil, thanks for your response.
I wrote "The VDC across R94 and R95 is 22.7-22.8".
When I wrote across, that meant from one lead of R94 to the other lead of R94. Ditto for R95.
I was just trying to provide additional readings (that made no sense to me)
Thanks aaron,, opps sorry yes my mistake. :-[
Maybe just start from the power supply and work forward to the low voltages and post findings here as I'm sure the clever minds here will pickup on where any problems may exist.
With neg probe on CP6 which is circuit common (Ground) verify the test points on the schematic.
TP1 and TP2 are ACV,, should both read close to 35Volts AC.
TP3&TP4 expect +/-40~ish VDC on those two.
TP5&TP6 obviously around +/-16VDC.
If they read ok and the amp is working then all is well. 8)
IF the heat bothers you then you can lift the hot resistors further away from the PCB so the heat won't transfer and melt the solder. You may wish to use new parts to get longer legs but I just add longer wire.
I've seen some of those fender amps where those big resistors have burnt the resin from the fibreglass PCB and still working. :o So yes those ceramic resistors run hot and the Zeners will also likely give a wandering finger 3rd degree burns. If you want to go further replace the 5 Watt resistors with 10Watt units but mounting becomes a problem and the Zeners will still run really hot.
Hope that helps, Phil.
Like Phil said, the DC numbers you measured across those resistors sounds normal. That works out to 1.6W each, which is fine for 5W resistors. As hot as they may seem, it should not melt the solder. Any other type resistors you put there will generate the same 1.6W of heat, but just spread it out more. You can buy aluminum resistors that mount on the metal chassis, but that will heat up the chassis. :(
The only glitch here is that you are unable to measure DC at either end of those resistors. I'm wondering if the board is not fully mounted so maybe the chassis is not getting ground connection?
Try using CP6 as ground for your black probe like Phil suggested. What are your DC readings for TP 3,4,5, &6 ?
Hi guys!
I would have sworn I posted a reply yesterday.....
What I ended up doing was cannibalizing a small heat sink from another circuit board, applied thermal paste, attached it to the resistor, and voila! Now I sleep better at night. (I hope the amp appreciates my efforts!) I put it on R94 b/c it sits right on the board as opposed to R95 that is elevated a 1/4" by it's own leads.
The reverb still has a hissing noise, but I'll leave that for when I get my strength back.
Thanks to all, I really appreciate your help!
BTW, my attitude is that not only are you helping me, but maybe somebody else in Google land will see the posts and it will help them too!
Quote from: aaron on June 20, 2017, 03:14:53 PMWhat I ended up doing was cannibalizing a small heat sink from another circuit board, applied thermal paste, attached it to the resistor, and voila!
I'd like to see a picture of that.
Good on you aaron. :tu:
Regarding Reverbs; you may have to live with the hiss as it's a common complaint. :grr
Reason; To save a buck they don't use a very strong drive circuit and just turn up the gain on the return PU circuit to compensate for the weak reverb drive circuit.
Result; is a lot of excess hiss and buzz is introduced.
Short of a complete redesign you won't get rid of the hiss completely. :(
How do I know this? Cause I've experimented and built a lot of reverb circuits. 8|
Phil.
Quote from: DrGonz78 on June 20, 2017, 03:51:47 PM
Quote from: aaron on June 20, 2017, 03:14:53 PMWhat I ended up doing was cannibalizing a small heat sink from another circuit board, applied thermal paste, attached it to the resistor, and voila!
I'd like to see a picture of that.
:cheesy:
It's already closed up.
Just imagine an Oreo cookie.
The bottom cookie is the ceramic resistor,
the filling is the thermal paste,
and the top cookie is the aluminum heat sink (it actually only had about 1mm of play on either side of the resistor)
:lmao:
YUM!
Quote from: phatt on June 21, 2017, 07:49:51 AM
Good on you aaron. :tu:
Regarding Reverbs; you may have to live with the hiss as it's a common complaint. :grr
Reason; To save a buck they don't use a very strong drive circuit and just turn up the gain on the return PU circuit to compensate for the weak reverb drive circuit.
Result; is a lot of excess hiss and buzz is introduced.
Short of a complete redesign you won't get rid of the hiss completely. :(
How do I know this? Cause I've experimented and built a lot of reverb circuits. 8|
Phil.
Well, that sucks!.....I'm a perfectionist!
That, ironically, brings us back to full circle where someone suggested I buy a reverb pedal !