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Author Topic: Pls help me fix Fender M-80 SS guitar amp  (Read 1019 times)

aaron

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #15 on: June 10, 2017, 05:10:35 PM »

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.
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Enzo

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #16 on: June 10, 2017, 06:23:17 PM »

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.
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Clyde

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #17 on: June 11, 2017, 03:07:44 PM »

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:
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phatt

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #18 on: June 12, 2017, 08:29:30 AM »


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.


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.
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aaron

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #19 on: June 14, 2017, 02:48:28 PM »

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
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DrGonz78

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #20 on: June 14, 2017, 03:01:56 PM »

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

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.
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A person who never made a mistake never tried anything new. -Albert Einstein

aaron

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #21 on: June 14, 2017, 04:57:23 PM »

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.
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Enzo

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #22 on: June 14, 2017, 08:42:05 PM »

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.
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g1

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #23 on: June 15, 2017, 12:34:51 AM »

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.
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aaron

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #24 on: June 18, 2017, 03:13:57 PM »

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

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phatt

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #25 on: June 18, 2017, 10:21:22 PM »

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.
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aaron

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #26 on: June 19, 2017, 03:11:42 AM »

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)
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phatt

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #27 on: June 19, 2017, 08:14:35 AM »

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.
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g1

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #28 on: June 19, 2017, 02:12:26 PM »

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 ?
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aaron

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Re: Pls help me fix Fender M-80 SS guitar amp
« Reply #29 on: June 20, 2017, 03:14:53 PM »

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!

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