Fender M-80 Chorus

[Dave G.]

My sons friend has a Fender M-80 Chorus he brought home for me to look at. I pluged it in and found it had a weak, very distorted sound. I plugged the pre out into my board and had a good clean sound so I determined it was the power amp. After removing the amp from the cabinet and looking at the power section I found R133 was burnt almost in 2. Also C65 looked like it had been hot, and R138 was open. We sent to Fender for a schematic and they sent us everything except the power stage. Does any one have a schematic, or is anyone familier with this that can give me some direction?

[tonezoneonline]

Email me .I may have what your looking for.
Mike

[Enzo]

And if he doesn't have it I certainly do.

Did Fender send drawing 037023? That should be a two pager with page 2 being the power amps and power supply.

There are two power amps that are identical, so you can always compare reisitance readings between them. Here is how I see it. R138 is the .47 ohm 5w ballast resistor for Q8. If that is open, it is a safe bet that Q8 is shorted - the output xstr. And if Q8 is shorted, a lot of stress was placed upon Q9, so replace them both.

Since Q8 was shorted, once R138 opens, the path for current is through R132,133. Check CR24 and Q6 for shorts. You know, for a 2 cent diode and an 8 cent 2N4401, I'd just replace them and be done with them. R132 = 1k, and R133 = 270 ohm.

If the full +40VDC got through R133 - possibly through R132, but could been through Q6 - that would certainly stress or ruin C65, which is only a 16v part.

NOTE: Look for R127, 129. They are the large 2w resistors in front of the output xstrs. If they are still 1k, chance them to 2.2k. This is a factory change to reduce bias current. AMp will run cooler and more reliably. Also on the other channel, similar resistors R154, 156. FIX THE BLOWN CHANNEL FIRST.

Look, the whole power amp is four transistors, a handfull of diodes, and some passives. Test all the xstrs with a meter set for diode. Looking for shorted ones. You are already replacing the output xstrs. Then go through the area and just test every diode for drop. A couple are zeners, but who cares? SHorted is shorted, and open is open. If a zener checks OK forwards as a diode, then chances are the thing zenes too. Especially don't forget the four diodes in the heat sink holes.

Don't forget to check R140, another .47 ohm 5w over by the speaker wires. It is in the speaker return leg and could have been damaged by DC on the speaker.

Which brings us to the speaker. verify the speaker is OK, It could have seen some DC for a time.

[Dave G.]

Thanks for your reply. Fender sent drawings #0038718000, 0037026000, 037027,and 038721. 037027 is the layout for the amp I'm looking at. I'll E-mail Tonezone and if he does not have it I'll let you know.

[Dave G.]

Thanks Mike for the drawings the M-80HMSeries looks like the same power amp only the one I have here is sterio so the reference numbers are different. I did recieve drawing 037023 from Fender but I see I have Page 1 of 2 so they didn't send me sheet 2. thanks

[Enzo]

I most definitely have it.

email is tmenzo at msn dot com

[Dave G.]

I checked everything out and found 5 shorted diodes along with the transistors and resistors, I ordered the parts last night. And the speaker is blown, the voice coil is open and there was a tear in the suround when we took it out. I have a friend in town who rebuilds speakers so were well on our way. I do have 1 question if you lower the bias current in a class B amp will that increase the chance of crossover distortion? I have no dought if Fender made the change it's the right thing to do I was just curious. Thanks again for the help

[Enzo]

if you lower the bias current in a class B amp will that increase the chance of crossover distortion?

That assumes it was right at the edge of Xover to start with.

Xover happens because one side isn't quite ready to tirn on when the other side turns off. As you increase the bias or idle drive, the xover gets less and less, until you reach the happy point where the two sides hand off seamlessly. But if you increase the idle current further, then the two sides start to both conduct at the same time. This makes the outputs run hot of course.

So when they back off the current, it matters where they are backing off FROM. There were service bulletins on the M80 series to reduce bias because too many of them were blowing up. That note on the drawing reflects the change order.

Here is a great trick. Or at least something I like to do. MAke a xerox of the page, and now use a yellow marker or something to highlight each bad part you find on this work copy. What results is the path of the failure current through the amp circuit. This part failed, and then the excess current flowed through This and That and then down through Here. etc.

The power of this is that it can be predictive. Sure you can find a lot of bad parts. but when you trace a line of destruction through a resistor, a transistor, a diode, another resistor, SKIPPING A TRANSISTOR, then another resistor. That seemingly good transistor sufferent the same stress as the other parts in the line. I would replace it. Even though it tests "good."

Same with a string of diodes. If there are four series diodes and three are bad and one still tests good. DO yourself a favor and replace it anyway.

It also suggests other potential failure paths or circuit sections that might have seen some stress.

Not only that, but it is a good record of what was replaced. I collect the bad parts in a container, but when little 1/4 watt resistor turn to charcoal, there is little to collect. So the work copy becomes a parts list for the repair.

[Dave G.]

I did notice 4 of the bad diodes were in a line(cr20-23). I did print the schematic and circle all the parts i'm replacing so the owner would have that record in case of future problems. I'll post the results when I get it back together, the speaker should be done middle of next week.

[Enzo]

And I just got my NEW Mr.Gearhead disc from Fender, and this year they have in fact included the second page of that drawing in the M80 CHorus file. Was that two part file clear enough?

[Dave G.]

it must be clear enough i can read it without my glasses

[ZIG317]

Hello:

I just got an M 80 cheap. I always liked this amp. Now, I have one. However, the Input jacks are all plastic , broken and cracked, I got the 9pin and 4pin jacks on ebay. Does anyone have a schematic for a fender M80? Althouhg the amp seems to work fine, I would like to know more about this board.