Fender M-80

[Enzo]

DIode tests both ways?

With the red wires off, there should be NO continuity between those two posts, and even ini diode test there should be no diode drop measured. If it checks between the two posts like there is a diode across them, that is not good, that is a shorted diode rectifier. if you meant it checks open even om diode test, then that is what we want.

There is a +HV rail and a -HV rail, and from each, the output power transistors connect to the speaker out. Their job is to condect EITHER positive or negative current to the speaker to make sound happen. But if both sides turned on at the same time then current would flow from the positive to the negative side directly. This will blow fuses.

In its simplest form, the bases of the + side transistors and the - side transistors are connected together. That way if the signal goes positive, it turns on th + side transistors, and when negative, it turns on the - side transistors. That is how it amplifies the current the speakers need.

In practice, we do not connect those bases together, instead we have a small circuit that allows the bases on each side to be slightly biased towards their ON voltage. SO instead of the opposing bases sitting at zero volts, our circuit biases the + side one at about +.5v, and the - side bases at about -.5v. Adding an extra half volt on eaither side, we have about +1v and -1v respectively at the pos and neg driver transistor bases.

Those TIP14x transistors - I am going to abbreviate transistor as xstr from here on - those TIP14x xstrs are darlington xstrs. That means there are two xstrs in one part, so they contain their own drivers essentially. SO I expect more or less +1v at the bases of Q3,4 and more or less -1v at the bases of Q5,6.

Look just to the right of th op amp and you see a string of four diodes CR7-10. They are between the + and - bases. At about half a volt each, that leaves the two volts (+1 to -1) total to hold those bases apart.

But what happens when one of those diodes opens? R73,74 will pull the bases of the + side xstrs towards +rail and that would turn them on fully. Likewise, on the negative side, R75,76 would pull the bases of the negative side xstrs towards -rail and that would turn them on hard. Pulling the base voltage of a bipolar towards its collector voltage is how to turn it on. Xstrs are really current devices and not voltage devices, but this description serves me better - put the voltage there and the current will flow.

SO as it sits, the current through R73,74,75,76 also flows through the four bias diodes. And their volatge drops add up to keep the xstr bases right on the edge of turn on. If that string opens, then BoTH sides turn on hard, and fuses blow.

I don't know that to be your case, but it merits exploration. A simple bypass should tell you. If we clip a shorting wire across the four diodes - basically short the base of Q3 to the base of Q5 - and power it up, and the fuses no longer blow, then we have found the area of trouble. If it still blows, fuses, then the bias string is not it.

[pchilson]

Ok, I read your post and was looking at the diode C7 - C10 and this is what I found. Never would have discovered it had you not mentioned "jumpering" across them. I pulled it up so it would be more obvious to see but it was just sitting there with no visual clue. I looked up what these diodes are in the schematic parts listing and it reads "Diode, BIAS REF". What is this diode? What do I ask for? If I put that in mouser search I get this but that looks nothing like it. What do I ask for at the electronics shop?

[Enzo]

No no no

At Mouser, enter "BYV26D" into the search - byv26d if you prefer - right off the schematic. Mouser only has stock onthe NTE version,but all it is is a fast recovery diode 1 amp and way more volts that this circuit could possibly generate.

You got any fast recovery diodes in your drawer? COnsidering there is less that 100v across this whole circuit, and not like that whole amount could ever appear across them anyway. a 200v part would be more than adequate. 800v is just silly - or what they have on hand from other use.

I hate NTE, and if you are ordering from MOuser already, it is only 75 cents, but look at Allied and real BYV26D is in stock for 8 cents each.

For static tests to see if everything else is OK, you could stick some 1N4007 in there temporarily

[pchilson]

Never looked on the schematic, only at the parts listing...
I have 2 of each: 1N4004 and UF4007
Will one of these work?
I can probably get the NTE575 tomorrow.

[Enzo]

The UF is a fast recovery type.


Never looked at the schematic...? wow...

[pchilson]

The UF is a fast recovery type.


Never looked at the schematic...? wow...

Of course I've looked at the schematic. What I meant was I just went looking at the parts list for CR9 instead of the schematic when looking for the part ID. I mean com'on, I'm not quitting my day job here...I'm trying to learn something from this. I screwed up and now I'm trying to fix it. I appreciate all of your patience and help. I wouldn't be able to do this with out it.

Once I get the replacement in I'll post back.

Thanks

[52 Bill]

Looking at the latest photo...
Where did you get the replacement output transistors? They appear to be too small.

I thought that the TIP147 and 142 only came in TO-3P cases, like the original ones in your first photos.

[pchilson]

Bought them from mouser. I bought these for the TIP147 and these for the TIP142

[Enzo]

They traditionally were in TO218 cases, but they have shifted them over to TO220s. I generally prefer to buy the TO218 versions myself.


Oh, I'm not picking on you, pch.

[Enzo]

OK, so the TIP142 is on the TO3P, at least from Fairchild. The Fairchild TIP142T is the TO220. The difference is the TO220 part is 80w while the TO3P part is rated 125w.

The Moto/On Semi TIP142 is in the TO218 and is also 125w.

ST makes the TIP142 in the even larger TO247, still 125w.

And as usual, NTE covers it, but charges 4-5 times the price of a real part.

[pchilson]

OK, so the TIP142 is on the TO3P, at least from Fairchild. The Fairchild TIP142T is the TO220. The difference is the TO220 part is 80w while the TO3P part is rated 125w.

The Moto/On Semi TIP142 is in the TO218 and is also 125w.

ST makes the TIP142 in the even larger TO247, still 125w.

And as usual, NTE covers it, but charges 4-5 times the price of a real part.

Ok, I see you don't like the NTE stuff. I learn again. But the question now. Is it ok or do I need to get another variety?

On to the diode. I found a replacement and installed it. Now when I power on with both rails connected it blows the fuse but if I unhook either rail it does not blow the fuse. However the 5w cement resistors, two per rail, R84,85 on the + rail and R86,87 on the - rail get exceedingly hot when the respective rail is connected. Enough hot that smoke is rising in a matter of about 6 or 7 seconds...

[Enzo]

That is because the bias string is still open. And those ballast resistors are all there is to limit current flow when all those transistors turn on to connect the rails together. if you disconnect a rail, then there is no longer a coplete circuit so no current flows. SO when powered up, there are in essence only those four .47 ohm resistors across the two rails. That means about 80v across two .47 ohmers in series - twice. Call it 80v over an ohm, for fun, calculate the power those resistors are being asked to dissipate.

If one of those diodes was burnt open, I don;t doubt they are all damaged. The same curent flowed through all of them. Check each one individually for opens. You don;t have to remove them, just flip it over and measure resistance for shorts then flip to diode test to see if they have a forward drop. And of course polarity matters. And just to see, you could connect the bases of the two end transistors together with a clip wire.

My beef with NTE? First off I never saw the attraction of paying 5-10 times the cost of a real part just to get something that was merely an approximation in a printed plastic baggie. YV guys have relied on them for decades, so it is not like they wil blow up aitomatically. But in power amps where more than one of them must cooperate, mixing NTE in with real parts is dangerous. The NTE sub is NOT the same thing as the original part, it was not made on th same lines under the same processes. When it comes time to share current, it may not. It may "hog" current meaning it turns on before the others do and tries to be the whole amp, or it lags in which case the other transistors never get it to contribute. This can blow up an amp.

If it is all you can get, or it is an emergency, or it is what the store down the street sells versus ordering something from Texas, well you do what you need to do. And as I said above, Mouser only had stock on the NTE diode. If I wanted the repair done now, then I'd setttle. NTE wanted 75 cents instead of 9 cents or whatever it was, but it is not like a dollar wil bust the budget.

The TO220 xstrs in your photo look to be Fairchild anyway, not NTE. Those are the 80 watt xstrs. This is a 100 watt amp into 4 ohms. Those two power ratings are not what to compare exactly, but I would be happier with the 125 watt parts. They were only a dollar to a dollar and a half each. Note the legs on your TO220s are splayed to fit the holes. The holes are laid out for the TO218 or TO-3P. Those legs would fit straight into those holes.

Fender used this pair of xstrs in a number of models of that era, and Crate used a ton of them in their solid state stuff for a long time, and not only that, but a number of Marshal solid state amps used them in the outputs, they are drop in replacements for the BDV64/65 xstrs Marshall used. If you do much solid state work, these are good ones to stock. I usually buy them 10-20 at a time for my shop.

[pchilson]

You are right about the BYV26D being burnt open. I will need to replace all four. Is there something equivalent that can be had from Ratshack or no?

[pchilson]

I replaced CR7, CR8, CR9 and CR10
Both rails hooked up - Fuse blows
CP7 alone - Fuse OK - ballast resistors very hot
CP5 alone - Fuse blows in about 4 seconds

[Enzo]

Your transistors are still all turning on. Try that clip wire between bases as I have mentioned a couple times. It will tell you if bias is the only issue or if there is more. It is entirely possible the loss of bias damaged the transistors that are i n there.

Possible bias problems are: burnt open trace somewhere, diode in backwards, sshorted transistor.