Customer returned fixed Fender Princeton 112 reverb: Suddenly fuse keeps blowing

[J M Fahey]

I would not trust the TO220 in that place.
You *can* find it in the regular (larger) case.

http://www.mouser.com/ProductDetail/...8IcBeHDc1LE%3d

http://www.parts-express.com/pe/show...tnumber=TIP142

http://search.digikey.com/us/en/prod...41-5-ND/603566

http://search.digikey.com/us/en/prod...2GOS-ND/920332

Good luck.

[kka]

ok, now that makes sense. I could locate a TO247. From the old transistors there is still a mica. Can I use that one and just add new thermal silicon?

[J M Fahey]

Micas cost cents.
Order 5 or 10 micas, and some thermal grease, and have it close at hand for next jobs.
Since you are already ordering the transistors, you will not pay extra postage for them anyway.
Order a few screw insulating nipples too, in case you get the TO218 case, which needs them.
The TO247 already has an insulated hole.

[km6xz]

This is a good thread, with a happy outcome...a low cost learning experience that will not be forgotten soon and the beginning of creating a routine of test-repair-and proof of performance habits that will save time and money in the future.
One thing not mentioned was the condition of the speaker. If one transistor shorted before the other, full supply voltage would have been present across the speaker terminals which could very well have fried the voice coil. Don't run the amp(any SS amp) into a load until you know the amp is stable and there is no DC offset. Test the speaker by measuring the resistance, if about 2/3rds the rated Z, the voice coil is not burned too badly. Next gently press on the cone with equal pressure on either side of the dust cap. Any grit or rubbing? When over heated a voice coil or its bobbin can distort its shape and rub against the pole piece of the magnet.

Silicon grease is used to thermally fill the slight surface imperfections of the mating metal surfaces. Just a tiny bit is needed, just to fill so if it looks white, without the metal color visible, you have more than needed to do the job.
Set up a burn-in bench. Get some good dummy loads, a spare old scope, a radio or CD player and set up every unit as a post repair torture test. Connect it all up and run music that peaks a bit over clipping. set the scope so it is visible to your workbench and let it run into loads for several hours. A good addition would be a small monitor speaker that has an attenuator so you can hear a quiet version of what is going on. When it is cold, while listening to to give it a good couple of thumps and shakes to see if there is any noise, crackles etc. Do it again after it is good and hot. Note idle current from the mains before burn-in and at the end. If there is much difference find out why. Mains current is a good thing to record for every unit that comes in, it is easy with a clamp-on current meter. After a while you will know just by putting a new unit on the bench and noting the mains current draw whether it is normal for that model. A quick 1 second test that gives a lot of info. SS amps of the same model will be very close to each other, tubes will have more variation but still, it will give insight into what you have.
If you really want to torture a unit, run a SS amp with a sine wave at 1/3rd to 1/2 power for extended periods, that will test the thermal rise more than full power will. Transformer thermal rise can be seen best at full power. To test bridge amps, use both channels of the burn-in scope, one lead on each output terminal of a channel. Put the vertical amp mode switch in A+B add mode. Select "invert" on one channel and set both deflection amps to the same sensitivity. That will give the true differential voltage swing and not be needing to ground reference anything which can't be done with bridging amps.
Once you set up for a number of simple routine processes, return rates will drop to about Zero. If you start shotgunning which many people on this forum seem to suggest, expect failure rates to zoom up and your reputation will drop. Avoid replacing any part unless you know what its contribution to the fault mechanism was, as cause or symptom. If any tech tells you on the forum to just replace a bunch of parts, such as replace all caps as a matter of course in a repair, you are advised to ignore everything they ever write again. They are incompetent as a diagnostician. Don't introduce more variables into the mix when diagnosing problems.
Good luck.

[52 Bill]

Some very good points there Stan especially about the speaker. Which also brings to mind that the other power amp parts need to be checked as well before power up, like the ballast resistors, driver transistors, etc. Anything that may have been damaged by the shorted outputs.

[Enzo]

Thanks for the info. I really can not find a TIP142 in TO-3 at mouser.com or anywhere else in the US. They all seem to be the TO-220 package or SOT-93. I have got a TIP142 in TO-220 in stock. The old broken on I can cut off leaving the legs in the PCB. I could connect the smaller TO-220 to those legs easily and could also mount it on the heat sink. I will keep looking for a TO-3,

TO3P - P P P - not TO3.

The TO3Pare are discontinued listings in Mouser.

But really, TO218 is the same thing for our purposes. The TO3P has insulated case around the hole, the TO218 is like a TO220 on steroids. It has a bare tab, so an insulating washer with screw would be required.

[kka]

Many thanks KM6XY. Very good advice which I will take to heart and put into action. I still need a lot to learn as I just started getting into SS technology. BTW, I live in L.A., but are originally from Germany. My dad has been into Ham Radio (DF7ML) since he was a teenager and I grew up with lots of radios and antennas all over the house, car, RV etc etc. He is a (now retired) electrical engineer and he helps me out as much as he can over long distance.
73s ;-)

[kka]

Understood. I ordered the TO218 ones. Many thanks.

[Chuck H]

That's not ambiguity, it's contradiction!

(and that was pedantry)

Getting back on topic, when you took the PCB out did you maybe disturb the insulating washers under the output transistors?

Since I believe both are true, that is ambiguity. It would be a contradiction if the two two statements were opposed. But they're not. They're mutually exclusive. At least that's how I saw it when I wrote that post with the intention of some small humor.

[km6xz]

Hi KKA Tell your dad 73s from KM6XZ portable in Russia. I moved the opposite direction, from Northern California to St Petersburg Russia 8 years ago. Having that atmosphere growing up will aid you in getting up to speed, the concepts are learned easily by small kids if they are around it. It just gets harder when one is older. If I had not started well before 10 years old I probably never would have gotten into it. I am sort of a retired electronics engineer also but this is my 3rd retirement, and this one has me working 12 hours a day in 2 small and 1 medium businesses I've started since "retirement". The concept of relaxing in an easy chair never caught on for me, when not working, I am out in the city socializing, clubbing, cultural events etc. It is a good thing I do not need much sleep.
On a practical side, when hams have their local or regional conventions, like the ARRL or local clubs have swap meets, that is a great way to get good test gear a lot cheaper than eBay which tends to have junkier or rougher condition gear for high prices. Since most hams are older, there are a lot of estate sales with radio and test gear so go to a ham club or get their news letters for "Silent Key" notices.
You can never have too much good test gear. When too much to fit on your bench, build another bench. It really saves time to move to another bench if you are waiting for an estimate approval on a unit all taken apart on the first bench. As it is now you probably have to reassemble a unit and put it back on the shelf if there is a approval wait or part wait. The most time consuming part of repair is often disassembly and re-assembly, particularly mixers that are not channelized(separate removable channel strips).
Another big help in shop logistics is getting AV-carts. They are bench high, strong, hard rubber with large wheels. Putting a larger unit like a mixer or keyboard on it when the customer brings it, and leave it on the cart out of the way until it is worked on. Roll it up to the bench and work on it still on the cart. I had dozens of them in my old shop and they paid for themselves in weeks.

[J M Fahey]

Excellent idea.
I think I'll dig my arc welder and build a couple.

[kka]

I received the new TIP147 and TIP142. I took very good care mounting them on the alu heat sink with Mica and thermal silicon underneath. I then put thermal silicon on both sides of the alu mounting block and screwed this block from outside the chassis all the way through the block into the heat sink so now chassis - alu block - heat sink are thoroughly connected with a nice layer of silicon in between.
I had previously checked the rectifier diodes and they are good. Also, prior to installing the new transistors I had checked both collector connections on the PCB and measured +41V (TIP147) and -41V (TIP142), so all seemed ok and the amp powered on fine and did not blow the fuse without the transistors installed.
So now that the t's are installed I had fired up the amp and the fuse does NOT blow, I can switch channels and all seems fine so far. The I connected a guitar and no signal comes through the speaker, amp is dead silent. I had hooked up a different speaker as well (one that I'm sure is good) and same thing, no sound. Then, when I turn the amp off while the guitar strings are still ringing you can suddenly hear the noise an amp makes while powering off, like a farting, distorted noise that fades away over a period of 2-3 seconds. So I only hear "something" coming from the speaker while powering off the amp.

That's where I am right now. I'm now checking my next steps and would appreciate some input and ideas of what to check next.

Here are the schematics of this amp: Princeton_112_Plus_schematic.pdf

Thank you.

[J M Fahey]

1) just in case, *still* use the lamp bulb limiter.
Turn the amp on withiut speaker.
The lamp should blink and then barely glow: red or not too bright orange ... or even not at all.
Measure +/-B.
You won't have full +/- 40V because of the loss in the lamp, but will probably have around +/-25V.
Still no speaker, no guitar, all controls on 0.
Measure the DC voltage at the junction of R65/R66, (you prevuiously checked that they are not open), you should have less than 50mV there, the voltage at the base of Q1 and Q2, on pin 6 of U5 (also less than 100mV DC) and at the bases of Q1 and Q2. (expect around + and - 1.2V )
Also voltage on pins 4 and t of U5.
You should also have measured previously (amp unpowered) that diodes CR 5, 6, 7, 8, 9, 10, 11, 12 are fine, meaning around 600mV one way, infinite or Over Range the other at your multimeter diode test.
OK: now you have enough to do.
Diode and resistor measurements amp unpowered; voltages of course powered, no signal, no speaker, always with the lamp limiter.

[kka]

I tested the diodes CR 5-12 and 9,10,11 are bad. I will replace those and should have checked them to begin with. I also ordered a MC 1436 just in case it is bad, too. I can not do more measuring until the diodes are fixed. I do have the lamp bulb limiter in place.

Thanks for all the help !

[kka]

I replaced the broken diodes. The amp powers on, but no voltage reading at Q1 and Q2 base, no voltage at pin 6 (and any other pin) of U5 (MC1436) and nothing at R65/R66 .
Prior to turning on the amp I double checked all diodes CR5 to CR12 and all are fine (work 1 way ca. 500-600 mV and not the other, tested with DMM).

I'm not sure if the MC1436 could be the culprit here. I have not gotten the replacement yet as they are not easy to find. Found one today. Before I order it I want to make sure that it indeed needs to be replaced. I will need to get the scope into the game to test the MC1436 .

Is there anything else I should to do at this point?

I'm good with tube amps and took on this SS amp only because it needed 2 pots replaced which was easy. The mistake I made assembling the heatsink back (slight gap) turned this into a learning experience which I am grateful for, but I hope that I will succeed in the end. ;-)

Many thanks so far for all the help !