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Thread: Fender Roc Pro 700 blowing fuses, bad TIP142/147?

  1. #1
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    Fender Roc Pro 700 blowing fuses, bad TIP142/147?

    I picked up this amp quite reasonable at $40 with the footswitch. It had just a bad input jack and bad reverb tank(output appears to be open) It played very well other than the noted problems and was easily too loud for garage playing. Being pretty much of an electronics novice I felt comfortable fixing an input, and the reverb should also just be a replace the part fix. Anything more would be a bit much.

    So, I dive in on the input jack replacement. Easy solder job, reassemble and start to test it out. It was late at night so I had the volume on the amp turned to almost nothing. It was working for about 10 minutes or so. I was getting a little noise from the guitar's input jack and had just touched the plug there and the amp let out a quiet thump and went dead. The fuse was blown and I had nothing to replace it with. So I just looked for failed parts and I pretty quickly discovered that I had been sloppy in reattaching the heatsink bar to the plate attached to the output transistors. DUH. I did not expect them to be that touchy, live and learn. Next day I picked up some 250V 2A fuses and tried one. Very brief hum from the speaker and the fuse blows within a second.

    I've done some searching and learning and I accept it will cost me the TIP142 and TIP147 transistors and new micas for my haste/inexperience. Also, this will serve as a good learning experience in real troubleshooting. I have a decent Fluke Digital MM, good soldering equipment, and plan to put together a series limiter bulb in the next couple days.

    Now my questions:

    What should I see testing the two transistors on my MM? It has a diode test function, but I'm more used to using it as a plain VOM and continuity tester. Is there an obvious "its toast" test I can do on the board now to confirm my diagnosis.

    Are there any other components I should test assuming the transistors are indeed fried? I'm just learning to really understand schematics, so I'm not sure which resistors are right in the output circuit and which are not. Schematic is linked below.

    What is a good source for the parts? I found ST brand transistors on Newark for around $2 each, but packaging info is a little confusing there. I believe what I have is Motorola brand TO-218 package and most of what I'm finding that is close is TO-247. Will I still need mica insulator for the to247 or is that just the wrong part to use?

    Anything else I should be worried about on this amp? Caps look good, no leaks/bulges. CR29 and CR30 have some heat yellowing on the board near them, but nothing else on the board looks obviously bad.

    Schematic can be seen here : Roc_Pro_700_schematic.pdf

  2. #2
    Senior Member Enzo's Avatar
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    The heatsink bar is not particularloy sensitive, but you do have to install it correctly. Can you image what would happen if you installed the spare tire on your car backwards? If the bar does not make good thermal contact, then the necessary cooling does not occur. MAybe a better analogy would be connecting the radiator hoses up wrong.

    TO218 and TO3P are close to the same, and either would work. TO247 is a bit larger, but ought to fit on the Fenders. TIP142/147 also come in TO220 - DO NOT get those. All of those packages have bare backs and need insulation unless you find them in insulated versions. Me, I'd stick with the TO218s if that is what is in there now. Mouser usually has a good selection. Nothing wrong with Newark, I just never use them as a supplier.

    Diode test on meters works the same on most all meters. If you google transistor testing you will find various tutorials. Basically we want to know the transitors three legs are not shorted together in any combination. I have never had the need to measure gain or anything, they pretty much are OK or they are bad. Oh sure, now and then I get something in between, but not usually when we are blowing fuses. The gist of it is the transistor acts like two diodes joined at the base pin.

    CR29,30 get hot normally.

    What to check? Look at the schematic, can you identify the TIP142 and 147? So what is connected to them? Between them are a couple 5 watt 0.47 ohm. They are certainly connected, and they should be checked for opens. DOn;t worry if your meter can;t measuer half an ohm very accurately, it can surely tell you if it is open or not.

    Now see to the left of them the 8 diodes? They are all involved, just check them as diodes with your diode test. ANy shorted or open? And from that bunch of diodes are a couple resistor trailing upper left and lower left, R92-95. They are probably fine, but they are on the edge of the possible damage zone, so check them.

    Now while google is hot, go look up Light Bulb Limiter. Very simple. Make one and use it.
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
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    What Enzo said, again.

    Plus: once you learn to do diode checks on transistors, it's worth considering spending the time to test every transistor in the power amp for function.

    One of the frustrating things about solid state amps is that when something fails, there is no coupling cap to keep the destroyed part from killing other parts. This "chain of destruction" is common in SS amp failures. If it were mine, I would test, and perhaps just arbitrarily replace the driver transistors as well as the output devices. Driver death is common, although not universal, in SS amp deaths.

    Never assume there's only one failure until proven otherwise. Fixing amps is sometimes like peeling onions. You remove one layer, get to the next layer, fix that, and so on, and there are frequently tears.

  4. #4
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    Quote Originally Posted by R.G. View Post
    Fixing amps is sometimes like peeling onions. You remove one layer, get to the next layer, fix that, and so on, and there are frequently tears.
    I think that we need to get t-shirts with this printed on the front!

  5. #5
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    Thanks to you both. I was hoping to see a response from Enzo. Searching up your previous posts has gotten me from seeing a dead pile of parts to looking at a repairable amp.

    I had space between the heatsink bar and the plate the transistors are attached to. It is more than a little ironic to me that I used to do thermal testing of heatsinks for CPUs for an upgrade manufacturer. The one part of this amp I should have known the most about I screwed up. Just to make the point again for the next person: Double check heatsinks when reassembling things. Heat must go somewhere or these parts self destruct. A simple mistake is making this repair 100x more complicated than it needed to be. Last check my simple fix is costing another $10 in parts with shipping, plus a lot of time.

    I picked up parts to build a light bulb limiter last night just after posting. Hopefully that will cut down on further collateral damage and future mistakes. I'm using a Fluke 79 Series II meter, but it hasn't been calibrated in years. It should be good enough for the .47 ohm resistors though, especially if I'm looking more for open and not precision.

    Thanks much for the guidance on what parts would be in the damage zone. I'll get to testing parts this evening and see if anything else took a hit before placing an order. I had forgotten about mouser as a source too, their price is better and their listings are easier to find.

    I have to agree with 52 Bill, that does sound like a good T-shirt slogan.

  6. #6
    Senior Member Enzo's Avatar
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    I would have suggested replacing drivers too, but on this thing, we have the TIP142/147 pair driven by an MC1436 op amp through those diodes. Not much to it. No drivers. Well, other than the extra stage inside the darlingtons.
    Education is what you're left with after you have forgotten what you have learned.

  7. #7
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    It took me a month or so to get back to this amp.

    I tested everything around the TIP142/147 and all seemed good. I picked up the parts from Mouser, two of each just in case. Pulled the old parts, and it seems that only the TIP142 tested truly bad once out of the board. Replaced them both anyway and reassembled the chassis, taking extra care to get the TIPs solid to the heat bar and the bar solid to the chassis. I plugged into my newly built light bulb limiter for the initial test and it fired up fine. A 40watt bulb in the limiter glowed bright briefly at power on, then settled quickly to a mellow glow. I finished putting the chassis back into the cab and closed the patient up. With a guitar plugged in it and playing it brings the bulb up pretty fast, and it started to cut out playing much above 3 or so volume. I kept it low for quite a while, and no problems. Traded in a 60 watt and was able to get it louder without the cut out, then a 100 watt and still no problems seen.

    After letting it run for a couple hours I turned it over to the teenage garage band and other than some problems with 60hz hum it is back up and working fine. The hum is probably just the messy pile of gear they had plugged in. As I recall it picked up a fair bit before the failure.

    Thanks again for the guidance and hopefully I will be more careful next time I work on something. Next I dissect the output coil on the reverb pan to see if it can be saved.

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