Fender passport repair

[fredcapo]

I had an older passport pa system that I hauled off of the curb. Was in protect mode. After reading a few posts here, and being in a slow week at the shop, isolated the problem to the power board. I then tested the transistors in circuit with my BK and several output transistors tested bad ( in circuit). So I decided to order all 8 as well as the 4 drivers from mouser...50 bucks or so. So I removed all 8 of the output transistors, and only one of them tested bad out of circuit...it had heavy leakage. All of the passive componentson the board seem ok, (though really Unsure about testing caps that may be marginal).
So, thinking perhaps the low resistance in circuit fooled the tester...

Question is, since these are bipolar devices (and not fet) should I only change the one that tested bad? Why did not the entire output stage avalanche like the Ampeg-slm svt pro does so well.

For extra credit...also wondering about the protection circuit...so one bad device is enough to trip it...how does it work? Thanks

[Enzo]

Look at the power amp schematic. There is a detection circuit at the output for protect. But the SMPS is able to protect itself as well. A current overload (such as the result of a shorted output transistor in the amp) will be sensed, and the power supply will shut down.

[fredcapo]

Thanks.

Let me ask about how you guys mount/remount power transistors... I peeled off some thin plastic insulation and compound. Just put that back? Or improve the insulation and bond with mica, film, etc and new grease?

Wondering why they don't use nylon screws?

Is any silicone grease ok? I have some old tubes of silicone grease...is any silicone grease electrically conductive (not desirable for this use)?

[DrGonz78]

It would be wise to replace any insulators that were on there previously. As for silicon grease for heat transfer remove the old stuff too, clean it up w/ a q-tip and some alcohol. Use 100% denatured alcohol and save the moon shine for the celebration. The grease should not be conductive and most are not unless it is like the silver type which you want to avoid in this instance. The grease you mention should do the job well as far as I can tell. Once you have finished replacing the outputs do a continuity test to makes sure they are on there correctly to the heat sink. Yeah the screws should not make contact to the transistors and should be insulated by the case or something. Just look at it carefully and it will make sense, ask more questions as you go too.

P.S. Rule of thumb... If the heat sink is connecting to the chassis ground then the transistors need to be completely insulated from the heat sink. Most of the time this is the case.

[Enzo]

Nylon screws do not have the strength.

[52 Bill]

Remember that the outputs are connected in parallel and if one is shorted the others that are paralleled with it will test shorted as well. The power supply does a real good job of detecting high current draws and usually shuts down before there is any additional damage done.

I personally don't like the gray silpad insulators that they use in those amps, but if they are not damaged I clean them and reuse them. If they are damaged, I replace with normal mica insulators.

The general rule is to replace all of the output transistors on the same side with the same type (and maybe run number for matching), so that they will share the workload evenly. I have had manufacturers tell me that if you use the factory selected parts, there is no need to replace any more than the one shorted part, as the remaining others will match the new replacement. YMMV

[Enzo]

That sounds good, but I cannot imagine fender is buying 100,000 transistors and then testing them all for some sort of matching. Matching? That is what ballast resistors are for.

[52 Bill]

That sounds good, but I cannot imagine fender is buying 100,000 transistors and then testing them all for some sort of matching. Matching? That is what ballast resistors are for.

I would imagine that is left to the manufacturer of the transistors to sort out before shipping.

[Enzo]

Well, my point was that no one is matching.

[Mick Bailey]

Thanks.

Let me ask about how you guys mount/remount power transistors... I peeled off some thin plastic insulation and compound. Just put that back? Or improve the insulation and bond with mica, film, etc and new grease?

Wondering why they don't use nylon screws?

Is any silicone grease ok? I have some old tubes of silicone grease...is any silicone grease electrically conductive (not desirable for this use)?

The big problem with silicone grease is migration. It's an excellent electrical insulator, though, but not thermally as good on its own as the white heatsink compounds or thermal pads. You'll notice that a lot of compounds are sold as non-silicone. I've pulled apart assemblies where silicone grease has migrated over the transistor legs and contaminated the board, making repairs difficult. Thermal materials are now a lot more effective than mica, but there's a bewidering choice and some are extremely expensive.

The main objective is to increase the effective contact area between the tab and heatsink, whilst at the same time maintaining insulation and thermal transmission. If the original material isn't damaged it can be re-used. Sometimes enough oozed compound can be scraped up to reapply. One thing I check every single time before powering back on is that the bolt and heatsink are isolated from the device tab.

The only time I see nylon screws used is just to hold the transistor down - often when they're mounted flat on a PCB and don't run hot. The problem with nylon screws is not only the strength - they either have a fine thread which strips, or a coarse thread with little core area - but also that they elongate with temperature and time in situations where the transistor runs hot and fail to maintain contact pressure. So the device then runs even hotter and eventually fails or melts the screw (or both).

[fredcapo]

Thanks, guys, for the valuable advice.

[ron vogel]

So, I'm guess the original owner probably didn't know the difference between a speaker cable and an instrument cable? There's a local music shop that rents PA's...they have a huge sign next to the rental counter with a fried PA on it reminding people...lol

[The Dude]

Also, whenever working on a Fender Passport, make sure no customers are within ear shot. You WILL swear frequently.

[flyingdutchman]

several output transistors tested bad ( in circuit)

With output devices in parallel (as they tend to be in larger power amps) if one device is shorted, all its siblings test short.

A trick I use in this case is to disconnect the emitter ballast resistors (by lifting one end) of all paralleled devices except one. The shorted devices (usually there'll be only one) can then be picked out in a matter of seconds.

should I only change the one that tested bad?

That's a difficult call. If the short was caused by overheating (defective fan or something) all devices may have suffered and the failed device was merely the first to go. If reliability is an issue, replace all devices. The difference in repair cost is often a small price to pay for peace of mind - both the customers' and yours.

Also, on older gear it is sometimes convenient to replace legacy output devices with more current types, e.g. MJ15015/16 now tend to be difficult to obtain, and costly. Replacement with MJ21193/94 can be convenient and cost-effective. Power transistor technology has advanced considerably in the last few decades and current devices tend to have better specs despite lower cost.

[fredcapo]

Also, whenever working on a Fender Passport, make sure no customers are within ear shot. You WILL swear frequently.

Since this was my own found unit, I took the easy disassembly road by breaking the plastic baffle that makes disassembly so difficult... Was pretty easy after that... Heh...