Repair time for Marshall MG100HDFX with TDA7293

[glebert]

I just started doing some part-time repair work for a local guitar shop. I mostly do it because it is fun, more social than working on amps in my basement. I am a contractor and get paid based on what they charge the customer. Sometimes I feel like the shop owner (who does repair himself) underestimates how much time things take because he sees repair work mostly as a way to keep customers happy. He is pretty much there the same number of hours if he is doing repair or not. So far it hasn't been too much of an issue.

I know that there is a Marshall MG100HDFX head in the queue that is blowing fuses. I'm assuming that will mean theTDA7293 IC will need to be replaced. Since I see these amps for sale used for $150 I'm kind of surprised someone is willing to pay a shop to fix it. I'm curious how many hours others would charge for this repair assuming it is just removing and replacing the TDA chip. The nice thing is the TDA chip should be on its own PCB, so the main board shouldn't need to be pulled. On the other hand IIRC unsoldering the old chip can be a PITA.

Thanks!

[Enzo]

I use a desoldering station, but if pulling that 15 leg IC is a problem, try this. Get some small cutters and snip off each leg as close to the IC body as possible. Leaves short pieces of wire sticking up from the board. Now heat each leg wire to melt the solder and pull it out with needle noses. Far easier than trying to extract the whole IC at once. Then clean off any excess solder and install the new part.

The amp could have god knows what wrong with it, but a bad 7293 repair usually happens in an hour labor. I don't know what the part costs now, but in my day I sold them for $8-10.

[glebert]

The amp could have god knows what wrong with it, but a bad 7293 repair usually happens in an hour labor. I don't know what the part costs now, but in my day I sold them for $8-10.

Thanks Enzo. That is about what I was thinking. I don't mind writing off a little time if I am learning something new but the owner had said something about 30 minutes and that sounded pretty optimistic to me.

Amplified parts has them for $12. I actually have a couple but I have no idea where I got them and am worried they might be fakes.

edit: I looked at the intake form and it say it is a warranty repair, maybe for that Marshall supplies the assembled power module which would make for a much quidker repair.

[Enzo]

Well WHO's warranty? If Marshall, are you an authorized shop> Or is your dealer? If it is a used piece but the store is warranting it, they will pay you for the repair?

I have a one hour minimum for repairs. I might charge just a few dollars to solder a loose wire in place or something, but a power amp IC swap? Full hour, boss.

30 minutes? Is that what he wants to pay for just swapping the part? Or does he want you to repair the unit?. I mean if someone hands me a part and an amp and says he wants to just pay me to swap out the part, I might come up with a discount rate. But that would NOT include any sort of guarantee that swap would repair the unit. I mean many times a shorted 7293 also damages the mute circuit back on the main board. And any such extra would not be included in that 30 minutes.

MOuser has them in stock for under $12.

TDA7293V STMicroelectronics | Mouser

[glebert]

I will find out more in the morning.

[Jon Snell]

To remove the TDA without damage;
Remove the PCB.
Use plenty of flux and reflow the IC pins going through the board with leaded solder. That makes it easier to remove as leaded solder melts at a lower temperature then tin.
Use solder wick to remove the solder on the pins.
Cover with more flux and lift each pin out at a time, cutting them close to the chip is a good idea, whilst heating with your soldering iron or use hot air to heat the pins up in one go or do as I do and use a wide bit to desolder all of the pins at once.
It takes longer to remove the board and refit the board than it takes an experienced engineer, to remove and replace the chip, if they have the correct tools. Otherwise get a replacement board from your Marshall dealership. They are common failure parts when people connect things up to the loudspeaker sockets, the loudspeakers have been changed and the impedance is too low.

[nosaj]

And don't forget to drain those caps if I remember correctly.
nosaj

[Mick Bailey]

I've only ever had the complete PCB assemblies from Marshall, whether warranty repair or otherwise. They are not component suppliers. I noticed the last batch of boards I had looked different to the earlier ones - can't recall if they are a different revision, though. I do know the spec sheet for the IC implementation changed and there is a fundamental difference that is reputed to reduce or prevent the early demise of the IC.

[Enzo]

There is more than one little module board. SOme have one connector, others have a shorter connector plus a three pin connector. Depends upon amp model.

When I did warranty work, I found it easier and more convenient to just stock 10-20 TDA7293 in a drawer than getting a stack of the modules. Marshall didn't care whether I replaced the module or the chip.

[nosaj]

about post 82 you will find relevant info to these IC's and how to avoid any pitfalls.
nosaj

[Enzo]

GAAAAAAHHHHHH!!!! Mode Four... EEEEEEEEEEEEEeeeee (Runs screaming into the night...)

[J M Fahey]

If you only have to replace a chipamp and thatīs it, to boot you already have it at armīs reach, 30 minutes is enough, more than half of that time being assembly and disassembly.

Now if somebody brings an amp in, they only say "it does not work" or "blows fuses" and YOU have to troubleshoot it, a job completely different from plain "replacing parts", yes, 1 hour is minimum.

If you depend on a shop forwarding jobs to you, then your options are accepting or refusing it.
If refusing, you can tell them "impossible within that time frame, I estimate xxxx" and then decision lies in their hands.

In a way, similar to factory authorized warranty where they allow "x" time, say 30 minutes and pay fixed amount for that.

I am quite certain factories have already studied and established certain "job tables", guess if above that they will refuse authorization and either issue a new unit or offer Factory repair.

Maybe the shop took note of earlier jobs and keeps that data as reference.

As of the often quoted "the amp is available used for $150" , yes, sure, but:

* itīs always "somewhere else" which involves freight band time.
* you can not personally inspect it, many cans of worms hiding there, lots of terror stories around.

I always suggest customers that if their unit was reasonably well kept and simply some part failed or they shorted output, repair will guarantee an "almost as new" amp, which they can trust.

EBay/Craigslist/Garage sale product? .... hope the Lord keeps an open ear for your prayers.

Worst is the dreaded : "yes ... it woooorkkksss .... sort of .... BUT ...."
You canīt straight send it back , and Techs may waste lots of time chasing ghosts, so I suggest spending *up to* "available elsewhere" price makes sense.
You are not blind gambling but knowing the horse you are betting on.
jm2c ymmv etc.

[nosaj]

GAAAAAAHHHHHH!!!! Mode Four... EEEEEEEEEEEEEeeeee (Runs screaming into the night...)

The several i have done are still working daily. probably due to that thread.

nosaj

[Randall]

I just replaced the chip in that model. No way in Hell would I accept only 30 minutes.

[glebert]

Well, this was all a big waste of internet time (although freaking out Enzo has to have some value), the model was actually a MG100FX, and it doesn't even have a TDA7293. Can't find a schematic for it anywhere, we will have to wait to hear back from Marshall if it is covered under warranty and what parts they could provide. So basically it wasted 30 minutes of my time opening and closing it, putting it on a variac to confirm high current, and looking for schematic, but at least I didn't do any actual repair work for free.

For this job I can say "it will take X time" and then the owner needs to approve that, get the customer approve it, do it himself, or just give it back still broken.