Fender Power Chorus Transistors

[greenmeanh1]

Hey guys. Been a while. I am working on a Power Chorus amp. Its a stereo amp driven with two pairs of TIP147 and TIP142 darlington's. Problem is i cant get the correct TO218 size as they are out of stock everywhere. Not ordering from China either. No TO218's so i had to get the TO220's which are a bit smaller.
Pretty sure i can get them mounted. I may have to drill and tap new mounting holes in the heat sink at worst if the legs are not long enough. Anyway ordered them from Digikey and they will be here in a few days.
Anybody see any issues with this mounting idea??

[The Dude]

I don't see a problem, but I'm curious. Wouldn't the TO247 have been easier? Digikey has them in stock.

[Enzo]

And Mouser also has TO247 in stock. They are spread apart on the Fender heat sink, there ought to be room for the slightly larger TO247 in place of the TO218.

TO3P is very similar to TO218, and should work as well.

Edit to add, I just looked. I have both TO218 and TO247 in stock, they are the same dimensions, the TO247 just has the tab enclosed within the package.

[The Dude]

Enzo,

I just mentioned Digikey because he already ordered from them. I thought maybe he could change the order if it wasn't too late. But, yes, there are other places you can get them.

[Enzo]

Oh, Digikey is fine, I just was pointing out other options were available.

The TO218 package seems to be history, as he reported. But I learned the TO247 really ISN'T larger than the TO218, just different.

[greenmeanh1]

Probably should of asked before i bought. TO 247's didn't look right or anything like what i have as the tabs were covered as mentioned where mine are not so i didn't think they would work. I have already ordered the TO220 parts so if there are no major issues with it i am just going to install the TO220's. I am replacing both pairs so at least they will be the same.
I am not even really sure what the original size was as it looks a though these have been changed out a few times and they are all different..Thanks. You guys are awesome!!
Edit Added a pic of the transistors. The 2 on the left tested OK the other 2 toast.

[The Dude]

Just an FYI: According to the datasheets, both the tab on the TO218 and the back of the TO247 are connected to the collector, so either would connect to the heatsink if such a connection is necessary. From memory, and I could be wrong, I believe that amp has isolation pads between transistor and heatsink, so it would be irrelevant. The collector connection is made by the center leg of the transistor.

[greenmeanh1]

So what was happening was the amp was going into thermal shutdown. Amp would run for several minutes and then the heat sink couple would pop. The IR gun showed a really hot 120 ohm resistor in the power supply along with one of the good TIP147's really hot as well. Approaching 200f. Other resistors in the area are 40 f hotter than the opposite channel. After several thermal shutdowns the fuse finally blew along with transistors in the right channel. Oddly not the channel where all the heat was coming from.
So now that the transistors are out the amp will power up and stay on but one side of the supply is still getting really hot.

[The Dude]

I've no idea what your level of expertise is, so excuse me if this is "oh duh" for you.

1) Transistors just go sometimes and there isn't a why. However, from your description of the slow ramp up, I'd say something caused them to go. So, be thorough. Check driver transistors, emitter resistors, and other surrounding components. It only takes one other defective part and you'll lose your new outputs.
2) Build a light bulb limiter to help prevent blowing the amp up right away if something is still amiss.
3) Don't hook up a speaker until the amp is repaired and check that there is no DC at the speaker output before connecting the speaker.

[J M Fahey]

1) get the TO247 which is "modern TO218"
FWIW I have been using TIP142/147 since the dawn of ages, almost 40 years now, and went through all the packages: rounded top TO3P > square exposed tab TO218 > square covered tab TO247 .

They are all compatible, meaning leg holes, mounting hole and spacing are always the same ; in fact are compatible with metallic TO3 and for the good reason that the new kid on the block wants to pick old guys customers, and wants to make it easier.

TO220 versions exist *only* because some people need current or voltage capability but not dissipation (in switching/low power/regulator duty), so are NOT fully compatible with big brothers in power amp designs and even less in a guitar amp.

To boot, youi already had thermal problems with the large ones , so ....
Keep those you ordered for other uses but here get the large ones, I mean it.

When one transistor in a parallel pair overheats as described, rather than it I suspect * the other* which might be open so not sharing the load, or cracked open track leading to it or open emitter ballast resistor.

Order a few of these, keep unused for future jobs, order with proper transistors so you pay postage only once, also order some new micas.

Even if a PITA, always test amp with transistors firmly bolted to heatsink, "just one minute" testing can kill them otherwise

[Enzo]

Work without a load - connect no speaker or dummy load to the output. Not until the amp is stable and not making DC on the outs.

If you were loading the amp, any chance the pairs of speaker wires have gotten mixed, as in + of 2 and - of 1 on one speaker and vice versa?

There are two 120 ohm 10 watt power resistors in the power supply. They are dropping resistors for the 15v zeners. it is normal for them to get hot, that is why they are 10 watt parts.

And do build the light bulb limiter that Dude mentions.

[g1]

Agree with Juan, TO-220's are often lower power versions of the bigger case styles. Check the spec. sheets.

[greenmeanh1]

OK guys, Thank you,, point taken. I ordered the TO247's from digikey today, Im sure the TO220's will come in handy someday. I don't mind having parts on hand so its not a big deal.
I am not a total newbie, been running and fixing my studio stuff for 30 years. I used to send stuff out that i was not able to handle but i am trying to change that and get better at diagnostics so i can do this myself. I am able to fix most power supply issues and cold solder joints. I am a freakin pro at replacing 9 pin Fender Jacks seeing as i have had to do so many of them over the years. I just bought a bag of 50 so i don't run out for a while. Getting some better tools together as well. I just got a electric desoldering gun which has sped up the process of parts removal big time. I also got a part tester to help me diagnose caps and transistors etc. which has been really helpful. So im pretty well set up, just have to learn a lot more!!
You guys have been very helpful.