Acoustic 470 fuse blower

[randomair]

hey doug, any word on that 470 transformer?

[randomair]

So, I decided to go through this amp from head to toe and make sure everything checks out before I go ahead and throw a working PT in it and found some issues. Well, more specifically, I found 1 of the driver transistors to be shorted. The transistor is: 480041 with heatsink. Unfortunately through searching online I can find no match for this part. The other two drivers on the board are 480040 and 40409. I know magic parts stocks the 40409 but these other 4800-- types I cant locate anywhere. Does anyone have any knowledge of replacements for these? gbono, enzo??? Thanks.

[gbono]

Yes, I believe you are referring to Q404 on the PA board? That would be ACC type # 48041 which is RCA type # 40410 the PNP compliment of the 40409. I don't know if MagicParts carries the PNP version but I have used the 2N4033 from Digikey - I drive out the old metal can from the heat sink using a vice and punch - carefully ream out the heat sink and press in the 2N4033.

Found this discussion regarding subs for the RCA type #'s in question:

Again the HK drivers are odd, the RCA design used the 40409/40410, and I stated in another thread that the MJE253/243 (or MJE15034/35) plastic parts are a better substitute 4A/100V/40W, good beta, good Ft, this is a current part. Use a small heat sink with them.

[Enzo]

400409 and 40410 are legitimate RCA transistor types, just as 2N3055 would be. Those 48xxxx numbers are just the Acoustic part numbers for them.

[randomair]

Hey thanks guys! Turns out Magic Parts stocks them both, so I am to just going to go ahead and replace them all on the PA board, probably replace all MPS-A06's too(if there is more than 1). I figure this amp needs an overhaul at this point.

[randomair]

Hey gbono! You had posted that you used 2n4033 from digikey and reamed out the existing heatsink of the 40410. Well I punched the 40410(480041) and now I have the heatsink here, quick question: after reaming out the heatsink did you slide the heatsink over top the transistor to fit like a hat? because that is the only way I can think to do it. It appears these fellas are originally stamped together? Thanks !

[Enzo]

The heatsink is indeed an integral part of the 40409 and 40410 - it is not something added. They came from RCA with it.

And that is another of many strikes against NTE. Look up the 40409 or 40410 and they recommend some part. But it lacks the heatsink. Unsuspecting young techs solder the thing in there anyway and wonder why their circuit burns up the next time they turn it on.

[52 Bill]

I've done the re-use the heatsink trick before. To the untrained eye the board will look factory original when your done.

Wait until you have the replacement transistor before you do anything with the heatsink. You will want the heatsink to fit over the new transistor with as tight a fit as possible to allow for the best heat transfer.

I used a tapered reamer to open up the hole in the heatsink just enough to fit over the new transistor case. I also applied a small amount of white silicon thermal grease to the case before pressing on the heat sink. Make sure that you align the transistor case correctly or the pins will not align with the pc board holes.

[randomair]

Great! Thanks for the tip! I am going to go ahead and give it shot today and I will report my findings. Thanks again!

[PRNDL]

It is possible to find replacements for those heatsink transistors (its not easy), but they are very expensive ($8-20 each). The drilling method is much less expensive.

I recommend replacing the output electrolytic cap can if this amp has one (the 370 does - it has 45 volts DC from the output transistors). That has a direct effect on tone.

[52 Bill]

Oh and one thing that I forgot to mention earlier is that the collector is connected to the heatsink/case on those original parts. I know that some boards (maybe not Acoustics) will use the heat sink to make the connections to the collector.

To be safe, check to be sure that the collector lead is also wired directly to the circuit and not only to the heatsink.

[randomair]

First off, thanks for the tips! I got the 470 off the ground and running! There was an open 1 ohm resistor in the PA board, the filter cap was toast, and I went the drill-out method for the heat-sink, everything seems to operating fine, except for channel A. I cleaned all the controls, jacks before I actually went ahead and really cranked it, but channel A is not responding well to signal. The 100k volume pot was real hard to turn even with a wash of cleaner (it felt really coarse when turning), so I switched out the pot because it just didnt feel right, I was hoping that was the issue but the channel only responds if the strings of the guitar are hit really hard. Basically it just farts at me. I will need to go through the board more thoroughly and make sure everything is up to snuff.

I will mention that before I decided to switch out the pot that channel A responded at a very high volume, but then the signal was gone immediately.

Channel B sounds great and responds really well to signal.

[gbono]

First off, thanks for the tips! I got the 470 off the ground and running! There was an open 1 ohm resistor in the PA board, the filter cap was toast, and I went the drill-out method for the heat-sink, everything seems to operating fine, except for channel A. I cleaned all the controls, jacks before I actually went ahead and really cranked it, but channel A is not responding well to signal. The 100k volume pot was real hard to turn even with a wash of cleaner (it felt really coarse when turning), so I switched out the pot because it just didnt feel right, I was hoping that was the issue but the channel only responds if the strings of the guitar are hit really hard. Basically it just farts at me. I will need to go through the board more thoroughly and make sure everything is up to snuff.

I will mention that before I decided to switch out the pot that channel A responded at a very high volume, but then the signal was gone immediately.

Channel B sounds great and responds really well to signal.

Glad your 470 is up and running - I think its the 170043 board that you will need to chase the A channel signal path. The pre-amp PCBs are kind of a pain to get to on this model.

[randomair]

Hey thanks gbono, yeah that board is a real treat to troubleshoot. I was hoping something like a broken ground or bad solder connection, but thus far, no luck. I am just glad she is alive. Its just divide and conquer from here.

[Enzo]

Without looking back through the whole thread to recap where we have been, the first thing I'd suspect would be small electrolytics in the signal path. Little caps like 1uf/50v or 4.7uf/50v. If only loud signal can press through for example, some DC level is probably squashing the signal.