Home » VintageKustom.com » Repairing Kustom Amps » Update! (No, I didn't shock myself!)
Update! [message #24363] Sun, 10 January 2016 18:51 Go to next message
Shaun_Musings is currently offline  Shaun_Musings
Messages: 323
Registered: April 2009
Location: Reading, MA
Senior Member

Okay, so the TO-39s are secure. That was why I blew the fuse; I didn't realize the little plastic washers were insulating the screws. So, those are new. Used the diode function to test for continuity.

Installed the new 4049s and 4010s, but without their heat sinks; I will be using press-on ones. Which means I had to connect a wire to where the heat sink was part of a circuit; it turns out the transistor itself is 'Base.' So that is all set; I am waiting for the press-on heat sinks.

Now, there is a resistor which started smoking and while it didn't burn up all the way, I'm not going to chance it.

There are a few other transistors on the board which need to be swapped; I'll take care of those in a bit.

The last piece is the diode. There seem to be two metal can diodes in a line, with the -->| pointing down toward the chassis; the next piece in line is the heat sink diode (1N4148TA) which I will be putting in tomorrow.

I may decide to change the big main filter caps in the future; they seem to be out of sync. I'll be going with 15,000 UF., 75 VDC...


Born on the Baaaaayou (Ok, in Massachusetts)
Re: Update! [message #24364 is a reply to message #24363] Sun, 10 January 2016 20:56 Go to previous messageGo to next message
chicagobill
Messages: 1959
Registered: April 2003
Senior Member
Sorry, but your post really doesn't make sense to me. TO-39 transistors are small like the 40410 without the heat sink. If you are talking about the large metal power transistors on the bottom, those are TO-3 cases.

The driver transistors like the 40409 and 40410 have the Collector connected to the case, not the Base.

If a resistor has burned, then something that is connected to it is drawing too much current. Maybe a shorted transistor.

It is very important that the three diodes are connected correctly. If you haven't moved the two diodes on the board, then they are in the correct orientation. The heat sink diode must be polarized correctly or the power amp will blow up. Those three diodes set the bias of the output stage and if the diode string is open or connected wrong the two halves of the power amp both turn on at the same time and will cause the transistors to short out.
Re: Update! [message #24365 is a reply to message #24363] Sun, 10 January 2016 21:15 Go to previous messageGo to next message
Shaun_Musings is currently offline  Shaun_Musings
Messages: 323
Registered: April 2009
Location: Reading, MA
Senior Member

I meant the TO-3 transistors on the bottom, yes. I replaced those. I also replaced the TO-39s, and right now they do not have a heatsink; I have ordered the press-on heatsinks, and they should be in soon. I have the little glass 1N4148TA diode, which has a band on it to indicate polarity. I'm assuming they all go in the same direction.

The original diagnosis appears to have been a bad output transistor. O don't have a variac to bring this amp up slowly, but is there anything I should do before I start it up? All that's left:

1. heatsinks on the TO-39s.
2. Install glass diode
3. Replace the resistor.


Born on the Baaaaayou (Ok, in Massachusetts)
Re: Update! [message #24367 is a reply to message #24363] Sun, 10 January 2016 21:33 Go to previous messageGo to next message
chicagobill
Messages: 1959
Registered: April 2003
Senior Member
Use a light bulb limiter if you have one. The driver transistor heat sinks are only needed for running the amp hard. For testing they can be left without them for now.
Re: Update! [message #24369 is a reply to message #24367] Mon, 11 January 2016 09:22 Go to previous messageGo to next message
Shaun_Musings is currently offline  Shaun_Musings
Messages: 323
Registered: April 2009
Location: Reading, MA
Senior Member

Okay, thanks... I don't have a limiter here, but I know I can build one easy enough...

Born on the Baaaaayou (Ok, in Massachusetts)
Re: Update! [message #24370 is a reply to message #24363] Tue, 12 January 2016 06:54 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4311
Registered: June 2004
Location: NY
Senior Member
You should also de rate the fuse by 50% for the first power up on the limiter.
Re: Update! [message #24371 is a reply to message #24370] Tue, 12 January 2016 15:56 Go to previous message
Shaun_Musings is currently offline  Shaun_Musings
Messages: 323
Registered: April 2009
Location: Reading, MA
Senior Member

Okay, I decided not to use a limiter, and give it a shot. Stupid me, but nothing exploded; it fired up nicely. Registered 8VDC at the output; which is a quarter of what it was before. Then about three minutes after I started it, a resistor started smoking right next to the third driver transistor (RCA 40410). Turned it off and checked the driver transistor. It looks like I didn't solder it down very well and/or the transistor became loose when I was installing the diode. Overall, however, looking at the VDC drop, I would say things are looking a lot better. I'm going to double check the transistor and take it off and make sure it's still okay and not fried, and go from there.

Born on the Baaaaayou (Ok, in Massachusetts)
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