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Kustom 400 transistor help [message #18512]

Mon, 25 March 2013 20:57

davohilts
Messages: 40
Registered: August 2006
Location: Portland, OR

Member

Hello,
I'm going to have to replace bad transistors on my Kustom 400 on the Kei-1 board PC704A. The transistors are called out on the schematic as 38736/38869 and 38737/38870 and require a clip on type heat sink. I removed one and it is pressed into the heatsink and looks like a factory job. Does anyone know of a source for these? I need two each.
Thanks!
Dave

[Updated on: Mon, 25 March 2013 21:01]

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Re: Kustom 400 transistor help [message #18516 is a reply to message #18512]

Mon, 25 March 2013 23:43

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Those driver transistors are manufactured with the heatsink welded on at the factory. The original generic part numbers are RCA 40409 and RCA 40410.

They can be replaced with a standard transistor with a clip on heatsink, or you can break off the original heatsink and solder in a new replacement transistor to the original heatsink.

I see NOS 40409 and 40410 transistors on eBay all the time. If they are reasonably priced, that could be a good source. But if they are asking too much for them, then it would be better to use modern replacements for a few dollars apiece.

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Re: Kustom 400 transistor help [message #18517 is a reply to message #18516]

Tue, 26 March 2013 00:43

davohilts
Messages: 40
Registered: August 2006
Location: Portland, OR

Member

Thanks for the info!
I'll check out ebay and see what I can come up with.

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Re: Kustom 400 transistor help [message #18564 is a reply to message #18517]

Fri, 29 March 2013 13:34

davohilts
Messages: 40
Registered: August 2006
Location: Portland, OR

Member

I guess I should just go ahead and replace all the transistors if I can find them. What I'm looking for is some way to get a modern part number for the transistors or some help in getting a number I can use to find a replacement.
On my PC704A schematic there are 10 transistors shown. I have replacements for 5 of them. I want to replace the rest.
1. Q700 This doesn't have any other info except a note which says 'unless noted all transistors are PET8002 or SE4002. Are these similar to eachother and can I assume that Q700 is one of these?
2. Q701 is 2N4249
3. Q702 is IN3754
4. Q703 is 38735 this number does't help me, any ideas as to a lead to find one?
5. Q707 is 2N3638
thanks for any help.
Dave

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Re: Kustom 400 transistor help [message #18566 is a reply to message #18512]

Fri, 29 March 2013 14:49

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Why do you want to replace all of the transistors?

Any unmarked NPN transistors on the schematic are the PET8002, so if Q700 is unmarked it falls into that group.

The 1N3754 is not a transistor, it is the diode that clips to the heatsink. I don't know why it would be marked as Q702. Diodes are usually marked as a CR.

The 38735 is a Kustom part number for another RCA transistor, the generic number is 40408.

If you search the forum, I posted all of the transistor numbers with generic equivalents a few years ago.

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Re: Kustom 400 transistor help [message #18567 is a reply to message #18566]

Fri, 29 March 2013 17:22

davohilts
Messages: 40
Registered: August 2006
Location: Portland, OR

Member

thanks CB!
When I opened up the amp I looked around inside and finally noticed the burned up resistor, a 100 ohm, 1/2 watt. The 1 K resistor next to it was also toasted and the board has a nice black streak up it where the fire and smoke went. The burned resistor is on the PC704A board and is next to a RCA 38736 CXJ 8L transistor.

I've gone thru and tested the transistors and diodes and have found that CR703 has failed as well as the associated transistor Q706. The two 38736 transistors Q704 and Q708 are associated with the area around the bad power transistor Q1 and the blown resistor R722 and I tested them in place. On Q704, I'm not sure which pins are which but with the neg. lead of the dmm on pin 1 and the positive on pin 2 I get no reading, when I switch them I get .64 VDC. Neg. lead on pin one and pos. lead on pin 3 I get no reading, when I switch the leads I get .87 VDC. Pos. lead on pin 2 and neg. lead on pin 3 I get .89 VDC and with leads switched I get .96 VDC. Does this seem to show the transistor is good? or is my wishful thinking.

I am replacing the transistors in one last effort to get the K400 working again. The one side of the 2-200 boards (PC704A) has failed and a friend was throwing it away. I told him I would take a look at it and have been trying to get the problem fixed. When connected the board makes my current limiter glow very brightly so there is a problem with it. I tested and found that one power transistor on each side Q1 and Q3 were bad and that CR702 had a broken wire. It was clipped to the board and I think this broken wire was the smoking gun that caused the failures.

Since I've got the board out I thought I would just replace all the transistors. I know it is crazy so maybe I won't replace things that test good.

I will look to see if I can find the file showing the generic equivalents.

Thanks for your help.
Dave

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Re: Kustom 400 transistor help [message #18570 is a reply to message #18512]

Fri, 29 March 2013 18:11

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

I'd agree that the broken lead on the bias diode could have caused the problem. If the lead breaks the bias string is broken and the two sides of the power amp will turn on at the same time causing things to get out of control.

What you need to start doing is to relate the circuit components with the parts on the schematic. Just because a resistor located next to a transistor has burned, it does not mean that the transistor is damaged as well. If you follow the trace connected to the resistor it will lead you to whatever transistor is probably bad. This will also help you to figure out what terminals you are testing on the transistors. After you've seen enough of these transistors, you will get to know what lead is the base, emitter or collector and it will help you when testing transistors.

Until then, look at the transistor that you are testing, and then look at the schematic. The schematic will show you what is connected to the base and what is connected to the emitter, etc.

The transistor you tested seems to be good, but the readings from 2 and 3 could be showing a short between the C and the E. But the low readings could also be caused by a parallel circuit connection. To be sure you should test it out of circuit.

You will also need to clean up the burned pc board and if needed, cut away any sections that have been carbonized. I use denatured alcohol and cotton swabs to clean off most of the soot and smoke and a Dremel tool with a brush or a grinder bit if something more agressive is needed.

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Re: Kustom 400 transistor help [message #18571 is a reply to message #18570]

Fri, 29 March 2013 19:17

davohilts
Messages: 40
Registered: August 2006
Location: Portland, OR

Member

Thanks CB, I had used alcohol to clean the soot off but nothing to get rid of the carbon.
I've done a search for you post concerning the generic to kustom numbers but didn't come up with anything. Do you remember which forum you posted it in?
thanks for the help!
Dave

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