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Old 6th September 2012, 06:36 PM   #1
wanders is offline wanders  United States
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Default Bad transistor in Mac Preamp

I have a McIntossh c-24 preamp with a shorted transistor in the power supply. I've done a good deal of research, but can't find a published off the shelf replacement. The unit was built around 1967 and is not one of the more popular Mac preamps, so there is not too much interest from hobbyists; and, Mac no longer supports it or has any suggestions.

I've attached a ps schematic, the component is identified as q-18. I also attached a brief summary of what I have found.

Briefly:

The Mac part # is 132-014 and, on a Mac cross reference list, I found Fairchild SE 7001 listed as a replacement. The circuit description is shown as well.

I've had no luck sourcing either of the two numbers shown and it has been suggested that someone, working from the schematic, could recommend a replacement that would suffice. I'd probably use anything that would work, but the transistor has a unique heat-sink attached. It's kind of a small, finned, aluminum cap that is glued to the top of the component. I could probably fabricate a different heat sink, but it would be very cool to keep the original.

Any ideas along those lines would be appreciated.

Thanks,
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File Type: pdf schematic.pdf (141.4 KB, 37 views)
File Type: pdf mac help.pdf (189.9 KB, 17 views)
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Old 6th September 2012, 08:22 PM   #2
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That transistor appears to operate with 85 volts on the collector, 75 volts on the emitter, and no more than 65 milliamps of collector-to-emitter current. So you need a transistor with ratings comfortably greater than 120 volts and 750 milliwatts of power dissipation. There are lots of inexpensive and widely available transistors which meet these requirements, so you can narrow them down by package type, pinout, and mechanical arrangement of heatsink. Why don't you post a digital photograph of the transistor+heatsink, showing clearances to nearby objects?

To name a few possible replacements that are electrically (perhaps not mechanically) suitable: BD159, MJE340, BU406, BUX85, 2N5657
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Old 6th September 2012, 08:25 PM   #3
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Default SE7001

Found this for you FAIRCHILD - SE7001
Alec
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Old 6th September 2012, 08:50 PM   #4
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Wanders,
It looks like you will soon be on your way. Glad you posted this here, you'll get much better information this way.

Steven
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Old 6th September 2012, 09:14 PM   #5
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Aah, FraserSoft's link suggests it is a TO-39 package (metal can) device. In that case, Digi-Key has a couple suitable replacement parts in stock. The 2N3440 is $1.11 for quantity=1

2N3440 STMicroelectronics | 497-2626-5-ND | DigiKey

datasheet:
http://www.st.com/internet/com/TECHN...CD00000896.pdf

Mouser has a suitable device for even cheaper: $0.80
2N3439 STMicroelectronics | Mouser

But since you now know that this part has failed once, maybe you'll decide to beef it up during repair and boost the margin-of-safety. . . . Perhaps replacing the TO-39 with a TO-220? Perhaps using a transistor rated for 20 watts instead of 1 watt?

Last edited by Mark Johnson; 6th September 2012 at 09:22 PM. Reason: link purification
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Old 6th September 2012, 09:56 PM   #6
wanders is offline wanders  United States
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Default Pic

Thanks to both posters for the information, looks like we're starting to close in.

Here are a couple of pictures of the transistor. It is mounted in a socket, the socket is soldered to the pcb. For reference, the glass fuse is normal size, around 30 mm long. The part measures around 8mm dia x 8 mm high. The heat sink is around 15 mm dia x 10 mm high.
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File Type: pdf trns-pic.pdf (157.8 KB, 28 views)
File Type: pdf trs-2.pdf (132.8 KB, 18 views)
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Old 6th September 2012, 10:16 PM   #7
wanders is offline wanders  United States
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I was uploading the pictures when the last posts came in. Looks like we've got the correct replacement. I'll order a few.

Changing the specs may be a good idea, but knowing what to use is out of my league. Transistormarkj, if you have some recommendations, I'd be happy to order them when I order the TO39's.

It'll be about a week before I get the parts, and I'll post results then.

In the meantime, thanks again.

Warren
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Old 6th September 2012, 10:17 PM   #8
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As long as you have it apart replace the caps in the power supply circuit. Age alone dictates they be replaced. I would go with the 2N3440.
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Old 6th September 2012, 10:42 PM   #9
wanders is offline wanders  United States
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Quote:
Originally Posted by Original Burnedfingers View Post
As long as you have it apart replace the caps in the power supply circuit. Age alone dictates they be replaced. I would go with the 2N3440.
Thanks, it is my intention to replace at least some of the caps, but I wanted to get the transistor replaced before I tackle those. From the schematic and one of the pictures I posted, you can see the large gray multi-capacitor canisters. I've read that those are naturals for replacement, but I think that exact replacements won't be available. I'm hoping to get some help with suitable substitutes, hopefully something that will mount where the original cans are now. If things progress, I'll start thinking about that in a week or so.
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Old 6th September 2012, 10:51 PM   #10
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Here is the official JEDEC mechanical drawing which defines the TO-39 package. You could compare these dimensions to your actual transistor (pulled out of its socket, heatsink removed) to be sure it is a match.
http://www.jedec.org/sites/default/f.../to/to-039.pdf

If you've got the huevos, an upgrade would be to unsolder the transistor socket and install a bigger, beefier, more powerful NPN transistor. The Fairchild TIP47, for example, is rated to dissipate 2 watts without a heatsink (if the ambient air is 25C). DigiKey and Mouser both sell it for under a dollar.
http://www.fairchildsemi.com/ds/TI/TIP47.pdf

However, the pinouts don't match. TO-39 looks to be a pin triangle ordered EBC. (Verify this by tracing the 3 transistor tracks on the PCBoard and comparing against the schematic!) TIP47 is a straight line ordered BCE. So you'd have to bend/extend the emitter lead of the TIP47 to fit it into the existing drill holes. Luckily the emitter is a low-current (65mA), low-frequency (DC!) signal, so the extra 8 mm of wire won't hurt anything.

Only do this if you've got huevos grande.

On the other hand if you decide to use the SE7001, you can improve heat transfer from transistor to heatsink. It would be wise to scrupulously clean and shine the inner diameter of the heatsink; if you have access to an ultrasonic jewelry cleaner tank, pop that little booger in for a few minutes of cleaning. Follow up with many Q-Tips and much rubbing. Then apply a thin film of heatsink compound (a/k/a "thermal grease") to the transistor case before mounting it into the heatsink.

Last edited by Mark Johnson; 6th September 2012 at 11:15 PM. Reason: bugfix
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