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need help crossing transistor


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4/25/2000 12:35 PM
jason
need help crossing transistor
I was wondering if anyone may be able to assist me in my search for a power transistor (TO-3 package).  
It goes to a PA (Dukane Medallion)used in a local industry (Bob, you have a call on line 2).  
Any way the numbers on the shorted transistor are 720 31 RCA H 7940.  
Anyone have a clue?  
Thanks,  
jason
 
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4/25/2000 1:45 PM
Stephen Conner

Hi Jason,  
 
It would be a RCA transistor type 72031 made in week 40 of 1979. They probably don't make 'em any more. It is probably not old enough to be Germanium so you will not have much trouble finding a modern equivalent. You don't need an exact equivalent, I don't imagine Bob will be too fussed about the THD n the receptionist's voice ;) so you should be able to suss it yourself from first principles.  
 
Motorola transistors are great for audio: the MJ802 (NPN) and MJ4502 (PNP) for amps up to 100W > 8 ohm and the MJ15024 (NPN) and MJ15025 (PNP) for bigger rigs.  
 
There is a cool amp fault finding page at:  
 
http://www.gmt.dircon.co.uk/repairfaq/REPAIR/F_Pwr_Amp.html  
 
Good luck.  
 
Steve C.
 
4/26/2000 5:43 AM
Steve A.

Steve:  
 
    The ECG catalog lists the ECG 130 as their replacement for the 72031, so that would probably be closer to the 2N3055 than the MJ802... but the MJ802's should work great and can handle more watts than the 2N3055's. (I seem to remember that SAE and Phase Linear were using the MJ802's and 4502's in their power amps almost 30 years ago...)  
 
 
For Jason:  
 
    It's usually a good idea to replace all of the output transistors, and not a bad idea to replace the driver transistors as well (especially if the old 72031 burnt up). BTW do all of the output transistors have the same number? Or would half of them be the PNP complement?  
 
--Good luck!  
 
Steve Ahola
 
4/26/2000 12:17 PM
jason

It's usually a good idea to replace all of the output transistors, and not a bad idea to replace the driver transistors as well (especially if the old 72031 burnt up). BTW do all of the output transistors have the same number? Or would half of them be the PNP complement?  
 
Thanks for the tip steve, that was my intention. They are all the same part number.  
 
Nice to have this new forum, I hate posting these kinds of questions on the amp board.  
 
Thanks to you both!  
jason
 
4/26/2000 4:03 PM
R.G.
Let me add one more bit of confusion.  
 
As background, I have a personal mania about old Thomas Vox amplifiers, and have written up a section of my web page with info about repairing and bulletproofing them. The most common problems with these things are blown output transistors, so I've been digging through ways to make fixes for blown transistors stay fixed.  
 
OT's (output transistors, that is 8-)) blow because either (a) they get too hot or (b) they exceed their safe operating area and a small part of the chip shorts internally. You can use "bigger" transistors, more rugged transistors, put in protection circuits, etc, but the best and usually cheapest thing to do is to put in more OT's in parallel. On the same heat sink, put double the number of transistors, using a new 0.25 ohm resistor in the emitter of each transistor, paralleling the bases and collectors.  
 
This is usually cheap, at most about $3 a transistor, down to about $0.60 a transistor for 2N3055's, and has the effect of doubling both the current capability and power dissipation capability of the output stage, while *halving* the equivalent temperature rise from the heat sink temperature on *both* transistors, and *halving* the exposure to safe operating area failures.  
 
Why don't manufacturers do this? Well, while an extra $6-$8 for another OT pair is small compared to us paying a repair bill, it is a HUGE increase in parts cost to them. The economy of scale is working against them. We can do it once, cheaply.  
 
If you have the room and the skill to paste them in, this is definitely something to consider.
 
4/26/2000 4:38 PM
Stephen Conner

You're dead right RG, in my 500 watt SS bass amp design I'm using 4 pairs of Motorola 16 amp transistors. That's obviously overkill from a current capability and power dissipation point of view. But hopefully it means the transistors will never see the limits of their SOA.  
 
That's the nice thing about building it yourself. You don't have to turn a profit at the end of the day.  
 
Steve C.
 
4/26/2000 6:48 PM
Reid Kneeland

Given that these are bipolars, which draw significant base current at audio frequencies, isn't there a potential problem with insufficient drive if you double the number of OTs? If you say it works on the Vox amps I'm sure it does, but in general simply doubling up on OTs may not work, right?  
 
Reid
 

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