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Author Topic: Help me compare these two power amp transistors  (Read 4644 times)
KE4JOY
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Posts: 1314




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« on: July 20, 2013, 11:50:22 AM »

I managed to fry (I think) a power amp the original part is MJE521

Data Sheet : http://alltransistors.com/pdfview.php?doc=mje521re.pdf&dire=_motorola

I selected a MJ200STU from mouser

Data Sheet : http://www.mouser.com/ds/2/149/MJE200-70233.pdf

They look similar except the replacement seems a little smaller in wattage.

My transistor checker cant deal with the big  power amp transistors so I don't really know if its cooked or not but I do know the bias resistors got a good cooking  Angry
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KE3WD
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Posts: 5694




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« Reply #1 on: July 20, 2013, 01:20:45 PM »

That really isn't a good sub here.  A good sub should meet or exceed the 3 main specs, and this one does not. 

The 521 is 40, 40 and 5

The 200 is 40, 25 and 8 

The 25V for the Vceo is lower than the other and that means this transistor would likely not survive in that circuit. 

Use your DMM set to Diodes scale or VTVM/VOM set to Low Ohms to check the three leads of the transistor against each other.  If any are shorted, bad power transistor. 

MCM Electronics lists the exact part you need, at twenty seven cents ea. 

http://electronics.mcmelectronics.com/search?cataf=&view=list&w=mje521&x=0&y=0

While there, pick up the Emitter resistors that opened as well: 

Use these two .47 ohm to replace the original .51 ohm.  Not a problem that they are slightly off of the original, what's important here is that both are in the ballpark and matched. 

Is there a Trimpot or two located on the driver board for this amp?  This could be important. 


73
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KE4JOY
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Posts: 1314




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« Reply #2 on: July 20, 2013, 01:29:22 PM »

No trimpot, there is a zenior in there I need to check as well.

The rig is a SB303 that up until this little screw up worked fine.

Here is a link to the schematic unfortunately it has a captcha thing on it so I could not directly link it.

http://www.vintage-radio.info/download/download.php?dir=heathkit&file=sb-303.zip&error=1
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KE3WD
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Posts: 5694




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« Reply #3 on: July 20, 2013, 02:53:01 PM »

Likely all that got hit were the output transistors and the resistors. 

In the ss tlo power amp, diodes only need to be replaced is shorted, which is very rare but can happen.  I don't see a zener in there, incidentally. 


And not likely anything could be damaged further back due to the coupling cap between driver stage and outputs. 


73
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KE4JOY
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Posts: 1314




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« Reply #4 on: July 20, 2013, 03:56:29 PM »

Your right it is not a zenior, my bad. still its an odd thing evidently adjusts bias to compensate for thermal drift.

http://www.ebay.com/itm/RCA-1N3754-D1300A-/400317486525

It checked out okay though.
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KA4POL
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Posts: 1864




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« Reply #5 on: July 20, 2013, 09:49:04 PM »

I would consider the MJE800, MJE801, MJE802, MJE803 as replacements, same case, better values.
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KE4JOY
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« Reply #6 on: July 21, 2013, 02:05:19 PM »

Those are darlingtons, any pnp equalivelant?
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WD4HXG
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Posts: 182




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« Reply #7 on: July 21, 2013, 02:06:55 PM »

In addition to the finals, check the driver
and replace the bias diode. Otherwise if the
diode is damaged it will shift the bias on the
finals, you will have crossover distortion
and the finals will fail fairly quickly.

Also your MJE521 is available here:
http://www.ebay.com/itm/MJE521-TRANSISTOR-TO-220-TYPE-NOS-/370550627576?pt=LH_DefaultDomain_0&hash=item564689dcf8
« Last Edit: July 21, 2013, 02:09:49 PM by WD4HXG » Logged
KE4JOY
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Posts: 1314




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« Reply #8 on: July 21, 2013, 02:14:09 PM »

The MJE700 & MJE800 look like a good pair.

Yea I plan to check the driver stage as well thanks for reminding me.

The bias diode is an odd thing 1N3754. It tested good with the diode checker but thought I would replace it anyhow but I am having trouble finding a cross for it. It has a metal can which mounts to the heat sink and adjusts current to the push pull pair.


I hate to go to different sites to order a few small components paying more for shipping than the components  Cheesy
« Last Edit: July 21, 2013, 02:39:10 PM by KE4JOY » Logged
KE3WD
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Posts: 5694




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« Reply #9 on: July 21, 2013, 03:22:09 PM »

IME you likely don't have to replace that diode on the heatsink in this case, because we know what caused the output failure and it was not due to overheating, at least not in that sense.  It was due to a short on the output. 

Don't put Darlingtons into holes that were designed for standard bipolar power transistors. 

MCM Electronics should have these transistors. 

or, Cross Reference to NTE.  Almost every parts house carries NTE.


73
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KE4JOY
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Posts: 1314




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« Reply #10 on: July 21, 2013, 03:49:55 PM »

Why not the darlingtons? They seem like a good fit, just wondering.
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KE3WD
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Posts: 5694




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« Reply #11 on: July 21, 2013, 05:27:50 PM »

Darlington pairs contain two silicon transistors inside for gain.  This is not only too much gain for the circuit you have, but the voltage drop from base to emitter would have to be set up in the power amp for twice what it is now. 

Not advisable to sub Darlington for the standard transistor. 

The amp would likely blow up and damage even more components than are damaged now. 


73
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WD4HXG
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Posts: 182




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« Reply #12 on: July 21, 2013, 07:27:17 PM »

Check the diode with current flowing through it.
Check the hfe and back into the anticipated current
for the base based on the emitter or collector current.
The diode may be ok but I would surely check it closely.

73

Chuck WD4HXG
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KE4JOY
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Posts: 1314




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« Reply #13 on: July 22, 2013, 08:23:25 AM »

Im pretty confident about the diode. I used a 'dynamic' diode checker that shows about .48V drop at room temperature. I warmed up the heat sink a little and the voltage drop decreased so it appears to be working okay. Its a good thing because I am unable to find an equal [110V .12mA] in the right (radial) package.

http://www.radiomuseum.org/tubes/tube_1n3754.html

Here is the tester I use.

http://support.radioshack.com/support_meters/doc7/7242.htm
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KE4JOY
Member

Posts: 1314




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« Reply #14 on: July 23, 2013, 12:49:23 PM »

Okay what about the BD437? as a sub for the MJE521

MJE521 : http://www.mouser.com/ds/2/68/mje371-49366.pdf

BD437 : http://pdf1.alldatasheet.com/datasheet-pdf/view/2928/MOTOROLA/BD437.html

It is slightly less wattage but looks (to me) to meet or exceed all other paramaters.

Sorry to be a pest but I'm trying to get all the components from one source.
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