Blown mje350 and led [REPAIR VID AT END] [resolved]

Mr. Self-destruct

Mr. Self-destruct

New member
Hi, Rob here. Newbie kinda. I have some questions.  The mje350 transistor and LED on the 4c snapped into smoke tonight. Could old age be the reason? Is there a usual reason?  I really dont push the amps,  it's for late night listening. Any insight is appreciated.  It's labeled cc525 mje350 Mar. Will I be ok with any mje350? I have 2 extra leds. what is the led part number? Thanks a lot!
 

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Rob Capizzi said:
It's labeled cc525 mje350 Mar. Will I be ok with any mje350? I have 2 extra leds. what is the led part number? Thanks a lot!
Click to expand...

you can reach out to Queen for another MJE350. I believe any of the variants should be OK - AFAIK. This is the part number for the LED:

https://bottlehead.com/product/hlmp-6000-leds-combo/
 
This is not going to help you problem , I just have a question. What is the tester you are using in the 3rd picture? Thanks.
 
The MJE350 would be expected to fail from a driver tube failure taking the collector to ground.  It's possible that if you replace it, the LEDs, and the 2N2907 that your amp will come back to life.  If it doesn't, then we will have to explore some options to get these up and running again.
 
Paul Birkeland said:
The MJE350 would be expected to fail from a driver tube failure taking the collector to ground.  It's possible that if you replace it, the LEDs, and the 2N2907 that your amp will come back to life.  If it doesn't, then we will have to explore some options to get these up and running again.
Click to expand...

Ok thank you.  LEDs meaning all 4? 2? Or the one? Also did you mean the mje350 or 2n2907a or both?
 
sl-15 said:
This is not going to help you problem , I just have a question. What is the tester you are using in the 3rd picture? Thanks.
Click to expand...
Hi drok transistor tester. I modded mine. I don't like the lead clamp so I opened and soldered 3 wires to 1 2 3 and added clips.
https://www.amazon.com/Transistor-DROK-Capacitor-Capacitance-Automatic/dp/B01MS1FOYM/ref=asc_df_B01MS1FOYM/?tag=hyprod-20&linkCode=df0&hvadid=642146280909&hvpos=&hvnetw=g&hvrand=17795736675563495445&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9003472&hvtargid=pla-906476891433&psc=1
 
Replace the LEDs on the side with the MJE350 that's blown.  Replace the MJE-350, 2N2907, and 2 LEDs, then see what happens.  50/50 that gets you back in business.
 
As I recall, the zener string can die if something else craps out. You can check each zener diode for resistance, before firing it up. (Check both polarities; one should conduct but the reverse should not.)

Unfortunately, the soft-start retrofit kit is long out of production.
 
PJ is right on here. When an LED explodes violently on a Paramount board there is most often a zener or two taken out in the zener string as well. Definitely check them for shorts and replace any blown ones before firing it up. I can't say that we came up with a definitive cause for this condition, but we have seen it happen a few times over the past 15+ years.
 
Man, I saw the MJE350 and thought this would be the B side of the C4S, but now seeing the 90.9 Ohm R1, this is the A side!

Yes, PJ is correct.  You probably have a 75V zener diode or several that are shorted out, just pull them all out and replace them.  The MJE350 needs to come out and be replaced by an MJE5731A with the metal tab facing the opposite way.  The LEDs get replaced, as well as the 2N2907.  When this is sorted out, be sure to go into your other Paramount and replace the MJE350 on the A side with an MJE5731A.
 
Oh right, MJE350. Pushing the limit at startup voltage. Just finally gave out after so many years. 5731A is the way to go.
 
Nicely done with the mod, thanks for sharing.
I bought some transistors on eBay once for my Foreplay and they crapped out after a month or so. If you do all that work I would spend a few more bucks and order from a reputable seller even if shipping is higher in the end than the parts.
This seems to be the same the only difference that I saw is the Watt rating which is 50 instead of 40: MJE5731AG

https://www.mouser.com/c/?q=MJE5731A

But Paul will know better, in case I am wrong.
 
I really appreciate all the help and knowledge.  I could however benefit from a parts list. I could make one but I don't know the zener diode part. I'd assume there are different ones? Thank you.
 
 
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