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

Rob C · 150159

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Offline Rob C

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on: November 06, 2023, 09:21:13 PM
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!
« Last Edit: November 25, 2023, 06:10:33 AM by Paul Birkeland »


Offline Happy Ghost

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Reply #1 on: November 07, 2023, 02:59:02 AM
Quote from: Rob Capizzi on November 06, 2023, 09:21:13 PM
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!

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/

Atul

Offline sl-15

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Reply #2 on: November 07, 2023, 03:55:51 AM
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.

Stefan Hampel
Soundsmith Carmen, modded Technics SL-1200mkII, Thorens TD 125 mk2 with SME V, Eros, Extended Foreplay III, BeePre2, Crack, Pioneer Spec 4, Sonus Faber Electa

Offline Paul Birkeland

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Reply #3 on: November 07, 2023, 04:50:04 AM
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 "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Rob C

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Reply #4 on: November 07, 2023, 03:58:11 PM
Quote from: Paul Birkeland on November 07, 2023, 04:50:04 AM
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.

Ok thank you.  LEDs meaning all 4? 2? Or the one? Also did you mean the mje350 or 2n2907a or both?
« Last Edit: November 07, 2023, 04:04:54 PM by Rob Capizzi »


Offline Rob C

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Reply #5 on: November 07, 2023, 04:01:30 PM
Quote from: sl-15 on November 07, 2023, 03:55:51 AM
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.
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


Offline Paul Birkeland

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Reply #6 on: November 07, 2023, 06:23:08 PM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Paul Joppa

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Reply #7 on: November 07, 2023, 07:03:20 PM
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.
« Last Edit: November 07, 2023, 07:05:11 PM by Paul Joppa »

Paul Joppa

Offline Rob C

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Reply #8 on: November 09, 2023, 08:39:00 AM
And obviously replace the driver tube? Is the 12at7 or 300b the diver? I prob should know that.
« Last Edit: November 09, 2023, 08:41:53 AM by Rob Capizzi »


Offline Paul Birkeland

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Reply #9 on: November 09, 2023, 10:05:32 AM
The 12AT7 would be the potential source for an issue like this.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Doc B.

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Reply #10 on: November 09, 2023, 10:54:09 AM
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.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline Paul Birkeland

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Reply #11 on: November 09, 2023, 11:04:12 AM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Doc B.

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Reply #12 on: November 09, 2023, 12:06:53 PM
Oh right, MJE350. Pushing the limit at startup voltage. Just finally gave out after so many years. 5731A is the way to go.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline Rob C

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Reply #13 on: November 09, 2023, 06:12:44 PM
Good to know. Thank you. can I buy what I need from BH?


Offline Rob C

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Reply #14 on: November 09, 2023, 06:18:19 PM
Quote from: sl-15 on November 07, 2023, 03:55:51 AM
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.

Vid of mod
https://youtube.com/shorts/MYkKOiIVswk?si=34S0M_MqxVvbeCf4


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