Paramount smoke out

[hpleung]

Hi,

I have acquired a paramount a few years ago. Its was already buildt (and seems to be built well).  Its the mono block, 2A3.

It worked wonderfully for many years. However i have not used it for perhaps 6 months.

Recently on turn on,, one monoblock released a lot of smoke and a chemical smell.

I quickly turned it off.

I am looking for some guidance on what to look for. I dont even have the guide as i did not build this.

I am attaching a picture.

Is there any resistance check i can do?

Thanks

[Doc B.]

We can sell you a Paramount manual that contains the voltages for both 2A3 and 300B versions. One possible failure mode of the Paramount circuit is a failure of the shunt regulator on that green PC board under the 9 pin tube socket (and partly under the board). Typically this will show itself as a blown LED on that board and you will find shorted or open Zener diodes in the string that is wired to a terminal strip under that PC board, next to the 9 pin socket. Unfortunately the resolution of the photo is not good enough for me to see if there are any blown LEDs on the green board.

If you don't see those things the best place to start will be getting the manual and checking voltages as you have suggested. Also look for electrolytic caps with bulged out end caps.

[hpleung]

Thanks Doc. Let me attach a few betters pics; i will also just get the manual for future reference.

I do see a slight bulge.in one of the caps.

I also see a crack in one of the component. Dont remember the name of it.

Let me know if you see anything obvious. Otherwise i will proceed with the manual and measurements.

[Doc B.]

Ah yes, those rectifers are toast. However they would have blown due to some other problem that needs to be sorted out. Is it possible that the tubes were out of the amp during the time is was not being used? That kind of damage could possibly happen if the 2A3s were plugged into the sockets improperly, i.e., rotated by 90 or 180 degrees from the way they should be plugged in.

[Paul Birkeland]

Also be sure that the Mundorf parallel feed cap in there doesn't have its metal lead touching the metal lead of one of the 22 ohm 5W resistors attached to the hum pot.  That will create the same situation as plugging the tube in improperly. 

There are some TO-220 packaged diodes that are a little more robust than the 1N5820 diodes we provided with the kit, and I tend to swap them out when I have one of these on my bench for repairs (though only the 2A3 is rough on these parts, not the 300B).

[hpleung]

What doc said is most likely what happened. I recently swapped in the EML 2A3; so likely i plugged it in wrong.

I am gonna get the manual so i can measure the resistance.

Are there any updates i should do besides the diodes that Paul mentioned? Since i am going to go inside, i might as well update.

I probably want to see if i can update the 300b socket so it isnt so easy to insert a tube incorrectly..

[Paul Birkeland]

I would suggest using the tube socket that fits the hole in the chassis that's there. 

https://www.mouser.com/ProductDetail/Vishay-Semiconductors/VS-20L15T-M3?qs=MLItCLRbWsy9VkphoLysiw%3D%3D

These are what I like for replacement diodes.  You'll need to be extra, extra careful that you put them in the right way. 

You have the soft start board rather than the older 12AT7 version of the driver, so that's all pretty robust.  Even the cathode bypass caps were chosen to handle an improperly inserted tube, so they are still fine as well.

[hpleung]

Can i ask if you can tell which version i have from the pictures? 1.1 or 1.0?

This is a question from Eileen as i am trying to get the manual.

Thanks

[Paul Birkeland]

You have what was a 1.0 that was upgraded to a 1.1 with the Soft-start upgrade package.  The 1.1 manual is accurate for what you have.