Resistance questions on Paramount 300B build...

[JP]

All,

Hoping you can help.  I've got some incorrect resistance readings on my Paramount 300B build, on both units.  I'll start with just one unit - hopefully I've got the same root cause on both units.  I've re-soldered the locations in question - no change in the measurements after trying that.  I'm measuring with a Fluke multimeter.

Location	Manual value	Measured value
Terminal 9	124K ohms	286.5K
Terminal 11	3.0M ohms	5.69M
Terminal 12	5.6M ohms	21.62
Terminal 14	9.2M ohms	OL
Terminal 15	15M ohms	OL
Terminal 19	128K ohms	287.7K
9pin - B4	130K ohms	288K
9pin - B6	177K ohms	289.1K
9pin - B8	*	OL
9pin - B9	*	0.6

There are also a few places that were supposed to show no connection that are showing 0.2-0.6 ohms - I'm assuming that should be okay.  Or should they really be showing no connection?

Link to complete list of measurements

Appreciate any pointers on things to try next / any requests for more information.  Thanks in advance!

JP

[Paul Birkeland]

I don't see anything too disturbing on there.  Your meter's infinity is a lot higher than others, hence the reading of 26,000,000 Ohms vs. 5,600,000.  The reading at B9 is also not an issue.

-PB

[JP]

Paul,

Thanks so much for the confirmation.  Can I ask you to review the measurements for the 2nd unit, since I got numbers that were a bit different from what's in the manual for both of these?

Thanks, and please let me know if there's any additional information required.

JP

[JP]

All,

Just curious if anyone else can take a look at those numbers and make sure it's okay.  I was hoping to get a confirmation before I power up the units and start testing the voltages.

Thanks!

JP

[Doc B.]

Both amps measure essentially the same.

[JP]

Thanks Doc - moving on to voltage testing!

JP

[JP]

All,

I got my 'right' unit working - the only issue I have is that I can only get the hum balance down to 5.1mV.  I've read on some other postings that this might be tube related, and that there are some different options to fix this - other tubes, changing resistors around the hum pot, etc.

My 'left' unit is still not working correctly.  I moved on to test the voltages, and the OA of the C4S board starts to go above 350V, at which point I shut things off.  I went back and retested my resistance values, and did some re-soldering, and then retested the voltages - same thing - OA of the C4S board goes above 350V.

What are the other things that I should be checking / where's the best place to start to continue my investigation?

Updated measurements

Thanks in advance

JP

[Paul Birkeland]

A very common cause for an issue like this is missing jumpers on the C4S boards.  The Paramounts have a lot of C4S jumpers, I would go back and triple check that they are all there.

-PB

[JP]

So I have a little good news / bad news...

The good is that after going over the unit, I found that I had not soldered the -reg (A side) from the C4S board to 17U.  I soldered that in and re-measured all of the resistance values - everything looked good.  I connected the tubes, plugged things in, and all of the voltages looked good until I got to measuring 17, and that's where the bad news is.  When I measured that one, I got a large pop/spark.  I don't know if it was sparking before I measured, but it was sparking a little bit afterwards.  I turned off the power.

I waited 20 minutes, and then rechecked everything on the unit to see if I had a short anywhere, or if there was any difference that I could see between the working and non-working unit.  After this, I re-measured the resistance again - everything looked good.  I powered up the unit again and OA of the C4S board was stuck at 0V.  In addition, I could still see small sparks coming off of 17 (without touching my probe to this location).  I shut the unit off.

Then I started measuring the resistance of a bunch of locations around 17 and OA.  And, based on all of that, I think I've traced the part that blew back to the MJE5731A transistor on the B-side.
Specifically, on the unit that works, I'm seeing a resistance of ~290K Ohms from OA all the way back to the MJE5731A transistor on the B-side.
On the unit that is sparking, I'm seeing a resistance of ~150K Ohms at OA, +reg (B side), IB, and MJE5731A B-side B; at MJE5731A B-side C I see OL, and at MJE5731A B-side E I get ~3.30M Ohm on both the working and non-working unit.
Complete measurements are here - see the last worksheet

So, a few questions:

• How do I get a replacement MJE5731A?  Yes - I know I'll have to pay for this.
• Would you agree that I've found the bad part?  Would this explain the 0V at OA?
• And, here's the million dollar question - after I replace the part, where else should I be looking for a problem?  My assumption is that the root cause of the problem was not with the MJE5731A - it's with something that I screwed up.  I'm also assuming that I don't have a problem behind MJE5731A B-side E since I get the same reading on both the working and non-working unit.  Yet, does the spark at 17 point to just a bad part?  (I'm guessing no.)  A bad solder somewhere else?  I know I'm asking you to find a needle in a haystack, and I'm going to do a review multiple times before powering things up, but any pointers would be greatly appreciated so that I don't run into this problem with the replacement part.

THANK YOU!

[Paul Birkeland]

How do I get a replacement MJE5731A?  Yes - I know I'll have to pay for this.

E-mail replacementparts(at)bottlehead(dot)com

Would you agree that I've found the bad part?  Would this explain the 0V at OA?

Check the zener diodes also, be sure they all measure high in impedance in one direction.  There has to be a cause for 0V at OA apart from the MJE5731, there has to be a short somewhere.  Incidentally, this short is what caused the transistor to fail in the first place.  (Too much voltage across it)

And, here's the million dollar question - after I replace the part, where else should I be looking for a problem?  My assumption is that the root cause of the problem was not with the MJE5731A - it's with something that I screwed up. 

Yeah, there's something else in there.  Your meter probe may have shorted OA to ground somehow, or there could be something else that's the real problem. 

-PB

[JP]

After multiple tests, it looks like I've blown a few things.  I'd appreciate a review, just to make sure that I'm not missing something.  Based on my testing (see the last two pages of the spreadsheet), I've blown at least:
A-side D1 HLMP-6000 LED
A-side D2 HLMP-6000 LED
A-side MJE5731A
A-side 2N29071
A-side R2 (330K Ohm)

I think I've tested everything and have verified that this is all I've blown, but appreciate your feedback / any recommendations on other things to test.

Thanks!

JP

[Paul Birkeland]

Have you checked your Zener diodes? 

[JP]

I checked the Zener diodes, and for the most part, they tested pretty well.
Problem unit: 2nd one is .584 VDC; rest are all ~.64 VDC
Working unit: All test around .64 VDC
I'm assuming that the 2nd one that is measuring slightly different is okay?

Complete details in the last 2 pages of my testing

Thanks!

[JP]

Paul, Doc,

Anything else I should check?  If not, I'll submit my request for the replacement parts.

Thanks!

JP

[JP]

I went back and measured the resistance of the zener diodes - the 2nd one coming off of the tube is measuring abound 3.2M Ohm (the same one with the different results from the diode test) - all of the others are measuring 6.2M Ohm, so I'm thinking that I probably need to get that replaced as well.

Thanks!

JP