Channel Sudden out (A Side) [resolved]

[svEnchantment]

My Stereomour- 2A3 (I built stock in 2012 or 2013) and has worked flawlessly until today. 1 Channel sudden quit playing, powered down and waited 30 minutes, powered up, channel still dead.

Swapped power tubes, same channel dead.

Got out the manual, multimeter and check caps, looked for any obvious damage, did resistance checks, all within range. Powered up and proceeded to check voltages - started  with IA=397, IB=398 (mains are at 123.5 - 125.0v), OA=385 and rising to 392, OB=178-179. Monitored for 2 mins, no change.HLMP LEDs lit. Kreg(s) at 2.49.

Stopped here since OA was so high. Powered down, pulled C4S board and reviewed closely for any damage, solder joints gone bad, etc. Nothing obvious.

Run a E81CC tube (12AT7 went bad about 5 years), lites up like it always has but unfortunately I do not have a replacement - have several 5751 if those would be worth testing but thought I would wait for advise.

Looking for guidance, feel like it is the tube or in the C4S board. Any advice or guidance would be appreciated.

Thanks

Bob S

 

[Paul Joppa]

I expect you're right - either the tube or the C4S. I would advise hunting up another 12AT7 - the 5751 will just make both channels silent. Fortunately it's a fairly common tube; many guitar shops even carry them.

 

[svEnchantment]

Thank you Paul, I will do just that. Will report back once I have have one in hand tested. I do appreciate the quick response.

Cheers

 

[Paul Birkeland]

You can rotate the C4S board 180 degrees and reinstall it.  If the voltage issue switches sides, that would indicate a cold solder joint on the center leg of the MJE5731A (most likely, or another similar soldering issue).  If the high voltage says on the same side, that would definitely motivate me to buy a new tube!

 

[svEnchantment]

Paul and Paul,

Well the new tube did not solve the issue. I did touch up the solder joint on the MJE5731A as well as any others that even remote looked suspect. Same results. Should I replace the MJE5731A (which I will need to order) or should I consider ordering a C4s card from you guys and just install new one.

Thanks again for continued assistance,

Cheers

Bob 

 

[Paul Birkeland]

What Kreg voltage do you see?

 

[svEnchantment]

Hello Paul,

The Kreg voltage on both sides is 2.49v.

Cheers

Bob S

 

[Paul Birkeland]

There's a 220 ohm carbon comp resistor going to pin 2 and to pin 7 on the 9 pin socket, I would strongly suspect one of yours is broken.

 

[svEnchantment]

Paul,

It has taken a bit to time to get back to you.

I checked the 220 ohm resistors running from tube pin 2 and 7. The Pin 2 resistor tested at 216.8 but pin 7 resistor test at 258.4. I dug around and found a handful of 220 carbon comp resistors and replaced the pin 7 to the terminal 9 resistor.

Fired it up and fundamentally obtained the same results with -

IA=401, IB=402 (mains are at 123.5 - 125.0v), OA=389, OB=172. Monitored for 1 mins, no appreciable change. HLMP LEDs lit. Kreg(s) at 2.50.  A silver lining is that I checked this with 3 tubes (including my original) and the reading are all close enough for me to declare my original tube as not failed !

Rechecked resistor and other connections just to be sure. No joy. So, looking for further guidance on repair advice steps.

Let me add, this was my first BH kit and my entry into "the journey" with tube based amps/preamps (2012-2013) and the build shows as I study it!.    I have since built a number of other BH kits -- S.E.X. 2.1 with upgrades, 300 BeePre, Paramount Monoblocks (300b), Smash/Smashup, Reduction and a non BH design known as the DG300B amp (source your own materials) from a company in Canada providing PCB's, schematics and design documentation.

My point for sharing the above is I have been studying amps to support a 45 build, which as you can expect lead me to the Stereomour II. I have followed a couple of threads here on the forum and the path to getting that up as a 45 based amp. But I have been thinking maybe I leverage all the great gear, documentation, etc I have in this original Stereomour and clean up my "learning kit" build and bring it back up as a 45 amp (45's have been on my list for a long while now).

Love you thoughts here as I would strip this kit for the most part, leverage proven BH documentation, update appropriate parts that don't take well to resuse, clean up OPT and Power transformer (clean, paint),  I have several of your wood bases (bought during one of your guys clearance sales in times past), lots of spare parts and access to all I would need through the interweb and so on.

Please advise what you think - specifically should we get this resolved first, replace the C4S which I am inclined to do regardless if I can still source that PCB from your spares/inventory.... or just get 'er back running and simply order a new Stereomour II and drive that to a 45 build --- but as an aside, I will say I have been holding out for the MonAmour 2A3 monoblock - which I have been on the notify list for a good while now.

Thanks in advance for all the consideration of the above and all the support you guys provide to this forum.

Cheers

Bob S

 

[2wo]

I too am a big fan of the 45 but whatever path you decide on, I think it is really worth your  while to get this one going, you seem very close.

And the more time you spend playing with this circuit, the better prepared you will be when you try to adapt it...John

 

[Paul Birkeland]

I think it would help to see some build pics.  The 2.5V and high output voltage from the C4S could possibly come from a cold solder joint on the middle leg of an MJE5731A, but this issue seems unlikely.  The grid stoppers being slightly different isn't a problem. 

 

[svEnchantment]

Paul,

Thanks for the continued support. Please find requested attached pictures. If you something more specific, please let me know.

Cheers

Bob S

 

[Paul Birkeland]

I would be extremely suspicious of the lower terminal solder joint where the shielded twisted pair drain wire lands.  This would be terminal 8L.

If one of the grids is floating relative to ground, that will give you weird voltages like you have.

 

[svEnchantment]

Paul,

Thank you for quick reply. I desoldered that "ball" running to 8L, cleaned up, re-soldered. Checked voltages again -- no help.

For reference IB = 398.1 OB =174.7,    IA=398.0 OA=382 - 384.0 through multiple measurements across 4 minutes.

A1, A4 at 68.3,  A2 at 362.1, A3 at 11.98.

At a loss, any suggestions on how to check for the grid floating to relative to ground ?

Again thanks !

Cheers

Bob S

 

[svEnchantment]

John,

Sorry I missed your post. Thank you for the comments and I agree, best to get this one solved and after years I am still learning and sometimes wonder if I will ever learn enough to really diagnose a problem!

Appreciate your thoughts on the 45, I have read bit on 45 Amps and the way they can drive mid range and horns. In my collection of speakers I have a pair vintage Cornwalls and a pair of Forte III's I would love to test with a 45. I know they sound great with the StereoMour (or did at least !). I also have a set of Zu Audio Omen II that I run with my 300B amp(s), but I have never been happen with that combo, I wonder how they might perform with a 45.

Anyway, I ramble, again I appreciate the note.

Cheers

Bob S

 

[Paul Birkeland]

Is A3 really 12V? (maybe 12mV?).  If you actually have 12V there, that could possibly point to part of what's going wrong with your kit.

 

[svEnchantment]

Paul,

Made me think I made a mistake on A3, so I rechecked as I do have an auto ranging meter - but my reading was accurate.

What starts at near zero rises within 3 mins or so to 12v but also continues to rise with time topping at 14.88

I made following other voltage checks on the terminal strips #'s marked as follows

1 = 365.4  2 = 384.2  4 = 14.87  6 = 0      16 = 69.8  17 = 399.4

Others I had listed in prior post(s) are still fundamental the same.

Paul, sorry progress is slow - I have reviewed and tested what I believe I know the check. I do appreciate your working with me on this.

Cheers

Bob S

 

[Paul Birkeland]

I would swap 2A3s and double check that you don't just have one with a lot of grid leakage. If the 12V moves to the other side, then this is just related to the tube itself.

 

[svEnchantment]

Good evening Paul,

Ok for the requested next test, I swapped the 2A3 tubes and the voltage problem on Pin 3 (A3) moved to C3.

C3 now starts at near zero rises within 3 mins or so to 11.95v but also continues to rise with time topping at 14.85. A3 voltage is pretty steady at 0.64v - 0.71v.

For further reference, the following voltage measurements were unchanged regardless of which tube was loaded in Side A or Side B.

IB = 398    OB =175,    IA=398  OA=382 - 384.0    through multiple measurements across 4 minutes.

So now I assume I do have a problem with my 2A3 tube (RCA Grey plate, circa 1958 purchased as NOS). This current pair has 6 years of regular use, estimating around 300 or so hours per year.

I do have several other pairs but have not loaded those, wanted to make sure this was the right step considering this problem started suddenly and we still have not identified the source of the C4S board voltage issue on the A side.

Ready for what ever next test or actions, large or small you think I should take. I feel that it is a failed component in the C4S board, although I understand this boards purpose, i am no where near a complete understanding of what can cause the 0A voltage problem short of a bad 12AT7 which I think we have ruled out with multiple tubes delivering the same results on the C4S.

Thank you again, look forward to hearing back.

Cheers

Bob S

 

[Paul Birkeland]

That 2A3 is getting gassy, so probably time to retire it. 

A while back in this thread, I mentioned that you could remove the C4S board, rotate it 180 degrees, then reinstall it. 

This would be a more definitive test to understand what exactly is going on in your amp, and if rotating the board 180 degrees still provides a consistent voltage problem (where OA still produces high voltage, even when flipped to the other side), then a replacement C4S board and parts would most likely resolve that issue.

If you rotate the board and OB then reads high voltage, the C4S board is not the issue.