First Time Builder / Flummoxed

[Ajak85]

Hi All

I'm a first time builder here for assistance.  I've just completed my first assembly of a Stereomour, and I afraid it's more like a Stereoless  .

Going into the resistance and voltage checks I was pretty confident in mu build, I took my time during the assembly and double and triple checked all of my solder joints.

I first ran into problems during the resistance check.  There were (4) points that were out of spec. (1,2,14,15).  The instructions didn't define exactly what these point should be, so I didn't worry about it too much, especially knowing that my DMM isn't the greatest.  However seeing that 2 and 14 didn't match raised a flag. 
1 - Out of range
2 - 15.5 M
14 - 11.2 M
15 - Out of range
All other points met spec.

I moved onto the voltage checks and that's where I really got concerned.  Me readings were all over the place.

HV (A) - 307  HV (B) - 351
1 - 169        11 - 52       A1 - 3.2      IA - 359
2 - 306        12 - 0         A2 - 205     IB - 329
3 - 0           13 - 0          A3 - -0.4    OA - 357
4 - 0           14 - 118      A4 - 3.2      OB - 124
5 - 4           15 - 329      C1 - 52.5     Kreg (A) - 2.5  (B) - 2.5
6 - 0           16 - 2.5       C2 - 332     -reg (A) - 0   (B) - 0
7 - 0           17 - 321      C3 - 0.1
8 - 0           18 - 0         C4 - 52.5
9 - 0           19 - 356     
10 - 0         20 - 52

I then tried some troubleshooting to isolate the problem, but soon realized that I should consult with the experts.

I could really use some input on where to start troubleshooting.  Also, instruction on possible bad components and how to test them.

Any help is appreciated, I'm excited to get it working and fired up.

Thanks,

[Paul Birkeland]

The first thing to address is why A1/A4 are really low, and not more like C1/C4.

The last time I saw this happen, one of the lugs on the hum pot on that particular socket was making contact with the chassis, which shorts out the cathode resistor and lets the 2A3 run away a bit.  For your voltage tests, you'll only want the amp on for 20-30 seconds at a time, as the plate choke feeding that tube could get pretty toasty.

Of course, there's also the possibility that the 2A3 in the "A" socket isn't heating up.  You can measure the AC voltage between A1 and A4 to double check this, it should be about 2.5V.

There is also an issue in the 9 pin area, which you have noticed with the 0V Kreg value and 357V at OA.  You may try reheating the center leg of the MJE5731 on each side of that board, just to ensure that you have a solid contact. 

Let us know what you find.

-PB

[Ajak85]

I took a look at the AC voltage between A1/A4 and C1/C4.  Both checked out, I measured 2.4V for each.

I also looked at the lugs on the hum pot and they looked clear from the chassis, no shorts found with the meter either.

Then I inspected the back of the C4S board and reheated the center pins on the MJ5731s.   

I proceeded onto the resistance and voltage checks.  Unfortunately I came up with basically the same results as yesterday.

The only observable difference that I noticed was a higher voltage on the A side HV position and positions 1 & A2.  Now I'm reading 340V on the A side HV position and 260V on positions 1 and A2, closer to the target values than yesterday.   I also noticed while checking these positions that my readings were just about at the target 380V and 350V when I flipped the power on, then shortly settled at 340V and 260V.

Any thoughts on what to investigate next?

Thanks again for your input.

[Paul Birkeland]

Try swapping 2A3's side to side to see if the voltages move with the tubes.

[Ajak85]

I just swapped the 2A3's and found the same result.  52V on C1 & C4.  4V on A1 & A4.

[Paul Birkeland]

Remove the wire from the "K" pad on the "A" side.

Do a resistance check between A1 and ground, A4 and ground, the "A" side hum pot center tap and ground, and T16 and ground.

-PB

[Ajak85]

I first tested those resistance values as is; A side "K" pad connected to T16.

Resistance to Ground: A1 - 1.18K, A4 - 1.18K, A Hum Pot center tap - 1.17K, T16 1.17K.

I then lifted the wire from the A side "K" pad and repeated the checks.

Results were the same, no change.
 

[Paul Birkeland]

If the red wires coming off the PC-3 chokes are allowed to touch adjacent terminals, this could explain your unusual voltages (partially).

Check to be sure that those wires are just all by themselves on their own terminals.

-PB 

[Ajak85]

I inspected those points (T17 & T19) and they looked clear of the adjacent terminals.

I measured from those points to the adjacent terminal, T16 & T20 and to the chassis.  Each measured >10 Mohms.

[Paul Birkeland]

Hmm, I must be missing something pretty major here.

Can you remove the screws that hold your C4S board down, tilt it to the side, then post a photo of the 9 pin socket area?  If possible, include both the 9 pin terminal strips.

-PB

[Ajak85]

Just to be sure, I put the tubes back in a fired up the amp for a moment.  Still reading 4V DC on A1 and A4.

I hope these photos help.  Let me know if you need any other shots.

[Paul Birkeland]

Is there a jumper between terminal 6 and terminal 8?

[Ajak85]

Yes, It's there.  It loops from 6L to 8L on the rear facing side of the strip, then loops from 8L to 10L on the front facing side of the strip.

I see what you mean that it's not clear in the photos.  Just to be sure I measured resistance between 6 and 8, it read about 0.1 ohms.

[Grainger49]

Just to be certain measure from T6 to T7, it should not be a short.  Then from T6 to T10, it should be 0.1 Ohms again.  Also from T8 to T9, it should not be a short.

I want to make sure that you didn't short anything you didn't intend to short.

[Ajak85]

I took those measurements.  Now I'm curious what they should be.

T6 to T10 is okay at 0.1 ohm.

For T6 to T7 I measured 1.5 ohms and T8 to T9 - 1.7 ohms.