Failed Voltage Checks and some other weirdness [resolved]

[petewebb]

Hi All,

I'm looking for some help, I got my mainline almost together today. I got past the resistance checks fine, however I failed on voltage checks.

Terminal
+275vDC on the Power Supply Board 271V
+6.3vDC on the Power Supply Board 6.3V
IA on the A side C4S Board 271V
IA on the B side C4S Board 271V
Breg Regulator Board (both sides) 217V
-reg Regulator Board (both sides) 0V
Kreg Regulator Board    A side= 4.62V,  B side= 8.8v

Only the A side on both C4S has LEDs lit up, however if I touch my red probe from my multimeter to terminal 20 the A C4S's Bside LEDs light up, and if I touch the red probe to terminal 30 the B C4S's Bside will light up. Also notice that if I dial the A side Bias screw way up on the bias board the B C4S's Bside LEDs will light up, but not on the A C4S's Bside LEDs.

The voltage reading on terminals 20 and 30 is about 55V, and adjusting the bias screws on A and B sides of the bias board doesn't appear to change the voltage very much if at all (nowhere near 145v)

All 3 tubes are lit up.

Any help would be much appreciated!

Thanks,
P.

 

[Paul Birkeland]

You have one 6C45P that doesn't appear to be drawing the current it should.  The good news is that the majority of what you've built appears to be working.

Touching stuff and seeing LEDs turn on tends to suggest you have a loose solder joint.  Be aware that it may not be something directly connected to those terminals, so you'll want to turn the amp off and tug on things gently with your needlenose pliers to see what moves around that should otherwise be well soldered.

 

[petewebb]

Thanks, PB.

Today I detached all wires from terminals 11-30 on the middle tag boards and resoldered them with new wire back to the tags, tube sockets and PCBs. All the voltage and resistance checks still look good except for the A side Kreg which is still reading only 4.62V. The B sides of the C4S board LEDs still won't light up, except for the B side of the CS4 above the C tube socket if I turn the bias screw for A side of the Bias board up high.  However, I no longer get either B side's LEDs on either CS4s to light while touching my red probe from my multimeter to terminal 20 or 30 (with black probe attached to terminal7) so I guess that is good.

I did notice that the MJE5731A on the b-side of the C4S board above socket A gets extremely hot, while the others are just slightly warm.

I also reflowed all the solder on both C4S' and the Bias boards for all through-hole components and wires, but still have the same results of no B side LEDs.

Any suggestions as to what to try next?

Thanks,
P.

 

[Paul Birkeland]

You can swap the big C4S boards side to side to see if the problem stays put or moves with the board.

 

[petewebb]

Neither of the B side LEDs light on either board, but I swapped them anyway. Unfortunately, the problem persists, and the low voltage on the A side Kreg is still about 4.2V.

While I had the boards detached, I removed and resoldered the black and red jumper wires on both C4S boards and the Bias board, but that didn't help either.

 

[Paul Birkeland]

Q2 solder joints on the high current C4S boards may be what needs help here.  If the boards are still out, having a look at the amp underneath them might not hurt either.

-PB

 

[petewebb]

Hi PB,

I'll resolder the MJE5731A's in the Q2 positions again.  By "have a look at the amp underneath" do you mean the tube socket connections?

Thanks,
P.

Q2 solder joints on the high current C4S boards may be what needs help here.  If the boards are still out, having a look at the amp underneath them might not hurt either.

-PB

 

[Paul Birkeland]

Yes, what you see with the amp flipped over and no PC boards installed.

Specifically with the Q2 transistors, the middle leg is not all that easily soldered and tends to require a lot of heat.

 

[petewebb]

I completely removed the transistors in all the Q2 positions, cleaned the pins and resoldered them back in place, making sure to get enough heat to the middle pins. The end result didn't change, unfortunately, still no B side on the C4S'

I also removed all wires and resistors/capacitors from the tube sockets and resoldered all those as well. Man those 0.1uF 100v caps are a royal pain 
I then checked continuity across all those connections. to the boards, which checked out. Unfortunately still the same behavior with no B sides lighting up, and the B-side LEDs only lighting up above the C socket C4S if I turn the bias screw way up.  The A-side Kreg is still at about 4.5v, while the B-side Kreg is about 9.5v.

All the tubes do light up when powered on (pic attached). A few times I forgot to add a tube back in after resoldering the tube sockets, and interestingly the behavior of the LED lights on the C4S is the same; no B-side LEDs lit.

I had an unfortunate issue where some of the OA solder pad lifted while removing the bias board, but was able to push it back down and use 2nd pad next to it instead. All still seems good there.

Pics of the sockets are attached.

Yes, what you see with the amp flipped over and no PC boards installed.

Specifically with the Q2 transistors, the middle leg is not all that easily soldered and tends to require a lot of heat.

 

[Paul Birkeland]

Can you double check that the 431 regulators are installed properly in the center board and not in the high current C4S boards? 

You can also rotate the center board 180 degrees to see if the problem moves.

Having desoldered everything and rebuilt the circuit entirely, there is a very, very strong possibility that new problems may be introduced, so do keep that in mind during the debugging.

 

[petewebb]

Hi PB,

I verified the 431 regulators are in the center board only. I flipped the center board around and still only the A sides of the C4S' light up.
I did a recheck of voltage as well. The B side Kreg now reads 5.83V, and the A side Kreg is 9.45v.  Also OB on the center board reads 3.48V while OA reads 2.13v. All other voltage reads as it should.

 

[Paul Birkeland]

The solder joint at 25L is the kind of thing that could make stuff like this add up.

The 55V coming out of the high current C4S board combined with the 3.48V on the center board tell us the 6C45 is not drawing any current.  Normally when I see something like this, my first question would be about how evenly the tubes are glowing, as a loose connection at terminals 4/5 would potentially cause an issue like this, and your issue appears to be specific to that side of the amp and not the boards themselves.

Can you carefully measure the DC voltage at pin 7 on the 6C45 socket on the bad side as well as the OB voltage on the C4S board on that same side?

 

[petewebb]

I double-checked 25L. Looks ok but I re-soldered it and I get continuity from the red wire back to the OT3. I don't get any voltage off 25, is that ok?

For socket pin 7, on A socket I get 212.7V  and 214.1V on the OB for that C4S
For Socket pin7, on C Socket I get 215.3V  and 215.5V on the OB for that C4S

 

[Paul Birkeland]

That would mean neither 6C45 is really drawing any current.  Can you attempt to turn down the center board trim pots till OA and OB measure about 1.5V?  I believe they may have to be turned clockwise to get that voltage to go down.

 

[petewebb]

I've got OA on the center board to 1.5V, however, I can only get OB down to 3.5V before the regulator bottoms out(clicks).

In this state, the C4S above socket A has A side LEDs lit, while the C4S above the C socket has A and B LEDs lit.

 

[Paul Birkeland]

What are the OB voltages on the outer C4S boards under these conditions?

 

[petewebb]

I ordered some spare LM431, 2N2222A, PN2907 and MJE5731A that come in this afternoon. Should I try replacing any of those that would make sense in the path of the issue?

 

[petewebb]

215V for both.

What are the OB voltages on the outer C4S boards under these conditions?

 

[Paul Birkeland]

If you have 215V on an outer board OB and 1.5V on the center board OA/OB terminal feeding that side, that's a bit of a contradiction.  Those conditions would only really happen with about 100mA of plate current flowing, and that would drag other voltages down and burn up the 270 ohm 5W resistor in the power supply in the process. 

It may be best to just proceed with a repair on this one.  There's just too much that isn't making sense to know where to go from here.  Pulling out and replacing all the transistors is just a guess that will be a lot of work to complete and may cause damage to the PC boards.  A shorted transistor can be measured with the continuity beep on your meter and testing each pair of legs on each transistor.

 

[petewebb]

I think I may have measured wrong on that last step you mentioned. 

Buttttt.....I replaced the LM431s and 2N2222As on the center board, B-side first... no effect, same issue. However, when I did the A-side all C4S LEDs lit up! I was able to Bias the tubes at 145V all the other power checks succeeded with the Kreg's on both sides reading 11.2V.  All the switches work and they should, and I have music playing through the XLR and 1/4" now. Sounds Wonderful.

Thanks for your help PB!