Troubleshooting Resistance Checks

[Doc B.]

Hmm, peculiar. It appears that the B+ supplying the plate at pin 5 of socket B shunted current thru the 237 ohm resistor between 5 and 6.  Somehow the 237 ohm resistor been shorted to the heater winding, thus also taking out the resistors on pins 3 & 4 of the D socket, which tie from heaters of both socket B and D to ground, and thus would be the path to ground for the short.

So the question is how did the screen at pin 6 in socket B get shorted to the heater winding (pins 3 and 4 on both B and D)?

[ksolis01]

Hmm, peculiar. It appears that the B+ supplying the plate at pin 5 of socket B shunted current thru the 237 ohm resistor between 5 and 6.  Somehow the 237 ohm resistor been shorted to the heater winding, thus also taking out the resistors on pins 3 & 4 of the D socket, which tie from heaters of both socket B and D to ground, and thus would be the path to ground for the short.

So the question is how did the screen at pin 6 in socket B get shorted to the heater winding (pins 3 and 4 on both B and D)?

Got the replacement resistors today. I checked around that area and when I was resoldered that area to try to get rid of any bad solders, I melted off the rubber on quite a bit of the heater wiring. The first two are pictures of the black heater wire in the B socket. My wiring ended up pretty tight, and the desoldered black wire part nearest to the end (first two pictures) was resting on B5 terminal. I'll fix it tomorrow, and am just posting along with pictures to confirm that this is probably the cause. Don't want to connect it tomorrow and end up shorting the resistors again.

To fix this, would it be fine to solder on a bit of extra length using leftover white wire or some of the black and red wire from leftover heater wire and wrap electrical tape on all the exposed areas? The extra wire to loosen up the area and decrease the chance that something rest on something.

[Paul Birkeland]

In order to fry that screen resistor on the 7 pin tube socket, you would either have to have a freakish short inside the tube that would've damaged other parts, or the pin and/or screen resistor leads had to be touching one of the heater leads.

[ksolis01]

I taped up all the possible ends that could have caused the short and added the burnt resistors back. I went through all the voltage test and essentially got the results that I got before, the little variation was within the decimals. I already tried resoldering all the connection under the problem board. Just gonna resolver again I guess.

KregA and  and KregB are significantly off, especially B.

Terminal     Voltage (DC)

Low Current C4S
IA        150V   (187.7)
OA 60-90V   (79.3)
KregA  3-6V   (1.936)
bRegA 150V    (187.7)
IB        150V   (148.1)
OB   60-90V   (54.6)
KregB   3-6V   (12.9)
bRegB   150V   (148)

High Current C4S (D socket)
IA         190V   (190.5)
OA        150V   (148)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (81.4)
bB        150V   (148.1)

High Current C4S (B socket)
IA         190V   (188.3)
OA        150V   (186.7)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (96.5)
bB        150V   (186.5)

Headphone Jack
Tip          0V    (0.01)
Ring        0V    (0.04)

[kgoss]

Are all of your resistance checks right?  Don’t apply power again till all resistance tests pass. They are there to prevent damage when power is applied to a build with wiring problems.

[ksolis01]

Are all of your resistance checks right?  Don’t apply power again till all resistance tests pass. They are there to prevent damage when power is applied to a build with wiring problems.

I redid the resistance checks. The four offs ones were suppose to be *, varying resistance, but came back as zero exactly. Like, there were some * terminals that also went to zero but varied in resistance first. However, these four went to zero directly.
Terminal   Resistance
10          0
20          0
22          0
23          0

[Paul Birkeland]

22 and 23 are the incoming AC line.  If those are 0 ohms to ground, your fuse will blow or the breaker will blow that feeds that outlet.

10 and 20 are the outputs of the constant current sink and the cathode of each half of the 6080.  Since you have at least half of the amp where the 6080 cathode voltage appears correctly, this 0 is also inconsistent with the performance of your amplifier.

[ksolis01]

22 and 23 are the incoming AC line.  If those are 0 ohms to ground, your fuse will blow or the breaker will blow that feeds that outlet.

10 and 20 are the outputs of the constant current sink and the cathode of each half of the 6080.  Since you have at least half of the amp where the 6080 cathode voltage appears correctly, this 0 is also inconsistent with the performance of your amplifier.

10 and 20 now have a fluctuating values in the mili ohms. 22 and 23 still measure directly to zero. I resoldered any joint that wasn’t perfectly shiny and also found that the white wire from the top row, middle lug, of the volume pot wasn’t connected to 12L. What else should I be looking for?

[Paul Birkeland]

milli-ohms or mega-ohms?

[ksolis01]

milli-ohms or mega-ohms?

Sorry, meant mega-ohms.

[Paul Birkeland]

A milli-ohm is 0.001 ohms, a mega-ohm is 1,000,000 ohms.  The "milli" is a short, the "mega" is nothing to worry about.

[ksolis01]

I ran another voltage test. The missing white wire disconnection changed two of the values from the previous measurements, the new ones are bolded. I'll give it another look tomorrow to try to find any other bad connection. All of them are mechanically sound but will check if the copper is cut. I had a white wire break off when running a voltage check so there is a possibility that another wire is compromised.

Terminal     Voltage (DC)

Low Current C4S
IA        150V   (187.7)
OA 60-90V   (79.3)
KregA  3-6V   (1.936)
bRegA 150V    (187.7)
IB        150V   (148.1)
OB   60-90V   (54.6) (71)
KregB   3-6V   (12.9)
bRegB   150V   (148)

High Current C4S (D socket)
IA         190V   (190.5)
OA        150V   (148)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (81.4) (91.1)
bB        150V   (148.1)

High Current C4S (B socket)
IA         190V   (188.3)
OA        150V   (186.7)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (96.5)
bB        150V   (186.5)

Headphone Jack
Tip          0V    (0.01)
Ring        0V    (0.04)

[Paul Birkeland]

A broken wire will indeed do a lot of strange things.

You still have the same regulator issue present on one channel though. 

I'd pull the 6AQ5 on that channel and recheck the voltage across the 2.49K resistor.

[ksolis01]

A broken wire will indeed do a lot of strange things.

You still have the same regulator issue present on one channel though. 

I'd pull the 6AQ5 on that channel and recheck the voltage across the 2.49K resistor.

Got the voltage. However, I decided to do a second check after turning the amp off and the value for B side came out different while A was consistent.
A side:
210 v

B side: 54, 69

[Paul Birkeland]

The voltage across the 2.49K resistor on the center C4S board above the 12AU7.  It should be well under 5V on both sides.