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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 4th February 2017, 01:08 AM   #11
Atk1d is offline Atk1d  Canada
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I wired up the PCB as I had before minus the volume pot. A direct input to PCB.

Readings.
Red/red 386 to 404
Yel/yel 5.3 to 6.4 DC. 470ac
Opt 464 /448
Pin 9 12at7 3.3 to 7dcv
Coupling caps 265/256 right side
97/446 left
For general knowledge the opt's are cxse25_8_5k

Going to change coupling caps and the 5w bias resistor Incase one or more have gone bad. With all the testing.
Also going back to 5ar4 rectifie. Though the Hammond can drive the 5u4gt with ease and it as a lower Ma DC than the edcor.
Will get back as time permit.
Thanks everyone. Frustrated I am, when I know the board will work.

Last edited by Atk1d; 4th February 2017 at 01:10 AM.
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Old 4th February 2017, 02:28 PM   #12
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It would help if you could post a picture of the board and the way you have it currently wired up. That way it's easier for me to see which parts you have and how to probe them. For now I will use component numbers and terminal connections.

Wild card (this has happened exactly once and could not be fixed remotely, the user had to send me his dead board, which I fixed in one minute after seeing it). A really good picture could have found it. Have you drilled out any of the holes in the board to accommodate parts with fat leads like boutique coupling caps?

I would hold off on changing the coupling caps. I also doubt that they are your main issue for now. It is possible that you have two problems. Let's figure that out before changing any parts, since that has a low probability of finding the problem, and may create another one.

You state that you are getting very low volume. Does this apply to both channels, or just one? Is the low volume sound clean, or distorted? Does the sound from both channels appear equal in volume and distortion. Does sound come from one or both speakers when signal is applied to only one input? Try each input separately.

Your voltage readings show a big difference between the two channels. Either I'm not interpreting what you are showing correctly, or one channel is completely dead from a DC voltage perspective. Lets fix that first.

Can you take several voltage readings with the negative voltmeter on a good PC board ground point, and the meter set to DC volts? A good ground is the end of R17 near the center of the board (R17 is one of the 5 watt cathode resistors). The input connector ground or the power transformer "T1-RED-YEL" terminals are also good.

Make the following measurements, with all tubes installed and warmed up for a minute or so. :

Test the power supply:

Each end of R1 or the L1 connector. R1 is the 5 watt resistor in the power supply.

If you have over 400 volts DC on each end of this resistor, the power supply is working properly. AC voltage readings are not needed if this is correct. There should be different numbers on each end. The end toward the rear of the board will be lower, and the difference will tell us how much current is flowing through the board.

Test the output stages:

Each of the 3 screws on the primary side of the OPT. These readings should all be within a few volts of each other, and roughly the same between each channel. Your readings show a channel / channel difference.

Each end of the other 5 watt resistors, R17 and R27, yes one side is grounded, and should have 0 volts, I'm just covering all the bases to determine if one of the tube sections isn't working. The readings on the ends closest to the output tubes will tell us if each output tube is working, and how much current is flowing through them. 30 to 40 volts DC are normal.

Test the input stages:

Each end of the 10K 2 watt resistors feeding the CCS chips, R14 and R24.

Each end of the coupling caps C11 and C21. This is where things are wrong in your posted readings, but I'm not sure how you made them. If they are done in this manner something is wrong. One end of each cap should have nearly zero volts, and the other should have about 100 to 200 volts. A 400 volt or so reading indicates that one side of the 12AT7 is not working.

The DC voltage at the end of R10 and R20 furthest from the board edge. These are the resistors closes to the small electrolytic caps on either side if the input connector. There should only be a few volts here, usually about 1 volt.

Note, I will be leaving on a road trip early Wednesday morning, returning the following Monday. (Orlando Hamfest). I will not have reliable internet access during that time, only phone connection.
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Old 4th February 2017, 03:07 PM   #13
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George, have a great time!!!
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Old 4th February 2017, 10:04 PM   #14
Atk1d is offline Atk1d  Canada
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Default Grrr

After some investigation,I found a bad resistor,replaced it.
Installed new bias resistor s, and coupling caps.
Double checked rest of PCB.
Checked out good (values in correct places)

Readings have not changed.
Rectifier b+488 b- 186 5ar4 sovtek
12at7 pin 9 - 30vdc
Pin 1 feeding right ch 256vdc

Coupling cap
Right 256vdc/0
Left 85vdc/0

6l6 drivers 480vdc
Bias resistor both 39vdc
Have look at photo, wired correctly.

I have clearly missed something.
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Old 5th February 2017, 01:45 PM   #15
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It is hard to tell but I'm looking at your input wires from the vol pot to the board, are the red wires the signal wires (wiper wires from the pot) ?
If they are then it appears they are connected to the ground terminals on the board. The terminals for the signal wires are the two outer terminals.
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Old 5th February 2017, 04:17 PM   #16
w5jag is offline w5jag  United States
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Quote:
Originally Posted by spendorite View Post
It is hard to tell but I'm looking at your input wires from the vol pot to the board, are the red wires the signal wires (wiper wires from the pot) ?
If they are then it appears they are connected to the ground terminals on the board. The terminals for the signal wires are the two outer terminals.
I agree.

Wiin W5JAG
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Old 5th February 2017, 09:19 PM   #17
Atk1d is offline Atk1d  Canada
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Default Top side

Here is top side showing most resistors, I've triple checked placement and values, all are correct.
No more signs of discoloration .Used a magnifying glass go check solder and for cracks, nothing bad there.
I did find that the speaker output Jack's were grounded to the chassis, drill holes were just large enough to allow post to squeeze thru.
Chassis was not built by me.... I have enlarged the holes. Maybe that was issue?
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Old 5th February 2017, 09:25 PM   #18
Atk1d is offline Atk1d  Canada
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W5jag ,spenorite the red wires are connected to the ground. From pot to the board.
Also going to re rout the input wires a little to close to the left out put
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Old 6th February 2017, 12:39 PM   #19
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Quote:
I did find that the speaker output Jack's were grounded to the chassis, drill holes were just large enough to allow post to squeeze thru.
Chassis was not built by me.... I have enlarged the holes. Maybe that was issue?
If all 4 jacks were touching metal that will cause your exact symptoms. It has happened to a few builders, and was one of the things that I suggested checking initially.

Quote:
Speaker terminals that are not isolated from the metal chassis (both terminals shorted to the grounded metal chassis).
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Old 6th February 2017, 01:18 PM   #20
Atk1d is offline Atk1d  Canada
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Default Test

George, Hope you had fun.

Following your test guide this is what I found. Put all back together

L1 connector.........493/481vdc

Opt primary........ L t..481/479/475
Rt. 480/478/475

R17...39.5 R27......39.8

R14.....385/480
R24.....444/480


Coupling caps
Rt.......5v/84
Lt........0/210

R10...... 2v
Checking the 12at7 pin 9 30vdc
8 0v,70v,6 88v,5 30v,4 30v,3 0v,2 0v,1 210v
Bad tube?

I have also fixed the speaker terminal issue. Now the only part grounded to chassis is a star gnd for safety and the input RCA

Last edited by Atk1d; 6th February 2017 at 01:33 PM. Reason: Fixed an issue.
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