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SUPPORT FORUMS FOR ZKITS >> ZKIT 1 Support forum >> Issues on start up.
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Message started by BlindLemon on 12/02/24 at 20:41:49

Title: Issues on start up.
Post by BlindLemon on 12/02/24 at 20:41:49

Older board, new build.

All together new startup.

Tubes in, no input, output into 8 ohm dummy loads and on a DBT.

Power on and after the tubes power tubes start conducting the b+ voltage does not stabilize at 350ish as it should, the voltage keeps decaying into the 200s before I turn it off.

Pull all tubes no issues. B+ 433vdc
Preamp tube in no issues. B+ 423vdc
Either power tube (SV83) in either position and the B+ starts cratering.
Does the same thing with EL84/6BQ5

Can't post a pic until I have 4 post.

TIA

BL

Title: Re: Issues on start up.
Post by BlindLemon on 12/02/24 at 20:43:26

Output transformers Edcor GXSE10-6-8K

BL

Title: Re: Issues on start up.
Post by BlindLemon on 12/02/24 at 20:45:09

Power transformer Edcor XPWR107-120

BL

Title: Re: Issues on start up.
Post by BlindLemon on 12/02/24 at 20:48:50

Well I think I figured out the PROPER way to attach a file.

BL

Title: Re: Issues on start up.
Post by BlindLemon on 12/05/24 at 13:40:18

50 views and no suggestions. I’m wondering if I posted this correctly? When I bring up this forum I don’t see my post; I only see it in the over all Forum Index as the Last post

BL

Title: Re: Issues on start up.
Post by Lon on 12/05/24 at 15:48:02

I think your post is visible. I'm afraid I am not able to be helpful. Hopefully someone who can will answer soon.

Title: Re: Issues on start up.
Post by Core32 on 12/05/24 at 16:55:04

I also can see your post in the correct ZKIT place.
No good idea on the possible issue as its been several years since I built mine as a point2point build without the board.
That being said it sounds like some component is the wrong value in your build or underpowered.
Did you buy and build the complete PCB or did you aquire it already built by someone else?
Checking carefully all the resistor values is a good start WITH ALL POWER REMOVED! And be sure there are no lingering high voltages before you start touching circuits with a meter or your fingers!

Title: Re: Issues on start up.
Post by Steve Deckert on 12/06/24 at 17:29:04

When the B+ continues to drop as you've described it means that something is drawing too much current -- likely something shorted or partially shorted to ground.

Whatever it is will be getting abnormally hot by the time your B+ drops below 200 volts so that may be one way to find it.

Otherwise you need to figure out what part of the circuit is drawing too much current. If you lift one leg of the 6.8K 2 W resistor you will be turning off the input stage. If when tested with no power feeding the input stage you still have the issue, then you know it is the output stage.

If it looks to be the output stage then I would lift one leg of the cathode resistor, which is 150 ohm, 5W. This will turn off the output stage and any current draw from the tubes. If by some chance this doesn't put the fire out then you have a short or partial short circuit somewhere between the 1K 5W and the transformer leads that connect to the circuit board and ultimately the plates of the output tubes. It could be a bad transformer.

You can lift one of the primary lead wires from each output transformer one at a time to see which of the two channels is causing the problem.

Measure the voltage drop across the 150 ohm 5 W cathode resistor and using ohms law figure out how many milliamps the output tubes are drawing. Typically 25mA per tube, so 50mA across the resistor. If it blows past 50mA, remove one output tube at a time to see which one is causing the issue.

Above all remember that the thrill of hearing something sound good that you made with your own hands and the long term satisfaction that comes with it is directly proportional to the amount of suffering that you put yourself through trying to get it to work.

Title: Re: Issues on start up.
Post by BlindLemon on 12/08/24 at 20:54:23

So it's in the left channel power section. With Left channel out (Left tube pulled or red B+ from OPT unsoldered) voltage looks good on right channel. Let it run for about 10 minutes to let the power transformer cook and B+ stayed stable, a little high but with one power tube out. I did try a couple different tubes in left channel.

Checked left channel 1K 2W resistor and it's good, replaced anyway. Tested again with B+ reconnected and same problem.

Unsoldered Left output transformer and wired up the Right output transformer on the Left channel. Same problem.

Left channel J2 is elevated off board and not touching any other component.

Just for grins replaced the .1uf from pin 1 to pin 3 on the board. No help.

Top of board (hidden) is tidy and clean as it should be.

The only other thing I can think of left to do I rig up a tube socket off the board and connected in before J2 (removed) to see if it is in the board.

Before I go to all that trouble any suggestions.

BL

Title: Re: Issues on start up.
Post by Gilf on 12/09/24 at 04:03:42

Greetings and welcome. Can you post a clearer photo? I can’t tell some of the fine details. Looks like maybe one of your diodes might be reversed but maybe it’s just the photo.

It looks like you aren’t using the two 20uf caps in series like the schematic shows, so what single cap are you using instead? And it looks like you have a resistor inserted at that point that is outside of the schematic specs?

This doesn’t really explain why you are experiencing a problem only in the left channel but I like to start at the foundation and work my way through a circuit.

Title: Re: Issues on start up.
Post by BlindLemon on 12/09/24 at 16:00:46

Thanks for the response.

Diodes in correct.

One 22uf 500V cap instead of 2 20uf 250V in series. Resister is a bleed down resister (220K to ground) to drain the PS caps after shut down. Essentially the same extra benefit as the as the 2 220k resisters in series across the 2 20uf 250V caps in series in the schematic.

Just took this picture. The board is in a state in which I left it after trouble shooting the left channel.

Oh, I also lifted the board to check for solder bridges and pulled the power tube socket for the left channel to make sure there was no solder bridges there also.

BL

Title: Re: Issues on start up.
Post by BlindLemon on 12/13/24 at 19:55:41

Had to double check to see if I posted in the correct forum for help.

BL

Title: Re: Issues on start up.
Post by JBzen on 12/14/24 at 11:51:50

Hi BL.

From your second pic, I would suggest to check all soldered joints. Some look cold.

Second place the two 20mf caps and 220k resistors in place as designed! Two 20mf caps in series = 10uf capacitance not 22uf as you seem two sumize.

I would think that is a good place to start before looking at an old board issue.

My thoughts.

Title: Re: Issues on start up.
Post by BlindLemon on 12/14/24 at 15:53:22

BJzen, thanks for the response.

Double checked solder - joints good/no bridges

As stated above the issue is in the power section , left channel. It is at a slow pace pulling too much current. At the point that B+, node after the ceramic 1K 5w resister in the power supply string, gets to ~225v I shut it down. The 1K 5w ceramic resister gets very hot, the 6.8k 2w does not or the 220k 1w bleeder resister.

The power tubes have been swapped out with different ones and the problem still exist. The OPT transformers have been swapped left to right and the issue still exist in the same channel. I even swapped in a spare OPT transformers grasping at straws. No love.

As far as the 22uf 500v cap being an issue, electrically it shouldn’t be. In most instances 2 series caps are used to increase the voltage capacity if you don’t have one with the required voltage capacity. 2 250v caps in series equals 1 500v cap. You are correct that the capsitance is halved of 2 caps of equal capacitance.

In this case (22uf vs 10uf) the only increase in current would only be a slight increase in inrush which would stabilize very quickly. Anyway that part of the circuit is PS filtering for the preamp tube. And as stated above the 2 power supply resisters after the 1K are not getting hot. Just the 1K.

I write all the above down not to be argumentative, but to state what I believe to be true in my mind. I am hoping there are some of you out there that are a lot smarter than I (not a high bar :D) and can explain to me where I might be wrong or can take a look at the provided picture and see something wrong.

If I’m not learning I’m going backwards.

I do appreciate all the input. The ultimate goal is to admire the glow while listening to good music.

BL

Title: Re: Issues on start up.
Post by Steve Deckert on 12/15/24 at 02:12:07

If the problem is the same with the output transformers swapped, then remove the transformer red and blue leads from the circuit board on the problem channel. Then check both solder pads for those leads with your meter to make sure they are not grounded.

Title: Re: Issues on start up.
Post by JBzen on 12/15/24 at 10:21:12

BL, no offense taken. I'm here to learn also.
Your thinking is correct. My question is how was it executed? Double check that! I often find that when we stray from the norm with a well thought out direction sometimes the execution is flawed leaving us in a cloud of smoke.
You got the right guy on it.
Hope you see those tubes glowing soon. Good luck.

Title: Re: Issues on start up.
Post by BlindLemon on 12/18/24 at 21:36:52

Been tied up, may be another day or two before I’ll get back on it.

Thx for the reply’s.

BL

Title: Re: Issues on start up.
Post by BlindLemon on 12/20/24 at 20:46:37

I'm stumped.

From SD;
Then check both solder pads for those leads with your meter to make sure they are not grounded.

Shows no continuity, not grounded together. Blue to ground = infinity / Red to ground ~250k which is about the same as the good channel.

Everything put back to "NORMAL". The B+ goes to below 250vdc before I turn the amp off. (Smoked 1k 5w resisters, replaced)

Pull power tube on offending channel. With power tube in the good channel the B+ stabilizes at about 350vdc.

Pulled power tube in good channel and place in bad channel and B+ craters with 1 power tube installed. Bad channel.

I don't know what else to do.

BL

Title: Re: Issues on start up.
Post by BlindLemon on 12/20/24 at 21:57:43

So I hooked the power tube up off board and the B+ still craters.

I find it very funny that the B+ doesn't start decaying until after the tubes warm up. You would think it would be a tube, but I have changed out power tubes.

Here's the power transformer specs, it ought to be stiff enough to hold the voltage up at idle???????

IDK

BL

Title: Re: Issues on start up.
Post by Gilf on 12/21/24 at 02:26:02

Oh that’s a frustrating situation. I don’t envy you.

The transformer you are using is spec. I’ve built several of these circuits with that transformer- certainly not an issue.

This is a wild guess but have you checked the tube socket on that bad channel? I have had cheap ceramic sockets go bad.

Otherwise, in this situation I’ve just gone through one piece at a time and checked every part, sometimes replacing as I go.

Good luck and keep us posted.

Title: Re: Issues on start up.
Post by Gilf on 12/21/24 at 02:31:16

You are sure all the parts are mounted high off the board? Especially the jumpers- that their are actually jumping and not shorting the path they are “jumping” over? Sorry if that is obvious, just can’t tell from the photo.

Title: Re: Issues on start up.
Post by Core32 on 12/21/24 at 04:18:30

My only builds were point to point, wire only.
Can you post a snippet of the schematic circuit area you believe is defective?
Or point me to the schematic link of the pcb.
If you can I will take some time to review and see if I can come up with a sequence of steps to help narrow it down.

Title: Re: Issues on start up.
Post by BlindLemon on 12/22/24 at 18:42:20

Here's the schematic.

Title: Re: Issues on start up.
Post by Core32 on 12/23/24 at 00:55:35

I first looked for differences between your schematic and the schematic I drew from the Decware designs before I did my point to point builds.
The first difference I noticed was in your schematic there is a voltage divider on each output tube consisting of two 1K 1/2W resistors. For some reason in my design, which I thought I used a lot of the ZKIT design to get values from, has the resistor to ground in that divider a 330K 1/2W, not 1K.
That is quite a bit of a difference. It makes no sense why one channel would work and the other one not because of this but it is a difference.
I am still looking but I would check the BOM to see if that divider should have different values than 1K || 1K.

Title: Re: Issues on start up.
Post by Gilf on 12/23/24 at 04:06:12

Perhaps here’s your problem. The 2019 schematic has some errors. Compare the power section of the schematic with the 2021 schematic:
https://www.decware.com/newsite/ZKIT1.pdf

I built to the 2019 schematic once and fried several components before realizing my schematic was incorrect. Could be the cause of your problem.