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Please Help! (Read 594 times)
Jinks
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Please Help!
08/02/14 at 02:26:56
 
Hi All, So I am really hoping to get some help with my Zkit1 build, I finished the board and installed it in the plinth which I laser cut at the school I teach at, I did the same plinth with my TT Lenco GL75 build, and I recently built a pair of the Madisound BK-12M folded horn kits, which sound awesome so I am super excited to get this great little amp to work, my pre-amp is a Croft Audio MKII from the U.K and is hosting twin Telefunkens and a Mullard.
So here are the problems I am having....test point A starts strong at 430 then drops to 230v and holds, test point B does the same and holds at 202v, point C starts at 11v then after 10 seconds rises to 28v and holds point D is at 1.8v and holds. If I leave the amp powered for about 60 seconds the 1K 5W resistor starts to smoke constantly and gets rapidly worse which is when I power down. Solder points all look good, the tubes are NOS Russian and were supposedly tested so are presumed good and they all light up fine; twin matched 6P15P and a 6N1P.
Any ideas?
Jinks.
I wanted to post images but this forum does not allow it yet!!!!!!!!!
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« Last Edit: 08/03/14 at 00:50:08 by Jinks »  
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Jinks
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Re: Voltages not right and smoking 1K 5W res
Reply #1 - 08/02/14 at 04:25:34
 
after testing the voltages and switching caps around and resoldering, nothing has changed, it looks like the smoke is however coming from the big 450V cap but it is hard to tell, I switched them and the voltage readings stay the same so I presume these are good, the 1K 5W is still getting very hot and could be the cause of the smoke, I dare not let it run too long. C and D points read 1.8V and are consistent in this, points A,B and C all start good and then hover at their correct voltages for maybe 10 seconds then start to fall quickly to the readings I give in the first post, not sure what to test next, I switched the output tube and nothing changed, the caps don't seem to be holding charge or maybe the transformer is losing voltage?
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Jinks
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Re: Voltages not right and smoking 1K 5W res
Reply #2 - 08/02/14 at 04:26:50
 
post 3
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Jinks
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Re: Voltages not right and smoking 1K 5W res
Reply #3 - 08/02/14 at 04:28:34
 
post 4
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Jinks
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Re: Voltages not right and smoking 1K 5W res
Reply #4 - 08/02/14 at 04:29:41
 
post 5
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Jinks
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Re: Voltages not right and smoking 1K 5W res
Reply #5 - 08/02/14 at 04:30:45
 
and hopefully pics-

https://flic.kr/ps/2JziDQ

Wink
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Jinks
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Re: Voltages not right and smoking 1K 5W res
Reply #6 - 08/02/14 at 23:26:45
 
Today I purchased a Capacitance meter online so I can check the caps, I tested the Edcor and it is a solid 320V on both Red outs, I went over all the parts again and they are all correct and test okay, no dry solders, still have no idea why the test points are so out of wack and the 1K 5W starts to smoke after a minute.
Just tested again all parts out of board and then back in, still getting the same results which at least is continuity..right Smiley point A is 230v POint B is 202v Point C is 23v and D is 1.65v from pin 8 on the input tube and 1.75v from pin 3.

Still waiting for any help from anyone or thoughts on how to precede.
If this does not work soon I am going to scrap the PCB and go point to point.
Thanks for reading......

m.
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« Last Edit: 08/03/14 at 00:49:24 by Jinks »  
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Steve Deckert
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If the 1st watt
sucks why continue?

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Re: Please Help!
Reply #7 - 08/03/14 at 00:59:20
 
Hi Jinks,

Welcome to the forum!

What a great looking amplifier!  Reminds me of my first amplifier project in the 1980's where I made a wonderful looking tube amp that didn't work.  I spent 6 hours a night troubleshooting it, 30 nights in a row before music came out.  

My guess is that your amplifier is oscillating which causes it to reproduce high frequencies way above human hearing range.  When an amplifier oscillates, it draws wild amounts of current and the voltages can change at the same time and often will.  That would account for the smoking hot 1K 6.5Watt dale resistor.

Cause of oscillation would be non-shielded input wires between the RCA jacks, Volume Control and Board Input.  

To find out if this is the case, short the input to ground on both channels at the input connection of the board.  If the resistor no longer gets excessively hot, then you'll know that was it.

Steve
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Jinks
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Re: Please Help!
Reply #8 - 08/03/14 at 01:52:02
 
Hi Steve, Thanks for your reply, it is beginning to feel like the 40 days and 40 nights of testing and re-testing! So I just now disconnected the RCA and speakers wires and volume wires and then tested the voltages again, the resistor now glows red hot and the voltages after about 30 seconds go to their resting places of 230, 202, 23 and 1.75
I feel like Dr House and it is never Lupus but suspects Thyroid Smiley but it's actually Rabies!
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Jinks
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Re: Please Help!
Reply #9 - 08/03/14 at 04:11:12
 
So I replaced all the input and output leads and volume pot leads with shielded audio cable and no change in the weird voltages and now red hot glowing 1k 6.5w resistor, pretty like an Edison bulb Smiley

The testing continues.

m.

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Jinks
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Re: Please Help!
Reply #10 - 08/03/14 at 19:51:44
 
Update-
Today I re-soldered everything using flux to get a really good solder then cleaned the board with alcohol. Next I shortened all of the input and output transformer leads to have clean signals, This unsurprisingly did nothing to my voltage test readings...still the same. So I am thinking it is something big, like a bad cap or diode or maybe the output tubes are bad or wrong, so I ordered some new tubes, new 300v caps and diodes and we will see if that does anything! This was supposed to be an easy build!
it's on a PCB after all!!!!
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Steve Deckert
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Re: Please Help!
Reply #11 - 08/04/14 at 00:59:02
 
There is no question that something past the 1K 6.5Watt resistor is shorted or partially shorted to ground.  

Let's remove the output tubes and see if that does it.  If removing the output tubes has no effect then start testing for shorts.  The second 47Uf cap could have been damaged.  You'll just have to start with your ohm meter and checking backwards from the 1K 6.5W resistor until you find the cause of the short.

Steve
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Jinks
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Re: Please Help!
Reply #12 - 08/04/14 at 02:59:05
 
Thanks Steve! I have something new to report! with the output tubes removed...the 1k 6.5w does not even get warm! and the voltages at A point are 423v at B-376v at C-0 and at D-3.3v and 3.2v
So am I right in thinking the output tubes are wrong or shot? I have ordered NOS 6P15P-EV's I have a feeling that the ones I have now are not EV just 6P15P would that make a difference?

Thanks again,

m.
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Steve Deckert
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Re: Please Help!
Reply #13 - 08/04/14 at 03:29:31
 
6P15P are the low end of the series and there are Chinese versions of this Russian tube floating around.  Clearly one or both of your output tubes has shorted.

What you want is carefully matched 6N1P-EV with the chrome plated pins and gold grids.  Alternately you can use a matched pair of EL84's of your choice.

-Steve
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Jinks
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Re: Please Help!
Reply #14 - 08/06/14 at 20:34:54
 
Thanks Steve for your diagnostics! I received my Capacitance meter in the mail and checked all caps and they are all good, I changed the 5W resistor and diodes just to see and of course no change, the tubes are definitely to blame, I ordered a matched gold plate pair from Russia and now will just wait for them to arrive!
I will let you know....fingers crossed.

Many thanks again,

Mathew.
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