Welborne Labs ST70 Mod

Welborne ST 70 Mod

I just finished my Welborne ST-70 - thanks to all of you who have helped me get this project under control. also, Ron sent out updated instructions which helped a lot. After tracking down a few bad connections, it fired right up.

And - it sounds really good - very pleased with the sonics.
However, the first couple of times I turned it on there was a very transparent plume of smoke - could barely see it. It seemsw to have gone away. Second, the power supply voltage is 445V instead of the maximum of 420 VDC according to the instructons. and third, it emits a very "electronic" smell after a few minutes of operation so I only run it for 20 - 30 minutes or so. I presume thatthis is connected to the high voltage. any thoughts about why it is so high (I tried another rectifier tube - same result). Maybe the choke? Bias is perfect and both channels are exactly the same measurement.

And now - the euphoria is over! After about 20 minutes, the fuse popped. A quick check noted that the power supply capicitors were relly hot, especially C105 and 103. Also, the caps in the bias circuit, C109 - 112 were blisteringly hot. So - HELP - ideas? Sugestions?
 
Cfranz, I recently ordered his remote attenuator (2 units) and had problems as it was not assembled correctly by them (I ordered assembled and tested unit). One of the units came w/o the remote, and when I asked for it, there was no reply...

Having read all complaints, I was terrified, but asked for technical help by e-mail (the mute did not function). To my surprise, Ron responded immediately and helped to track the problem by sevral e-mails (a missing jumper wire).

PM me if you think I could help, though I am no expert.

It's really pitty to see so many complaints about otherwise a good resource.
 
Welborne ST70 instructions

If anyone needs a copy of the latest ST70 instructions (rev D) please email me and I will be more than pleased to send you a copy. They are quite good in fact. But, my amp is still dead - anyone know why capacitors would get really hot?
 
I emailed this sugestion but I'll post it here as well. It may be possible (room in the chasis permitting) to place 1 ohm resisters in series with the caps that are heating up. (Don't want to leave them in or run them very long). Measuring the voltage accross the resistors will give you a direct reading of the current through both the resistor and the cap. Knowing the current passing through the device may be informitive as to what's going on.

From Ohm's law: V= IxR

if R = 1 then V = Ix1.

I sometimes find it useful to know how muct current is passing. Try to use High wattage ones, 5 to 10 watts if you can, no sense inviting a problem with overloaded resisters.

Beyond that idea, without a schematic I couldn't tell you a thing on this.

Good luck with it.

Shelly_D
 
Brio84 said:
And now - the euphoria is over! After about 20 minutes, the fuse popped. A quick check noted that the power supply capicitors were relly hot, especially C105 and 103. Also, the caps in the bias circuit, C109 - 112 were blisteringly hot. So - HELP - ideas? Sugestions?
Click to expand...


Any further progress on this?

By the way, Thank you for the documents, they arrived yesterday. I can see how my sugestion on the resistors in series with the hot caps would be very hard cause they are circuit board mounted. Unfortunatly you may end up having to remove those caps from the board anyway to find out what's going on but right now, just let us know how you are doing and where you are in this. I'd like to see you get it up and running.

Shelly_d
 
Welborne ST 70 Mod

Shelly, glad you received the documents. I found two blown zeners in the bias circuit but still have not found the problem. Capacitors still heat up and the fuse blows. As both channes are affected it must be a common problem, e.g. a power supply issue ????? Also, I just noted that with the inputs shorted there is 9 vac output on one channel and 12 vac on the other. and, the power transformer now vibrates indicating an excessive current drain! It sure would have been helpful if Ron had annotated the schematics with voltages! Anyway - completely frustrated at the moment!
 
Your experience almost exactly parallels mine. I eventually gave up and went with Triode El's mod. I may also try Mottram's mod to see which I like better.
 
Does Wellborne advertise on some secret audio web site where new audio modders/tweakers exclusively flock? It seems on every board I go to and for quite awhile, there has been a running thread on how crappy Wellborne is to deal with and yet.....people stil do. I just don't get why people continue to do business with this firm.

:dunno:
 
Brio84 said:
Shelly, glad you received the documents. I found two blown zeners in the bias circuit but still have not found the problem. Capacitors still heat up and the fuse blows. As both channes are affected it must be a common problem, e.g. a power supply issue ????? Also, I just noted that with the inputs shorted there is 9 vac output on one channel and 12 vac on the other. and, the power transformer now vibrates indicating an excessive current drain! It sure would have been helpful if Ron had annotated the schematics with voltages! Anyway - completely frustrated at the moment!
Click to expand...

hey-Hey!!!,
The symptoms you describe sound extensive. It could simply be a rectifier diode installed backwards in the bias circuit. I don't like the idea of regulating the bias voltage if the screen supply in the power stage is not also regulated so as to maintain a more stable bias current in the face of varying line input.
cheers,
Douglas
 
PakProtector said:
hey-Hey!!!,
The symptoms you describe sound extensive. It could simply be a rectifier diode installed backwards in the bias circuit. I don't like the idea of regulating the bias voltage if the screen supply in the power stage is not also regulated so as to maintain a more stable bias current in the face of varying line input.
cheers,
Douglas
Click to expand...

In my case I double, triple, and quadruple checked the orientation of every diode and electrolytic cap in the circuit. Nothing helped because they were all installed properly. I wonder if there's a flaw in the board because 2 of us had the same symptoms.
 
Brio84 said:
Shelly, glad you received the documents. I found two blown zeners in the bias circuit but still have not found the problem. Capacitors still heat up and the fuse blows. As both channes are affected it must be a common problem, e.g. a power supply issue ????? Also, I just noted that with the inputs shorted there is 9 vac output on one channel and 12 vac on the other. and, the power transformer now vibrates indicating an excessive current drain! It sure would have been helpful if Ron had annotated the schematics with voltages! Anyway - completely frustrated at the moment!
Click to expand...

With the inputs shorted there should not be any ac voltage at the output. The fact that there is says that there is an ac signal getting into some stage where it should not. The fact that the power supply caps also get hot suggests to me that the AC is coming from the power supply when it should be DC. From this point on I would do all my troubleshooting using a Variac or put a light bulb in series with the power cord to limit how much current you draw. You want to trace the trouble with out risking blowing anything.

I now suspect that you have a rectifier or diode in the circuit that is not doing what it should. Either of the bias diodes (D1 or D2) could be either open or shorted, or either of the 2 sides of the tube rectifier (v5 - 5AR4) could be faulty. The tube could check good on tube checker and end up being bad anyway. It only takes one half of the tube to go bad to end up with a great deal more AC ripple out of the power supply then the amp is designed to deal with. That could cause the output voltages you are seeing.

At this point I would check D1 and D2 in the bias circuit and if they are ok I would try replacing the 5AR4 and bring it up slowly on a variac just enough to get signal through it. If you can get output then short the inputs and check to see if there is any output. If those voltages have gone away then I think we will be making progress.

You will have your work cut out for you on this. You can't let those caps get too hot but you must keep yourself safe while yo monitor their temp and do the testing.

Good Luck

Shelly_D


Please excuse any smileys in this - I can't get rid of my 13 year old son.

PS: Sorry to take so long getting back to you, I had to prepare the sader for Passover on Sat and then research a company I was going to interview at today on Sunday.
 
Welborne ST 70

Thanks for the excellent suggestions - I had not appreciated that the rectifier tube could partially fail. I have ordered a variac and as soon as it arrives I shall see if I can solve this puzzel. I did do a quick
AC measurement on the high voltage and measured almost 30 volts on the 445 VDC! so, maybe it is the rectifier tube.

Question - if an electrolytic capacitor fails, will it measure as a short or will there still be meaningful resistance when measured by a DVM?
 
Brio84 said:
Question - if an electrolytic capacitor fails, will it measure as a short or will there still be meaningful resistance when measured by a DVM?
Click to expand...

I'm sorry - That one I just don't know. :scratch2:

It might be easier to use a light bulb in series with the power line and just order a new rectifier tube.

(I know - now I tell you - After you placed the order. - Sorry for that.)

Shelly_d
 
Brio84 said:
I did do a quick AC measurement on the high voltage and measured almost 30 volts on the 445 VDC! so, maybe it is the rectifier tube.
Click to expand...


That is WAY :yes: to much ac on the high voltage. That will produce an AC Current through your filter caps in the power supply and explain all the heat. It will also explain the AC at the output.

The question now is where is it coming from - failed filter caps (the electrolytics) or a bad rectifier.

I think you've narrowed it down finally. The only 2 places that could be coming from is the rectifier or a bad cap.

Good luck

Shelly_d
 
welborne st70 Mod

Hi All,

I recently completed my own Welborne st 70 mod. I have checked all connections and parts orientation again and again and cannot see any errors. I fired it up measured a b+ of 268v and 269v on pads G & H (the manual states 410v +/- 10v. The outputs biased t0 1.5v and the amp passed signal to test speakers - sounded great. I monitored the bias voltage and the voltage started quickly increasing to the point that I could no longer maintain 1.5v. I noticed a small plume of smoke rising from the pcb board so I shut it down. The 4 zener diodes had melted the solder at their respective pads. The 6 small electrolytics c109 - c114 were blisteringly hot as well as r46. I have emailed Ron but as you can imagine I have not received a response. I have the revision E manual and will forward it to anyone who needs it. If anybody has any thoughts I would love to try and get this thing running.

regards, Daryl
 

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I believe I've read before to remove the zeners for the reason you mentioned. I'm sure someone more helpful will chime in soon.

But in the meantime, how about a picture of the case it's in?
 
welborne st70 Mod

Hi,
Here is a pic of the case. Cherrywood with aluminum top plate.
I wish it played as nicely as the wood looks!
 

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Lets start at the very beginning:

What are you getting out of the power supply? Compare voltages at each test point to what you should have.

Also, are all of your rectifiers working properly, both sides of the GZ34 and all the SS diodes?

Also, if you have a scan of the schematic it would help all of us in the troubleshooting.

Good Luck
 
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