Cannot adjust the voltage of EHT unit by the rotary control

Hi,

We have an EHT unit purchased by Philip Harris but its rotary control doesnt work anymore while the rest is fine. We did check the fuse but it seems like problem is not from there. already sent email to the supplier but havent received respond yet. Please would anyone be able to give us advice for this situation?

Thanks so much.
Tuyen
 
Really difficult to give advice on this, there are dangerous voltages involved (up to 5KV), So UNLESS you have extensive knowledge of electronics including voltage multiplier circuits LEAVE IT ALONE. High voltages can stay on components even when switched off.
That said.
Is it one of the Space saver designs with a LCD of voltage? (these are the ones I have)
Is the voltage zero or 5KV? Is the rotary knob actually turning the potentiometer shaft?
I'd start looking inside (having discharged any capacitors) at connections to the front panel pot.

Tony
 
Really difficult to give advice on this, there are dangerous voltages involved (up to 5KV), So UNLESS you have extensive knowledge of electronics including voltage multiplier circuits LEAVE IT ALONE. High voltages can stay on components even when switched off.
That said.
Is it one of the Space saver designs with a LCD of voltage? (these are the ones I have)
Is the voltage zero or 5KV? Is the rotary knob actually turning the potentiometer shaft?
I'd start looking inside (having discharged any capacitors) at connections to the front panel pot.

Tony
Hi Tony,

Thanks for getting back to me.

Yes, it has 3-digit LCD indicating the output voltage. The issue is that voltage shown in that LCD is stable at zero although we rotate the knob. We unscrewed it to look inside to see if we can find any loose connection or wire, but it is not.
 
OK. I've taken the top off one of mine (I hate these space saver cases)
If (only if) you feel confident going further.

There are two red leads going to the main PCB near the top (Next to the black rectangular component which is a bridge rectifier) I read 14.5V AC.
Be careful not to short out to the screw.
There are two black leads further down also reading 14.5V AC
I read 16 V DC across the outer two wires (at the circuit board) of the potentiometer (control fully anti clockwise) this drops to 12.5 V DC with the control fully clockwise.
and (looking from the back) a voltage proportional to the position of the dial on the left and center connections, some where between 0 and 12V DC.
Further trouble shooting will require disassembly of the unit (lots of screws, swearing and researching Voodoo to get back at the deign team.
T
 
OK. I've taken the top off one of mine (I hate these space saver cases)
If (only if) you feel confident going further.

There are two red leads going to the main PCB near the top (Next to the black rectangular component which is a bridge rectifier) I read 14.5V AC.
Be careful not to short out to the screw.
There are two black leads further down also reading 14.5V AC
I read 16 V DC across the outer two wires (at the circuit board) of the potentiometer (control fully anti clockwise) this drops to 12.5 V DC with the control fully clockwise.
and (looking from the back) a voltage proportional to the position of the dial on the left and center connections, some where between 0 and 12V DC.
Further trouble shooting will require disassembly of the unit (lots of screws, swearing and researching Voodoo to get back at the deign team.
T
Haha, I will try and hope I wont need to use lots of swearing. Thanks so much for your advice, Tony :)
 
That happened to us with an older model of Unilab EHT and I sent it to Techlab to be repaired. They'd also found a couple of other issues and fixed them too.
 

CovTech

Lvl 37 Alchemist
COMMITTEE
Yup. But being oversea is a bit inconvenient for sending to the supplier or trouble shooting :(
Granted that would be a pain but if you contact them they should be able to identify a local supplier/repair company they'd endorse to do the work
Someone must be doing it for them wherever you are
 
As Tonytech says, is the rotary knob actually turning the shaft of the potentiometer, maybe check that the grub screw is tightened up first before you start taking it apart.
 
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