Help with a HHSCOTT LK-72

[vishalk]

Best way for me is to go over the manual and note the changes, and mark down the value of the components.

From there I can post what I've got here and then work my way though it safely and slowly.

Bear with me please.

[vishalk]

Hi Please see below, i blue pen mark is currently what i have.

55vac goes into the rectifier, then I measure 75V.
This then goes straight into the Capacitor which is 4 x 100uf@100vdc

Now I thought before it goes to the cap there should be a dropper resistor?

From there there are three resistors tailing of to the other caps. 4.6k, 68R and 55R.

No matter where I measure DC volts I get around - 74vdc

Which means the 12ax7s are seeing 74vdc, so this is already wrong right?

[vishalk]

I then measured around the rectification and smoothing section.

Transformer voltage when loaded with the 5ar4 is 391vac.

After rectification (when tube is warm) i get a lovely 496vdc (just the 5ar4 in) at 243vac input (my mains voltage)

Now after that it goes through a 80R resistor and through a series of caps and resistors. At the smoothing cap (c204) i measure 496vdc.

After the 80R I get 499vdc.

Theres a 1.2k resistor where I measure 458vdc and another resistor where I measure 465vdc

Again without load and just the rectifier tube installed.

My concern is this, from there voltage is taken off to the output transformers which see 499vdc?

I mean doesn't that all seem too much?

[Station X]

Originally Posted by vishalk

Which means the 12ax7s are seeing 74vdc, so this is already wrong right?

No! No! No!

We been around this loop several times. The heaters won't see 74VDC because there'll be a volt drop across resistors R207, R208, R209 and R210 all of which are 18 Ohms. Put you meter, on a volts range, across each of these resistors in turn and you'll see what I mean. Whenever current flows through a resistor there'll be a volt drop across it.

The question is has an additional series resistor been added to drop more volts to compensate for the reduced forward volt drop across a silicon rectifier?

If you measure the voltage across C207 it should be around 43->45 VDC, but your earlier figures suggest the figure is around 38VDC

[vishalk]

Aarrrgghhh sorry Graham. Yes I'm going round in circles, how frustrating.

OK got it now. 12ax7s tubes are in.

[vishalk]

The value of the resistor giving 37vdc is 55R

[Station X]

Looking good.

[Station X]

Originally Posted by vishalk

The value of the resistor giving 37vdc is 55R

Where is the 55R resistor connected in the circuit?

[vishalk]

Originally Posted by Station X

Looking good.

OK so now we figured that out, I need to change that value resistor? Makes no sense I might aswell move the wire to the other pin in the capacitor showing 44 dc?

[vishalk]

Or just replace it!

[vishalk]

Originally Posted by station x

where is the 55r resistor connected in the circuit?

[Station X]

This is making no sense. What do the handwritten figures refer to? You haven't quoted units. If they're resistance figures they don't tie in with the resistors being 18R. What's the colour code of the resistors?

[vishalk]

The pictures show the area we are discussing. The resistors are 4.6k which goes to the bias trimmers. Then 68Ohm to 55ohm. 55ohm is where the voltage feed comes off the the 12ax7s

[Station X]

I'm not interested in the bias resistors at this stage.

Can you draw out the circuit for the DC heater feed showing the values of the resistors please. Looking at the second picture attached to post #22 it appears that additional series resistors have been added.

[vishalk]

Niether am I.

I don't see the difficulty here



This picture is the rectifier and capacitor for the 12ax7s. Taken from the lk72 manual which helps you put the parts together.



This is a picture if the actual area you will note the rectifier, and the capacitor with the resistors.

A common modification from what I've read is adding bias adjustment, this is also taken off the same capacitor and power supply as the 12ax7s. Note the two black trimmers.

The resistors are no longer 18ohm. They have been changed to 4.7k, 68ohm and 55ohm.

[vishalk]

It looks confusing, the prior tech put three resistors in series to make 55ohm (look at picture)

[Station X]

That circuit has been heavily modified for reasons best known to whoever did the mods.

Considering the DC Heater supply, a red wire runs from the rectifier to the four section smoothing capacitor. Current then passes through a resistor (value unknown) and then through the combination of three resistors in series which are said to total 55 Ohms. From there there's presumably a wire to the heaters, but it's unclear from the pictures.

The original arrangement would have resulted in a voltage of 45 VDC across the heaters which are wired in series. What voltage are you seeing? You need to measure it between chassis and the end of the 55R triple resistor chain. That's more of less the South West tag on the smoothing capacitor.

If the reading isn't around 45 VDC you'll need to adjust the value of the 55R resistor(s). Unless of course whoever did the mods opted to change the heater voltage.

[Station X]

What you appear to have is this.

[ms660]

Going by the 1st schematic posted, the total resistance in series with the heaters is 87 Ohms with the original selenium rectifier, in the OP's amplifier the total resistance in series with the heaters is 123 Ohms according to the OP, the extra resistance (36 Ohms) is most probably there to compensate for the lower forward voltage drop due to the fitting of a silicon rectifier.

Lawrence.

[Station X]

I wouldn't disagree.