Ferguson 621 U woes.

[poppydog]

I have replaced r28 with a 120k and nothing has really changed apart from a bit more volts on the other side of r28, 77.1v compared to 42.1v before the change.
R34 is in spec.

C54 has been replaced with a 4.7nf 600v polyprop type as stated in post 1, do I replace it again in case i cooked it with the soldering iron.

If I short grid1 to chassis and measure the HT at r37 and its rises to near what its suppose to be, it still wont tell me which I have, a internal short or a conductive board will it?
Is it worth trying to find a half decent ul84 to try?

poppydog

[Nuvistor]

No need to change C54 just yet.
In post #13 you had 0 volts on one side of C54 and +6v on the other connected to G1 of the UL84. One side of C54 is connected to the r28 that as 77v , the anode of the UABC80 triode.
If that is still correct check the continuity from C54 to the junction of r28 and the triode anode in the UABC80.

[poppydog]

Continuity is good all the way to pin 9 of UABC80, C54 to r28 up to the UABC80 is 76.7v

poppydog

[Nuvistor]

Any thoughts on the 0v Reading obtained in post #13 or was it just a bad reading, which most have done from time to time?

[poppydog]

Maybe me putting test probe where I should not have..
Have I led you up the garden path??

poppydog

[Nuvistor]

I have led myself up the garden path many times so not to worry.

[Nuvistor]

Unless I have missed it you have not said if the radio receives stations and it’s the hum and rustling noise that is the only problem, is that correct?

Is that +6 volts still on G1 of the UL84? That would point to problems as stated before with the valve/PCB/valve base. Out of interest what voltages are on the other electrodes of the UL84?

[poppydog]

With regards to me shorting grid 1 of the UL84 and hoping to see the HT rise, it didn't.

poppydog.

[poppydog]

Originally Posted by Nuvistor

Unless I have missed it you have not said if the radio receives stations and it’s the hum and rustling noise that is the only problem, is that correct?

Is that +6 volts still on G1 of the UL84? That would point to problems as stated before with the valve/PCB/valve base. Out of interest what voltages are on the other electrodes of the UL84?

It picks up stations on lw/mw but they are very distant, Fm is pretty good but it hisses and distorts etc if you go anywhere near the radio. The hum gets louder the longer you leave the radio on and the rustling after a while is constant. The hum on lw/mw you can make it go louder via the volume pot.

I will have to have a look at the voltages tomorrow now Frank , thanks for your help so far.

poppydog

[poppydog]

I’m hoping I have got the pin IDs correct for the ul84.
Voltages on the ul84 are as follows- (chassis neg)

Pin 1 IC 6.47v
Pin 2 grid1 6.47v
Pin 3 cathode 14.64v
Pin 4 heater 0v
Pin 5 heater 0v
Pin 6 IC 158.8v
Pin 7 anode 143.7v
Pin 8 IC 144.1v
Pin 9 screen grid 154.2v.

The longer you leave the radio on the higher the grid 1 voltage gets.

poppydog

[poppydog]

voltage on heater 45v AC.



poppydog

[Nuvistor]

So the 6v on G1 is still there but we have approx 14v on it’s cathode which is correct from the circuit. The resistance from cathode to chassis is 270 ohm, I wouldn’t be surprised if that is much lower either the resistor low in value or the capacitor leaky.
It still wouldn’t account for the 6 volts being on G1 but would account for the more or less correct voltage on the cathode while passes far too much current.
If you could check the cathode circuit if not already done.

[poppydog]

C59 has already been replaced as stated in post 1, r35 is in spec.

poppydog

[Nuvistor]

Well I am rather lost at this point.

[ms660]

If it were mine and I didn't want to buy a UL84 to prove a point, I would splash out on a 450 ohm 10 watt resistor (or there about) remove the UL84 and sub the heater with the 450 ohm, connect the two g2 and the two anode connections on the valve socket together and fire up and monitor the voltage on the g1 connection on the valve holder.

Lawrence.

[GeoffK]

The anode and screen grid voltage should be within a few volts. The G1 grid at +6V instead of maximum 0V is pulling down the anode voltage and increasing anode current.

[Nuvistor]

That’s a good idea Lawrence, much better than the one I was having about just pulling the valve and if the voltage dropped immediately on G1 it’s the valve if slowly faded away it’s the PCB/valve holder.
The resistor is a much safer idea.

Only problem there could be if the leak on the PCB is between Cathode and G1, this test may not show that unless I am missing something.

[poppydog]

Thanks for all the replies and suggestions. I can probably cobble up some big resistors in series that i have here so I will give it a go and perhaps also try pulling the valve and watching the voltage.

poppydog

[Herald1360]

If you're careful, you can use the UL84 as the dummy heater by connecting just its heater pins to the set via Croc clip leads.

If the cathode voltage is correct and the cathode resistor and bypass capacitor are OK, then at 14V Vk the UL84 can't be taking too much current. It can be knackered from doing just that historically, though!

[poppydog]

The 2 G2, I take are pins 9 and 8, pin 7 and pin 6 anode?

poppydog