Trying to get my phono 3 going again; HT problems

[Steebo123]

Hi

I have come back to look at my Phono3/PSU3 after leaving it in frustation a few months ago.

PSU checks out fine for voltages. However, HT problems on the Phono3; no voltage on the + pin of large cap C17 (referencing the Phono chassis ground), 320+V if using PSU ground terminal. More worrying is full 324V between the two chassis grounds! Valves come on ok, and heaters burn normally.

All Ground connections seem ok, and r35 is ok at 10k.

Perhaps C17 has gone?

Any help gratefully received.

Steve Vaughan

[bob orbell]

You may have the HT pos & neg connections reversed in the unbilical cord, go back and cheak it all again, this could give you a very nasty shock. Switch it OFF. BOB

[Steebo123]

Hi I have checked that, and it is off now, only turned it on to try and diagnose fault.

[John Caswell]

Hi Steve
Starting from basics, connect umbilical but not power, DMM set to continuity or 200R range,measure between HT out of PSU and HT in of Phono.
Ditto earth out of PSU to earth in of Phono ( cable to cable NOT case to case). In both cases it should read 0R.
If it does not then you need sort out why, broken cable perhaps or crossed cables (doubtful)
Contrary to what Bob says if the HT cables were crossed over the capacitor in the Phono would draw excessive current get pretty hot and possibly explode. You would know all about that believe me!
You then need to check the wiring on the Phono to see that HT and Earth go to the correct points on the PCB.
Report comments back here.

John

[Steebo123]

Cheers John and thanks Bob. I didn't explode...

I am away on business for a day or two but will try and get around to this.

Cheers

[Steebo123]

As it has worked previously I suspect an umbilical cable connection issue.

Will let you know and thanks....

[Steebo123]

Hi John

Checking things now. HT to HT has continuity, but black - ve out of PSU that goes to earth on Phono (Pin 3 of the socket pins viewed from inside of phono 3) is not connected. So, looks like a loose wire in umbilical.

I will investigate further.

Thanks

[Steebo123]

All working now John thanks. Loose earth solder joint in umbilical. All sounding good.

I do notice if I put my ear close to the speaker with no record playing and volume at 50% I can hear a faint squeal, sounds a bit like radio break through. Any comments?

Cheers

[John Caswell]

Does it disappear when you turn the radio off
Bear in mind that there is a fair amount of gain here so it will amplify anything going, and there will always be a background hiss from valve noise/Johnson noise/shot noise, q.v.
Remove pickup cables and fit a pair of phono plugs with anything from a s/c to 100R across them. has the interference (if it is that) gone?
Some of the noise can be alleviated thus:-
Selected valves,
Low noise Rs in the anode, cathode and grid circuits of the first two stages. I have fitted bulk foil resistors but at £14.00 ish each cough and hold your wallet.
Try these and report back

John