Bigbottle Phonostage Builders thread.

[patrickblue]

Hi
I've just replaced the 7805 with a a Texas Instruments LM340T-5 from Farnell.
Things are getting stranger. The 7805 is reading 3.3v, not 5v. There is 8.3v going into the 7805.
There is now voltage at the heaters when the valves are in, which are becoming vaguely warm, but only 3.24v on the 7v valves, and 2.82 on v2.
The low voltage input is 10.5 at both inputs when measured across the centre tap.
Plus the relays have stopped working.

I'm at a loss.

 

[Bigman80]

Hi
I've just replaced the 7805 with a a Texas Instruments LM340T-5 from Farnell.
Things are getting stranger. The 7805 is reading 3.3v, not 5v. There is 8.3v going into the 7805.
There is now voltage at the heaters when the valves are in, which are becoming vaguely warm, but only 3.24v on the 7v valves, and 2.82 on v2.
The low voltage input is 10.5 at both inputs when measured across the centre tap.
Plus the relays have stopped working.

I'm at a loss.

Put a pic up of your PCB.

A good one so we can see what's on there if you can

 

[Flikoman]

8.3V input for 7V out is too low. What is your input Voltage without tubes?

Regards

 

[patrickblue]

Just under 9v, 8.97 to be precise.

 

[Flikoman]

Transformer secondary voltage? 8.97Vin is too low, I think for regulator to work optimal.

 

[patrickblue]

Secondary is 10.5 volts.

 

[patrickblue]

I'll get a few pics up shortly.
The specs say input needs to be 3 volts higher than output.

 

[Flikoman]

Secondary is 10.5 volts.

10.5VAC is secondary? Strange that with no tubes you have only 9VDC input in regulator ...

 

[patrickblue]

Photos attached


Or maybe not

 

[patrickblue]

Try again

 

[Bigman80]

ZD1P....is that a 2v zener?

 

[patrickblue]

It is.

 

[Vivant]

You say you have 10.5 'across the centre tap'
Is it 10.5-0-10.5? i.e. 21V across the two outer terminals?

 

[patrickblue]

Yes, 10.5-0-10.5, and 21v across the two outers

 

[Flikoman]

ZD1P....is that a 2v zener?

Wasn't that 2V7 in BOM?

@patrickblue
How much current can these secondaries withstand? Soecs of secondaries for heaters?

 

[Vivant]

The zener has been changed to 2V to give 7V on the heaters. Only Philips valves are 7.6V, my bad for the original spec.

The secondary for the heaters should be rated at 1A.
Patrick what voltage do you have on the 3300uF C3P?

 

[Flikoman]

Ok,

I'll change to 2V, what about series resistors to 12AX7? They have to be reduced too, for 6.3V.

For patrick, it is strange that he has that low voltage on input side in regulator without load (tubes).

 

[patrickblue]

The zener has been changed to 2V to give 7V on the heaters. Only Philips valves are 7.6V, my bad for the original spec.

The secondary for the heaters should be rated at 1A.
Patrick what voltage do you have on the 3300uF C3P?

2.8v without valves. Do you want me to measure with valves in as well?

 

[Vivant]

You should be getting around 14V on C3P without valves.
Sounds like something is wrong.

Measure with the multimeter set to AC volts, if you have a big reading it could be one of the rectifier diodes is faulty.

 

[Vivant]

Ok,

I'll change to 2V, what about series resistors to 12AX7? They have to be reduced too, for 6.3V.

Yes reduce the 2 x 8.2 ohm down to 4.7 ohms.