Croft 25 Basic Preamp

[pastaman]

Hi everybody,
I have a weird problem with my Croft 25 Basic preamp. It randomly produces a buzzing noise on the line stage that last a second maybe 2. When I switch to the phono input it produces the same noise followed by a loud bump. So far tested all the capacitors, but everything seems OK. Tried to change the valve on the line stage but the problem stays. Does anyone have any insight what it could cause this problem?

Thanks!!!

 

[JonSnell Electronic]

Could be a bad solder joint to a ground connection ... maybe a smoothing issue .... I would research with the schematic and see where the issue is.
If you have access to an oscilloscope, you may be able to find it quite quickly.

 

[pastaman]

Thanks for the quick reply. I'll ask my repair guy to check those possible issues. I am in Italy and it's difficult to find a valve specialist....

Thanks again

 

[Alllensoncanon]

What Frequency is the buzzing noise? Mid, low, or motor boating? Could be a feed back oscillation issues induced by aging tubes (output tubes included).

 

[pastaman]

Hi there,
thanks for the reply. The noise is around 100 Hz and (if it helps) happens even with the volume pots at zero. The Croft 25 basic has only 3 valves: 2 for the phono stage and one for the line stage. I tried already to change the line stage valve (ECC83) with a brand new one but after few minutes the buzz happened again, so I ruled the valve out.

Thanks for the help I am very grateful

 

[Alllensoncanon]

100 Hz sounds like power supply noise. If you have a scope, put it on the power rail. May be the filter cap is gone.

 

[analog_sa]

If it happens at volume zero it means it is not the phono section. If both channels have it, then it is definitely the PS. Most probably a cap around the regulator, per example at the base of the pass transistor.

 

[pastaman]

Thanks for all the help guys. Tomorrow morning I'll ask to check the PS and the caps around it.
Glad it won't be a serious issue as I love my little Croft.

 

[analog_sa]

Most probably a cap around the regulator, per example at the base of the pass transistor.

Correction: only saw the internals now... no regulator for B+ and obviously no pass transistor.

It is so simple, yet i cannot even begin to guess what the problem may be...

 

[pastaman]

>>>It is so simple, yet i cannot even begin to guess what the problem may be..<<<

Exactly what my guy here says... 🙂

 

[pastaman]

Hi guys, after testing all components my technician gave the preamp a clean bill of health. After hours of painful trial and error I reached the conclusion it's actually the electricity rings in the farmhouse that are at fault. I placed a linear transformer (in 230V out 230V) between the main plug and the Croft preamp and most problems seems to have been cured. Now I only get a soft buzzing sound (still around 100 Hz) maybe once a day. I was thinking maybe would be a good idea to place a filter either before or after the linear transformer. Do you have any suggestions for the filter (placement, type & cost), or maybe a unit with linear transformer and filter all in one that will not break my bank... I already spent all my hard earned money to purchase the Croft....

Thanks in advance!