Croft Micro/ Croft Super Micro (improve) help please

[jamdown]

Hello guys
I am new here
My Croft Micro preamp which has given me much pleasure for all the time I've had it started playing up, with a strange slow oscillation, with my lack of tech knowledge, I diagnosed the fault as a doggy ecl85, and promptly purchased said nonculprit (3 off) ebay.:doh:
Well! you know what happened next, it embarrassed me in front of guests. Cut long story thanks to kind advice from webb below where also found schematics
http://www.southampton.ac.uk/~apm3/diyaudio/Croft_preamps.html
found neon regulator true culprit.
well its kinda hard to know (and i have purchased all various lookalikes from RS, Maplin, and Cricklewood) which is the right one? there is no marks to ident between various.
Have now ditched just replacing neon with unknown?? , and have got me most of the parts needed, but there are still a few things to make me do a headscratch.............so
Has anyone got a Croft Super Micro mk 3? and is there a chance that the hood is up?
I would if posible love to ask up a few question re components if at all posible, please.
Or is anyone else familier with this preamp? CROFT Super Micro mk3.
TIA

bernard

[quadsugdenman]

Hi Bernard,

E-mail Glenn Croft, he will tell you exactly what it is!!! brilliant guy.

Glenn Croft <glenn@croftacoustics.co.uk>

He's sorted my 20 year old Super Micro A on a number of occassions.
He might be away at Yorkshire cricket, nonetheless send him an e-mail.

regards

Garry

[jamdown]

Hello Garry
I hesitate,
He may be more than happy to tell me source of neon (only one I haven't bought is Farrell as £20 min order),but not sure he would accommodate my other questions ie:-
What zener diodes and wattage are used after the two 400mf cps in b+ before regulation.
There is a third zener connected to the cathode of the cv4004 the positive of the zener is connected to ground (odd?), should the + terminal of the 220mfd that parallels the zenner also be connected to ground?(I understand least this part of the circuit) which is taken from half of the diode bridge, series trough a 10mfd (polarity?)and it looks like the two diodes are blocking it. (its hard being a novice at these matters, (you ask the questions but cant answer them)) :)
I my mind i am thinking, if i can get any knowledge from other users so much the better alternative.
I think although Mr Croft has his email available, I have to assume that he maybe a very busy man manufacturing all amplifiers, from what I've read he doesn't work from any drawings, all in his head so may not suffer half semmi technical amateur like myself asking as above.
A lot of manufacturers these days (unlike quad and leak) just tell any one with queries to go jump.
At the moment I am trying to combine the circuit that I have traced from my pre amp and combine it with the super micro mk3 circuit, with the differences in red and any differences of component values in brackets.
first time i have used any program like paint, so learn on the job.
I have today received the rest of components from RS, enough to start rewire, will leave queried components till later.
Made some compromises with capacitor values as those in schematic appears to be un available , so the two 400mfd becomes two 390mfd, 600mfd now 470+150 620mfd.

thanks for answer Garry

regards

bernard

will try and post drawing later

[jamdown]

:)

http://i1240.photobucket.com/albums/gg489/wassyme/CROFTPREAMPCIRCUITB.jpg

oops not the size i expected, well icant read it

[dowser]

You will find Glenn very helpful - so helpful in fact that when i contacted him about a Micro II I bought a few years back I ended up shipping it to him to do the work rather than trying to bodge it myself :)

He services all his old equipment, and at very reasonable rates.

I don't know the value of the neon, but this is a common problem and causes HT regulation issues. Standard approach nowadays would be to replace the neon with a zener of the correct value (Glenn can advise), although I had my faulty neon bulb swapped with a neon reference valve.

Good luck - great sounding pre-amps, and Glenn has excellent customer service (although agreed, he will not answer all of the question you have I guess). My approach was simply to get Glenn to bring it up to functional spec (I also had output stage modified to latest version he uses at same time as repair), with a view to me modfiying myself after. All I've done though is change the smoothing caps. Doesn't need much else doing to it :)

Richard