LF,

I have built the fullwave rectified version of the design shown on page 10.33 
of the RSGB Handbook 10th edition.  It reads correctly at 25kHz but only show 
1/3 of the correct value at 136. 

Not having any 1N4148 diodes I used 1N4007s.  Might this be the cause of the 
problem?

73
John F5VLF