Avo RF Signal Generator HF 134

[Yorkie]

The only downside to Avo equipment (or Taylor, or Advance etc) is its age, but that's almost certainly going to be the situation whatever you go for, and to get it into good working order will usually need a little bit of work and TLC, but not too daunting. Generally, (assuming the phantom fiddler hasn't been at it!), the only work needed whatever the model - be it Avo, or later (Taylor) tends to be the need to replace smoothing caps in the power supply. The tuned circuits usually had high stability components such as silver mica caps. Allegedly the HF134 is almost identical to the 135, and looking at the circuit for the 135 it uses 3 x 10uF/450V smoothing caps quite possibly in one can. (8uF would be fine as replacements if need be). The might well be OK if re-formed, but it's handy if you can check if their ESR has gone high and if they measure reasonably close to 10uF. If they're not OK, there are physically small modern replacements which would fit inside the can. There may also be one or two 'waxy' paper caps which would benefit from replacement.

The AVO uses a metal rectifier, and given its age that's quite possibly a contact-cooled selenium one which may be OK, but if not, 4 x 1N4007 silicon diodes would be fine in its place, possibly with a series resistor to lower the higher voltage which silicon diodes would produce. In line with many signal generators of that era including Heathkit designs such as the RF1U, The AVO uses two twin triodes - a 12AT7 and a 12AU7.

Whatever make or model of signal generator you obtain, ideally a frequency counter is useful to check the calibration, and a scope to observe the output waveform with the internal modulation switched in/out. The good thing about signal generators is that unlike old valve radios, few of them had a lot of use and spent more time on the shelf than on the bench so usually, the valves are fine, leastways in my experience.

Just my thoughts - others may have a different take on things.

Avo were of course designed for use by professionals, but there are others to look out for, notably the Heathkit RF1U and the Tech TE20D, aimed more at hobbyists, though perfectly adequate. I've attached a pic of a Heathkit RF1U which I use, standing on top of my homebrew nixie frequency counter (1976 vintage), showing a 100MHz signal.

Rob Vine, who goes by the name of 'Bushbaby' on this forum, and 'Guitarist28' on the other side of the wall put a query out on that forum in Feb 2011 about the HF134 - I don't know if he still has that equipment. He was seeking a manual, and someone came up with the circuit of the HF135. I think it's considered discourteous to 'lift' attachments from other forums, but you'll find the circuit at this link:

http://www.vintage-radio.net/forum/showt...hp?t=66016

If Rob doesn't spot this post you might like to drop him a PM on here to see if he can help further Tim.

Hope that helps a bit Tim.