Radford Low Distortion Oscillator Series 2

[audiomik]

looking for any service info or schematics for this Instrument., pic below.

Output metering and attenuation is working, 'Low Distortion' and 'squarewave' switches when engaged give no output and the sinewave output has asymetric clipping as in the 'scope image below.

Any info or assistance in restoring this appreciated
Mik

[dchisholm]

I can't point you toward any documentation, but I hope you find it!

Seeding a search engine with "radford low distortion oscillator" turns up several interesting pages. Radford test gear seems to have been highly regarded for its performance despite lack of physical beauty. Apparently, there were several evolutions of this instrument. What I find most interesting is that it was tuned via a custom-built, tapered, gang potentiometer. Also, at some point, the design changed from a Twin-T architecture to state variable (I think that's what he means by "biquad").

It may take some effort to find the source of the clipping, especially without documentation. The most obvious place to start is a power rail that's not up to its specified voltage. (Not necessarily the negative rail - there may be a net phase inversion between the origin of the clipping and the point you are observing.) Another possibility is degraded sensitivity in a level detector circuit, perhaps due to failed electrolytic coupling capacitors.

Keep us up to date!

Dale

[dchisholm]

Is it worthwhile to join the Yahoo groups "radford" at < http://tech.groups.yahoo.com/group/radford/ >, or "radfordfiles" at < http://tech.groups.yahoo.com/group/radfordfiles/ >?

Dale

[audiomik]

Dale
thanks, am intending to have a good go at this as a christmas/new year holiday project.

Radford Instruments used to be based in Bristol when these were made, had one of these LDO2's and an LDMS new from them many years ago; also some of their later Instruments as well when I was manufacturing some specialist pro Audio products back in the70's/80's.
However I never had to get inside any of these instruments.

Can't access the Yahoo Groups from here as they need a Yahoo ID setting up - think it's blocked by blacklisting somewhere due to spamming on Yahoo! Email! addresses!

Anyway, will get the cover off in due course and see what's inside, at least the special Wire Wound ganged pot for frequency tuning seems OK....
Mik

[dchisholm]

The "Radford Files" group has documentation for the LDO-2, but the files are too large to upload here.

Dale

[audiomik]

Well, have looked at the info which Dale kindly forwarded only to find that the LDO2 I have isn't the same as in the circuit description which shows an FET for amplitude control.
The LDO2 I have uses a thermistor, mounted on the left of the switches at the bottom of the pic below.!

Anyway, have so far succeeded in getting the Low Distortion output sine wave working as far as I can measure it, since my HP333A or TF2331A distortion meters doesn't measure anything like low enough.

Next is to look at the square wave pcb - shown in the pic below with it's 3 transistors.

Mik

[jackinnj]

Originally Posted by audiomik

Anyway, have so far succeeded in getting the Low Distortion output sine wave working as far as I can measure it, since my HP333A or TF2331A distortion meters doesn't measure anything like low enough.

Consider Bob Cordell's "Distortion Magnifier" -- there are PCB's available:
CordellAudio.com - The Distortion Magnifier

It's also described in his book.

[audiomik]

jackinnj
thanks for that, however I will have access to a Rhode and Schwartz machine (similar to this: R&SUPV Audio Analyzer (Rohde & Schwarz UK - Products - Test & Measurement - Audio Analyzers) )
in the new year so will do any final tweaks on the Radford when all is working together with testing some Digital LMS's.
Mik

[DRC]

The asymetric clipping may not be an issue.
At what amplitude does it start clipping ?

dc

[audiomik]

Originally Posted by DRC

The asymetric clipping may not be an issue.
At what amplitude does it start clipping ?

dc

Not as bad as it was at first as have found the level presets attached to the thermister needed 'working' a little to get them back into full function.
It happens just on the level set using the amplitude preset(s) that gives full scale for the meter when the step attenuator is at -10dB. (This is at 10volts pre the Attenuator.)

Voltage output is correct at ~3.2V and switching in the 600ohm source makes the -ve half cycle clipping worse.

Am looking at the EL822 and it's cathode resistor/coupling cap to start with....
Mik