Radford Low Distortion Oscillator Series 2

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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
 

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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
 
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
 
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
 

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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.
 
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
 
I'd switch your meter to DC and take a look at offset. Both with the AC output turned up just below clipping and down to almost no output. Might point you into correct section. If it was Solid state I'd look for a soft -V cap. But with tubes I couldn't say. An output transformer should negate any offset.
Doc
 
Thanks, the circuit is a 'Bridged T' in the feedback loop with additional bootstrapped feedback to the first Valve (EF184) in a pair with the second one (EL822) doubling as the Output in Cathode follower mode without a transformer.
Thinking something may have gone 'high' in the cathode follower output stage to reduce negative half cycle current availability.....

No full schematic yet so doing this the 'hard way' at present from the generalised cct below:)
Mik
 

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Yup, need to spend some more time circuit tracing and component checking!

With date stamps on the Caps of April '67; things will be expected to have drifted in value which is the most likely cause.

Anyway interesting to do as haven't seriously worked with Valves since the '60's

Mik
 
I was trying to point out that a cathode follower (or even emitter follower) would not be biased for symmetric clipping if you want to get low distortion. I would assume a capacitor coupled output so I would check that cap for leakage.

Is the O/P voltage, at clipping, 3.2V RMS ?
What's the max indicated O/P voltage ?

dc
 
At the output of the cathode follower, there should be a bit over 10volts of signal available with ability to drive the attenuator.

In the test I am running with 10dB of attenuation; it isn't quite achieving the 10volts without the asymetric clipping happening. The attenuator loading causes the clipping which appears just below full output. Have so far checked the output cap for ESR and it appears to be OK.

Haven't had time to investigate further yet, but biasing of the output cathode follower could well be one issue, in which case the anode or cathode resistor(s) of V1 might be high.....

Mik
 
Since my last post, I've ordered a pair of new Valves for the Oscillator circuitry and when they arrive in a couple of weeks will have had the opportunity to properly check the associated passive components and acquire replacements as necessary.

So hopefully some further results to post in a couple of weeks or so

Mik
 
posting on the yahoo Radford group

Is it worthwhile to join the Yahoo groups "radford" at < radford : Radford Electronics >, or "radfordfiles" at < radfordfiles : Radford Electronics Files and Database >?

Dale


Thanks Dale,
but has anybody sucessfully posted a thread or guestion on these sites. unfortunately I have not been able, but it may be it's me and not finding the correct procedure.
i am still looking for information on the control element in the RADFORD LDO3 oscillator, and wanted to expand my search field but had my querry fail with the following statement:-
"Your message was not delivered because it was sent to an
announcement-only group, where only the moderator may post."

jimb0s
 
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