Just wondering Folks, has anyone ever heard of a "Roberts Valve & Circuit Analyser" - made by - London Sound Labs Ltd" back in the late 1940's ?
Yep, there is a mention in the Radiomuseum site, and I've a thread running in the VRR&R Forum. However, I'm still desperate to find a circuit diagram, as there is no technical data in the wee A5 users handbook. Its in an aluminium clamshell case something similar to the CT160's, but is about 12" sq. instead of oblong, and not as sturdy. It has no internal mains derived power supplies, but has a selection of adaptor leads which plug into any valve holder in any radio.
I've actually got a couple(both partially duff), with the intention of making a go-er of one.
Regards, David
I'm pretty sure David that my friend in Sorn Ayrshire has at least one of these.
After lockdown I'll take a run over and find out. A phone call would be pointless as his own health is not so good and he cares for his ill wife, they are both over 80.
Good on yer Trevor. Thanks for that. Me thinks that London Sound Labs built these things out of the vast abundance of post-war surplus stuff. Then hoped to make dosh by marketing them through Kelly's. But they didn't last long & went bust. Regards, David
Hi David.
I was at Bob's yesterday and mentioned about your tester. Unfortunately he didn't have one and he thought that there was one many years back, it could have been borrowed and not returned or perhaps he had on for repair. So I'm sorry I can't help in this instance.
None are complicated. The AVO VCM163 is complex because it adds an audio signal and measures the gain of that. It won't take long to reverse engineer a schematic.
Most use un-smoothed half-wave, so the voltages on the g1, g2 and anode switches are the equivalent voltages and thus the measured currents on the built in meter, not what you'd measure with a normal meter. The filament / heater voltages are the only ones that match reality.
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attachment=19779]Thanks Mike for your reply. I'm afraid that both Roberts are beyond redemption, and are presently being incorporated into a hybrid project. Roberts' idea of examining a valve's voltages whilst in-situ in the original radio is an interesting one, but their circuitry was flawed. They have no internal power supplies, and no method of determining Gm. Hopefully, my hybrid will remedy that.
Right enough, AVO & other VCM's & Valve Testers rely on 20mS half-wave pulses to determine mA/V. Having repaired & maintained RAF CT160's back in the '60's, and more recently owned a couple plus a MK3, I'm afraid I'm guilty of boring the pants off VRR&R Forum folk, over a number of years, with heaps of "Banging-On" thread posts. So will refrain from annoying chums on this Golborne Site.
I've put out a heap of pictures on the VRR&R (Test Equipment) Forum relating to the Roberts Hybrid, but will add some here. Basically, the hybrid uses a trusty AVO 8/6 for Voltage & Current measurement, a series of latched on-off-on toggle switches in-lieu of an AVO's great Thumbwheel Switch, and several salvaged Roberts' wafer switches for choosing the test functions.
Regards, Dave
For my version of Ronald Dekker's µTracer, I have a row of source sockets and a circle of sockets that are wired to the too many valve bases. Two sockets in the middle are for top caps. I then use short 2mm patch cords. Some cords have resistors in them (needed for Magic Eyes).
I'm going to rebuild it with ONE socket for an adaptor as the µTracer pulse width modulates its 20V supply to generate the heater voltage. The little beads / wiring inductance then makes lower voltages on many sockets too low.
Then also I can fit oscillator parts with a buffer and see what minimum filament/heater/HT is needed for oscillating.
Good on you, Mike, for your Curve Tracer project. Of late I've been encouraging folk to build their own homebrew valve testing equipment. Along with "re-purposing" existing vintage test equipment. I use my AVO MK3 for ordinary valve testing, but built a DC Tester several years back for standardising valves & producing drawn (hopefully) accurate Ia/Vg Graphs. Some of AVO's family of VCM's/Testers are now approaching pensionable years & attract horrendous speculative internet prices. Plus now cost an arm & a leg for original Pots, Meters, etc. I've been saying for a while now that folk can build a decent homebrew DC Tester for 200 quid or less. And, I take my hat off to those folk who are now building 21st century software driven digital testers & tracers. That's the way forward, compared to my 1960's/70's analogue mindset.
Regards, Dave
I replaced the innards of AVO meter in one of their Valve testers, I forget which model. Edited scan of old scale to suit the new meter's deflection angle and added an LM358 op-amp to convert from original resistance and 30uA FSD to drive the 50uA meter. I used the 2nd op-amp in the package as a voltage follower / virtual earth to null offset. As offset can be + or -, I used a pair of TO92 regulators, +5 and -5 from the raw DC via 1N4148 diodes and a small clock radio transformer. The offset adjust pot fed by +600mV and -600mV from two diodes, so +/- supply tracking, otherwise the offset pot won't work!
The circuit will allow anything designed for a meter from 15uA to 100uA to drive any meter from 20uA to 1mA. One resistor to simulate old meter, a resistor to set basic gain and a pot on the output to fine tune. Obviously mechanically zero the meter with power off and then null offset with no input connected.
The owner had also two AVO transistor testers as possible donors for meters as it uses the same movement, though the internal ballast may be a little different. Both O/C. No protection circuit. The original was sort of sticky above 1/2 FSD, which gave the illusion of reduced sensitivity. I think it's the worst movement in any AVO instrument, ominous that "servicing" and "faults" of the meter is in the manual.
Right enough Mike, the meters in AVO testers are a particular weak point. As I've said many times - 'they don't like it up em' - being whanged to FSD repeatedly. Therefore the recent trend to fitting the "Schottky Diode" mod certainly helps to relieve that. Back in the 60's, with RAF CT160's & their TT sibling - the CT446, phooked 37.5uA meters were a common fault. Some years back, a VRR&R chum gave me an op-amp which enabled me to use an AVO MK1's 444uA fsd meter in a CT160. Worked a treat. I also used a 37.5uA meter, supplied from Benchmark in Ludlow, in a CT446 TT, and that fitted ok & worked well. Back then & still now, I'm appalled at the cost of a genuine AVO VT's replacement meter - £200. Likewise, some mercenary internet wheeler-dealers were asking £200 for a CT160's RV2 Vg Pot.
Looking at other V/R Forum sites, there is a current spate of phooked AVO MK3's, MK4's, & CT160's, and the occasional VCM 163. Hence, despite having an affinity for AVO T/Eq., I'm encouraging folk to build homebrew Testers & Curve Tracers, particularly if they can re-purpose redundant but working other items of T/Eq. into their projects.
Regards, Dave