Mysteriously Mute Maestro

[tiv48]

have you checked the trimmer cap c8?
if you pm me your address i will send you a uch81
rob t

[Joe]

Thanks, Rob. I have checked C8 and that seems to be all right.

Chris has already offered me a UCH81 so hopefully that will sort it out. If it does the next challenge will be the case as that needs re-painting and I haven't touched aerosol paint for years and have never done much with it.

I'll report back in due course.

Thanks again for your offer.
- Joe

[Yorkie]

You've certainly given it a good going over Joe, so hopefully, the new tested valve from Chris will do the trick. Maybe he could check out the suspect one for you?

Over the years, Pilot Little Maestros came in many guises with many circuits and mods.

With you mentioning aerosol sprays, I guess that yours looks similar to the one that I have, which works but needs a respray and restoration. I've have the paint for it for some time. I guess that when bakelite went our of fashion the makers decided to 'jazz up' the bakelite cabinets by spraying them in what at the time was termed 'contemporary' colours, but what we'd now call gaudy. I don't dislike it though, for all that, and I think the dial is very attractive.

I've also got a 1939 Maestro woody, but not the one with the rounded corners - it's square. I need to make a back for that as the old one has disintegrated, so I'll need to make a router jig to make the replacement back or it will take forever with a drill and scroll saw. I've added some pics of that. I think it's going to be a full re-furb on that one. Forum member Colin (Retired) wrote an excellent artiicle in the BVWS Bulletin about his total stripdown, rebuild and French polished 1939 woody.

Best of luck for a successful conclusion Joe!

David.

[Joe]

One of the wooden ones is on my wanted list but I am a bit nervy about having to do something about the dropper cord.

I'm not a current BVWS member so didn't see Col's article. I am seriously thinking of joining in the new year though as the data CDs would be darned handy. If I'm thinking of getting a particular set it's nice to see what's what first.

All the best
Joe

[Yorkie]

Replacing the lin cord with a Wattless dropper using a motor cap has been well documented, so don't let that put you off Joe.

David

[Yorkie]

Just a point about voltages on the T105:

Loooking at the ERT Service Chart it states that the readings listed were taken using an AVO model 7, (whereas most service sheets state that an AVO 8 was used). Although either meter will read that actual voltage present when the meter is applied to the circuit, those voltages will be lower than they would be with the AVO not applied to the circuit, due to the loading effect of the meter. The loading will be less pronounced at higher voltages, but will be much more pronounced when using an AVO7. An AVO 8 has an input impedance of 20,000 Ohms per volt, but the Avo 7 in only 1,000 Ohms per volt. Even the cheapest and tackiest modern digital have an input impedence of 20 MegOhms, so won't load the circuit to any extent. Hence, the readings will be much higher, and a more accurate representation of what the voltages are before the operaration of the circuit is affected by applying say an AVO 7 or 8.

To give an example of how this would affect the voltage measurements on the Little Maestro T105, consider the potential divider comprised by R14 (47k) and R15 (1 MegOhm). If we were to apply an AVO 7 on the 100V range across R15, this would have the same effect as applying a 100k resistor in parallel with that 1 MegOhm resistor. From Ohms Law we can calculate that the resistance at that point would fall to 90.909K - a tenth of what it would be if a modern cheapo digital voltmeter was place across the 1 MegOhm resistor, and a tenth of what it would be before the AVO 7 was plonked across it. The effect of this would be to stop the circuit from working.

That's why on the ERT Service Chart for the 'T105' it states: 'Readings were taken on an AVO model 7. The voltages below are the only ones that can be taken with the receiver operating without upsetting its performance'.

It then lists Voltage readings at just four test points on the circuit. For example, V1 pin 1 85V; V1 pin 8 75V.

I've no idea how much higher the voltages at those points would be if taken with a modern digital multimeter with a 20 MegOhm input impedance as compared to an AVO 7 at only 1,000 OPV, or even an AVO8 at 20,000 OPV. Not perhaps as high as those observed by Joe, but definately higher.

I can uderstand that many restorers like to use AVOs when referring to service sheets which had readings taken by the same model of instrument, and I can understand why many hold them in high regard and have a great affection for them as robust instruments, but certainly on solid state gear, for the reasons stated, they're of little practical use. I've mentioned before that if we consider a potential divider consisting of two 100k resistors with 12 Volts applied across it, we know from Ohms Law that the true Voltage at the junction would be 6 Volts, and that's what you'd see with a digital voltmeter (20MegOhm input impedance). If howwever, we stuck an Avo 8 (20,000 OPV) across the junction to ground, it would read just 4.8V - an accurate reading of the voltage at that point, because of the loading of the AVO8 on the circuit reducing the voltage. With an AVO7 (1,000 OPV) it would be even worse - it would measure just 1 Volt and would doubtless stop the circuit from working.

I'm not 'dissing' AVOs, and as I said, they do have their uses when taking measurements and comparing them with circuit data on which the measurements werre originally taken with the same model of instrument. However, beyond that, especially where solid state gear and low voltages are concerned, they're best left on the shelf as fond reminders of how life used to be - not as a serious piece of test gear for today's purposes.

Sorry to have drifted off topic to some extent, but I felt that I had to go into some detail to justify why I made the iinitial assertion about the voltages on the Service Chart having been taken with an AVO 7, and the implications of that, be it on the T1105 or any other set if the same instrument isn't used when checking voltatges.


David

[OldTickTock]

when I first started restoring I was having all sorts of nightmares with meters, especially on my TV's I tried AVO 7, AVO 9, a TakedaRiken
True RMS bench meter, a Marconi 1044K, Heathkit V7AU. I finally settled on a Fluke 75 which I use all the time and for me is the dogs danglies and i've never looked back since. The others now just gather dust.

Chris

[Joe]

I know what you mean as when I first came across digital meters I wondered why the read high. You can often see or hear some effect of connecting a meter. I've had even more problems when using my old 2000Ω/V multi-meter to measure current as that was like sticking a resistor on the circuit and I'd get things working and then they'd go wrong when I took the meter out.

I agree that there's room for both and that it's 'Horses for courses.'

- Joe

[Andy - G8MNM]

I've been awaiting the outcome of this project and cannot wait any longer!
Has it spoken yet?

Andy

[Joe]

Has it ΩϙϿέᾇὥ.

The new valve made no difference at all so next time I'm ordering any others I'll get replacements for the capacitors which were likely suspects and see if changing them improves matters.

I've recently been concentrating on more co-operative sets. It'll probably end up like Brunel's Clifton Suspension bridge in that it was an early project which wasn't actually finished 'til after his death.

Thanks for your interest anyway.

- Joe