Info Needed for Little Maestro 10 AC/DC

[John G8MWF]

The AC/DC set is 'more or less' working now and the project is becoming a parts hunt.
The 250K pot is, as you would expect, has had it, and is playing havoc with the AGC.
Another working rectifier is being tracked down and a 8uF high voltage capacitor.
The cardboard back has crumbled into several pieces due to heat damage.

My intention is to seek parts out at the Tonbridge Audio Jumble next month and hope that I am lucky in finding what I am after.

So for the moment I have put the AC/DC set away and place a list of jobs inside it so that I don't forget.

In the meantime I have started the AC set which has become my first smoker . . . . .

[Sideband]

The 250K pot is, as you would expect, has had it, and is playing havoc with the AGC.

Haven't got the circuit to hand but shouldn't that be a 1Meg pot?

[John G8MWF]

According to Trader Service Sheet 913 and measurements I have made it should be and is 250K.

However, some versions such as my AC set is marked up as a Model 10 but the chassis is almost identical to the Model 8. In this setup they use a 1M, R6, pot as a volume control to feed the AF to it and bias V3s grid from the slider.
I must admit that threw me for a while!

On the down side I am now looking for two values of replacement pots that are getting harder to find, more expensive, and in good condition.

[Sideband]

My Model 10 has a 1 meg pot fitted. As far as I am aware, the AC version should be the same. If you are looking at the RTV circuits, I don't think their model designations bare any resemblance to the actual model of the set. For instance my Model 10 is the same circuit as their Model 7. I think what they have done is printed 10 'Little Maestro' circuits and just listed tham as models 1 to 10. Trader sheet 913 is not quite the same and doesn't have the delayed AGC circuit.

[John G8MWF]

My AC/DC set matches TSS 913 verbatim. The AC set looks nothing like TSS 913!

I think the information that I have managed to scrounge has come from a Radio Serving book of some description, and it is by no means complete, but it does clearly refer to model numbers and not example numbers.

Please have a read the extract that I have included regarding the values of the volume control - there are two distinct methods used by these sets!!!
I have also included the diagram that I am working from and some scribbles but there are NO component values and some of these values DO vary from the AC/DC set - so I cannot just copy these.

A very simple radio with lots of variations and misleading information.

Hope that helps?

[Sideband]

The model 8 circuit you show is basically the same as mine except of course for the power supply. Model 7 above it in the RTV book is exactly the set I have. it happens to be the 7th circuit in the series so doesn't relate to the actual model number. The notes that you show say that the volume control is 1 megohm which is what is fitted in mine.

The problem is that you don't know for sure if the volume control has been changed at some time. If you have the RTV book, I'm sure that there is a components list for each model on a separate page.

[John G8MWF]

I do not have access to the RTV book, I am still a relative newbie and don't know about it, so I would really appreciate a copy of the component values or just those for R11, R10 & R19.

The volume and waveband switch have both been replaced as the D shafts have been crudely filed down to allow the control knobs to fit.

[John G8MWF]

Originally Posted by Sideband

It is most definitely a 35Z4GT....and it's the original as well. I can say that with confidence as I am the only one who's kept this little set going throughout its 63 year life. Its only ever had two faults....the first in 1967 when I got it was an O/C heater in the 12Q7GT. Apart from changing all the waxies (which I don't count as a fault because it was still working at the time) it then worked right through until 2009 when the dropper failed. I replaced that with a 2.22uF 400V AC capacitor with a considerable reduction in heat. That is still in place and I took the opportunity to check the heater voltages while the set was out of its cabinet. They are fine with exactly 35V across the rectifier heater and 12.4V across the 12Q7GT....the easiest ones to check.

Sorry for the delay in commenting on this but after getting the AC/DC set working I am not at all happy with the heat that the dropper resistor generates!

I have not tried to use capacitors as a dropper before and I am a especially wary of trying this on the mains supply.
If I understand things correctly then I should be able to get hold of a 2.2uF AC Motor Run Capacitor and replace the drop with it?

[Sideband]

Basically yes. 2.2uF is a 'rounded' value and you may have to trim it with a small parallel 'X' capacitor. I fitted a .22uF X capacitor in parallel with a 2uF motor run capacitor just to get the heater voltage spot-on at 230V. A motor run capacitor is a nice size to fit the existing dropper housing. Alternatively you can make the value up by using parallelled 'X' capacitors. The important thing is that the capacitor needs to have an AC voltage rating of 300V or more. Note that a 'motor run' capacitor is designed for continuous operation, a 'motor start' capacitor is designed for intermittent operation. If you use a motor start capacitor as a dropper, you may find yourself replacing it again in a few years time.

Don't be wary of doing this.....it works. Chances are there is something operating in your house that is using a capacitive dropper....my central heating timer is (I know because a few years ago it failed because the capacitor was dry-jointed)! Also my three-speed fan used in the summer uses one as well.

Mine has been giving good service for around 8 years. The idea of using a capacitor as a dropper is by no means new and Thorn famously used the idea in one of their portable TV's.

[John G8MWF]

That is excellent information - and timing!
I must admit I was pushing the AC/DC set further back on the shelf due to the shear amount of heat that it generates.

I also found the enclosed Excel sheet that you may find interesting just as your reply arrived.
It would be interesting to see how close the values that it arrives at compares with the working result!!

[John G8MWF]

Just looking at the calculations I can see there is a Surge Limiter Drop Voltage - presumably there is a resistor in series with the capacitor?

[Sideband]

I think there was a discussion on the need for a surge limiter some years ago and it was generally felt that it wasn't needed. There is a 100 ohm in series with your rectifier anyway so it isn't really necessary. I never used one but of course there is no harm in fitting one.

I also remember that a few years ago the heater section of the dropper in my Pye 11U TV failed and I used two or three 'X' capacitors in parallel (can't remember the value off-hand) to replace the faulty section. It continues to work well some three years later...no surge limiter used on that either.

I use the same spreadsheet!

[John G8MWF]

I am finding this aspect of the task really interesting!

The spread sheet is excellent and based on you comments do I need to reduce the surge limiter voltage to zero in the equation?

A 2uF cap + 2 X 0.1 X caps have now replaced the big green dropper and it is now running nice and cool.
The heater voltages on pin 2 of V1 & V3 is approx. 12.3V AC for a 245V mains input - does that sound OK to you?

The layout of my radio does not permit easy access to V5 so I am reluctant to measure the heater voltage across it.

I would prefer too stay a little bit lower on the heater voltage to try and preserve the heater life of the valves.

While this fix does not keep to the radios original design it will certainly help reducing damage to the back cover and possibly other components.

I am also considering making a similar modification to my White DAC90A which gets cracks in its case due to the amount of heat generated inside of it.

One other observation which is not really relevant in todays world - this set is now strictly speaking a AC only set!!

[Sideband]

Originally Posted by John G8MWF

based on your comments do I need to reduce the surge limiter voltage to zero in the equation?

You can do. I don't think it makes much difference to the final value.

Originally Posted by John G8MWF

A 2uF cap + 2 X 0.1 X caps have now replaced the big green dropper and it is now running nice and cool.
The heater voltages on pin 2 of V1 & V3 is approx. 12.3V AC for a 245V mains input - does that sound OK to you?

Those voltages are fine. Remember that the original design was for 200-250V and the value of the dropper was probably calculated for the higher end voltage.

Well done on trying this! The heat reduction by the 'wattless dropper' is substantial and as long as the capacitors are adequately rated, there should be no problems. These capacitors are designed not to fail s/c and at worst will only lose capacitance over a long period of time which will actually reduce the heater voltage.

How about some pictures of your capacitor solution?

[John G8MWF]

Picture enclosed of the temporary setup by Messrs' Bodge It and Leg It!

It is reassuring to know that the capacitor fails to the O/C state. I was thinking about using a fuse resistor in series with the input.

The other thought I had was if there is an issue with the surge current on a cold start up?
I 'suppose' that the scale lamps and the shunt would offer some protection but I must admit I haven't seen any evidence of a surge in the lamp brightness when I switch the set on.

Any thoughts on this?

[Boater Sam]

Would these smaller capacitors do? They are 99p 4uf (other values are available ) 450v AC. From the usual auction supplier.
CBB61 4uF AC 450V Ceiling Fan Motor Running Capacitor

[John G8MWF]

Unfortunately not in this case.
The capacitance value is fairly critical and after using the excellent calculation tool the value then has to be 'tuned' in 0.1uF steps until the correct voltage is arrived at.

[Sideband]

It's worth looking out for some values between 2uF and 4uF and those CBB61 types are excellent for the job...and they have test approvals. The one I used was from a florescent light fitting but I've since built up a small supply from various scrap items that come in at work. I tend to rescue any 'X' type capacitors from new scrap power supplies and control units in values between .1uF and 2.2uF.

Nothing wrong with that temporary hook-up John. That cap looks like it has a stud fixing so you could either find a convenient hole in the chassis to use or it may even fit the existing mounting. It isn't even necessary to use heat resistant wire.....

With reference to surge.....I don't think there is one. When my Model 10 had its dropper, the pilot light would be bright for about 10 seconds and then reduce as the heaters warmed up. Now the capacitor is fitted it stays the same brightness and in fact there is only 2.5 volts across it

[John G8MWF]

Yes, I am really starting to get into this idea.

I have purchased a 2uF CBB61 type for 99p on eBay which should arrive this week. The only thing I don't trust is the Chinese safety approval stamps on them!

Maestro - The stud mounted cap is going to go in place of the RS dropper which I have removed and I am drilling a 8mm fitting hole on its mounting base.

It is a bit of a relief not to be tracking down heat resistant wiring especially for use on mains circuits. The heat shrink stuff is good but the voltage rating of it is really not safe for this purpose although you can 'get' away with it.

I didn't think that there was a surge problem but with the ever increasing price and rarity of components it does pay to be careful.

Another future project is to modify a DAC90A to use this technology and there is a nice space on the chassis where the mains filter X2 cap was located that should do nicely. I can also leave the existing dropper in place for cosmetics.

Time to tidy up and then find where I have safely put my metal drill set . . .