Info Needed for Little Maestro 10 AC/DC

[John G8MWF]

Hello,

I have just started looking at a 'dead' Little Maestro 10 A.C and it is obvious that someone has 'had a real go' at the power stages and not put them back the way they should be!

So far I have found the power switch O/C, R15 200 ohms not 100, C17 blown apart and I am not sure if R16 is original or wired up correctly.

Could someone let me have some pictures of the underside of the radio and R16 on the top side so that I can try and put it back in the original order?

Kind regards

John

[John G8MWF]

Update - this model should be the A.C/D.C model - thank you.

[Nuvistor]

Do the circuits available from the Service Data at the top of the page not help?

Frank

[ms660]

The position of R16 is shown on the Trader sheet, also the position is shown here:

http://www.radiomuseum.org/r/pilot_l...0acdc10_a.html

Lawrence.

[John G8MWF]

The trader sheet diagram is good but it doesn't show the physical wiring routing.
True I can wire it up so that it works but I would like it to look like it was as much as possible.
I will post some pictures of the alterations later today.

[Sideband]

There were some variations to the Model 10 AC/DC. I have one but it's layout is different from the Trader sheet. The circuit is mostly the same except for a difference around the output stage which uses a 'split' cathode resistor and part of this voltage is used for delayed AGC.

[John G8MWF]

Two pictures of my chassis are enclosed.

As previously mentioned, I have the diagram for the AC/DC set but a lot of work has previously been carried out on it.
Apart from wiring mistakes the routing has most likely been changed and I would like a chassis picture to compare my set against so that I can put it into order.

My other concern is R16, the big green dropper resistor that looks to have been replaced and may be adjustable - any thoughts on this one?

[Radio Tech]

John, if I were you I would change all of those wax paper caps, they are notorious for leaking, you can get polyester types which are rated at 630V in most of the common values from Jellyfish Audio. Hope this helps.

Regards

Ken

[David G4EBT]

Originally Posted by John G8MWF

My other concern is R16, the big green dropper resistor that looks to have been replaced and may be adjustable - any thoughts on this one?

R16 is the heater ballast resistor which has to drop the excess voltage over and above that which the valve heater chain requires. The heater chain amounts to 3 x 12.6V plus 2 x 35V - a total of 107.8V. The value of R16 is stated as 830 Ohms and the heater chain draws 0.15A, so 830 Ohms will drop 124.5V. Added to the heaters, that's 232.3V (not taking account of the 100 Ohm dial lamp shunt resistor), which sounds about right. At 124.5V x 0.15A, R16 will dissipate 18.7 Watts.

It's no surprise that droppers often gave up the ghost, but the replacement one in your set looks well up to the task - you just need to check that the tap is set to 830 Ohms or thereabouts. Assuming that the set is working - if only after a fashion - you can check the voltage across the heater chain to see if it amounts to about 108V, and if it's higher than that, you can adjust the tap on the replacement R16 so that the heaters aren't being over-run.

As to the position of R16, it's exactly where it's shown on the diagram on the Trader Sheet.

If R16 has another tap, that could well be R15 - the dial lamp shunt. Just guessing).

Hope that helps a bit.

Good luck with the set John.

[ms660]

Don't forget the HT load current and ripple current dissipation as well....

Lawrence.

[Sideband]

Hmm....yes yours is different from mine inasmuch as the dropper is in a different place. Underneath it looks pretty much the same. The dropper in mine is on the right hand side with the rectifier valve behind it.

Definitely change those waxies. They will all be dud by now. I've had my model 10 for 50 years (1967) and it was the first radio I repaired when I was 14. I changed all the waxies a year or so later for polyester types and it transformed the performance. It was used as my bedside radio for many years and worked completely faultless until 2009 when the dropper failed. That has now been replaced by a capacitive dropper with a huge reduction in the amount of heat generated.

[ms660]

I'm sure I've seen a big green dropper like that on this forum before unless I'm mistaken

Could it be original? Live in at the bottom, tap to bulb/heater chain, top to rectifiers anode?

Lawrence.

[Sideband]

Originally Posted by David G4EBT

If R16 has another tap, that could well be R15 - the dial lamp shunt. Just guessing).

No the dropper is a single resistor. The dial lamp shunt is the big 100 ohm carbon stick resistor (bottom left in the piccy above). I had to change the one in mine.

[John G8MWF]

Thank you all - this is all usable information.

The reason I keep on about having a look at an AC/DC picture is because the person who wired it up had the surge limiter, R14, connect directly to the mains WITHOUT going through R16 - so the live end of R16 and R14 went straight to the mains. needless to say R14 is fried.

I am also pretty sure that the rectifier valve has bit the dust as well.

Naturally I will replace the dreaded wax caps and their replacements are in the project box.

To cut a long story short whoever repaired this set missed out a piece of wire, possibly when they were replacing R16? It is the only way that I can get this back to the diagram in service sheet 913.
So unless there is some kind of mod to do this post the 913 sheet then a piece of wire must be added.

It would be nicer to follow the way the manufacturer wired it up by seeing the picture of a 'good' one but, saying that, there have already been some changes when electrolytic caps have been replaced.

So do I run a piece of fibre glass sleeved wire from the dropper or a piece of insulated wire from the valve base?

I also suspect that once the HT is sorted out that there will be 'other' problems as most of the valves are looking delicate!

[ms660]

I'm looking at Trader sheet 913...Something odd to me, why have a rectifier surge limiter of 100 ohms after what is in effect a surge limiter of 830 ohms....Look at the rectifiers anode voltage given in the valve analysis, it's 220 volts...not going to get that after the surge limiter and the 830 ohms because that voltage is set by the total heater chain/bulb voltage which is approx. 111 volts....I think the surge limiter was connected to the live and not via the heater dropper.

Lawrence.

[ms660]

Re: Above, meant to say the surge limiter should have been connected to the live in that schematic.

Lawrence.

[Herald1360]

The R&TVS circuit (9 in a general Little Maestro sheet) is more informative. The surge limiter is indeed connected to live, but there's also a shorting plug option to run the set from 110V supplies.

[John G8MWF]

Ok this machine is the 200-250 AC/DC model and there are no straps.

Do I trust sheet 913?
These sets circuits do seem to change over their production life!

Looking at my KB10FM that has a 113 ohm (R13) ballast resistor and then my DAC90A has a 150 ohm ballast plus 950 ohms and heater chain!

With the price of valve rocketing I would REALLY like to get this one right especially as I picked up the set with a fried ballast resistor and rectifier!!

[Sideband]

If I can I'll have a look at mine tonight. I'm pretty sure there is a 100 ohm resistor in the anode of the rectifier. At least mine hadn't been got at at all. I'm the only one who has kept it going.

[ms660]

If you want it as original then use the dropper and surge limiter values in the schematic(s)

If you want it optimised for a nominal 240VAC operation, then do a heater/bulb chain/dropper calculation, working out a value for the rectifiers surge limiter is not straight forward due to the pulse nature of the rectifiers conduction time (much less than 10ms) Most folks use a trial and error method.

When selecting the value of the rectifiers surge limiter resistor, start high, say 500 ohms and see what that results in and then take it from there.

According to the Trader sheet, total load current is approx. 50mA so use that for IsqrdR then multiply that by 3 for approx. surge resistor power dissipation (load and ripple current) then double it for a more cooler running resistor in the final installation, eg: suppose it ends up being 300 ohms for the correct voltages then 0.05*0.05*300 = 0.75 then 0.75*3 = 2.25 (actual watts dissipated approx.) then *2 for reliability and a bit cooler running = 5.5 watts. min.

In the absence of any other voltage measurements from other sources you'll have to use the ones on the Trader sheet for the target voltages unless someone can oblige.

Hope this helps.

EDIT: Post crossed.

Lawrence.