Info Needed for Little Maestro 10 AC/DC

[John G8MWF]

Originally Posted by usradcoll1

I first found the valve base skirts cracked and appeared the same way. Brand new one in the carton. They're made of brass!
All I did is lined up the crimps with the bakelite base and wrapped wire tightly around the base and soldered it together, where the crack is.

I just repaired the skirts by first securing them with a tie wrap and soldering a piece of wire across the gap.
The one thing that I did notice was additional hairline fractures in new positions on the skirts. But overall the repair was very successful.

[usradcoll1]

That's a better idea yet! It's been a few years since I did one! I'll keep that in mind.

Regarding the 35Z4 rectifier valve, possibly originated in the US, but we didn't use it much. Shortly after the 35Z4 came out, the valve makers came out with the 35Z5, which had a tapped heater for use with the dial lamp. Naturally, it was used as a cost saving, eliminating the use of a dial lamp shunt resistor.

[John G8MWF]

Due to the ever increasing cost of valves I was considering using a 1N4007 across the faulty valve - an idea that I was not at all keen on. I have purchased a used 35Z4GT which is hopefully coming in the post soon.

It will be interesting to see what the situation is when HT is restored.

[usradcoll1]

I seldom seen a rectifier valve, that wouldn't rectify if the heater is good. Most of the 35Z4/5 types were open heater. The 35Z5 heater tap would open when the dial lamp would burn out. Some manufacturers would shunt the dial lamp with a 10 or 22ohm 1/2 watt resistor, to try to minimize that problem.

[John G8MWF]

OK, while I am waiting for the rectifier to arrive I would like to try and identify my other Maestro Model 10 AC set and obtain the trader sheet or a diagram for it.

Trader Sheet 912 does not resemble my set in a lot of ways - see enclosed picture.

Plus when you measure the external aerial to chassis it shows as zero ohms so it was no wonder someone has cut it off!!

Any input, as usual, would be very much appreciated.

[Sideband]

Originally Posted by John G8MWF

And just for a sanity check are you using, I suppose you must be, a 35Z4GT rectifier?

I do NOT have a heat shield on my dropper which explains why the back cover is burnt and crumbly and broken.

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.

The HT at the reservoir capacitor (cathode of the rectifier) is 270V and the main HT on the smoothing is about 207

With regard to the crumbly back cover....mine's the same. The dropper generated a huge amount of heat and was deflected out through the back which has fallen to bits in one corner. I have ideas of fabricating a new one out of pegboard. Any new back cover should last forever because of the great reduction in heat.

[John G8MWF]

If you could knock up TWO backs while your at it . . . . .!

You don't hear of many valve radios with their original owners AND service engineer!

I may well try the capacitor when the set is working. There are way too many variables with this and the AC set that I am working on and they are both fighting me VERY hard to stay not working.
There has been a lot of work on these sets which makes it a bit more difficult to work out what is going on.

The back cover of mine had been in direct contact with the replacement dropper and during the course of moving things around it snapped in half.

The decision now, after a cup of tea, is if I should get all of the valves tested to avoid guessing their status and in doing so save time and money. With the price of valves it may be a good idea.

[Sideband]

If they haven't been overrun, it's likely that the valves will be OK....with the exception of the reccy perhaps. Only one has been changed in mine, the 12Q7GT but all the others have been checked by substitution. I find these octal valves very rugged and reliable.

I'm not quite the original owner. It belonged to an old lady originally and when it went wrong, she asked by brother (TV engineer) to have a look. The new valve cost 17/6d in old money back in 1967 which was actually quite a lot of money for an old lady back then....about 90p now, so he asked her if she still wanted it repaired. She did but unfortunately she went into hospital soon after and never came out.... so as there was no-one to return it to he hung on to it in case a family member asked but after a year it was still unclaimed so that is how I ended up with it. I bought a new valve after saving up several weeks pocket money and I've been its guardian ever since.

[John G8MWF]

The rectifier starts off OK put gradually drops off by40 volts or so after a couple of minutes and then I turn it off to avoid possible damaged.
The safety lamp does not glow any brighter so I don't think there is anything dragging it down and it is just a case of the valve fading.
Output valve V4,6V6GT, has working heaters but does not appear to draw HT current when the HT is available and there is no hum or pops or crackles from the speaker.

I met the seller of this radio who told me that it had belonged to her recently deceased grandmother and that it had always been at her house.
She even blew it a farewell kiss when its was handed over to me so I feel that this one is definitely for rescuing !!

[Boater Sam]

Output transformer primary open circuit? Hope not. Cathode resistor open circuit? Easier.

[John G8MWF]

Both are OK replacement xfmr 460 and R10 290 ohms. Original xfmr 1200 ohms so there may have been a burn out at some time taking the 6V6 with it?
C17 had to be taken out of circuit (wax) as that was drawing current and warming up quite nicely! You could watch the safety lamp getting brighter as it did.

The new plan is to get the valves checked on a tester and go from there. Hopefully without having to go too deep into my pockets.

[Sideband]

Originally Posted by John G8MWF

The rectifier starts off OK put gradually drops off by40 volts or so after a couple of minutes and then I turn it off to avoid possible damaged.

Output valve V4,6V6GT, has working heaters but does not appear to draw HT current when the HT is available and there is no hum or pops or crackles from the speaker.

What Model 10 are we talking about here?? We started off with the AC/DC version with the 35Z4GT which was suspected of being low. Now we've introduced a 6V6GT which will be in the AC only set!

The AC/DC version has a 35L6GT....which set is suspected of having the low rectifier?

As suggested, make sure the output transformer primary is OK. A resistance check should show several hundred ohms or so and with luck, a crackle in the speaker as well.

[John G8MWF]

My apologies - will wait until I finished the AC/DC set first.

[John G8MWF]

Ok, I finally got HT working but the set is still not receiving . HT on the Anode is around 200V as I am running a 60W safety lamp is series.
Audio stages getting a nice mains hum when I touch TC on V3.

Nice loud scratching noise when I touch TC on V2 with meter probe but very light/nothing when I do the same to V1.
There is a very light mush if I attach an aerial to the TC of V2.

HT getting to V1 and I am not sure if the oscillator is working at the moment but I would expect it to be a bit more lively when I scratch the TC even without the oscillator working.

ALL wax caps have been replaced.

I suspect V1 but as usual it is one of those valves I do not currently have.

Are there any other suggestions that I could try?

[Sideband]

Originally Posted by John G8MWF

Ok, I finally got HT working but the set is still not receiving . HT on the Anode is around 200V as I am running a 60W safety lamp is series.

200V on which anode?

Check the anode volts on the mixer and of course the triode oscillator section. Take the 60W lamp out of circuit as it's unlikely that you'll need it now anyway since most of the set is working (unless you are using it as part of the dropper....).

Nice loud scratching noise when I touch TC on V2 with meter probe but very light/nothing when I do the same to V1.
There is a very light mush if I attach an aerial to the TC of V2.

HT getting to V1 and I am not sure if the oscillator is working at the moment but I would expect it to be a bit more lively when I scratch the TC even without the oscillator working.

Are there any other suggestions that I could try?

Make sure the screen grid volts are present. Usually if the oscillator is not running, you will hear any background 'mush' get louder as you tune LF (towards 550 metres). Make sure the wavechange switch is clean. You'd normally hear a crackle when it's turned anyway. If all seems OK, this is where I would get the RF generator out...... If your model 10 has those cheap paxolin valveholders, check the contacts and if in doubt carefully squeeze the contact from underneath.

12K8GT is quite plentiful and not too expensive (I bought one for another set a few years ago for a fiver). There are several on eBay at present.

[Boater Sam]

Get it off the lamp limiter now that its not drawing too much current, you can't expect it to work with it in series.
Check the oscillator grid, if it's negative volts with respect to chassis, it's oscillating.

[John G8MWF]

It's alive! - thank you.

The cathode of V5 is now 220V DC which is a little low but acceptable?
R13 measures 1150 ohms slightly lower than the 20% tolerance. The volt drop across it is nominally 40V, I calculate the HT current to be around 35mA - so nothing is frying at the moment.

Sensitivity is not bad but the AGC line is only reaching 0.4V which seems a little low to me.
Audio is low but I haven't replaced C14 the 50uF cathode decoupling capacitor yet.

[Sideband]

It's obviously working with low HT at the moment (it's about 50V low) but if the mixer is borderline it might struggle to keep going.

Not sure if your output stage circuit is the same as mine but the 50uF is rather critical. If the cathode bias of the output valve is made up of two resistors (22 ohm and 270 ohm) then your set has delayed AGC and the bias for the AGC is developed from the voltage at the tap of the 22 ohm. Basically what it means is that the AGC only becomes active on stronger signals. If the 50uF is O/C or otherwise faulty, it will affect the AGC action. It did on mine.

[John G8MWF]

50uF replaced and the old one, Dated October 1949, only measured a 1000pF!
There was no noticeable difference in performance but the low audio problem has been traced to a high primary resistance on the output transformer 1200 instead of 480 ohms. I am looking at one of the old RS universal transformers to replace it with but I am not sure about the mechanical size of it and the mounting holes on the speaker and I will have to find it first.

If the manual is to be 'believed' the set should work on as low as 200V AC but that probably assumes everything is 100% - and the manual also states that the cathode should be 240V DC. So the HT on my set is less than 10% under.

And just to make things a little more interesting the used replacement rectifier purchased from eBay went no output on me so I was force to use a 1N4007. So I may need to put a series resistor on it before giving it the full 240V.

[Sideband]

I wonder what conditions were used when measuring that cathode voltage? I would expect more that 240V HT when the set is fed with 240 volts mains. 235Vac in actually gives me 262Vdc on the rectifier cathode. Main HT is around 202Vdc with about 190Vdc on the anode of the output valve.