Masteradio D111

[ViragoRider]

I found this in a charity shop near me and had to rescue it. I like the art deco look of it and its size makes a statement.

Internally, it looks mainly untouched apart from one of the cathode bypass caps that have been replaced by 2 "Radiospares" caps in parallel. These can be seen on the bottom left of image 2. I have also circled a wire that appears to have been cut off from somewhere. I've yet to trace where that should go to but I think it could be the pickup input.

It is missing its original knobs. The one it has is not an original one as that just pushes on whereas according to the Trader 829 Service Sheet, the original ones were a screw fit.

The case is also rather damaged, having a long crack in the top as though something had been dropped on it at some point. You can see the crack in image 3. I plan on reinforcing the top with a couple of aluminium plates and the proposed fix is shown in image 4. I think this will prevent any further damage to the top but I'm not sure how to make the mend invisible. Any suggestions on how to restore cracked Bakelite are most welcome.

The original valves look good but as can be seen in image 5, the metallic coating is peeling off the bottom of the valves. The PEN45 valve is also loose on its base as I discovered when I tried to remove it from the chassis. It's not super loose but it definitely moves although I don't think it's gone to air. Time will tell I guess. I haven't powered it up yet as being a charity shop find, it has had its cord cut off and I'm in no rush to power it on.

I had to restring it as the dial cord was very frayed and snapped on me while I was checking out the tuner operation. Other than that, I'm really impressed with the quality of construction.

[ViragoRider]

I've been making progress with this set - at least to the case. I have cemented a couple of aluminium plates to the inside of the case to repair and reinforce the top where it is cracked and I am now in a position to begin the electrical restoration.

I understand that if I remove the rectifier valve and bring the set up slowly on a Variac and dim bulb I will be able to check out the mains transformer windings. If everything checks out with that, would it be safe to repeat the procedure with the rectifier in place? I would like to see if the set has signs of life before I start recapping it or would that be a bad idea? I'm not sure what would be best for the set so I would welcome a little advice on the way forward from this point. This is the first set with valve rectification I have dealt with and also the oldest, so I want to be ultra cautious with it. I wouldn't want to do any harm to it so if the best advice is to recap first then I will do that, but I know of others that have powered sets on before recapping.

As always, thanks in advance for your advice and expertise.

[Simon Gittins]

C23 is the audio coupling capacitor; you could monitor the voltage at the junction of C23 and R19 as you bring the voltage up and then switch off as soon as you see any positive voltage if you want to prove to yourself that it is faulty. I would be inclined to replace it before powering up.
C25 is from V4 anode to the tone control, if this is leaky the tone control could burn out.
C26 is a mains aerial coupling capacitor, I suggest removing this altogether as it won't work well under today's conditions.
With these three dealt with it should be ok to power up with the variac and see if it works to some extent.

[PJL]

That is an unusual crack and maybe a manufacturing fault.

The first thing to do is to check the continuity of the mains and output transformer windings.

[Mr 1936]

Hi

A nice looking just postwar set, maybe using a prewar design.

I would agree with Simon, deal with the three caps he mentions, and get the set running first. If you decide to carry on changing caps, do them one at a time.

The HT has a smoothing choke (I think) so you could temporarily isolate it and check that its not O/C before powering up.

I have had success with bakelite cracks by reinforcing underneath (as you have done) then melting furniture restorer's brown wax sticks into the crack. Scrape off gently when set, and polish to finish off.

Many radios that were small and light enough seem to have been put away in the loft at some stage. A proportion will have been working but no longer needed, and put away because they cost a significant amount of money and the owners couldn't bear to throw them out. The rest will have developed a fault, and the owners will have put them away in the hope that they would somehow mend themselves.

[ViragoRider]

I changed C23 and c25 and removed c26 as as recommended and put a mains flex on it as this had been cut off by the charity shop. The first power on was a little scary as my dim bulb was glowing rather too bright for my liking at only 100v output from the Variac. I shut the power off and started double-checking the mains transformer windings. The resistances checked out so I tried again, no rectifier valve in place to be on the safe side. I couldn't see any signs of life, no heater glow and the dial bulb was not lighting up despite it being good - I had tested it some time ago for continuity. I then noticed that the dial bulb holder, which attaches to the chassis via a push on clip was shorting to the chassis. I moved it to remove the short and the dim bulb looked a lot healthier after that.

At this point i went for a proper test with the rectifier inserted and slowly brought the set up to 240v mains input. All the heaters were glowing as well as the dial bulb. i could hear static so that was a good sign. Using a 3 foot length of wire for an aerial, I started moving up and down the MW band and could hear stations although they were feint.

Moving the wire around and holding it improved reception so I need a better aerial. I'm really pleased to report that reception is good on all 3 bands, LW, MW and SW. This is my first SW capable radio I have done and I'm looking forward to discovering what I can pick up when I've finished the restoration.

Tomorrow, I'll start changing the wax caps and I'll check the voltages before and after the cap changes to see what effect it has. Tomorrow will be an enjoyable day I think. For now though, it's time for a beer to celebrate.

[Ed_Dinning]

Hi, you may find that when you recap the set (paper types only) reception will improve a lot.
This can be due to the caps on the AVC line being leaky and as they are in a high impedance circuit (1 meg plus feed resistors), they will make the set insensitive.

Good luck with the rest of the restoration

Ed

[Herald1360]

Won't leaky AGC decouplers tend to reduce the AGC's capability to wind back the gain with increasing signal, rather than cause low gain?

Leaky screen decouplers can reduce the gain by reducing the screen voltage though.

[ms660]

Originally Posted by Herald1360

Won't leaky AGC decouplers tend to reduce the AGC's capability to wind back the gain with increasing signal, rather than cause low gain?

There's an easy way to find out.

Lawrence.

[ViragoRider]

I finished the recapping and the set is waking up now. On checking the voltages before and after the recap I note the Anode voltages have gone up slightly, but the screen voltages have shot up in comparison.

The Anode voltages before and after are as follows (using the Trader 829 servicing sheet)

V1 TH41 247 255
80 85
V2 VP41 251 255
V3 HL42DD 47 54
V4 PEN45 234 247
V5 UU6 237 247 (ac on each anode)

The screen voltages before and after the recap are

V1 TH41 96 130
V2 VP41 94 130
V4 PEN45 205 250

I'm a little concerned about the screen voltage of V1 and V2 now as the trader sheet quotes them at 87v. They are fed by R6, 25K resistor which tests at over 33K. I'll order some replacements and swap it out. Would this be the cause of the higher screen voltage? I think I need to check the other resistors too now, since R6 was so far out.

[ms660]

Were those voltages you measured taken under no signal conditions?

Lawrence.

[Mr 1936]

Hi

The voltages you measure can be pulled down a bit depending on the loading of the meter. This is most noticable when the effective source impedance is high, for example a screen grid.

The Trader sheet may quote the meter assumed

Here's a few examples of meter resistance:

Modern DVM (all DC ranges) 10 Megohms
AVO 8 on 250V DC range 5 Megohms
AVO 8 on 100V DC range 2 Megohms
AVO 7 on 400V DC Range 200 Kilohms
AVO 7 on 100V DC range 50 Kilohms

If you are using a modern meter this may be one reason the readings are a bit higher than the Trader sheet. High mains voltage is another.

[ViragoRider]

Measurements were taken under no signal conditions and I used an Avo 8. I note the Trader sheet measurements were taken with an Avo 7, so that might account for the discrepancy.