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Old 21st Jan 2019, 11:35 pm   #1
Davewantsone
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Default Roberts R66 diode MR2

I have recently acquired a honey coloured Roberts R66 in a awful state. The back had completely come apart including the rexine! The 3 knobs were fast on the rusted shafts. I had to saw the volume one off!
Well I have now got it working with a new volume control and several replacement components. I had to replace all the LT (3) capacitors C28-9-30 and resistor R14 which had a 5.6 ohm fitted but was reading 12 ohms. The diode bridge MR2 (listed as FSW1392A) was replaced with 2 1N4007. The reason for writing is that the heater voltage was now 1.72V This could be due to the less voltage drop produced by the 1N4007, however I did note an extra wire going from the C29 capacitor +ve back to MR2. This is not indicated on the two circuit diagrams I have (Trader 1273 and Radio Sevicing page232). When I connected this lead back to the defunct MR2 with the ground wire of MR2 also connected the LT supply dropped to 1.42V. Is the MR2 acting like a zener diode? Why no indication on the circuit diagram?
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Old 22nd Jan 2019, 1:25 am   #2
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Default Re: Roberts R66 diode MR2

That extra wire doesn't sound like it should be there. I suspect that the extra volts are as surmised down to the lower Vf of the 1N4007s. Connecting C29+ back to the dead MR sounds like it's just loading the supply a bit and the voltage is being dropped in the series smoothing resistors. The fact that it's now about right is probably just fortuitous.

C29+ in the Trader circuit is the output end of the heater filter- I would be inclined to connect that stray wire via two more 1N4007s in series to C29- to give a better defined "zener" to clamp the filament supply at around 1.4 volts.

The filament supplies in these mains/battery sets with parallel filaments were always a bit "iffy"- a failed filament could cause a voltage rise which wouldn't do the remaining filaments any favours. Modern options like basic diode clamps or low voltage dc regulator circuits simply weren't available to the designers. Other designs used series connected filaments and powered the chain either via a dropper from the HT side or a 7.5V dc supply. Either way an open circuit filament simply stopped (or at least reduced, there were filament balancing resistors to bypass the cathode currents from the valves higher in the chain) the filament current with no risk to the remaining good filaments.

BTW MR2 isn't a bridge (4 diodes) it's a full wave rectifier or half bridge.
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Old 22nd Jan 2019, 8:42 am   #3
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Default Re: Roberts R66 diode MR2

Just a note here.
The OP says he replaced the LT caps, C28, 29 and 30.
I guess he is working from the 'service data' red book diagram and not the 'trader sheet'. They are not quite the same numbering. Nor the dropper chain R numbers.

Davewantsone did you read the Modifications section of General Notes on the Trader Sheet?
It refers to an additional length of resistance wire used to build up the value of R16 (trader number) to 5.4 ohms from 3.9 ohms.
That could be the extra wire you have noticed perhaps.
If that is the case it needs to be included in the ''series chain'' to drop the LT volts.

I would carefully check the chain is wired correctly - someone might have altered it in the past to boost the LT volts due to MR2 problems (or get a little more life from the DK92?).

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Old 22nd Jan 2019, 10:38 am   #4
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Default Re: Roberts R66 diode MR2

I believe in some later European sets they used a rechargeable cell to keep the filament supply stable and low.

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Old 22nd Jan 2019, 10:56 am   #5
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Default Re: Roberts R66 diode MR2

Yes, DEAC or as we would say NiCd cell was used. At ~1.25V discharging it's fine for the filaments and it would only rise to about 1.4V max on charge. It's another possibility as a "protection zener" too.
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Old 22nd Jan 2019, 4:19 pm   #6
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Default Re: Roberts R66 diode MR2

I have had a look on my other maroon R66. There is a wire that goes from the third smoothing LT capacitor( C29 Trader 1273) back to MR2 Just like the honey R66. MR2 has 5 connections arranged when looking from back at 11, 1, 5,6 and 7 'o'clock. 6 and 7 are ac in, 5 is rectified output to C31(Trader). 1 is connected to ground and 11 is connected to C29 the third LT smoothing capacitor. So it does not look like a stray wire and like I said it does lower the voltage to around 1.4V
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Old 22nd Jan 2019, 7:36 pm   #7
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Default Re: Roberts R66 diode MR2

I am beginning to think your original post is correct. That part of MR2 must have a voltage regulator effect?

I have an R66 in front of me and it conforms to the diagram in the Trader sheets. No extra wires on mine, a just a silicon replacement for MR2 and an extra dropper to match the voltage...
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Old 22nd Jan 2019, 11:53 pm   #8
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Default Re: Roberts R66 diode MR2

Is your original MR2 still present in the radio? If so does it have 5 connections?
I have reconnected the original MR2 in the Honey coloured R66 and it seems to be working just as good as the pair of 1N4007 I had replace it with. I have left MR1 in place but have replaced it electrically with 1N4007. The three LT smoothing are also in place. I have done some testing and managed to get 2 to give satisfactory performance with only one now being electrically replaced.
Does anyone have a solution for removing a sawn off pot shaft embedded in the volume knob which also has the grub screw stuck as well!
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Old 23rd Jan 2019, 12:57 am   #9
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Default Re: Roberts R66 diode MR2

Quote:
Does anyone have a solution for removing a sawn off pot shaft embedded in the volume knob which also has the grub screw stuck as well!
PlusGas, left handed drill bits, a bit of heat and a lot of swearing may work.

There are some more sophisticated approaches which have been described in the forum that you may find with a bit of searching- try "seized grubscrew" in the search box up top.
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Old 23rd Jan 2019, 11:11 am   #10
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Default Re: Roberts R66 diode MR2

Often a shunt selenium was used to have close to the recommended 1.35V. You need to add a series resistor so voltage is about 1.4. Then add 2 x 1N4007 (not any other 1N400x) in series across filament supply as otherwise if half of O/P valve filament fails the voltage would rise too much.
1.7 - 1.4 = 0.3V. Assume 130mA (the 125mA is nominal), so 300/130 Ohms = 2.3, so use 2.7 Ohms
1.35V is philips/mullard recommended.
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Old 23rd Jan 2019, 5:34 pm   #11
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Default Re: Roberts R66 diode MR2

Well that's what I thought was in the MR2 but why no indication on the circuit diagrams? (The voltage remains around 1.4V with or without the valves fitted).
If the recommended LT supply is 1.35V, would the valves not be overdriven if the radio had been battery powered with a 1.5V LT cell?
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Old 23rd Jan 2019, 8:10 pm   #12
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Default Re: Roberts R66 diode MR2

Mod during production run? Not seen as significant enough to update literature, perhaps because any contemporary repair would tend to replace unusual parts like MR2 with like for like.

Zinc carbon dry cells didn't sit at 1.5V for long once in use so the filament overrun would have been too short to have much effect on life of the valves. Particularly on a set which would see little if any use on batteries anyway!
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Old 24th Jan 2019, 3:11 pm   #13
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Default Re: Roberts R66 diode MR2

#11
Then add 2 x 1N4007 (not any other 1N400x)

Why is this?
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Old 24th Jan 2019, 5:20 pm   #14
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Default Re: Roberts R66 diode MR2

This is because if an internal short in a valve or when faultfinding the HT voltage could be applied to the LT supply. 1N4001 only have a PIV of 50V, 1N4007 of 1000V. The HT could be over 100V so a short-circuit could blow a 1N4001 diode allowing full HT across the valve heaters and zap the lot!
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Old 24th Jan 2019, 7:12 pm   #15
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Default Re: Roberts R66 diode MR2

I think it's more likely because the 1N4007 will have the highest Vf of the group. You don't want the clamp turning on too soon. If HT+ shorts to LT+ it will be clamped at around 1.5V, low enough to be safe for the filaments but of course the diode is forward biassed. If HT+ shorts to HT- it'll be clamped more firmly by a short circuit!
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Old 28th Jan 2019, 7:51 pm   #16
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Default Re: Roberts R66 diode MR2

Quote:
Originally Posted by Davewantsone View Post
If the recommended LT supply is 1.35V, would the valves not be overdriven if the radio had been battery powered with a 1.5V LT cell?
The 1.5V Zinc Carbon battery isn't 1.5V!
It's nearly 1.6V with no load and freshly made. It droops with age and load.
For motors & torches the cell endpoint is taken as 0.9V
Battery valve design was most critical for the Mixer/Osc. The DK91, DK96 etc. The first was an Sylvania octal in 1938 and 1st B7G was the RCA in 1940 that the DK91 is the Philips copy of. There were German metal Y base types that were designed for parallel only and NiCd use. The Sylvania / RCA / Philips all designed for either 50mA series or later 25mA series and nominal 1.4V (1.35V on mains operation). Usually the most critical is the heptode or octode Mixer oscillator and typically designed to work down to 1.0V. As it ages the minimum oscillation voltage might be 1.1V filament. That's actually the endpoint off NiCd and not far from endpoint of Zinc Carbon, maybe about 20% wastage.

The average over time of the Zinc Carbon is around 1.4 to 1.3V depending how fresh from factory the cells were.

A mistake is to put a fresh Alkaline cell in each time you demo. Let the Radio use up the cells!
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Old 28th Jan 2019, 7:56 pm   #17
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Default Re: Roberts R66 diode MR2

So HT of a 90V nominal set might be designed for 100V down to 54V. Certainly 75V to 85V is fine for mains derived HT or using disposable lithium cells (tricky!). Typically HT is less critical so per cell endpoint is 0.9V per cell.
Maximum volume will gradually degrade with falling HT.
The set will simply stop with too low LT on the mixer/oscillator.
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Old 28th Jan 2019, 8:03 pm   #18
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Default Re: Roberts R66 diode MR2

Quote:
Originally Posted by Herald1360 View Post
I think it's more likely because the 1N4007 will have the highest Vf of the group. You don't want the clamp turning on too soon.
I've tested some diodes with If of 5mA to 20mA. The 1N4007 has different construction to the other 1N400x. It's got the higher Vf needed. It also makes a cheap RF attenuator or switch because it's MUCH slower to switch than the others, behaves like a PIN diode. It also means that with high capacitive load it doesn't need the snubber capacitors you sometimes see on bridges to minimise RFI from the current spikes. It's too poor a Q to use as a VHF varicap, but it will work as a varicap for AFC (LW to SW) or tuning bandspreaded SW tuning. Too small a change for MW/LW.
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Old 28th Jan 2019, 8:55 pm   #19
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Default Re: Roberts R66 diode MR2

Quote:
Originally Posted by Mike. Watterson View Post
Quote:
Originally Posted by Davewantsone View Post
If the recommended LT supply is 1.35V, would the valves not be overdriven if the radio had been battery powered with a 1.5V LT cell?
The 1.5V Zinc Carbon battery isn't 1.5V!
It's nearly 1.6V with no load and freshly made. It droops with age and load.
For motors & torches the cell endpoint is taken as 0.9V
Battery valve design was most critical for the Mixer/Osc. The DK91, DK96 etc. The first was an Sylvania octal in 1938 and 1st B7G was the RCA in 1940 that the DK91 is the Philips copy of. There were German metal Y base types that were designed for parallel only and NiCd use. The Sylvania / RCA / Philips all designed for either 50mA series or later 25mA series and nominal 1.4V (1.35V on mains operation). Usually the most critical is the heptode or octode Mixer oscillator and typically designed to work down to 1.0V. As it ages the minimum oscillation voltage might be 1.1V filament. That's actually the endpoint off NiCd and not far from endpoint of Zinc Carbon, maybe about 20% wastage.

The average over time of the Zinc Carbon is around 1.4 to 1.3V depending how fresh from factory the cells were.

A mistake is to put a fresh Alkaline cell in each time you demo. Let the Radio use up the cells!
A zinc carbon cell when the voltage falls to just under 1.5V under no load is usually exausted. My original question was the heater voltage would be higher than the rated 1.35V for the valves. Could valves fail due to this? The MR2 has some form of shunt to limit the voltage on a.c. A alkaline cell has a slightly less output than a Zinc carbon cell when new but a more constant output voltage until exausted. The best cell would seem to be a Nicad only delivering 1.4 V fully charged dropping to its rated !.2V when nearly flat.
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Old 29th Jan 2019, 6:17 pm   #20
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Default Re: Roberts R66 diode MR2

No, an exhausted Zinc Carbon is 0.9V, though design limits can be for 1.0 or even 1.1 V.
An Alkaline cell has a flatter discharge and higher end point, a bit more than 1.1V

Only Lithium. mercury and silver based primary cells have a flatter discharge than Alkaline.

The valves are designed for longest life at reasonable performance at 1.35V and are nominally 1.4. In general the life time is halved for each 10% of increase of voltage above rated value for a tungsten filament. Ordinary lamps have nitrogen. Halogen lamps have halogen gases at higher than atmospheric pressure. Valves would have a short life as bright emitters as the filament is in a vacuum. However the battery valve filaments are coated. The first B7G 50mA series used nickel filaments. The later 25mA filaments needed much more advanced accurate diamond dies for the tungsten. It's more brittle, so there is a spring to maintain tension as it heats up. Replacing the LT frequently with new cells or overrun mains PSU will shorten life. They will run for some while at 1.6V, but obviously about 1.55V is half the life of just under 1.4V, 1.49V might be close to half the life of 1.35V and 1.64V may be about 1/4 life of 1.35V operation. They certainly won't fail rapidly at 1.65V if NOS, but life would be short.

In theory a battery filament might be longer life than indirect as they are run cooler but they are more fragile being thinner and exposed direct to vacuum instead of in a ceramic material inside the coated cathode tube. Also mains might be more stable.

Hence check mains PSU volts on load. If it's series, then check across the DAF91 or DAF96 or equivalent. That's usually the 0V end of the chain. Typically the o/p valve is first on a series chain so providing the -ve bias can be simply 0V.

Generally only parallel is used on later UK models with a rectifier as 1.35V regulator (along with a series resistor in smoothing), or on German sets with a DEAC (NiCd). The earlier German valves had different filament currents so unlike the Sylvania (1938) and RCA (1940 B7G) couldn't be used in series. The 1939 Philips/Mullard P8 edge connect are just the 1938 Sylvania Octal types with a different base. Pin order identical.
The older 2V filaments (1.7V to 2.1V from a lead acid*) used up to maybe 1952 in houses without electricity in table models as it was cheaper for LT. HT cost about the same. Philips/Mullard started converting existing design 5pin and 7pin types to Octal. By 1945 all the 2V types in Octal.

(* The 2V filaments never intended to be connected to the Lead Acid cell while it was charging. A mains PSU would replace the cell. The cell would be charged at a cycle shop. That's how Curry's Cycles got into radio. The lead acid cell voltage would rise too high and if the cell disconnected or gassed too much then the charger would likely burn out the filaments).
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