3rd May 2017, 7:49 pm | #261 |
Dekatron
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Re: Restore a Beau Decca
Take a copy of the circuit and check each component is wired correctly marking up on the copy as you go. Once you are sure it is OK, attach your DMM across C33 and turn on. If the voltage comes up to about 275V you are OK, otherwise switch off.
Once you are done with this, you can start measuring voltages and comparing with those on the trader sheet. |
11th May 2017, 7:51 am | #262 |
Pentode
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Re: Restore a Beau Decca
Thanks Station X and PJL. The voltage across C33 is 350- which looks a bit high. I also carried out voltage checks on the 2 valves V1 and V2 (see photo). The expected voltages from them data sheets are shown for each pin with- the actual measured voltages are in brackets. Pin 4 on V1 and V2 is lower than expected. Pin 6 on V1 is slightly higher 86V instead of expected 70V. Both heater voltages measured around 5.6v AC.
On the LW and MW settings the radio is making the type of noise you would expect from a tuner- white noise and hum which varies slightly as you move the tuner- but no stations are picked up in either case. |
11th May 2017, 10:39 am | #263 |
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Re: Restore a Beau Decca
The low voltage on pin 4 (screen grid) of V2 suggests that C16 is leaking electrically or R10 has changed value.
Pin 4 of V1 ia also low on volts, it's fed from a potential divider, so check R2, R3 and C4.
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11th May 2017, 7:13 pm | #264 |
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Re: Restore a Beau Decca
Thank you Station X.
I checked the resistances with power off, then powered on and checked voltages. Here are the readings: Valve 1 R2 spec value 36K, measured value 58K, voltage across 103V R3 spec value 36K, measured value 59K, voltage across 102V Valve 2 R10 spec value 91K, measured value 108K, voltage across 114V I also measured R22 R3 spec value 5K, measured value 8.8K, voltage across 92V Not sure if surrounding circuit is affecting R2 and R3. Should V across R22 be so high if resistor is only 8.8K (or should be 5K)? I guess a next step could be to put C4 and C16 back to original condition unless there are better suggestions?? |
11th May 2017, 7:40 pm | #265 |
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Re: Restore a Beau Decca
R1 and R2 are more than 20% off value and if I were in your position I'd change them. I'm not saying it will cure the problem, but it will eliminate one possible red herring. 36k is not a preferred value, so use 33k or 39k.
One proviso though. It seems strange that the two resistors have approximately equal values, so I'm wondering if there's been a design change. How are the resistors colour coded? Orange-Blue-Orange denotes 36k R10 is just within 20% of the specified value, but if you're having to buy resistors why not change it to 100k. You speak of R22 and the R3. I assume you mean R22? That's also off spec and will result in low HT, since more voltage will be dropped across it. I see no merit in changing C4 and C16 again if they're the right values and in the right position in the circuit.
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12th May 2017, 7:59 am | #266 |
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Re: Restore a Beau Decca
I don't think we've had any voltage readings for V1 and V2 cathodes. They'd show whether these valves were passing any current.
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12th May 2017, 11:43 am | #267 |
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Re: Restore a Beau Decca
Thanks Station X
I will measure V1 and V2 cathodes and post result. I can confirm that the resistors are orange-blue orange. However I think the circuit may be affecting the readings- and my multi-meter! I re-measured R3 today with the set fully cold. With the -ve meter probe on the capacitor end of R3, the reading instantaneously reads 34K then drops to 24 after about 1 sec the rises slowly over 30 secs or so up to 39.8 and stays steady at this. With the +ve meter probe on the capacitor end of R3, the reading instantaneously reads 36K then drops to 32 after about 1 sec the rises slowly over 30 secs or so up to 44.8 and stays steady at this. Any reason why the difference?- it is repeatable, I've done it several times. |
12th May 2017, 12:38 pm | #268 |
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Re: Restore a Beau Decca
It'll be the HT electrolytics charging up from your meter, to measure R2 and R3 properly it's best to disconnect one end of each resistor then measure them.
Lawrence. |
12th May 2017, 8:01 pm | #269 |
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Re: Restore a Beau Decca
Thanks Station X and ms660. I have measured the cathode voltages of V1 and V2- they are 4.5V and 3.3V. I've also measured other voltages surrounding the valves on attached photos.
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12th May 2017, 8:51 pm | #270 |
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Re: Restore a Beau Decca
From that, I make the hexode's anode current approx 2.9 mA, at least two and a half times what it should be, that might account for the high cathode voltage, what DC voltage is on the hexode's control grid (the valves top cap connection) and is it negative or positive when measured with respect to chassis?
EDIT: Also what's the voltage measure on the triode's grid (pin 5) it should be negative with respect to chassis, ideally the meter probe should be applied to the triode's grid via something like a 10k resistor, that will help the probes lead capacitance from upsetting or stopping the oscillator which can sometimes happen if you don't use a resistor. Lawrence. Last edited by ms660; 12th May 2017 at 9:03 pm. |
13th May 2017, 12:17 am | #271 |
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Re: Restore a Beau Decca
This time I will make sure I check my logic before saving the posting!
The V1 readings have changed from your last measurements: Ha 227 199 (220) Hg2 94 114 (135) Ta 86 43 (70) k ? 4.5 (2.5) When the triode is oscillating, the AC signal is fed back to the triode grid. The AC signal on the grid is large and during the positive cycle it is rectified by the grid, causing the grid voltage to go -ve. This reduces the anode current and gain of the valve and self-regulates the amplitude of the AC signal minimising distortion which leads to harmonics. During your first set of measurements it looks like the valve was oscillating as the anode voltage was higher than the service sheet indicating a lower anode current. In your second set of measurements it looks like the valve is requiring more gain as the anode voltage is lower so the valve is taking more current (3.12mA). The high cathode voltage is directly attributable to the high ta current. The low Hg2 voltage will be due to the R2/R3 value difference and the increase Hg2 in your second reading is attributable to the heptode bias changing due to the higher cathode voltage. Measure the triode grid volts as already suggested. My guess is you have selected a different waveband and it is not oscillating as the datasheet for the ECH35 suggests that 3.12mA @43V is about what you would expect if the grid were at cathode potential. I suggest you try switching to MW and check the triode grid volts. I would not be concerned about the other voltages at this stage as they would not stop the set from working. Last edited by PJL; 13th May 2017 at 12:23 am. |
13th May 2017, 12:27 am | #272 |
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Re: Restore a Beau Decca
You say the V2 cathode voltage is 3.3V but you have marked it as V3 in the drawing?
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13th May 2017, 10:33 am | #273 |
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Re: Restore a Beau Decca
Extract from Service Sheet.
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14th May 2017, 10:02 am | #274 |
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Re: Restore a Beau Decca
Thanks Station X, ms660 and PJL for the explanation- really helpful and starts to make sense of whats happening- you also pointed out that I marked the readings on V3 instead of V2- they were made on V2 but copied onto the wrong valve.
As suggested I put the radio on MW and then measured the hexode's DC control grid voltage- it is positive 0.37V (0.47V when selectivity set to OFF) Also the triodes grid (pin 5) using a 10K resistor- it is also positive 0.37V (0.52V when selectivity set to OFF) |
16th May 2017, 6:49 pm | #275 |
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Re: Restore a Beau Decca
I've just re-read the earlier posts. Please ignore my last post- it is incorrect. What I have done is:
Set the radio to MW and selectivity to 4. On tuning along the waveband, the radio puts out buzzing and noises associated with tuning (and reacts to the close proximity of the light near the coiled aerial) but no stations can be heard. I have now re-measured all V1 voltages and the results are shown in the attached table. The hexodes control grid is positive (+0.75V) and the triode's grid (measured via 10K resistor) is also positive (4.64V)- so not negative wrt chassis! |
16th May 2017, 7:15 pm | #276 |
Nonode
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Re: Restore a Beau Decca
Hi!
The voltages on the Hexode section of the valve are close enough to those shown on the service sheet, so you now have a straightforward case of local-oscillator failure! It's normal for you to pick up odd buzzes, hisses and squeaks with a duffy local osc, and it suggests the amplifying chain from the Hexode grid to the detector-diode anodes in the 6B8 is basically alive! Begin by checking the small (usually silver-mica) capacitors coupling the anode and grid of the triode section of the ECH35 to the anode feedback reaction-coil and the oscillator tuned circuit - if these test OK or substitution fails to effect a cure, you'll have to plough thro' the wave-change switching, coils and padder-capacitors, etc., against the service-sheet I'm afraid! My experience is that coils in very old receivers were really well made and it's unlikely you have a fault in one that would totally stop oscillation on all bands - only the band the faulty coil is used on would be affected! Treat the set as a MW-only one first, ignoring everything else for the other bands, and try to get MW alive first! Chris Williams
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16th May 2017, 8:19 pm | #277 |
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Re: Restore a Beau Decca
PS!
It's virtually standard practice to parallel-feed local-oscillator anode feedback reaction coils, series-feeding seems to be limited to text-book circuitry and the small all-dry valve battery type of set - the reason is that the 'Q' of the oscillator coils is reduced by passing the d.c. anode-current thro' them which is liable to make the set sluggish or low-sensitivity at one end of the band! If you trace back from pin 6 of the ECH35, you should have a resistor of 22k (red-red-orange), or 33k (orange-orange-orange), or 39k (orange-white-orange), or 47k (yellow-violet-orange) - this is the oscillator anode-feed resistor leading up to the tuner h.t.+ line, then leading off pin six will be a small capacitor - typically flat mica - and another similar type of capacitor leading off from pin 5 - both of these will disappear towards the wave-change switch wafers and the general undergrowth - these are the two capacitors I'd like you to check! You might be able to get American made mica capacitors off ebay - these are quite good if the import-charges aren't too ruinous, alternatively put a request in the "Wanted" section -I'm sure other Members'll be able to assist with these! I haven't seen the undersides of a Decca Beau tuner but looking at the size of it from the pictures you've posted the oscillator circuits look like it's going to be fairly dense undergrowth round the coils & switches! Could you post us the section of sheet round the ECH35 Triode bit please? Chris Williams
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16th May 2017, 8:30 pm | #278 |
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Re: Restore a Beau Decca
PPS!
There's just a slight possibility the triode-section of the ECH35 might be defective and running into a lot of grid-current which would stall the oscillator, but it's not all that common an occurrence, and from the readings you've quoted I wouldn't immediately suspect low-emission of the triode section either! Do you have another set using this valve or one with a 6K8 or X61 etc, you can try the ECH35 valve from the Beau Decca in? Chris Williams (sorry for several posts - I keep thinking of something else to suggest as I write one post!)
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16th May 2017, 8:58 pm | #279 |
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Re: Restore a Beau Decca
Chris has it covered and the only things I would add are:
1. Make sure the waveband switch is properly cleaned 2. Check your wiring of the replacement caps around the oscillator |
18th May 2017, 4:02 pm | #280 |
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Re: Restore a Beau Decca
Thanks Chris55000 and PJL. Excellent stuff.
I've cleaned the waveband switch selector and checked the new wiring around the oscillator. Unfortunately I don't have another unit with a ECH35 or similar. I've traced the wiring as you suggested. From Pin 6 there is a green brown orange (51K?) resistor -R8 that the spec says should be 50K. From pin 6 there is also a small silver micra capacitor. From Pin 5 there is a 120 ohm resistor and connected to this is a 500pF micra capacitor. I assume these are the ones you refer to. I've de-soldered them both, but not too sure what checks I can carry out as I only have a standard multi-meter and I don't have any replacement capacitors for values this small. I have checked for open circuit. Both show infinite resistance on the 20M range of the meter. Are there any other checks I can carry out with the equipment I have? I've attached diagrams for clarification. |