Hi Marion,
255 volts at the CRT A1 does prove the the line timebase is working to a fashion but according to the voltages table on page 394 of the Newnes Radio & Television servicing book the voltage at pin 2 of the CRT should be 420. This set has a finned metal HT rectifier W4. HT volts at the anode of the boost diode V13 (U153/PY81) should be 220. If it is much less than this figure then the HT rectifier is faulty. A silicon diode can replace it.
C51 (0.015mfd) should be replaced, it 'is almost certainly leaky, refered to as "that capacitor " in audio amplifiers.
The attachment is of the timebases of the HMV1824.
Till Eulenspiegel.
I'll take another look in tomorrow, but for now, I am at least satisfied that there are still signs of life in this set.
I'll need to backtrack my earlier boo-boo, and disconnect just the EHT smoother, without disconnecting the heater winding.. The EHT smoother is a waxy! So I would doubt its viability in any case.
It's going to be slow, because I'm only working on reduced power anyway but all other things aside, I think i'll just go one small step at a time.
Marion
... If it is much less than this figure then the HT rectifier is faulty. A silicon diode can replace it ...
... but only if you add a series resistor to compensate for the low loss of a silicon rectifier, otherwise you are going to have rather a high HT rail ...!
When all else fails, read the instructions
That's right, if it is necessary to replace the old 14A96 metal rectifier with a silicon diode, a series resistor of say 21 ohms 10 watts should be added.
Till Eulenspiegel.
I'm not happy with that resistor value but, despite an intensive web search, I can't find any data on the original selenium rectifiers to help formulate a definitive answer to the problem.
If we take the HT current to be an arbitrary 250mA, a 21Ω resistor will drop a mere 5.25V (and dissipate about 1.3W) which doesn't sound enough.
A 50Ω resistor, on the other hand, will drop 12.5V (and dissipate a little over 3W) which might be closer.
Does anybody have any accurate data?
In the days when the silicon rectifier (BY100) first became available I saw many of them soldered directly across the original selenium rectifier with consequent high HT and over running ...
I'm pleased to say that I worked in an environment where bodging in any form was banned! Accordingly, we continued to replace the relatively short lived Westinghouse originals with superior Automat equivalents - I never recall having to replace any of the hundreds of these that were fitted in a wide range of sets.
Using my marvellous powers of hindsight, I regret never taking the opportunity to investigate the possibility of correctly using a suitable silicon rectifier/ resistor combination as a replacement ...
Is it possible that someone might have a NOS selenium rectifier (of any type) in their stock? In which case, is there possibly a data sheet in the box ...?
When all else fails, read the instructions
Hi Terry,
It's most likely that the metal rectifier is OK. There is a similar one in my Pye FV1 and that's got to be over sixty years old. In fact one of the only parts in the HMV 1807 that did not not give any trouble was the metal rectifier.
That 21 ohm series resistor does seem at first sight a bit low but you have to consider the ripple current of the reservoir capacitor. Bear in mind that only one half of the mains waveform is used so the capacitor has to charge the same time the set is drawing it's 250mA HT current.
If one was look at the values of a typical TV mains dropper resistor the HT section always has much lower values compared with the heater section. For example, you'll see 66 ohms sections for the heaters and the HT sections as low as 20 ohms.
I hope these notes clarify the situation.
The attachment shows a typical mains dropper from a TV set.
Till Eulenspiegel.
Okay..
Here's my plan for today.. I'm having an earlier start (and probably, an earlier finish) today.. I'm going to undo my boo-boo from yesterday, and then look again for rectified EHT from the U151.. If all else fails, I have some spare EY51's (somewhere).. Whilst I'm at it, I will have a look at the HT, and see what I'm actually getting off the selenium rectifier.
I was wondering... When I said that the line whistle was almost unltrasonic, is it possible that by disconnecting the EHT rectifier's heater winding, I've inadvertantly altered the loading on the LOPT, and thus the line frequency?
Marion
Hi Marion,
Overloading the line output stage will not alter the timebase frequency although an overloaded line stage does seem to have a different pitch to the whistle somehow. Old hands used to say it sounded "strained"
The frequency of the oscillator should be checked if possible. Generally when the frequency goes low the EHT tends to rise. It might be worthwhile checking the values of resistors R53 and R54 in the line oscillator, both are 330Kohms.
I've included a 0.015microfarad capacitor with the bits I've just sent off an hour ago. The capacitor will replace C51. It's the coupling capacitor between the oscillator and the grid of the N152/PL81.
As mentioned before expect an HT line voltage of 210-220. Pin 9 of the boost diode is a good test point.
Till Eulenspiegel.
Hi David,
"Strained" woul be one way to describe it Adjusting the line hold pot brought the frequency back to an 'audibly correct' pitch, but seemed weak (lacking in loudness).. I don't think it would be a case of overloading though.. I'd inadvertantly disconnected the heater winding, which would have unloaded the LOPT, wouldn't it?.
Many thanks for the extras you've added to the parcel - They will be in invaluable as I have very little in the way of spare components.
I'm just about to make a start, so will be back soon with an update.....
Marion
Situation update - I hope I'm wrong, but it doesn't look good...
I've reversed my previous boo-boo, and now have about 7kV of rectified EHT, somewhat below the specified 11kV (EHT rectifier heater is dim).
On the HT side of things, I've tested the voltages at the rectifier (which has eveidently had attention in the past).. 245VAC entering the rectifier - about 155VDC exiting to the smoothing cap (measured at the rectifier).. There are obvious signs of previous work being carried out, not least of which is the poor soldering at the rectifier's 'solder pads', and there has been at least one resistor that has been replaced in a poor repair.
What is worrying me though, is this mark on the CRT nipple.
It looks like a hairline crack in the glass, and is quite visible on close inspection.. In the dark, it doesn't show, but with daylight all around, it is impossible to miss.
If this is a crack, then I'm stumped as I have absolutely no spare CRTs at all, and not much chance of finding an Emiscope 4/14, though David's reference to a Mullard MW36/24 would hold out a feint glimmer of hope - if I only could find one.
I'm assuming that there is no way of knowing if the CRT has gone to air? I can't see any gettering in the neck of the tube, so can't use that as a guide.. Any ideas, anyone?
Marion
Measure Cathode current?
If it hasn't gone to air and you get a faint glimmer eventually you could clean the nipple with soapy water, then alcohol and put regular (not fast setting) epoxy over it.
Hi Marion,
The EY51/U151 heater imposes only a light load on the line output transformer <0.6watt.
A simple test to dertermine if the boost capacitor is SC is to disconnect the top cap of the U153/PY81. If EHT is apparant at the anode of the EY51 then the capacitor is SC or leaky. No EHT, then the capacitor is OK.
Nevertheless, the capacitor should be replaced as soon as possible, it will fail eventually.
Out of interest, examination of the circuit diagram will reveal that the boost diode and capacitor are effectively connected in series across the scanning coils. The valve should really be called by it's correct title, the damper diode. Energy from the scan coils at flyback is stored in the capacitor.
Till Eulenspiegel.
Hi Marion,
A simple test to dertermine if the boost capacitor is SC is to disconnect the top cap of the U153/PY81. If EHT is apparant at the anode of the EY51 then the capacitor is SC or leaky. No EHT, then the capacitor is OK.Till Eulenspiegel.
I'll try that now, just before starting on my teatime meal
Marion
Another check would be to check the heater voltage when the set is on. If the tube is to air the voltage will be low due to the air convection cooling effect on the heater.
If the CRT heater glows then at least there is some sort of vacuum in the tube.
Even the once common MW36/24 CRT is now quite rare. I'd reckon there is more round tubes about than this one. Any 14" 70 degree magnetic focus tube will work in this set. Cossor 141K, Mazda CME143 or CME144. The pentode gun Mullard/Philips MW36/44 is another.
Some later EMI sets used an electrostatic and focus tube, the SE14/70 which is similar to the AW36/20/1
Till Eulenspiegel.
OK.. Test done.
With the U153 top connection removed, I can draw a very feeble spark at the U151's anode (using ether, my EHT meter, or an insulated screwdriver) - Leaky boost cap then? At this test, line whistle was almost extinct.. Also, beneath the chassis there is eveidence of an overheated R56, though I should point out that this appears to be an old 'injury'.. The resistor is not giving off any smoke or smell of burning.
The CRT heater glows strongly, and appears normal.. There is no evidence of anything untoward going on inside the tube, i.e. no arcing or flashover.
Marion
That boost cap has to be changed. R56(4.7Kohms) is the PL81 screen feed resistor and does run hot. Can go low in value.
Till Eulenspiegel.
That boost cap has to be changed. R56(4.7Kohms) is the PL81 screen feed resistor and does run hot. Can go low in value.
Till Eulenspiegel.
"Roger that" - As the guys at NASA would say.
I'll have a look at what I can put my hands on, not forgetting that still I need to check those other two resistors as well (R53 and R54).
Marion
Just look at what the Postman delivered today.....
I'll be beavering away again at this project, shortly. Many thanks, David, for your generous donation to the project On the HT front, I have some 1N4006 diodes that I could use in lieu of the selenium rectifier.
The circuit diagram shows the rectifier like this..
Is there any reason why it is effectively two diodes in parallel? Or any reason why it should not be replaced with a single diode?
Marion
The 1N4006 has a currant rating that it about four or five times the rating of the old rectifier so it can be fitted as a single with a series resistor of 20 to 50 ohms.
