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 Post subject: Repairing EM4?
PostPosted: Jun Mon 10, 2019 2:07 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
I've got two EM4 tubes. One is totally dead, but the other one WAS working fine when I started my restoration but annoyingly something has happened (no idea what but I think physical whilst out of the radio rather than electrical).

It now only works on one side. The other side doesn't move (it's full on green all the time). If I gently shake it I can hear something moving inside.

Is it at all practical to open it up and try to fix it or should I be 'happy' it half works and get another at some point?

It's a red Philips Miniwatt EM4.

Thanks


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Mon 10, 2019 3:33 pm 
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Joined: Aug Thu 20, 2015 3:09 pm
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Location: Albion, CA, USA, 95410
Nope. If you open the tube you will let out the magic smoke.

Actually, of course, you will let air in to the tube. As it is a vacuum tube air is the death of it. Even the tube manufacturer didn't repair tubes, with the exception of CRTs. Start looking for a replacement.


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Mon 10, 2019 3:35 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
madlabs wrote:
Nope. If you open the tube you will let out the magic smoke.

Actually, of course, you will let air in to the tube. As it is a vacuum tube air is the death of it. Even the tube manufacturer didn't repair tubes, with the exception of CRTs. Start looking for a replacement.


That's what I presumed. Thanks for confirming.


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Mon 10, 2019 4:46 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5374
Location: Sunbury, Ohio 43074
Unless you dropped it, if it WAS working, I'd be more apt to suspect some electrical problem with your radio or whatever it is being used in. Perhaps some voltage measurements are in order, unless a defect has fried one side of the tube. It would be an odd defect that could accomplish such a thing ;-)

Do you have a tube tester? You can check eye tubes with most tube testers. They should glow when tested.

Edit: I would not be too concerned about slight rattling when shaken, unless you are shaking it up with a paint mixer. While it *could* indicate something came loose in there, it's more likely not the case. But if you do keep shaking it, eventually prophecy will come to pass..................

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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 10:46 am 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
Yes, I think you are right Barry.

But I've replaced all three components on the holder (the two resistors first, then tested, then the capacitor and tested again) and it's still not working.

When I turn it on though, it does start open and then closes (or, at least, the faulty side closes - the working side goes to whatever level the signal strength indicates), so there is movement on both sides.

I could do with some help learning how to test this please.

I've measured the voltage on a1/d1 and a2/d2 using a multimeter with negative to pin 6 and positive to pins 5 (a1/d1) and 8 (a2/d2).

But the magic eye stays half-open on both sides when I do this.

I'm getting readings in both ac and dc. Can you tell yet I haven't got a clue what I'm doing? ;)

Ac readings

D1/a1 low of 16.2 untuned station to a high of 17.4 strong signal
d2/A2 low 10.4 to high 12

Dc

D1/a1 low 26.2 to high 27.4
D2/a2 low 17.3 to high 18.5

From reading about it here (http://www.r-type.org/exhib/aai0133.htm) I was expecting 0 to 5 and 0 to 16v.

Obviously I am doing something wrong.

The key things I would say (for what that is worth) is:

1) the tube starts fully open on both sides and then closes when first turned on
2) tube tube stays half open on both sides when measured

so those two facts suggest the tube is working

3) I've replaced all three components in the holder (2 resistors and capacitor) and all are correct

4) I don't know how to measure it properly.

Can I deduce that it seems likely the problem is before this stage?

Thanks for your help. I'm learning every day with this!


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 11:48 am 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5374
Location: Sunbury, Ohio 43074
I would say based on what you have observed, that the tube itself is OK. If by "holder" you mean the thing the tube plugs into? That is called the tube socket.

Your observations indicate that the problem lies somewhere else in the radio. I would STOP replacing components until you know a lot more about what you are doing. Time to do some reading (book learning, in other words) about the basics of how radios work. There is plenty of material out there on this.

It might be even better to start with a book on basic tube theory .. you will learn a lot. Any time you come across a term that you are unfamiliar with, stop and go look that one up. It may seem tedious at first, but the knowledge will come. There are no shortcuts. Try those, and you'll end up with a radio fit for the trash bin:)

Good luck

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Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 11:59 am 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
Thanks Barry but that really doesn't help. This is the last piece of the restoration and I'm trying to (and have already) learn.

Maybe someone else is more willing to share knowledge?

Having looked at a discussion about a SS magic eye replacement schematic by CR Boggis using an EM34, I see he is taking pins 2 and 7 which on that tube are the filaments. He's then putting it through a rectifier.

I've just done the same on the EM4 using pins 2 and 3, and voltage is coming in around 5.3 vdc after the diode drop.

But it isn't changing when I tune into a station, so I'm presuming the EM4 works differently?

Any actual help, rather than being told to wait 20 years, greatly appreciated. Thanks.


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 12:01 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
Barry H Bennett wrote:
I would say based on what you have observed, that the tube itself is OK. If by "holder" you mean the thing the tube plugs into? That is called the tube socket.

Your observations indicate that the problem lies somewhere else in the radio. I would STOP replacing components until you know a lot more about what you are doing. Time to do some reading (book learning, in other words) about the basics of how radios work. There is plenty of material out there on this.

It might be even better to start with a book on basic tube theory .. you will learn a lot. Any time you come across a term that you are unfamiliar with, stop and go look that one up. It may seem tedious at first, but the knowledge will come. There are no shortcuts. Try those, and you'll end up with a radio fit for the trash bin:)

Good luck


ps yes Barry, I know what a socket is, but on this radio the socket is on a metal bracket that slots in to the front of the radio, that's what I meant by holder. There's really no need to be so patronising.


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 12:13 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5374
Location: Sunbury, Ohio 43074
ummmmmm .... I wasn't being patronizing... I was responding at the technical level at which I believed you to be, in an effort to help. Perhaps if you are asking for help you shouldn't allow your feelings to be hurt so easily. Seemed like from your words, you were replacing parts on the "holder" so I made my comments accordingly in an effort to educate.

Good luck, and I. truly mean that. I am unsubscribing this topic now, since you don't want help apparently.

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Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 12:45 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
Very disappointing Barry. First time I've had anyone respond like that on this forum. It's always been fantastically helpful.

Hopefully someone else could offer some advice?

Thanks


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 1:11 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8923
Location: Long Island
The EM4 is a dual sensitivity indicator. The first side opens when the grid is from 0 to -5 volts and the other side opens when the grid is between 0 and -16 volts, assuming the 1-megohm plate load resistors and 250-volt plate/target voltages are right. One side responds to large voltage swings while the other responds to small voltage swings so you can fine-tune strong or weak signals.

It’s pretty hard to imagine anything that could happen to one of these tubes that would “break” one side but not affect the other. Not impossible, but hard to imagine. On the other hand it is very easy to envision all sorts of things that could happen in a restoration effort that would affect operation of these circuits.

What you need to do is measure the grid voltage on the EM4 with a vacuum tube voltmeter (a FET VOM or DMM with a 10-megohm input could be used instead) as well as the voltages on the anodes. Unless you are getting proper anode voltages and a reasonable grid voltage swing, the problem is likely to be elsewhere.

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 Post subject: Re: Repairing EM4?
PostPosted: Jun Tue 11, 2019 5:13 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
Thanks Chris.

This is the schematic of the radio - https://www.dropbox.com/s/eh86ygl2qhvtr ... m.pdf?dl=0

Measurements with my DMM are (with negative to pin 6)

b+ in before either resistor is 233 vdc

b+ goes to pin 9 where it is connected to R36 and R40.

R36 (1M) goes to pin 5 (a1/d1), R40 (2M) goes to pin 8 (a2/d2).

other measurements are as above.

Does that help?

Thanks for your time and expertise.


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Fri 14, 2019 7:00 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8923
Location: Long Island
Use pin 4 (cathode) of the tube as the negative terminal for your measurements. It connects to chassis ground.

Your readings do show that the voltages on the anode/deflector electrodes change with tuning the radio. That is good, as it shows the tube is working on a basic level. However, your voltage readings of 26-27 volts DC on A1/D1 and 17-18 volts DC on A2/D2 do not appear correct. They may have resulted because you were measuring to pin 6 (grid) of the tube. Measuring to the cathode, you should be getting approximately +70 volts DC to one anode/deflector and +35 volts DC to the other, going by the schematic. Note that AC voltage measurements are not needed. The tube is essentially responding to the AVC or AGC voltages which are DC. The target voltage is on pin 7. This should be about +225 VDC.

The main voltage of interest is pin 6,which is the control grid of the triode. It should vary between zero and negative (-) 16 volts with respect to the cathode as the radio is tuned. Note that the AVC diode voltage is used when the set is on AM and the AGC voltage is used on FM, so you'll get different results between the two bands. The grid voltages are taken through some very high value resistors so you have to use a VTVM, FET-VOM, or 10-megohm input DMM to read this. An ordinary DMM or VOM will probably give false readings here.

When the grid voltage is about zero with respect to the cathode, the triode section is conducting fully, and both anodes will draw maximum current through their respective load resistors. This makes the corresponding deflector voltages significantly lower than the target, so the deflectors repel electrons and create 90-degree shadows on both sides. When the grid voltage is around negative (-) 4 to (-) 5 volts, the high sensitivity section of the triode should be cut off. Its anode/deflector voltage should then rise to almost the same as the target. With no voltage difference it no longer repels electrons, and the shadow closes on one side. As the grid voltage is made further negative, (-12 to -16 volts), the low sensitivity section then cuts off. Its anode/deflector voltage rises to near the same as the target, and the shadow on the other side should close.

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"Hell, there are no rules here--we're trying to accomplish something!"

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 Post subject: Re: Repairing EM4?
PostPosted: Jun Fri 14, 2019 10:35 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
:( I posted a detailed reply but I don't know if I pressed Preview by mistake or something else has gone wrong but it's not here. So... second time around!

Thanks for the detailed and informative reply Chris, it's really appreciated.

I'm afraid I do have a cheapo DMM so...

Testing with negative on pin 4...

Pin 7 gives me 233.

a1/d1 is mainly 86 to 114v BUT... in the middle of the band (FM) it drops quickly to a low of about 21. This is about 3cms in the middle. Once it hits 40, the magic eye works on both sides. It opens more (or does it close?!) as it goes down to 21, and then goes up again, closing at 40, and then back up to 86 to 114v and no movement. On the side that doesn't normally open, it opens to a max of about 25 degrees in total (either 12.5 either side of the middle if that makes sense).

I did this with the radio having been off for a day. I've never noticed it before.

I turned it off and after a minute or so turned it back. This time it seemed to only drop for about 1cm in the middle. Having thought about it, I thought it could be the capacitor being charged so I turned the radio off leaving the DMM on to drain it. Then turned it back on and it was back to about 3cm.

But the next time I turned it off without draining the capacitor, when i turned it on it was still 3cm. So no idea what that is about?

Voltage on a2/d2 is a fixed around 121 to 123 all across the range.

As i say, cheapo DMM so not reliable but voltage on pin 6 was -1 to -6 only. And it didn't change at all in that middle band when the a1/d1 voltage dropped.

So, given that I have this middle part of the band where the voltage drops and the magic eye works on both sides, and given the voltages are otherwise out of whack, it seems fair to presume the eye is fine, and something else is wrong. Any idea what could be causing this?

Does this get us any closer?

Thanks again
John


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Sat 15, 2019 2:34 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8923
Location: Long Island
According to the Philips EM-4 data sheet (available online) the two sides of the tube are not designed to open the same. The low sensitivity side will not open as far as the high sens side. Presumably this is a consequence of deflector placement or some other factor in the construction of the tube. They also mention the shadows do not fully close if higher voltages (250-V or more) are applied to the target.

As for what’s going on with the amount of opening, the grid of the tube is responding to changes in the AVC or AGC voltages that the set develops. If those grid voltages are not going negative enough the tube will not operate over its full range. That’s why it would be nice to be able to measure the grid voltage with the right kind of meter. If the grid swing is proper, the EM-4 could be weak. Not enough grid swing would mean issues elsewhere in the set. There could be a number of things to check in that case: weak IF tubes, misalignment, incorrect plate or screen grid voltages, and other things.

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 Post subject: Re: Repairing EM4?
PostPosted: Jun Sat 15, 2019 7:52 am 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
Chris108 wrote:
According to the Philips EM-4 data sheet (available online) the two sides of the tube are not designed to open the same. The low sensitivity side will not open as far as the high sens side. Presumably this is a consequence of deflector placement or some other factor in the construction of the tube. They also mention the shadows do not fully close if higher voltages (250-V or more) are applied to the target.

As for what’s going on with the amount of opening, the grid of the tube is responding to changes in the AVC or AGC voltages that the set develops. If those grid voltages are not going negative enough the tube will not operate over its full range. That’s why it would be nice to be able to measure the grid voltage with the right kind of meter. If the grid swing is proper, the EM-4 could be weak. Not enough grid swing would mean issues elsewhere in the set. There could be a number of things to check in that case: weak IF tubes, misalignment, incorrect plate or screen grid voltages, and other things.


Thanks Chris. I think it's fair to say you've given more than enough help - for which I am very grateful - and we've reached the point we will struggle to progress without using a decent DMM. I'll see what I can do about that. In the meantime I will do another pass over the rest of the circuit to check voltages. I have replaced a number of capacitors (and the whole power stage), but tested everything bit-by-bit. Of course, some capacitors have changed value (5uf to 4.7uf etc etc), so I need to make sure none of those are the cause of the problem.

On this set (compared with the first one I was working on), I've not opened up the FM tuner section as there was nothing that needed replacing in the first one but I will do that to check.

Finally - I'm putting a raspberry pi inside the radio for other reasons that I will document when done, but I've got a 240x240 full colour lcd that fits perfectly where the tube goes. Last night before I went to bed I got a jpg image of a magic eye (grabbed from the net to test).

I'll very probably switch to using this rather than the original tube as it will allow me to be more flexible with what I display and won't need replacing when the tube eventually dies as they all do. I'm looking to get a watch glass (either single or doubled domed) to go over it - or try to cut the top off my other totally dead EM4 if I can...

I've attached a couple of pics (excuse the dirt under my thumbnail :shock: )


Attachments:
Webp.net-resizeimage (4).jpg
Webp.net-resizeimage (4).jpg [ 185.01 KiB | Viewed 721 times ]
Webp.net-resizeimage (5).jpg
Webp.net-resizeimage (5).jpg [ 183.2 KiB | Viewed 721 times ]
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 Post subject: Re: Repairing EM4?
PostPosted: Jun Sat 15, 2019 1:25 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8923
Location: Long Island
That's a good use for a Raspberry Pi! :D Turning one into a tuning indicator "tube" is a really cute idea! Normally when I work on antique radios, I am trying to keep or make them as original as possible, but it has to be admitted that many kinds of tuning indicator tubes are becoming too scarce and too expensive to use on a regular basis any more. Modern technology to the rescue!

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 Post subject: Re: Repairing EM4?
PostPosted: Jun Sat 15, 2019 2:40 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
Chris108 wrote:
That's a good use for a Raspberry Pi! :D Turning one into a tuning indicator "tube" is a really cute idea! Normally when I work on antique radios, I am trying to keep or make them as original as possible, but it has to be admitted that many kinds of tuning indicator tubes are becoming too scarce and too expensive to use on a regular basis any more. Modern technology to the rescue!


Thanks. All good fun. I've seen a few people over the years have used leds to make SS tubes and they are really neat, but a screen gives me more options. In fact, one of the things I'm going to do is carefully video my own EM4 and use that as one of the display options (obviously, given the fact it only half works, I'll have to post-process it to mirror the working side!)

I'm keeping the core of the radio as original as possible with the exception of the obvious such as modern capacitors instead of paper caps etc. But I'm also planning to add a few extras that won't impact on the original nature but will modernise and improve it for daily use. The eye replacement is by far the most obvious change - although when set to display a magic eye I'm hoping it would take close inspection to notice - the other additions are less intrusive. Again, I'l post about them when (if) I get them finished!


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 Post subject: Re: Repairing EM4?
PostPosted: Jun Wed 19, 2019 10:35 pm 
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Joined: Mar Thu 28, 2019 9:44 pm
Posts: 89
It seems I've managed to get it working.

I opened the FM Tuner stage and the resistors were out (the 2k was nearly 4k) and a 5n cap looked worse for ware as did a 500pf. So I replaced those caps and all resistors (except one which I may look at tomorrow actually)

Just before calling it a night I reassembled it and did a quick test... success 1 - radio still works, success 2 - the magic eye is now opening and closing on both sides.

It's still reading odd measurements so not ideal but both sides are working.

There's still something wrong overall though that I need to trace. I'm getting a sharp popping sound as I tune through the stations. It seems loud too, as if there is too much power but main b+ is right, heater elements are right.

Apart from the EM4 voltages being odd, the other one that is odd is one of the anodes of the ECC85. One reads fine at about 220 give or take (230?) but the other side is measuring much higher, so much so my multimeter can't read it. I did read that it could be interference causing faulty readings, so I soldered a fairly long piece of wire and took it out of the tuner stage, put the casing back on, and tested it there. It was still jumping around a lot, often going to 300, 400, 600, 700 even. Only briefly though. This is what led me to replace parts tonight. I haven't checked it the same way with a lead outside the casing yet, but testing it directly was too much for the multimeter so I suspect it's still dodgy. I'll trace back tomorrow but if anyone has an ideas that'd be great.

Two other things. This may be obvious, but I only realised it last minute and have saved myself a lot of work. To get in to the FM tuner properly, I had to disconnect the dial wheel from the shaft. This was gong to cause the string to come off and... been there before. Nightmare.

So I carefully unscrewed it, removed it from the shaft while holding the wheel, the put a bolt in to the center of the wheel and screwed that in. Then rested that where the shaft would be and it held it together perfectly while I did the work and then put it back together! Phew!

And finally, I managed to convince my wife to let me take apart an old pendent she doesn't wear Lovely thick dome glass. Fits perfectly over the oled display. Really makes the screen look realistic, especially as it slightly enlarges it all.


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