Philips A5X93A tuner

[Antracite]

Hello to all,

Its my first thread here in this forum

I recently bought a pair or vintage philips valve equipment, an AG9016 amplifier plus a A5x93a stereo tuner.

Fortunately both were in near mint condition, and after a recap and the replacement of two ECC83 valves the amplifier was up and running.

The radio didn't switched on, and after a quick check I noticed that a small clip was missing in the transformer. Bingo! It turned on, and it was working... some minutes later the EZ80 valve rectifier and the EM84 magic eye started loosing power and died.

So, I guess this isn't very good news. I tested all the valves, and all are ok (minus the ones I mentioned before). And tested the big 3 x 50uf cap - it reads +- 43uf.

Any of you had a similar experience, what should I check first? I have the service manual.

Thanks in advance

[SteveCG]

Welcome to the forum Antracite.

I don't know this particular tuner but ...

What does the 'Small Clip' do? Is it part of a mains voltage selector for the tuner? If so, have you got it set on the correct mains voltage for where you live? (I imagine 220 Volts)?

[Antracite]

Hi Steve,

Thank you for your quick response. Not my picture but I marked the spot where the two copper parts were loose, the top part is connected to the on/off switch.

The main voltage is set for 240v

By the way, I re-tested the EM84 valve and it is OK... but the EZ80 its dead.

[SteveCG]

I tried the picture link but just got a black screen.

[Antracite]

Originally Posted by SteveCG

I tried the picture link but just got a black screen.

Solved

[Andrewausfa]

That 'small clip' is an over temperature cut-out to protect the transformer windings. Quite common on Philips transformers. They are soldered with low melting point solder, not regular stuff.

If it's come apart it may suggest something is very wrong somewhere.

Andrew

[Antracite]

Yes, I can find the same clip in the AG9016 amplifier transformer.

There is a bump wooden cabinet, so I thought that a fall could have broke loose the clip.

But it looks something wrong with the transformer and that explains why the valve died. Any ideas?

[Herald1360]

Perhaps more likely that a problem which was overloading the EZ80 initially caused the transformer to overheat and the trip to operate. After replacing the trip (using ordinary solder?) the same fault then caused the EZ80 to fail rather than the trip to operate.

First thing I would try is to run the unit with EZ80 removed. Check all other valves' heaters are on and check for correct voltages from transformer outputs. If that's OK, leave it on for a while and monitor the transformer temperature to see whether any internal transformer fault is hiding.

Have a careful look at R1 and R2 for signs of overheating. If they're OK I'd be suspicious of the first section of C1.

Check the dead EZ80 to see if it has a heater cathode short.

For info to any other curious onlookers, the manual is available from Elektrotanya- wait for Get Manual to appear in the paragraph below the preview picture.

[Andrewausfa]

If you remove the rectifier is the transformer getting hot? Have you checked the voltages on the secondary?

Then on the 3 x 50uF can, have you checked each capacitor for short circuit?

Nice little sets, I've never seen one before, valves and transistors.

Andrew

PS Oops! crossed with Chris.

[Antracite]

Andrew and Chris,

thanks for the ideias!

Some quick questions.

In the service manual, where can I find the output voltages from the transformer?

I should find a short in the capacitors with an ohmmeter right?

After the EZ80 died I tested the set once more and I can confirm that all the other valve's heaters were on.

Thanks in advance. I will do further testing tomorrow. (I have a huge Akai X355 reel to reel lying in the workbench right now hehe)

[Herald1360]

Haven't got the manual handy now, but ISTR the TX has only the one heater winding which should deliver 6.3Vac within about 5% after allowing for any mismatch between input tap setting and actual a.c. supply voltage.

The centre tapped secondary will be about 250-0-250Vac but more importantly both halves should be very similar voltages. Any significant difference is bad news. If voltages look OK and it doesn't get hot after half an hour or so it's probably OK.

Low voltage ohmmeter checks of the caps may not reveal much but if the TX is OK try pulling all the signal valves and fire up with a temporary 10K 10W resistor between EZ80 cathode and the first cap. If the cap(s) are leaky there will be a noticeable voltage across the 10K and it may even get hot. If the caps are OK they should get close to the peak value of half the HT secondary voltage on them.

[Edward Huggins]

What a lovely piece of kit. It's a long time since I've seen any Philips valve Hi Fi gear. Edward

[SteveCG]

I agree with Andrew and Chris that the first test I would do is to run the tuner with the EZ80 removed and see if the mains transformer gets hot after a few minutes. If in doubt switch off !

[Antracite]

Done.

I turned the tuner on without the EZ80 for a couple of minutes. All the heaters went on and the transformer was really cool.

Time to check the big caps and voltages?

Thanks!

[julie_m]

If you have not done so already, make yourself a lamp limiter, a.k.a. the Poor Person's Variac. This is simply a light fitting with an old-fashioned tungsten filament lamp, in series with the appliance under test. If the set is trying to draw too much current, the lamp will limit the current -- and at the same time, warn you by lighting up brightly. An experienced restorer can read the changes in brightness and get a good idea what might be wrong with a set. A "good" set should show very dim as the valves warm up, getting brighter as the reservoir and smoothing capacitors charge, and then settling down to a bit brighter than at first but still dim once the capacitors are charged. If it stays bright, that indicates faulty smoothing capacitors.

If you can't get hold of another EZ80, for a temporary fix, try joining together two 1N4007 diodes, cathode to cathode; and adding a 220 Ω, 2 W resistor in series with each anode. Poke the two resistor ends into pins 1 and 7 of the EZ80 valveholder, and the cathode into pin 3. (Valve pins are numbered exactly like IC pins; increasing anticlockwise from the gap when viewed from above, or increasing clockwise from the gap when viewed from below.)

To avoid potentially expensive damage, do not power the set up with any kind of rectifier fitted, but without a lamp limiter, until you are sure all is well.

[Antracite]

I've been thinking about building one of those for some time. And now it looks like the perfect time! [ I'm a lucky guy considering the old stuff that I plugged in without checking ]

Although I often find cheap tubes in the fleamarket, the diode option looks very nice. I will look in to it after I have the limiter done.

Thanks!

[julie_m]

I can thoroughly recommend building a lamp limiter. You probably will use it on every radio and record player you restore. It is especially good at showing up faults due to something taking excessive current. It can also show you whether or not capacitors are reforming; if so, the "bright" phase will get shorter each time the set is turned on. (That's an idea for something to do when bored: Make videos of lamp limiter with a good set, and with various deliberately-induced faults. I'm sure there was a record given away with a magazine once, with clips of what various faults sounded like; probably as an accompaniment to an article or series of articles on fault-finding radios.)

Another simple piece of test equipment that is worth building is a signal injector: an astable multivibrator built using just about any two identical transistors you have in your parts drawer, running at about 1 kHz; the probe tip coupled via a 10 nF / 400 V capacitor to one of the collectors, and a pair of leads terminating in crocodile clips to "steal" some power for it, either from the battery of a transistor set or from across the cathode resistor of an audio output valve. The square wave output is so rich in harmonics that it can be thought of as a broadband signal modulated with audio; it should break through any working IF stage and produce a tone from the speaker. Just work backwards through the valves / transistors until you find a grid / base that doesn't make a noise, and sort of spiral outwards from there until you find the faulty part

[Antracite]

I have one similar to this one laying around, never tested it.

Its a signal generator from the 60's - it was a kit sold by a electronic correspondence school here in Portugal.

Would this work as the signal injector you described?

[Antracite]

Hello to all,

So, I followed julie_m instructions and advises and builded a lamp limiter.

It worked perfectly! In a plug-in and plug-out rampage I managed to find a problem in the transport circuit of a reel to reel player, and double check the health of some of my hi-fi stuff!

I found that the 100w bulb was too much for the philips valve amp and tuner, so I switched for a 40w and it was glowing (dimly).

Afterwards, I did the temporary fix suggested, the two diodes/resistors. And it worked - small flash of light, going dim afterwards. I didn't noticed the variations mentioned, so I guess there is something wrong with the caps.

But even so, its a small victory!

[julie_m]

Originally Posted by Antracite

fterwards, I did the temporary fix suggested, the two diodes/resistors. And it worked - small flash of light, going dim afterwards. I didn't noticed the variations mentioned, so I guess there is something wrong with the caps.

Actually, that is not really a bad sign. I'd expect the whole show to be over much more quickly with silicon rectifiers than the original EZ80.

A valve rectifier has to warm up. The cathode won't even begin to emit any electrons at all until it has reached a certain critical temperature; and then the number of electrons available for conduction begins to increase until it reaches a maximum. Since an ampere can be defined as a certain constant number of electrons passing a checkpoint per second, it follows that a valve rectifier is a current-limiting device. Also, the current drawn by the set will be slightly less without the rectifier heater in circuit.

This means the current drawn by the set will be initially high for a brief instant (as the cold heaters have a low resistance), then low until the rectifier begins to conduct, then rise to a high value until the reservoir capacitor is fully charged, then fall back to a steady low value (still higher than while the rectifier was warming up, because now some current is being drawn by the anode circuits).

But a silicon diode is ready to conduct as soon as a voltage is applied in the right sense (i.e., the moment you switch the power on, or 0.01" later). So the reservoir and smoothing capacitors will charge much more quickly in this situation. And also, your tuner does not have an amplifier feeding a speaker, which draws the lion's share of the HT current; so the final brightness may not be noticeably more than the initial brightness anyway.

Before you plug the tuner into an amplifier for a good old listen, you can do some more tests with a multimeter, as follows:

1. Select your highest AC volts range. Power on, via lamp limiter (40 W). Black probe to chassis, red probe to pin 1 of rectifier socket
2. Move red probe to pin 7 of rectifier socket
3. Probes across the resistor connected to pin 1
4. Probes across the resistor connected to pin 7
5. Select your highest DC volts range. Black probe to chassis, red probe to C1 positive terminal
6. Move red probe to C1A positive
7. Move red probe to C2 positive
8. Power off.

For tests 3 and 4, you may drop down a range, if the reading is small enough to fit within the next range down. Readings 1 and 2 should be within a few volts of each other, as should readings 3 and 4. Reading 5 should be somewhere between readings 1 or 2, and that plus half as much again. Reading 6 should be a bit smaller than reading 5, and reading 7 a bit smaller still.

Post your results; but if you get something like 250, 250, 40, 40, 300, 275, 250, and you feel especially daring, you can try connecting the outputs of the tuner to a known-working amplifier and loudspeakers and seeing if you can tune in a station! Of course, be ready to switch off at once if anything untoward is happening.

By the way, seeing as I can never resist a flutter, if you still have the old EZ80, I bet you will measure an open circuit between pins 4 and 5, and a low resistance between pins 3 and 5. Because what I think happened was: The EZ80 developed an internal short between the heater and cathode, near the pin 5 end. The full force of the HT was then applied to the section of the EZ80 heater between the short and pin 4. The extra current flow caused the heater to fail open-circuit. (Highlight text to reveal -- or tilt screen and stare obliquely at it.)

EDIT: The signal generator picture is too small to read the figures on the scale; but if it goes up to at least 470 kHz (typical intermediate frequency), and preferrably 1.6 MHz (top of MW band), it will be usable for radio work. And if it covers 20 Hz to 20 kHz, it will be usable for audio work. Does it have the ability to select sine or square waves, or is it just a sine wave oscillator?