AVO All Wave Oscillator rectifier replacement

[Radio_Dave]

Hi,

I want to change the rectifier in my AVO All Wave Oscillator. It is a strange thin cardboard tube thingy about 4 inches long which I know nothing about .

I want to replace it with an 1N5404 but I am worried about increasing the DC voltage. I have a circuit diagram but it only shows the AC output from the transformer, no other voltages are shown.

Can anybody tell me what type of rectifier the AVO has and how much, if any, the voltage is likely to increase by fitting the 1N5404

Many Thanks

David

[ronbryan]

David

The rectifier in the mains powered AVO All Wave Oscillator is a metal rectifier type H50. It is similar construction to types manufactured by STC, and used in old Telequipment 'scopes but I can't confirm it is an STC type.

By coincidence, I have recently plotted the characteristics of the H50 in my AVO 'AWO' (circa 1944) using a couple of meters and a 30 V DC power supply, so here are the results.

Current (ma) Forward Voltage (V)
1 mA @ 12.6 V
2 mA @ 16 V
3 mA @ 19.4 V
4 mA @ 22.4 V
5 mA @ 25.4 V
6 mA @ 28.4 V
6.5 mA @ 29.8 V

The forward resistance can be calculated from the above data, but it varies from 12.5k at 1 mA to 4.5k at 6.5 mA

The current taken by my 'AWO' varies between about 3mA to 6mA according to frequency and whether the modulator is on or off.

Strangely enough, in my instrument at least, the poor regulation of the power supply (caused by the high forward resistance of the rectifier) appears to help retain amplitude stability over the tuning range, rather than the reverse, as when the oscillator takes more current, the HT drops, reducing the amplitude.

My suggestion is to use a silicon diode with a 5k6 resistor in series.

Ron

[Radio_Dave]

Thanks Ron,

Your information is excellent


David

[Radio_Dave]

Hi,

I've replaced the metal rectifier (type H50) with a 1N5404 and a 4.7K resistor the HT is between 71 and 126V depending on whether the modulation is on.

I temporally re-installed the metal recifer and found that the voltages were 88 to 119V

As the metal rectifier seems to give better voltages I'm tempted to put it back but I have no idea how reliable these items are! I rejected it in the first place because I couldn't measure any resistance from it However it seems to be working.

What I need to know is how do I check a metal rectifier and are they reliable?

Many Thanks

David

[ChristianFletcher]

I have found these old rectifier to be very reliable unless they have been severely over run and they often stink of garlicky in this case.

They also seem to suffer from damp storage and corrosion problems. I would probably still replace this with a diode and suitably rated resistor.

I have never tried to test one of these by doing the normal forward voltage measurement it probably would not work anyway. The best test is probably the circuit its installed in. You could always wire a lamp across the output and check the dc.

You did not state if the AC output from the transformer was also constant while you tested the DC output.

[Ed_Dinning]

Hi Gents, looking through some theory last night I came across the following:
Copper Oxide rectifiers are reliable and withstand much abuse, do not age, but have a low reverse breakdown voltage.
The successor to them was the selenium device, the one that stinks when it dies. This had a much higher reverse breakdown voltage but aged and required re-forming (much as an e-cap)without constant use. They are also moisture sensitive.
So I shall look at copper oxide rectifiers more closely in future and not just replace with silicon and a resistor. I've already done this on my 1932 Pye " S " , but I'll check the rectifier and see how good it is. From memory a similar unit in a Pye " G " was still operating when I repaired that set many years ago and I thought I'd just been lucky.

Ed

[jim_beacon]

Ed,

I've always found copper oxide units to be very reliable - they are the basis of AC measurements in AVO meters and the like, and I don't think I've found one that has failed yet.

However, as you say, they have a very poor PIV rating, and quite a high forward resistance (but a very low forward volt drop - somewhere around 0.1V) so this may be more of a reason for their quick exit from the radio world.

Selenium also has a low forward voltage drop - they are very difficult (if not impossible) to replace in instrumentation circuits. They do have a better PIV rating, though still not really high enough for valve work (hence the series stacks), and they do tend to expire on a regular basis.

Jim.

[Ed_Dinning]

Hi Jim, are they still made or have AVO's gone over to Schottky's now. My text book suggests that they are manufactured by a simple process.

Ed

[ronbryan]

David

You can test a metal rectifier by measuring the forward and reverse characteristics. All you need is a voltmeter, a milliammeter and a variable source of DC (at least 30V). Compare the forward volt drop at integer values of current with the figures I posted earlier. I suggest you don't use currents greater than 10mA as I am unsure of the rating of the H50.

The reverse leakage also needs to be tested. On my unit with 30 V applied leakage was about 10 microamps. Ideally this should be tested at around twice the HT voltage as the reservoir capacitor will be at +90V and the AC input will be at 1.4 x the secondary voltage on negative half cycles.

I came to the conclusion that the metal rectifier was best left alone and that provision of a well stabilised HT supply was unnecessary.

Ron

[GMB]

The forward resistance can be calculated from the above data, but it varies from 12.5k at 1 mA to 4.5k at 6.5 mA

From your figures, I would say that was about a 3K resistance with a 10V voltage drop.
So you could model this with a zener a silicon diode and a resistor.

What I have done with OKish metal rectifiers is just left them in the circuit but with an added silicon diode in series to make sure there is no reverse breakdown.

I'm not sure about a 0.1V drop Jim. That certainly isn't my experience.

[Radio_Dave]

Thanks for all the replies . I have re-instated the metal rectifier as it shows no signs of distress and appears to be working fine.

I have made a note of Ron's measurements as I'm sure they'll come in handy in the future

Regards

David