Hi Folks,

I have acquired an Airzone bakelite tombstone style radio model 682V which is missing its vibrator power supply.

This is a dual wave unit using 2 volt battery valves 1C7G, 1M5G, 1M5G, 1K7G, IL5G.

I was wondering if anyone has a circuit diagram and any service information for it.

Thanks in advance.

Regards,

Michael.

The schematic can be found in the AORSM Vol. 5, page 28 & Vol. 6, page 52.

It may be a bit hard to read as the quality of some of the AORSM circuits are pretty crappy. Until the original manufacturers schematic turns up this will have to suffice. I also checked the Peal versions & they are just as bad.

Airzone 6852V Circuit Diagram

Here you go: http://www.radiomuseum.org/r/airzone_peal_6852vp6852.html

It may be more practical to fit that with a conventional transformer power supply?

Hi Folks,

Thanks MonochromeTV for the circuit diagram and to Garyoz for the link.

It doesn't look likely that another vibrator power supply for this would be found easily.

Marcc, that looks like a good idea. It should be easy to knock up a power supply to provide
a regulated 6 volts DC for the filament string and 135 volts for the HT.

There is enough information between the 2 diagrams to work out the details.

By the look of the photographs on the Radiomuseum site, it may be easy to make a bracket to fit into the
mounting brackets on the side of the cabinet, and to mount the power supply in the same spot as the original.

Thanks,
Michael.

Hi Folks,

I have been able to power this radio up. I am using a 6 volt power supply using a LM317T for the LT supply and a HT supply of 135 volts.

The component values on the diagram were hard to decipher but I was able to redraw the circuit by hand, and between what was on the original diagram and what was in the chassis I was able to determined the component values.

The speaker and output transformer was also missing. I was able to find a speaker and a 10 K to 2.5 ohm output transformer.

This radio has all of the original components, heaps of old leaky ducons and cracked moulded mud capacitors. Also lots of perished rubber wiring.

After replacing the capacitors and providing a speaker, the radio plays . It was a little weak, but aligning the IF improved it considerably.

The only problem with it is that it is horribly deaf on the shortwave band. From the dial markings it seems to cover from about 6 MHz to 21 MHz, or 42 to 13.6 metres.

There is no noise at the low end of the band, but about the middle there seems to be some activity and then nothing at the top end of the band.

Voltage checks around the converter are the same as for the broadcast band, except for the screen voltage, which is lower.

Osc grid is hard to measure as putting the meter probe on the pin causes the oscillator to stop on the SW band, but I can measure about -4V on the BC band.

Voltages around the converter are: HT 135V, Plate 129V, Screen 52V on the BC band 47V SW band, Osc grid -4V BC 0V on SW, Osc anode 106V on both bands.

I have changed converter but it did not make any difference. Is there any thing else that could contribute to the low performance on shortwave (except sunspots of course!).

Reards,

Michael.

There is also the option of not bothering to work it all out, as someone else has. There are quite a few that manufacture PSU units to for battery units made up, or as kits and given the circuit should be able to supply something appropriate.

Marc

Hi Marcc,

Yes, I notice on another thread somebody has already done the hard work.

That may be the way to go.

The original lash up was to determine wether it was possible to make the old radio work again.

This radio was a shed find, it had been there since I was young and I remember that it came from one of my friends who lived on a farm.
Since then it had been plundered for bits evidently....


Michael.

OK...

So I cranked up the CRO. Put probe on Osc plate.

Broadcast oscillator is as you expect. nice and strong.

On SW there are oscillations from the bottom of the band to the top of the band.

The level is not as high as the broadcast band.

So the oscillator seems OK.

It looks as if it is in the IF or the aerial circuit.

Grrrr.......

Hi again,

Turns out that the I.F. channel was faulty. This set has 2 IF stages.

I didn't think that the fault would be there as the broadcast band seemed to work OK.

So, anyway, I thought that I would check the voltages.

Plate volts checked OK, but the screen volts on both valves was about 20 volts, just a tad low.

Both screens are fed by a 100K ohm resistor bypassed by a .05μF paper capacitor.

The resistor had gone high. It read about 5 megs.

Easy fix, a replacement 100K resistor and a new bypass capacitor and plenty of noise on SW.

The increase in I.F. gain definitely improved the performance.

Onwards and upwards......

It is always interesting to note the different ways things are done. I have dealt with several battery sets & have a 1924 Gilfillan in the line waiting for the owner to sort out the woodwork. Interesting: They have a voltmeter for the batteries. It has a hole where the voltmeter should be, however I did manage to procure a genuine one to plug the hole with.

Even it is a battery set, I will replace the wax paper caps and electrolytic's, if it has them and check resistors as I go. I have made some PSU's and also bought commercial ones. Do be careful with solid state regulated ones. The way the power is wired here causes them severe issues when the negative, or positive is directly earthed. It has to be earthed via a 0.01 line cap.

People forget that if there is an AGC / AVC it, when switched in, can deliver (weak) signal to the 2nd detector & give the illusion that the IF is working.

I find the Signal Generator at IF frequency, aided by an Oscilloscope an expedient way of finding lost signal, distortion & dead oscillators. I normally check & set the IF before moving to the Oscillator & Antenna adjustments. I am past being shocked at the number of IF transformers I find, that have been tampered with. Their misalignment can make sets very "deaf". Using the CRO as the visual voltmeter when calibrating, also helps spot distortion: Then you use it to find the source.

I am not impressed with the latest article in SC where the Wax Paper caps were not replaced. 50+ years of experience says not a good idea. The only info I have is from Lafayette. They state that for a screen decoupling cap below 50Meg resistance ("Megger" or Insulation tester) and 200meg for a coupling cap, is a fail.

Rarely do any test past 100's of K. That has to impinge on bias & performance. I had a plate bypass cap recently test 3K (set dead).

Looking at the circuit diagram, a ghost of a printed circuit on the other side of the scanned page can be seen. This effect can be avoided by placing a black sheet of paper or other object over the back of the page to be copied on the scanner.

Hi Marcc,

Yes. It looks as if I got caught by the AGC thingy. When I was aligning the IF I was monitoring the AVC voltage.

It looked like there was gain there and as it came into alignment I reduced the sig gen level accordingly.

Just that there wasn't enough gain due to the reduced screen volts.

I suppose the moral of the story is to not get distracted by red herrings and stick to the fault finding procedure.