Good Afternoon All, and thank you for this opportunity.

My father and I are, after about 25 years, finally nearing completion of a restoration of an Airzone 609. We put it off forever, until about three years ago we decided to actually get in and start it. In the last three years, on Sundays when we could get together, we identified and replaced all of the electrolytic caps with modern day ones, bridging out the originals that are in cans, and replaced all of the paper caps with polyester. All of the original carbon resistors look good, but we haven't checked them all with a meter, but will do so if it proves necessary.

We have it operational, but the audio is quiet, until we hold our hand over, or actually touch the cap on the 6A7 tube in the second stage. When we do that, the thing comes to life with plenty of volume. This tells me that the power supply, bucking transformer section, and the rectifier are working fine. I am suspect about the antenna through the first 6D6 and into the 6A7.

I have included links to both the video of what we are experiencing as well as the cct diagram. Any advice greatly appreciated.

Regards
Aaron

https://photos.app.goo.gl/Tc2tVghbkCjifoYWA

https://drive.google.com/file/d/1FM0uERbn6JUvMHDupSlbtg2bwtun34f0/view?usp=sharing

The signal simply isn't making it from the antenna terminal, through the 6D6 and to the 6A7. Nothing weird about it.

Things to check:

- you have a long wire connected to the antenna terminal? You can test it with your finger.

- correct voltages on the 6D6 (and does it light up?)

- continuity of all the coils before and after the 6D6 (old coils eventually go open-circuit)

- continuity of the switch gangs before and after the 6D6 (these switches are unreliable and may need cleaning etc)

- AGC voltage being correctly filtered and applied.

Thanks Robert. Can check all of those things out, but unsure how we would verify the AGC. Not sure where that resides in this cct. Many thanks

The AGC voltage is in the line at the bottom of the diagram (where it says R4, R11). It should be slightly negative, and goes more negative on stronger stations, and is present at the top cap of the 6D6.

However, it's best measured at one end of R4, a 100k resistor.

So, hook your meter to R4, note the voltage, perhaps -1 volts. Then put your finger close to the top of the 6A7 to make the volume increase, the meter should go more negative, maybe -3 volts.

On the diagram, you can see that this voltage passes through various coils and switches before making its way to the valve top cap. You need to make sure that this path is still conducting.

If your 6E5 tuning indicator is working it will show the AGC varying as you tune through a station, ie with signal strength. But it should not vary with sound from the speaker.

By Bridging out all the old can capacitors I hope you mean completely taking them out of circuit. Ie no wires connected?

Sitting at the back of my bench is a 609 waiting for the owner to come and collect it. It is the version without the tuning indicator. This is the only radio where I have had an IF fail because none of a 6D6's leads had been soldered and the envelope literally fell out of the tube base, when the chassis was turned sideways?

The one featured on Radio Museum is the one sitting here.

I would suggest shrink tube on those bad fly leads. I had to replace one. Seek advice if the valve top cap is lose.

What equipment do you have? for the purpose of alignment? Not sure of your experience?

One thing that can go wrong that needs confirming albeit I don't think it is the problem; Is the reception of a weak signal from a strong station, which cannot be tuned. I.e. that's the only station it gets? In that scenario the signal gets to #75 by way of the AGC. That is normally caused by a dead oscillator, or IF.

6D6 later 6U7 is the worst tube ever invented for picking up trash & using it to destabilise everything. If you look on radio museum you will note that you should have four shielded tubes. You only have two. Only the rectifier & Output tube are to be without shields. That can result in an oscillating stage whereby the oscillation can generate enough AGC to shut down every tube connected to it. That in your case being 609E and having an eye tube, may be reflected by its behaviour.

The next issue, which I suspect is manifest in yours, relates merely to tuned circuits. The the IF (456 kHz) is a separate entity to the front end tuning. It must be tuned with the least amount of signal (abt 50uV) from the Signal Generator into the signal grid (Top cap) and DC blocked (series cap). Too much signal will cause the AGC to cut in and you will never get it right.

As you have a TRF stage before the 6A7 Frequency Changer. There can be a major fall down here and it applies to all bands. In order to function properly all of the antenna coils from the antenna connection to the 6A7 signal grid, have to be on the same frequency. This is part of alignment. Should they not be on the same frequency you will get massive signal attenuation (which fits your issue). There are trimmer capacitors to achieve this.

My normal procedure is to run an IF alignment first. That tells me that the set is working between the frequency changer(6A7 & speaker. If it isn't you go back and fix it. My combination is a Signal Generator and Oscilloscope: Formidable if things go wrong with signal. Only when that early lot is sorted do you touch the front end.

I will not close couple to the front end if its avoidable, only the antenna. Do not have #80 sideways. Mechanical adjustment of the dial is first to ensure the pointer is tracking the gang. Identify the antenna trimmers which are side trimmers in the front cans. One lot BC the other SW. and the third oscillator.

Like the IF with top trimmers behind the gang next to #75; Oscillator adjustment screws will be alive, (B+) as will be the ones on the plate of the 6D6's. Do not use a driver that will short them to the can. Shrink tube has been invented.

Carl is right: No old Electrolytic cap should be left in circuit: Especially "wets". Isolate, or remove.

Grid resistors on audio tubes and plate resistors on 2nd detector first Audio tend to have an attrition rate. Resistors that have gone high on the Frequency Changer may cause it to stop, or cause poor sensitivity (low IF modulation) in the radio. Most Pentagrids are voltage sensitive: 6A7 & family are not the world best frequency changer tube (RCA).


Isn't fixing fun.

It's obvious from the video that the RF stage isn't working.

Check the voltages around the stage for a start. Missing plate voltage for example means O/C coil primary.

Never unusual for a primary winding in a TRF stage to go open. What is rare is for both coils to go open on set, but it has happened to me. What is often over looked on a BC / SW set is to check the set on SW to see if the problem is only common to BC band.

One hopes, if its faulty, is that it is an untuned primary. That can often be sorted. without throwing away the coil. Tar / Bitumen are the enemy along with rodents and corrosive breaks at nodes.

Rodent breaks tend to be on the outside. They like the wax.

Delete wrong area

OK, so after completing the tests Robbbert recommended, I have found the following.

AGC cct - R11 reads 102kohm, R4 reads 112Kohm. The voltage across R4 with the radio on and receiving was +1V (not -1V as suggested). Putting my finger close to the cap of the 6A7 only dropped the voltage to +.05v so looks like the bias of the 6A7 is not correct.

I note the next suggestions from Robbbert. What is the best way to check the continuity of the coils? Can the cans be lifted without major surgery? I see that the end of the cans closest to the base are rivetted, and some more accessible than others. Would prefer to avoid having to drill those out if at all possible. If they were open, surely, I would not be receiving anything?

Dad says that 14kohm high on R4 shouldnt matter. Is that true, or should we look to change it out?

Many thanks again for your help.

Aaron

I just received an email with the above content, so reading it now.

The resistances of R4 and R11 are reasonable, no need to change them out.

When you say voltage across R4, is that from one end of R4 to the other (incorrect)? or from one end of R4 to ground (correct)? If it's from R4 to ground then you'd have to find where the rogue voltage is coming from, since it will be bad for all the valves it's connected to.

You don't need to open any cans or even to disconnect anything in order to measure continuity of the coils. Turn the radio off and unplug it from the wall. Then, by looking at the schematic you can see the multi-pole switches (says 1.2.3 near the 6D6 and 6A7), you can trace continuous paths from each valves grid, through the bandswitch, through the selected coil, then down to the area around R4 and R11. Make sure your multimeter shows continuity at each switch position, for each valve.

Additionally, on the output of the 6D6, there's another part of the bandswitch to select one of 2 cans - use the schematic to choose the proper measurement points for that.

And lastly, there's yet another part of the bandswitch to select a coil for the aerial. Simply measure the ohms between the aerial and chassis for each switch position.

You do not drill out the rivets at the bottom of the round can, nor do you un-wire it with out cause. One is in with spring clips. Looking at the condition of the wire on the fly leads they need replacing. Mine has cloth rubber and it was in the main OK. The fly lead of the square can is replaceable from the top. The shielding braid if ok can have new wire run, or dragged into it otherwise its shielded audio cable.

V1 & V2 with the big cans are fun as they are side trimmers. I had to fit new fly leads and that was done without un-wiring which would be a nightmare as they are between the chassis & wave change switch. You can have entertainment by removing the side nuts & then sliding the cover off. The entertainment lays in getting it back on.

You can always tell a dead HV primary going to a tube plate: The plate will have no voltage. I toss resistors over 10% high especially if its a commercial fix. You will never get the thing stable without shields on all RF / IF tubes Mine is:-

https://www.radiomuseum.org/r/airzone_656_ch_609.html.

Running Pentodes, Tetrodes & Renodes, with open plates & a live screen grid; Can destroy The screen, as it becomes the plate which cannot handle the current.

Beware: There is what looks like a link between two gangs on this one: That is a capacitor.

Marc

Re the AGC voltage being slightly positive when it should have been negative.

I had this once on a set.
The cause was the AGC bypass capacitor itself, the one the F terminal of the aerial coil.
When removed from the set and the capacitor replaced with a new one the normal negative values returned.
The capacitor itself was generating a positive voltage across its leads.
This was a well known effect to old radio repair guys.
The capacitor had some kind of internal breakdown issue casing it to act like a cell.

I would simply replace any AGC bypass with new to avoid this weird issue.

Fred.

I do consider fault finding in a set with old wax paper, oil filed & tired electrolytic caps (anywhere) an exercise in time wasting. The waxed papers will rarely not leak, to the point that 99.999% will be duds. Coupling ones will feed plate voltage onto the grid.

If I get a Mica cap with its end out of circuit, I will leakage test it 500V with an insulation tester (tube radio only) rare failures.

Non Polarised caps in tube radio: At test / rated / working voltage. Screen decoupling: less than 50Meg dud. Coupling less than 200Meg dud. If it leaks at all here: Bin.

Marc