DAC 70 Dial Lamp etc.

[Neil Purling]

Is the dial bulb of the Bush bayonet or screw fit?
I wondered why a microwave bulb would be reccomended. They look to be of smaller dimensions even though the wattage is still 15. So, does it boil down to proper clearance? What should this be?
I will experiment with increasing the value of the resistor in series with the lamp. So long as I can read the dial I will be happy with reduced illumination if I know the dial is no longer at risk. I've heard that these things are like bedside lamps as standard.

BTW When I was looking for a FM radio one of the sets I looked at was it's brother, the VHF 90.

[paulsherwin]

Small Bayonet Cap, the miniature version of the standard British bayonet bulb.

Many modern bulbs will be smaller than the original. The important thing is to keep a good distance between the bulb and the plastic surround, and this is easier to do with a smaller bulb. It's perfectly possible with the original though. You'll see what I mean when you examine the radio.

Paul

[Neil Purling]

Sorry lads. I spent so long on my post you had told me what bulb it has.
I just watched that advert. Is the dial really as bright as all that? The only local collector I know doesn't have a working example of the breed.
Has anyone used a larger value series resistor?

[paulsherwin]

The bulb is quite bright, yes. A 15W bulb was used because it is the lowest power 240V bulb easily available, but it is much brighter than needed, hence the series resistor. It's still bright with the resistor. You could increase the resistor value or add a diode in series, though this may produce visible flicker.

It's easy to experiment without removing the chassis.

Paul

[radioman]

How about trying a 24V 5W SBC lamp (type 150) as used in trucks, fed via a 3uF capacitive dropper ?
I just lashed one up to the mains and its nice & bright with 22V across it. ( Don't forget ~220k discharge resistor across the capacitor )

Andy

[Neil Purling]

I will try using a larger series resistor first. They are only 40P each.

[Neil Purling]

Take a look at the image of the Bush provided by the vendor. A lovely layer of dust, which says The Phantom hasn't had a fiddle.
The vendor was lucky to get away with energising this. Do you see the state of the mains filter waxie below the switch-pot? It looks like it is just starting to melt.

[Steve_P]

Well it's arrived safety then so we can all heave a sigh of relief.

Chop it out for now. Replace with an X2 rated component. There is one cap you must change - you know the one! Then bring it up on a bulb limiter (see the other site: http://www.vintage-radio.com/projects/lamp-limiter.html ) and see what it does.

It'll probably work!

Cheers,

Steve P.

[Neil Purling]

Well, I haven't got it yet.......
It is reassuring to see the thick dust. The filter capacitor I know I already have, I will have to take a polypropylene 0.1uf out of something else or use one of some 0.1uf @ 1000V PIO milspec. The smoothing can may have survived the vendor energising the radio. So what if it didn't I still have the discrete capacitors to re-fill the old can.
Like I said before I have the other caps. It's something to keep me occupied.

[Neil Purling]

There's one thing that's troubling me. I'll not be able to use my signal gen or 'scope with this 'cos it's a live chassis. I don't have an isolation transformer. Why do you think I was relieved that The Phantom hadn't paid a visit to my DAC70?

The UL84 g1 voltages: When we talk of the thing with a + g1 voltage due to That Capacitor where'd you have the - meter lead: cathode or chassis?
I wanted to see how bad it'd be with the briefest of tests, enough to allow the HT to start to come up.
Of course I would do this with the filter cap snipped and my test lamp in circuit.
What'd you expect to encounter?

[magnastat]

Pos lead of DVM on g1, neg lead of DVM to chassis.
You wouldn't want to see more than a few mV, tops.

[paulsherwin]

You don't need an earth connection to inject RF. If you did, you could isolate the connection with an X2 capacitor.

I can't imagine what you'd use a scope for fixing this set.

When you power up, the dial lamp will light and the limiter will glow *very* dimly. As the rectifier and output valve warm up the limiter will get brighter but should still be quite dim. If it's any brighter than this you have a problem somewhere, most likely smoothing caps.

There's a better than even chance it will 'just work', especially if you change those Mouldseals.

Paul

[Neil Purling]

Its arrived safely. Now the fun starts. Its dismantled safely, boy was it filthy inside.
Theres a few of those lovely Mouldseals to deal with. Two being 3000pf @ 400V, 20% tol.
One is on g1 of the UL8f to get rid of RF I was going to change that for 1000pf to change the tonal response. The other is in the local oscillator.
That isn't a preferred value in use today.
The RF filter across the mains is 0.05. Usually you see 0.1uf. Will the larger value make any difference as I have 0.1uf @ 1000V.

[petervk2mlg]

Isn't it best to use a mains (AC) rated capacitor (X class) in this position?
Peter

[Steve_P]

Yes, and 0.1uF is fine.

Cheers,

Steve P

[Neil Purling]

I have just about finished the DAC70 except I am not sure iof how sensitive the set ought to be. Are there any other forum members who can offer comment on how sensitive their DAC70 is. One of the coils has moved on the ferrite rod. There is a wax blob on the rod, but not one on the coil former for me to line them up again. The coil is is currently set against the wax blob. Should you be able to get Arrow Rock at the top end of MW on the ferrite rod from where I live? Otherwise I might have to experiment with adding a coil of wire placed on the top of the radio to give inductive coupling

[paulsherwin]

Remove any remaining wax from the ferrite rod and slide the coil around to peak the signal while listening to a weak station. There should be a very obvious peak. Lock the coil in the new position so that it doesn't move again.

I'm not sure Arrow Rock is still on the air. The signal was never listenable here in Oxford, certainly not on a domestic 4 valve superhet.

Paul

[Neil Purling]

I know that I am getting images with this radio, whether this is by reason of the design or misalignment I have my local radio appearing faintly at the top of the dial at just over 400 metres when it is only on 202 metres. In such circumstances it is hard to tell a weak original station from a image. there is also the BBC World Service around the top of MW.

I have just looked and Arrow Rock is not on MW any more. My trf kit radio managed it with a long aerial but never after early evening.
Setting the coil with a weak station works, no matter wherabouts on the dial it is?
The loose coil is the one wound in fine wire. theres another that is of a much coarser gauge.

[paulsherwin]

BBC World Service on 463m would be a good choice for MW alignment. RTE R1 would be good for LW. There is more to RF alignment than just peaking the ferrite rod coils, but correct coil positioning will make a huge difference.

You can deliberately weaken a signal by rotating the radio so that the signal is partly nulled out.

For best possible performance you should do a full RF alignment as described in the service data.

Paul

[Sideband]

You should peak the MW aerial coil at the LF (500 meter) end of MW. In the London area, there is a local station near 550 meters and I sometimes use that as a quick check for aerial alignment when assessing performance and then use the generator later and do a proper job.

If you feel like experimenting, there is a way of finding out if you need to adjust the aerial coil. Firstly you need to know that if you slide the aerial coil towards the centre of the rod, you are increasing the inductance and if you slide the coil towards the end of the rod, you are reducing the inductance.

OK simple equipment. You need another ferrite rod and a small loop of wire. The loop of wire should be just larger than the diameter of the rod in the radio being tested so it will slip easily over the end of the rod. It's easier if you can fit this small loop to the end of a plastic knitting needle so you can prod around safely inside the set without touching anything with volts on it! Note that this must be a closed loop (so-called shorted turn)

Testing: The second ferrite rod should be brought up towards the ferrite rod in the set. Keep the second rod parallel to the one in the set. If the signal increases this indicates you have to increase the inductance of the coil i.e move it towards the centre of the rod. If the signal reduces, take the ferrite rod away then pick up the loop and slowly slide this over the end of the rod. If the signal increases then you need to reduce the inductance i.e move the aerial coil towards the end of the rod. If the signal reduces then the coil is correctly adjusted.

It all sounds complex but you will get the hang of it very quickly. Basically with a correctly adjusted aerial coil, you should get a reduction in signal when the second ferrite rod or the shorted turn are introduced to the aerial being tested. Note that these tests should be carried out with the receiver tuned to the LF end of MW (500 meter end).

Exactly the same tests can be carried out with LW with the set tuned to somewhere around 2000 meters.


Hope all this is clear.


Rich.