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Old 11th May 2017, 4:46 pm   #21
FERNSEH
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Default Re: Marconi 702 advice

Three more EMI capacitors get the treatment.
There's plenty of TCC waxies available but the EMI ones are a bit scarce.

DFWB.
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Old 11th May 2017, 11:59 pm   #22
peter_scott
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Default Re: Marconi 702 advice

Excellent!

Peter
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Old 12th May 2017, 12:15 am   #23
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Default Re: Marconi 702 advice

Tonight I have checked AC voltages, with up to 50 volts applied to the EHT system
and 140 to the rest (no rectifiers) and they looked OK. The rest of tonight will
be (with filaments disconnected) slowing raising the AC up to 140 volts (a
variac on 120 AC, no 120 -> 240 transformer) while monitoring the current, rectified by
a 1N4007.

It turns out one "World" Aurora was still available, so I will get it.
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Old 12th May 2017, 8:19 am   #24
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Default Re: Marconi 702 advice

At that age and rarity certainly worth re stuffing without a doubt.I do it on rare radios.
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Old 13th May 2017, 1:06 am   #25
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Default Re: Marconi 702 advice

Tonight I first ran 4.7 kV (all a regulated and current limited lab supply
could put out) into the EHT caps and resistor string for 15 minutes
and all was OK at the expected .9 mA.

Next I installed a temporary stepup transformer and obtained
a dim but visible raster. It woke up slowly over a period of about two
minutes. So did the horizontal (line) scan, which grew from a wavy three
inches to essentially fill the mask.

No smells, and both the meters on the filament and HT lines
were rock stable at 4.00 and 280 volts.

Next I will once again disconnect the EHT/CRT filament fuse and use a scope
on the rest of the circuitry to check for cap leakage and bad waveforms.

Well, tomorrow; the rest of tonight is for a beer. Its best to proceed slowly and stone sober.
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Old 13th May 2017, 8:49 am   #26
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Default Re: Marconi 702 advice

Hi Doug,

If your raster is dim it would be worth measuring the CRT heater current. The base connectors can give unwanted resistance. The current should be about 1.3 amps.

Peter
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Old 13th May 2017, 2:00 pm   #27
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Default Re: Marconi 702 advice

Thanks for that tip. It (the heater) does look a bit dim, but I did have 4.00
volts from my Sencore CRT tester on it and it looked the same;
I had cleaned the contacts on the tube.

Is the screen supposed to be bright with the brightness all the way up? I
have not yet checked the cathode-grid voltage.

Remember that there was no signal .. the Aurora has not even been shipped.
Since it is white positive video and 100% DC coupled I don't expect much
brightness.
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Old 13th May 2017, 3:43 pm   #28
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Default Re: Marconi 702 advice

My 702 hasn't been switched on for many months, nevertheless, it displayed a picture after a surprisingly short warm up time.
The TRF unit still doesn't supply sufficient video to the CRT grid, the peak positive going video including syncs is only 8volts. The high slope CRT needs at least 12 volts of video for a fully contrasted picture.

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Old 13th May 2017, 4:06 pm   #29
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Default Re: Marconi 702 advice

I am working on my 702 TRF unit and also get low gain - 12V into the grid from an unattenuated Aurora. I will try powering it up after its installation this afternoon and see what the picture is like. The CRT hasn't been used for many, many years so could take a while to warm up.
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Old 13th May 2017, 11:45 pm   #30
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Default Re: Marconi 702 advice

I investigated the TRF this afternoon. It appears rather narrow,
the 3dB points being 43.5 and 47 MHz. With the sensitivity control
all the way up it required about 10 mV of RF to get 12 volts CRT drive.
The Aurora can make much more than this.

The CRT grid was of course at 0 volts with no signal. The brightness
control range is 8 to 24 volts, so the dim raster was at -8 volts
on grid with respect to cathode. The actual cutoff was well above that.
I have not measured the HV with the actual PS unit on it.

Even more oddly it appears the sound carrier wants to be at 43.5 MHz.
Presumably the latter is just an adjustment of the audio
frequency changer setting.
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Old 14th May 2017, 12:20 am   #31
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Default Re: Marconi 702 advice

Apart from the sound tuning knob the trimmer TC1 has screwdriver access with the side panel off.

Peter.
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Old 14th May 2017, 4:27 am   #32
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Default Re: Marconi 702 advice

I let the CRT run for 45 minutes and it woke up quite a bit.
It also has a very serious about .7 cm dia turn off burn in the center.
But no ion burn.

I aligned it. It was very sensitive to oscillation. The RF connection was a kludge,
with a ground wire 3 cm long. I never got it fully stable until I disconnected
the connector and attached another piece of coax with a shielded connector on it.
I will install a modern connector and shield with copper foil. I aligned it to get a much wider bandpass. It should have adequate sensitivity for the Aurora.

The screen behaved very strangely when I had the EHT on. The contrast control
has very little effect. This was traced to a knob reversal on Sensitivity and Contrast.
The very old paper .25 uF cap connected to the contrast control
turned out to be cunningly disconnected and used only as a tiepoint.

Does "sensitivity" mean sensitivity to impulse noise? It seems to have
only a 1 volt range and only on one stage.
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Old 14th May 2017, 7:10 am   #33
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Default Re: Marconi 702 advice

Sensitivity is just stage gain. Too much and you'll run into saturation.

Peter
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Old 14th May 2017, 2:20 pm   #34
dtvmcdonald
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Default Re: Marconi 702 advice

Ah! Just what I thought ... I think the saturation is intentional, and
"sensitivity" does mean "sensitivity to impulse noise". White speckles
of the usual sort around here (very very bad at 40MHz) were prominent
until I got the connector well shielded. It seemed to sync
to something 60Hz, whether the noise or hum.
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Old 15th May 2017, 9:03 pm   #35
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Default Re: Marconi 702 advice

A question. What does the original antenna input look like? I saw at least one
picture that had a long piece of coax with exposed shield dangling out the back.
This picture

http://www.vintage-radio.net/forum/a...9&d=1494356758

earlier in this thread shows what mine looked like. Its at upper left. The coax is vinyl coated and about
1/2 inch in diameter. The polyethylene insulation is hollow so that the center
conductor rattles around in it, somewhat like in 125 ohm coax. It had a rather
clunky connector that looked very unperiod on it, so I replaced it with an F connector
on copper coated circuit board with a small shield over it.
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Old 16th May 2017, 12:00 am   #36
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Default Re: Marconi 702 advice

Hi Doug,

The original antenna cable has an inner conductor covered in gutta percha then covered in two layers of tinned copper braiding.

You can see an original cable and plug in the photo of the HMV901 set in the National Museum of Scotland in my website. Further down the page there is a photo of the original cable in my own set. The disassembled aerial plug in the photo is an accurate replica of the original plug shown on the left.

The cable is not detachable from TRF unit (leastwise not without suitable tools.)

Peter
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Old 17th May 2017, 11:02 pm   #37
dtvmcdonald
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Today I got my Aurora.

I attached it after setting System A Channel1 and
instantly got a quite nice set of "color" bars. In sync, in focus!
Not even touching focus or the hold controls.

Attaching a signal source ("American Pickers" rerun) got a perfectly
nice picture. Its a little dim (before blooming) with the sun
leaking through the blinds, but will be just fine at night.
I will be able to assess the picture better after dark. The
linearity is much better than some pictures I have seen,
very acceptable.
My calculations as to correct RF amount were correct.

However ... no sound. Setting volume all the way up
and tuning resulted in a very weak sound. I then turned off the set and
removed the EHT fuse and turned back on with brightness
and contrast all the way down. Turning up the RF 20 dB resulted in
weak sound. A scope showed about 0.2 volt audio at the
first audio grid. That seems very low with volume all the way up.

The IF tube seemed a bit cool but not cold. Removing it
I noticed the top cap was loose. It had been fixed at some point with a
small wire soldered to the top and the top glued on with rubber cement!
I fixed this with silicone. The pins were filthy and were cleaned.
It will be several hours before the silicone is set enough to try it.

In the meantime I decided to check the two audio tubes. The first one
had filthy pins and was cleaned. The audio output tube would not come out!
It turns out that one pin was soldered in to get around a broken contact!
It was not so filthy so I just added a little DeOxit to the pins and plugged it
back in. The audio chain remained as before ... weak sound at 0.2 volts to
the first audio from an external audio source.

How much should the first audio need for normal room volume? American
radios with 6AV6/12AX7/ECC83 typically need only that much,
but this tubes specs less gain. I suspect the IF stage ... is the reasonable?

If I need to service the audio chassis, it looks hard to get out, though I
likely can do some soldering from the bottom, especially that soldered pin!
It also looks like if unscrewed it can be tilted enough to service. The
speaker is in the way!


How do you remove the audio chassis?

If it can't be fixed easily (i.e impossibe to remove) I might
just give up on audio or install and hide
(under the wooden bottom ... it will fit) a tiny transistor amp that
will easily amplify 0.2 volt.

Any specific suggestions for audio, pending reinstalling the
IF amp?
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Old 18th May 2017, 12:08 am   #38
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Default Re: Marconi 702 advice

I haven't had mine out recently but I think you need to disconnect the mirror lid prop, carefully fold it back so the lid is horizontal with mirror face up.

Remove the two screws at the front of the CRT wooden frame then push it up from below and remove it. Disconnect scan coils at the sync chassis. Disconnect the CRT bases and EHT connector, also the "push-about" coil.

Carefully lift the CRT out of its metal cradle then lift out the cradle. Unscrew the two screws in the back panel that retain the remaining cradle support and screening plate. Disconnect and remove the speaker. It should now be possible unsolder/detach the RF input to the sound chassis at its point of entry. Also disconnect the sound power connectors at the paxolin terminal board on the power supply chassis. Then unscrew the screws beneath the sound chassis that retain it to the cabinet brackets.

These instructions are just from memory. I think I normally also remove the sync chassis but that is probably not essential.

Peter
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Old 18th May 2017, 2:56 am   #39
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Default Re: Marconi 702 advice

I got a photo, attached. Its not perfect, there is a reflection of my kitchen in the glass.

The big black splotch is bad silvering in the glass. The small round spot
in the center is an electron burn.

Replacing the audio IF tube did not fix the sound. I will test tubes tomorrow.
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Old 18th May 2017, 9:09 am   #40
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Default Re: Marconi 702 advice

Looking healthy although difficult to assess on the cartoon image. It would be good to see Test Card C on it.

Peter
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Last edited by peter_scott; 18th May 2017 at 9:24 am.
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