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Old 4th Jul 2020, 10:31 am   #21
WaveyDipole
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Default Re: Advance type 63A

I think that the circuit is very similar and they cover the same frequency ranges, but the key differences I have observed so far are:

Type 63:
CW/FM2 (external FM?)
AM1
AM2
FM1
XTAL check
Male RF connector
Mains F.M. deviation = 150
Dial has external adjustment

Type 63A:
30% AM
FM 22.5kc/s modulated with 1000kc/s
FM 75.5kc/s modulated with 1000kc/s
FM mains (50hz modulated), Ext FM, CW
XTAL check
Female RF connector
Mains F.M. deviation > 150
X-Sweep Phase/Output
No external adjustment for dial

I still have the covers off so was able to access the crystal check socket from the rear. The output was approximately sinusoidal at a frequency of 5.14MHz and an amplitude of 250mVpp. The insertion point is about 3/8in wide but the connector inside looks like typical 4mm banana size so a millimetre or two smaller. I wonder whether anyone has or has seen a "Probe Unit P.N.10670" ?
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Old 4th Jul 2020, 11:12 am   #22
OldTechFan96
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Default Re: Advance type 63A

The FM2 socket on the 63 is where the sweep output comes from.

Do you have a dual channel oscilloscope? You could feed the the output of the crystal into channel 1 and feed the RF output into channel 2 so you can 'see' the beating procedure. I don't know how well this would work, just a random idea.

It might be worth posting in the 'Wanted' section to see if anybody has a service manual for the 63A, or the probe unit.
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Old 5th Jul 2020, 11:43 am   #23
WaveyDipole
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Default Re: Advance type 63A

I experimented with connecting a high impedance crystal earpiece to the PHONES (CAL OUTPUT/XTAL CHECK) connector and turning the Frequency knob. A high pitch whistle with varying pitch akin to the heterodynes on a radio are emitted when the pointer is near a multiple of 5MHz marker on the dial. There is a null point in the centre which presumaly indicates when exact synchronisation is reached with a multiple of the frequency of the crystal. Since the crystal itself was a little of the 5MHz mark, the null points didn't quite line up but they were pretty close on all bands.

Note to self - make sure that the modulation knob is set to the XTAL CHECK position!
Otherwise a VERY loud whistle is emitted!

I have replaced one cracked Hunts 0.02 capacitor at the X-Sweep connector. I intend to replace the one at the Ext FM input as neither of these are critical to the oscillator. The remainder will require careful consideration. I think the two at the crystal check output will be safe to replace, but I think any others will need to be done one at a time monitoring the effects, or perhaps are best left undisturbed for now.
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Old Yesterday, 3:38 pm   #24
WaveyDipole
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Default Re: Advance type 63A

I have been looking at the two capacitors marked on the attached circuit diagram. The capacitors used in these positions are Hunts Mouldseals.

C10 is a single 0.005 but in the position of C11 there are actually two capacitors: a 0.05 and a 0.005 in parallel. I have a couple of questions:

Firstly, is the purpose of C11 filtering or bypass? Is the presence of two capacitors meant to deal with two different frequency bands, or was this a matter of factory selected correction? The 0.05 capacitor has a pretty wide 20% tolerance. Both the 0.05 and 0.005 seems to be original factory fitted Hunts black Mouldseals.

I have a suitable 0.047 to replace the 0.05, but my second question is whether ceramic capacitors of appropriate voltage would be suitable to replace both of the original 0.005 paper capacitors? I have a number of decent 0.0047 or 4700pF at 1kv, but might the micro-phonic properties of ceramic disc capacitors pose a problem here? I rather suspect that it shouln't be a problem for C10, but am not sure about the C11 pair.

There is also the matter of the adjacent C33 which is a 0.04. This seems a rather specific value, but it is also a Hunts paper capacitor, so 20% tolerance or perhaps 10% at best for these black ones? It is not stated in the parts list or on the capacitor body. Would this have been specifically factory chosen?
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Old Yesterday, 4:23 pm   #25
ronbryan
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Default Re: Advance type 63A

C11 tunes the frequency of the 1kHz modulation oscillator. The 0.005uF cap was probably added in parallel to the 0.05uF 20% to get the frequency closer to 1kHz.

Ron
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Old Yesterday, 4:43 pm   #26
Electronpusher0
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Default Re: Advance type 63A

Wow, I never realised quite how much work would be involved in restoring it! You are doing a magnificent job, it clearly went to the right home.
Peter
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Old Yesterday, 6:55 pm   #27
WaveyDipole
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Default Re: Advance type 63A

Quote:
Originally Posted by ronbryan View Post
C11 tunes the frequency of the 1kHz modulation oscillator. The 0.005uF cap was probably added in parallel to the 0.05uF 20% to get the frequency closer to 1kHz.

Ron
Ron, thanks. In that case I dare say that the value is probably not that critical although I can well imagine that at the factory they would have wanted to get as close as possible to the 1000c/s value stated by the control on the front panel. On this unit the pitch of the modulation tone changes slightly when switching between AM, 20% FM and 75% FM in any case.

Quote:
Originally Posted by Electronpusher0 View Post
Wow, I never realised quite how much work would be involved in restoring it! You are doing a magnificent job, it clearly went to the right home.
Peter
Having taken out all those screws to get to the innards, I figured that I might as well do a thorough job on it. As we know, those Hunts Mouldseals are unreliable and the odd one was cracked so it seemed sensible to replace all of them while the unit is dismantled, although the ones inside the oscillator unit do need a little thinking about. That is why I am proceeding one feature at a time and observing the impact of each change. In any case, I am in no hurry.
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