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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 4th Aug 2020, 9:18 pm   #21
frsimen
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Default Re: TF995A help please

Hello Mark,

That sounds like progress, which is encouraging.

You can replace the 0.1uF 250v paper capacitor with a 0.1uF 250v or more polyester capacitor. Look for one with axial leads, as that will make a neater replacement. Bear in mind that if that capacitor is faulty, other similar ones will probably be bad too. I had to replace several in my TF995A/1 when I first got it.

Paula
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Old 4th Aug 2020, 9:45 pm   #22
Boulevardier
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Default Re: TF995A help please

I've often wondered why Marconi chose to put the output on a flying lead on that range of generators. A fixed socket on the front panel would have been a lot more secure.

Mike
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Old 5th Aug 2020, 7:39 am   #23
3pinplug
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Default Re: TF995A help please

Hi all

Ordered the replacement capacitor online for C54 so will be a few days before I have it.

I have opened the unit and looked at the lead and this appears to be one full length of lead, it seems to disappear inside the attenuator control.

Is it a case of removing the knob at the front and then there will be a nut I can undo to remove the attenuator, also what grade cable do I need to replace this when I do it.

Wondering if the cable is broken where that exposed braid is as it looks to be at a funny angle, once i manage to get the whole lead removed I will test this out and see where the short / break is.

Attach some pics so you can see what I am seeing in case different to other peoples units.
Thanks
Mark
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Old 5th Aug 2020, 10:38 am   #24
David Simpson
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Default Re: TF995A help please

What a number of 995A owners do is replace the loose lead with a fixed BNC plug outlet on the front panel, as I've done on my A/5. Those leads are prone to being phooked, so will need replacing anyway.
Like Cpl.Jones said 'they don't like it up em' attenuator-wise, if in the past someone has signal injected an Anode circuit with a hefty HT on it, or shorted the leads with max o/p selected. Some folk who do a lot of signal injection/tracing also make up safety signal probes for both RF & AF.
In general, with this range of Marconi Sig Gens, it best to get in the habit of starting out with attenuators set at the lowest o/p, then carefully rise to the expected level. Aerial i/p circuits handle microvolts, roughly speaking, and oscillator & IF circuits - millivolts, regarding domestic radios. Only if you were using a S/Gen as a VFO source in a Transmitter, might you need the 10mV x 10 max attenuator setting. Then, at the end of the day, before switching off - return knobs & pots etc. to minimum.
Quite frankly, for basic radio testing, I just use my trusty AVO AWO for LW, MW & IF tests, as it jolly stable, but I keep it monitored by a reliable but cheapo(Dong Kwang) modern chinko digital counter. HF & VHF - I use the 995A/5.

Regards, David
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Old 5th Aug 2020, 5:32 pm   #25
3pinplug
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Default Re: TF995A help please

Hi all

Attenuator off and lead removed, now off I can confirm the short was without doubt where that exposed braid was clamped to the metal plate.

I have managed to get the end of the wire out the attenuator, there was a single grub screw which I removed and the cable pulled out, however it came out with two metal shells (see pics) and a component soldered to the end.

Can I ask a few further questions before proceeding.

1. Can I use RG58/U cable as a replacement cable with a BNC on the end, I already have a cable, so if i cut one end off I could fit it straight in the attenuator and it comes with factory fit bnc connector so one less thing to worry about there.
2. Where do the two metal semicircular shells connect to inside the attenuator when putting the cable back in?
3. Do I simply push the cable back in with that component on the end or does that component connect to something?

Please see pics.

Thanks

Mark
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Old 5th Aug 2020, 7:24 pm   #26
frsimen
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Default Re: TF995A help please

Hello Mark,

I had another look at the attenuator arrangement and the figure of 121 ohms I quoted yesterday is incorrect. You should measure nearer 75 ohms, confirmed by measurement on my TF995A/1. That being the case, you will need to use 75 ohms BNC connectors and 75 ohm coax, preferably a flexible type if you are retaining the existing arrangement.

RG58 is a 50 ohm cable, which will not be good for the accuracy of output level/impedances, particularly on the highest frequency range. It will get things going again, in the absence of anything more suitable.

Possibly the way around the problem is to cut the existing cable where it is faulty and reterminate it.

Paula
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Old 6th Aug 2020, 6:22 am   #27
3pinplug
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Default Re: TF995A help please

Hi Paula
I have ordered a 75ohm bnc cable, plan to cut one end off and fit inside the attenuator. Waiting for parts at present so hopefully be here for weekend. Do you know how the two metal shells fit when pushing the cable back in the attenuator, assuming these cover the screen and the end of the component must contact something inside at end of recess. Thanks for all your help its been great.
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Old 6th Aug 2020, 7:58 am   #28
Dickie
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Default Re: TF995A help please

That component at the end of the cable must be R99, a 37R5 1% resistor. I would be surprised if it was simply a push fit into the attenuator body, more likely it will be soldered. I suspect you might have to remove and dismantle the attenuator to access the solder joint. You might need to find a new R99, depending on where the fracture has occured. So the process would be to re-insert the cable with shells over the exposed braid, position it correctly, solder it all up, and then clamp the cable by refitting the screw.
Not having a TF995 to look at, this is all guesswork.
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Richard, BVWS member
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Old 6th Aug 2020, 9:20 am   #29
frsimen
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Default Re: TF995A help please

Hello Mark,

I don't know the answer but what Richard (Dickie) said above seems sensible.

I have attached what little information there is in the TF995A/5 manual regarding dismantling the attenuators, in case that is of any help.

Paula
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Old 7th Aug 2020, 7:38 am   #30
3pinplug
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Default Re: TF995A help please

Hi Richard and Paula
Yes seems a bit strange it has the component on the end, the attenuator has another cable joined to it as well but that one i can see the core and braid and how they are fitted so i will get the thing free and before taking it apart have a good 360 look around and see if i can repair it without dismantling the whole switch. If i do have to take it apart i will post some pics as if anyone else has to do this then at least we have some do’ s and don’ts plus pics. Will look over the weekend and come back with my findings. Hopefully i can repair it but if not then i still have the other unit to look at as a back up plan.
Regards
Mark
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Old 7th Aug 2020, 10:55 am   #31
David Simpson
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Default Re: TF995A help please

That's a great idea Mark, posting pictures of a repair project for other to follow. There must be heaps & heaps of Marconi's range of 1950's,60's Sig Gens out there in the vintage radio fraternity which are now showing their age. Once you've sorted out your 995A, perhaps the Moderators might consider making it a "Sticky" Thread for others to study.
I feel guilty, having serviced & repaired RAF Marconi's S/G's from the horrendously heavy TF144G's up to the equally heavy TF801B's, back in the 60's, for forgetting so much, and not keeping personal notes/records. Back then, I just considered them another job on my bench. But back then, even though they were newish or SOTA, they still got phooked by trainee's & supposedly qualified servicemen. Nowadays they'll need lots of TLC to keep them going. When I left the RAF in '75, I never, never, expected to be involved with avionics, test equipment, domestic radios etc., ever ever again. Other more interesting & totally different horizons beckoned.
Keep at it Mark.

Regards, David
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