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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
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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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Old 10th Aug 2020, 7:48 pm   #32
3pinplug
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Default Re: TF995A help please

Hi David

Thank you for the kind words, making some progress but still running into "opportunities" as I like to refer to the ongoing problems!

So heres where I am...
Struggling to desolder the sielded wire that connects to the adjacent attenuator, want to try and get this off in one piece if i can, so it can go back as is, otherwise i am going to have to venture into the other attenuator and I could end up in the same position I am in with this one. Managed to desolder the center core of the lead but the solder on the braid simply will not melt with my soldering iron see first pic.

I ordered a 75ohm BNC lead and thought when it arrived I could simply cut the plug off one end and fit the lead, how naive was I!!!, the cable diameter was minimal at only 2.5mm so it was too thin. I have measured and you need a 5.4mm outer on the lead in order for it to fit in the recess with the two half shells when you assemble it. I have done my homework this time and now found some 75 ohm RG59u coax which looks ideal at 5.4mm and its for RF applications.

Once I get the attenutaor out I will be able to fully look in the recess and work out if there is an opening at the end where the leg of the component pushes through as I can't imagine at present how that all fits inside that recess. So i will continue trying to remove the solder from the braid.

Now heres where I need some further help from the forum, I noticed some caps leaking "oil"? - does this mean I should replace them - see attached 3rd pic!

The caps are REF: C1, C37, C91, C16,C17,C18 and are all paper 0.1uf +/-20% 350v DC - do i replace these with polyester type capacitors as well?

Thanks

Mark
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Old 11th Aug 2020, 7:52 am   #33
frsimen
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Default Re: TF995A help please

Hello Mark,

There is one slight problem with RG59 coax. It has a solid copper coated steel centre conductor, which means that it isn't very flexible. That would be fine if you were making a connection to a panel socket, but isn't ideal if you are retaining the flying lead approach used in the TF995. A better choice would be URM70 coax, again 75 ohm but with a stranded copper centre conductor, which makes it much more flexible. URM70 is 5.8mm diameter.

I ended up changing all the paper capacitors, as most of them were electrically leaky. I don't know if yours are leaking oil or if it is some sort of breakdown of the outer sleeving. Either way, it's worth considering changing them now to avoid future trouble.

You will need a high wattage soldering iron to tackle the chassis joint. The option is to cut the braid close to the old coax, so that you can solder the braid of your replacement coax to the stub of braid that is left. That won't need as much heat. Also, have you considered just reterminating the existing coax, it will only be a few centimeters shorter than it is now if you do that?

Paula
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Old 11th Aug 2020, 11:10 am   #34
David Simpson
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Thumbs down Re: TF995A help please

Mark, Paula's advice is spot-on. Don't fanny about with a stiff co-ax fly lead - fit a BNC outlet on the front panel.
Your capacitors are leaking electrolyte, so definitely change to polyester. The electrolyte is also mildly corrosive, so give everything nearby a good clean. In fact, whilst the chassis is out of the cabinet - have a good shufti to see if there are any other suspect capacitors. The 995's have a super detachable PSU - give that a good check-out whilst you're at it.
The normal range of temp for day to day circuitry soldering is approx. 340 to 360 deg. Hard soldering to chassis points might need 360 - 380 deg. Also remember, most vintage test equipment will have been soldered with good old 60/40 flux-cored solder. If you are using "lead-free" solder, you'll encounter problems. In days past, for tinning wire ends & basic soldering jointing, a lot of folk just used a simple mains plug-in soldering iron which just had the "Wattage" stamped on them. Say - 20W or 30W or whatever, and it took us old-uns hours & hours of practice to judge how many seconds to apply the iron. Nah, nah, these days I would advise vintage radio enthusiasts to use a decent TC soldering station, with a selection of interchangeable bits. Particularly if you are delving inside test equipment. Bodge a soldering joint in a Ferranti 145 or DAC90, or whatever - no problem, easily repairable. Bodge a joint in a Marconi S/G or HP 'scope - you're asking for trouble.

Regards, David
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Old 12th Aug 2020, 12:42 am   #35
Silicon
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Default Re: TF995A help please

I worked on a TF995 recently.

Those metal clad capacitors with clear plastic sleeves are mostly paper capacitors.
I think the oily appearance of the plastic is due to some kind of chemical ageing effect.

A similar clear plastic tube used as a flexible conduit for the wires to the power supply unit became very rigid and 'smokey' in appearance.
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Old 18th Aug 2020, 7:07 am   #36
3pinplug
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Default Re: TF995A help please

Hi all

The work continues! I have managed using a torch and my phone to locate where in the recess the end of the resistor fitted to the cable that pushes into the recess attached to, I have morked the remainder of the leg. I have managed to get a pic which I attach for you to see. My next dilemma is how to get at this to fit the repalcement resistor as not sure at this point where it terminates and obviously need to get at it.

It appears to go into the back of the attenuator so looking at the unti on the front the shaft is held on with a circlip, the rear of the attenuator has a pin and screw. I am thinking first off to remove the circlip which will hopefully allow me to then push the shaft through the block it is fitted to and remove it for further inspection, i don't fancy initially trying to remove that pin or undo the screw as I have no idea what they are holding.

Has anyone else ever taken one of these apart to get at the resistor an replaced it, I am thinking the answer may be no but thougth I would ask before I remove it in case theres anythign I need to be aware of.
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Old 18th Aug 2020, 7:38 am   #37
frsimen
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Default Re: TF995A help please

Hello Mark,

I've no idea how to remove the cover or circlip.

Do you actually need to change that resistor? Measure the resistance from the stub to the chassis. If it is close to 75 ohms, you don't need to change it.

Paula
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Old 18th Aug 2020, 8:01 am   #38
3pinplug
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Default Re: TF995A help please

Hi Paula

Yes that stub should is the leg of the resistor which should be attached to the resistor which is soldered to the center of the 75ohm cable. I will get another pic and post a bit later how I believe it should go but that is only my guess.

Regards

Mark
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Old 18th Aug 2020, 8:34 am   #39
frsimen
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Default Re: TF995A help please

Hello Mark,

Looking at the photo of the resistor side of the attenuator all the 16 attenuator resistors are shown clearly. That stub is either the coax end of R99 or the point where the attenuator end of R99 should be soldered to. Have you already removed R99? If not, that's where the coax connects to. Is that hole large enough for a soldering iron to reach the stub? If it is, you probably don't need to dismantle the attenuator any further unless R99 is still in place and faulty.

Paula
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Old 18th Aug 2020, 10:32 am   #40
David Simpson
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Default Re: TF995A help please

Mark, & folks, Paula has highlighted a servicing/repair problem that many VR enthusiasts might encounter - the right size of soldering iron bit for whatever soldering task is being encountered. If you are just using a bog standard "Solon" type iron with a 1/4" single slope bit, then awkward recesses etc. are going to be difficult. One might need a tapered bit or longer narrower bit, and so on. That why I, & quite a few others use & recommend using a temperature controlled soldering station, with an iron that accepts interchangeable bits.
They can be bought from RS & other suppliers. Or eBay. Or they come up for sale on the Forum at times. Or at BVWS Swapmeets.

Regards, David
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