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Old 31st Jul 2020, 8:40 pm   #1
3pinplug
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Default TF995A help please

Hi all
I have just aquired a:-
TF995A/2M
TF995A/3/S
I am looking to get a 9.8mhz, 1000hz, 30% Mod from the unit but i have some questions i hope fellow owners can help with:-
1.can you connect a frequency counter to these to check the output, if so can you let me know where to connect to.
2. I know it covers 9.8mhz but how do I get the “1000hz”.
3. Which is the better of the two units and what is the difference if any between the two units?
Thanks
Mark
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Old 1st Aug 2020, 10:01 am   #2
Dickie
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Default Re: TF995A help please

Hello,
There are several TF995 owners here so I'm sure one of them will be along in a while. To answer your questions:
1. The only place to connect a counter is on the main RF output. But the gen. does have a crystal check facility for calibration purposes.
2. The 1000Hz 30% is AM so you need to set it for internal modulation, 1kHz, 30% AM.
3. The "better" of the two units will more likely depend on their past history and treatment, rather than their technical specs. when they left the factory. By the nature of things, the TF995A/ 3 is likely to be a development of the /2, but the differences will not be great.

If you haven't got one already, I suggest you download a manual, available here:
https://elektrotanya.com/marconi_tf9.../download.html

As you can see its for a /5 variant, but most of it will be applicable to your models.
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Old 1st Aug 2020, 10:33 am   #3
David Simpson
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Default Re: TF995A help please

Mark, did your TF995A's come with a two way terminating unit - TM5551 ? This could be used to monitor your signal o/p. Or use a BNC "T" .
Some Digital Freq. Counters don't like modulated signals. Best to just us RF initially to check the frequency. As Dickie says, the 995A series, like the TF801B(CT394B) & the TF114H(CT452A) have an excellent crystal check facility. Acquiring a Manual will greatly assist you to understand setting-up procedures. If you are planning a signal injection project - switch on your sig gen before breakfast. This range of Marconi valve S/G's benefit from an hour's warm-up, stability-wise.
Most of the Hameg range of Oscilloscopes, as well as Goulds, Waveteks, and many other common 1970's to 1990's (ex college)scopes usually have a minimum bandwidth of 10MHz & some go up to 60 or 100MHz. Would be worth your while getting one, as they are handy for checking Mod depth. Modulation Meters can be jolly expensive.

Regards, David

Last edited by David Simpson; 1st Aug 2020 at 10:35 am. Reason: Alteration
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Old 1st Aug 2020, 5:20 pm   #4
3pinplug
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Default Re: TF995A help please

Hi Dickie
Thanks for the link to the manual.
So if i understand correctly if i select Internal Am then this produces 10000hz?
Thank you for your help, greatly appreciated.
Mark
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Old 1st Aug 2020, 5:24 pm   #5
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Default Re: TF995A help please

Hi David
Yes i have the tm551 so i will get a bnc an take a reading o a T, will do it without any mod and report back with my results.
Thank you ever so much!
I have the 2 units but the A/3/S has a piece of cable coming out the front cover with a BNC on the end, dont suppose you know what this is for as it is not labelled at at?
Regards
Mark
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Old 1st Aug 2020, 8:32 pm   #6
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Default Re: TF995A help please

Quote:
Originally Posted by 3pinplug View Post
Hi Dickie
Thanks for the link to the manual.
So if i understand correctly if i select Internal Am then this produces 10000hz?
Thank you for your help, greatly appreciated.
Mark
Hi Mark,

Yes but 1000Hz (c/s). Mine is the TF995B/2M but I have the original Marconi manual for the A/2M if you get stuck. By the way, you should be able to hear the 1000 Hz tone coming from inside the unit, I can on mine.

Andrew
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Last edited by Andrewausfa; 1st Aug 2020 at 8:45 pm.
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Old 1st Aug 2020, 9:31 pm   #7
3pinplug
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Default Re: TF995A help please

Hi Andrew
Sorry typed too many zero’s there! I will try with the frequency counter and see if i can hear the tone when the counter is not connected and let you know, thanks for the tip!
If you have the manual available to email yes I would very much like a copy, thankyou!
Regards
Mark
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Old 1st Aug 2020, 9:52 pm   #8
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Default Re: TF995A help please

Hi Mark,

I don't have a pdf copy, it's the real paper copy.

Andrew
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Old 2nd Aug 2020, 10:35 am   #9
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Default Re: TF995A help please

Aye Mark, the BNC lead is for connecting to the Terminating Unit. David
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Old 2nd Aug 2020, 11:05 am   #10
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Default Re: TF995A help please

Quote:
Originally Posted by David Simpson View Post
Aye Mark, the BNC lead is for connecting to the Terminating Unit. David
Or, if you haven't got the Terminating Unit, it's simply just the output connection for the generator - that's if it's the same as my TF995B/2. On mine the cable emerges from the case via the right-hand handle recess.

Mike
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Old 3rd Aug 2020, 1:26 pm   #11
3pinplug
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Default Re: TF995A help please

Hi all
On the A2 model i found that the lead emerging from inside the right handle had the wrong bnc plug on the end to fit to the terminating unit, luckily i had a T adaptor as suggested and managed to hook it up. I ran an unmod signal straight into the signal generator and “ nothing”, i also tried a mod signal but no tone heard. The bnc on the cable from the right handle looks to have been fitted by someone so will have to check that.
The A3 unit has a kettle power connector so i have had to order a lead, when that is here i will test and see if i get the same result!

Last edited by 3pinplug; 3rd Aug 2020 at 1:27 pm. Reason: Typos
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Old 3rd Aug 2020, 8:04 pm   #12
Dickie
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Default Re: TF995A help please

I ran an unmod signal straight into the signal generator and “ nothing”

Need a bit of clarification here. The only signal you would ever run into a signal generator would be an external modulation signal.
If you meant you ran the unmod signal generator output straight into your counter that makes more sense. If the counter registers "nothing" with the output level at maximum you have a problem!
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Old 3rd Aug 2020, 8:39 pm   #13
3pinplug
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Default Re: TF995A help please

Hi richard
Yes thats what i did with unmod signal as thought i had to do it that way, will try it again as you suggest.
Thanks
Mark
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Old 3rd Aug 2020, 8:50 pm   #14
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Default Re: TF995A help please

It's well worth checking the integrity of that BNC output line socket. I spent ages looking for a "no output" problem on my TF995B when I first got it, and it turned out to be just a mess of broken connections in the BNC line socket. Just unscrew it and look inside - only takes a few minutes to check!

Mike
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Old 4th Aug 2020, 6:49 am   #15
3pinplug
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Default Re: TF995A help please

Hi all
Quick update on few things i have tried and found:

Andrewausfa- on the A2 unit i can actually hear the 1000hz tone as you mention, it is also there on the A3 unit but a lot quieter.

I have checked the bnc plug for continuity with the end of the cable and all god to the pin and outer of the bnc plug and no shorts. I opened up the A2 unit and where the bnc cable goes into the unit there was a piece of exposed braid at a mounting point ( looks factory) so i checked continuity from there to end of cable at bnc connector and all good. Haven't managed to check continuity for the core of the bnc cable as to do that i have to remove a cover on the back of the front plate the cable goes into .
Still getting no reading on my frequency counter on either unit , i have a 100mhz oscilloscope could i try that to check for output at the terminating unit, its weird the frequency counter not showing anything at all on both.
Any ides on next step very much appreciated and thank to you all for your help to daye its very much appreciated.
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Old 4th Aug 2020, 8:11 am   #16
Dickie
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Default Re: TF995A help please

Yes, do try the 'scope! Much more diagnostic than a counter.
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Old 4th Aug 2020, 11:28 am   #17
David Simpson
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Default Re: TF995A help please

Mark, have you had a shufti through the Manuals setting-up procedure? Have you got "Set RF" & "Set Mod" readings on the Meter? This range of Marconi Sig Gens can also have O/P Attenuator problems which might need attention.

Regards, David
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Old 4th Aug 2020, 5:26 pm   #18
3pinplug
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Default Re: TF995A help please

Hi all

Right some further progress, tried scope on the output - nothing! So doesn't seem to be putting out a signal, however I followed the manual as suggested and found this!
1.Set Mod Selector Switch to Int Mod AM.
2. The manual didn't mention this but I flicked the Carrier switch to on - assuming this was right?.
3. Adjusted Set Carrier to Set RF mark on the meter.
4.Then held the Meter Reads switch in the AM position - needle immediately pegged to the right side of the gauge and stayed there even when moving the Set Mod knob, it didn't move the needle at all - is that supposed to happen as the manual appears to suggest that I should be able to adjust the meter needle at that point?
5. When doing this I can hear the tone and the Set Mod knob at the two extremes of adjustment either turns tone off or I can hear the tone.

Does this sound like an O/P attenuator problem as mentioned or something else from now following the manual procedure?

Thanks

Mark
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Old 4th Aug 2020, 7:28 pm   #19
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Default Re: TF995A help please

Hello Mark,

You have more than one fault here, by the sound of it. The first thing to do is to check that you have some sort of continuity in the output lead back to the attenuator.

With the TF995A switched off, set the attenuators to maximum output. Connect your DMM set to read ohms between the outer of the BNC connector and the pin. You should measure something around 121 ohms. If you see a short circuit or an open circuit that will explain why you can't see the output on the oscilloscope. The attenuator could be faulty but more likely is that the output cable isn't connected properly either at the BNC plug or at the other end.

The meter shouldn't peg on the end stop when you try to measure the AM modulation depth. It should move as you adjust the modulation level control as you expect. I would check C54, which is probably a paper capacitor and likely to be leaking. The leakage will provide sufficient current to the meter to cause the problem you have described.

Paula
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Old 4th Aug 2020, 8:32 pm   #20
3pinplug
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Default Re: TF995A help please

Hi Paula

Many thanks for this it has identified a problem that I missed earlier on!

I checked outer to inner on the BNC as you suggested and got 167ohms initially, however with the probes still on I just moved a bit and noticed i then showed 1800ohms, 1600ohms, 1700ohms then a short.

Randomly moving the cable around and moving it to certain positions slowly the resistance was fluctuating all over the place and I could hold it in certain positions with a complete short showing.

So definitley either the cable or the plug its very intermittent, probably bes t to replace the whole lot if that is possible.

I will now hunt C54 and see what it looks like in there, the manual says

C54 = paper 0.1uf +/-25% 250v DC

Could you please advise on what modern equivalent I can replace a paper cap with whilst I am checking C54 in the unit, as not overly familiar with paper caps or do i have to get one of those.

Thank you for your help, everyone has been so great with their assistance and patience as I am still learning.

Mark
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