Re: TG501 is missing 1 ns output


Bob Koller <testtech@...>
 

The output circuit may be misadjusted. It requires a good output cable and termination, a good scope, and cre to adjust. Also clean the push button switches.


From: raymonddompfrank
To: TekScopes@...
Sent: Sunday, July 22, 2012 5:22 PM
Subject: [TekScopes] Re: TG501 is missing 1 ns output

 
Hi Tom,
Thanks for your reaction.
I'm using a 7904+7A19. I know the 1ns signal is sort of a sine. I have no problem seeing 2ns, 6-8 divisions no problem. 1ns output is completely flat line, which convinced me there's no signal. You made me think though, I'll have to measure attenuation with SG504 @ 500MHz and 1GHz respectively, taking into account >750mV and > 200 mV (when circuits tuned) and see if i made a mistake expecting a certain amplitude. With my SG504, I can see 1 GHz as well, although heavily attenuated.

--- In TekScopes@..., "Tom Miller" wrote:
>
> What are you using to see the 1 ns waveform? If I remember right, it is just a 1 GHz sine output.
> CR515 is a schottky hot carrier diode. You should be able to find one.
>
> You can get 100 of them for $12 here. This one is in a SOT-23 package but should go in just fine.
>
> http://www.ebay.com/itm/On-Semi-Schottky-Barrier-Diode-SOT-23-MMBD101LT1-100pcs-/400025535496?pt=LH_DefaultDomain_0&hash=item5d23614408
>
>
> Tom
>
>
> ----- Original Message -----
> From: raymonddompfrank
> To: TekScopes@...
> Sent: Sunday, July 22, 2012 6:31 PM
> Subject: [TekScopes] TG501 is missing 1 ns output
>
>
>
> A few days ago, I purchased a TG501 time mark generator. It is fully functional, with the exception of the 1 ns output, which is completely silent (duly disclosed by seller).
> I have no reason to assume otherwise than that it just quit working; the relevant areas on the PCB and all components on it seem clean and dry (no cleaning residue), in good condition and untouched by rough hands. Especially, the tuning capacitors seem undamaged (no damage or shorts visible under magnification). Changing their settings has no electrically visible effect. I am aware of the critical importance of the mechanical condition with this circuit.
>
> All signals > 1 ns are available and their amplitude is according to specs, especially 2 ns (> 750mv pp).
>
> Although I have very limited knowledge of HF circuits, it seems to me that CR515 is by far the best candidate to put the blame on. Since the solder pads for it are covered by a large sheet metal screen, albeit with a little hole where the pads are, they are difficult to reach. Removing the screen by removing the solder (wick) and lifting it doesn't look very attractive to me because of the risk of damage to the PCB.
>
> I have the following questions:
>
> 1. Am I right in assuming that it's quite likely a defective CR515 and if not, where else to look - and how?
> 2. Any special hints re. how to replace CR515? Of course, in general I know how to replace a TO-92 package that's soldered flush with the PCB.
> 3. One of my maintenance docs mentions CR515 as an MBD101. This is difficult to procure. I can easily get a MBD301. Would it be silly to try it, mostly because its capacitance is greater or other reason?
>
> Thanks in advance!
>
> Raymond
>



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