开云体育

I got the distinct feeling that the stuff I am working on has been gone through. It dawned on me it was, by me ! But it also was before me. this is one we set aside a couple years ago to get a few other things done.

This one had no trace. The -8 supply was out with a shorted bridge rectifier which I took care of and still no trace except for in beam find. This one was a bit scary because it kinda looked like one of those unobtanium ICs might be bad. Luckily they weren't.

So as I get the weird ones as usual, I find the beam is banked of course and set out to find out why. I am not finding much so then I started unplugging things, one was a short. It was the chop banking from J349. The little wire on the end was bent over and shorting to ground. I corrected that and I got channel 2 but not 1. The symptom set was initially a bit confusing because in XY there was no spot and I know it unblanks on that setting. It was deflected (far far) off screen.

So looking around I had no choice but to just go through the stages, it had high DC offset so I was looking for that. Well I found it. One of the diff pair was missing, Q182 to be exact. I temporarily replaced it with a 2N3906 and she worked.

I would rather have the right transistor in there, it calls for an SAB4113. Google disavows any knowledge of such a part number, at least for anything like this.

The new 2N3906 has long leads and I don't want to cut them anyway, plus I don't really want it left in there. I found a couple of transistors in an old 422 carcass which seem to work fine. However I can't be sure of their frequency response. In the manual for the 422 it says "replaceable by 2N3251". That transistor has an FT of 300 MHz which should be adequate BUT, replaceable by does not mean that's what it is. The 422 is only a 15MHz scope so who knows ?

I can confirm that it does produce a waveform at 83MHz, the highest generator I have at the time but is not suitable for aligning this. It is also not a clean sine wave and I can see odd order harmonics on it so it is confirmed to hit 200 MHz, but is the response flat ? Better to just use the right part if possible.

Thing is I know the frequency, it is obvious. Silicon, PNP all the basics but I have no idea of the gain. I think it is 200 for the 2N3251 but that again does not mean that the original transistor had that gain. Nothing indicates that it is a Darlington, they almost always mention that so I am confident it is not.

So does anyone know of a "recommended" replacement for that SAB4113 or perhaps more detailed specs so I can find one ?

Next problem is a slight overshot in channel 1, I doubt the FT is too high causing that. They wouldn't hand pick for FT would they ? I looked for things like open variable resistors or whatever that could cause overpeaking but all of them I found can only cause underpeaking if open, and they don't short. Usually...

The overpeaking is occurring on the vertical preamp board indicated by the fact that it shows up at T178. For reference I checked and it is not there at T278 which is the channel 2 equivalent, using the same input.

Now on this one I found I had to clean the attenuator. On that recent 465B there was at least one bad solder connection, the first I had ever seen in a Tek. Cleaning was not an issue.

You may remember that I need the attenuator for the 465B because the PC foils are not in the greatest of shape. I am not all that bad at soldering but this thing did not want to cooperate, it is as if there is a contaminant on the foils that was there before the original soldering, something I HAVE seen before but not in a Tek. (the solder balled up on the pin of the /100 module)

Originally I thought the attenuator boards were not compatible. I saw the section later in the vertical which on the 465, 465M and a few other similar models does the /2 and /4 functions downstream whereas the /10 and /100 is handled right at the front end. The 475 looked like that but it isn't. It is just like the 465B except that the 2mV/div range is added. The board is the same but the cam has an extra position which leaves the main attenuator in 5mV/div but kicks up the gain later in the chain.

That means if anyone would be willing to part with that part from a 475 I can use it in the 465B.

Other than the transistor and attenuator, I would like to know if there is a way to check the frequency response. Not necessarily to align it, just to check it. I suspect it will be fine once I find the cause of the overpeaking, about which I am accepting suggestions as well. I have not turned up anything as of yet.

What the hell can give it too much high frequency response ? Maybe a filter after a decoupling coil on a power supply line ?

Any ideas on that are welcome. In fact I have other ones with not so perfect response and eventually I would like to see all them perfect as well, whether they are to be sold or kept.

Thanks in advance.