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 Post subject: B&K 415 Sweep/Marker Generator
PostPosted: Oct Tue 27, 2020 11:53 pm 
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Location: St. Louis, MO
I got this thing 2 months or so ago and I've been working on it when I have time. I recapped it and replaced all of the out of spec resistors, probably 30% of them. Now it's time to adjust this thing back to working order. I didn't get the original test probes with it and the original 4 outputs in the middle bottom were Switchcraft microphone connectors, so I swapped those 4 for BNC so I could attach all my other cables and probes to them. I'm trying to restore this thing to working order.

I'm on section 12.4.1 of the book by the way, Marker Oscillator Adjustments. I'm not sure how to connect this thing's "horizontal output" to a modern scope like my Rigol DS1054Z, so I drug out my old Eico 470, which does have a horizontal input. I hooked up vertical to vertical and I did get markers like I'm supposed to... after I figured out that I'm supposed to adjust the Center Frequency wheel to see them all. I'm an idiot. Anyway, I don't have all my markers. I'm missing 41.67 MHz and 42.75 MHz. I know that 41.67's transistor is bad. It failed against the voltage chart in the book. I noticed that the transistors are the same for each marker frequency, so I swapped in one from another frequency and 41.67 worked again.

Now to get a replacement transistor. According the the manual, it's part # 176-005-9-001. I can't get any good info off the transistor's label, but according to my searches, an equivalent is NTE108. In fact, the parts list in the paper manual included with mine had "108" penciled next to that part number. I think I found a modern equivalent to NTE108 as this one at Mouser. 42.75 however, I'm still working on why I get no output from it. It's voltages matched the chart.

So I got several questions.

1. How do I set up a modern scope to be able to use an external horizontal input?
2. Is that transistor a good equivalent?
3. Do I need the test probes? The demodulator probe is going on eBay for about as much as I paid for this thing, plus I'd have to convert it to BNC. The switchable 75/300 ohm output probe and the 10kohm isolation probe shouldn't be too hard to work around.
4. Anyone got any tips for what to check for 42.75?

After I typed that, I realized that Digikey actually has NTE108 in stock...


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Wed 28, 2020 1:01 am 
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The NTE108 is available from everywhere, for about $1. Digikey, Allied, Newark.... others. Probably not Mouser tho.

It's a general purpose RF transistor and should work fine. https://www.newark.com/nte-electronics/nte108/rf-transistor-npn-15v-600mhz-to/dp/29C4417

You can make your own probes. There's nothing particularly magic about them.

As far as using your modern digital scope as an X-Y .... every one is different, and some apparently (from other threads) can't do that at all. But you'll have to read the manual for yours, or hope someone here has the same scope and has done it. It's simplicity itself on any older scope, but seems to be a mystery on modern gear for some reason. There should be a switch somewhere to allow you to switch one of the normal inputs into the X channel input. Likely a software switch in yours.... buried 400 levels deep in some arcane menu.

As far as your missing 42.75 ... there's nothing special about it. It's the same as the others. Replace the transistor. You can check the other parts. Might be a bad crystal I suppose...rare but it happens. Maybe try resoldering all the parts in that marker circuit. You could swap a crystal from one of the others to see what happens. Then you'll end up with TWO that don't work lol.

Don't overlook the SWITCH for that marker as being the culprit.. dirty or oxidized. Exercise it a few million times.

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Wed 28, 2020 4:23 am 
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Location: St. Louis, MO
It WAS the crystal! Wow. I would've never in a million years suspected that. I swapped out the crystal from the 41.67 section and 42.75 MHz came alive again.

So now I need a 42.75 MHz crystal according to the parts sheet. Any idea where I could find one?


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Wed 28, 2020 10:27 am 
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Put the bad crystal back in. It may work now lol.

Finding one may be difficult. You need to know a little more about the type of crystal it is. I'm no expert but I do know that there are different cuts. Fundamdntal, overtone etc. Don't know if it would even matter in this thing.

Best bet might just be someone with a donor instrument.

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Wed 28, 2020 4:45 pm 
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I believe you would need a third overtone, AT cut, parallel operation crystal. You would have to determine the physical crystal holder package type, HC48, HC49, HC50 or HC50.
Finding one of the right frequency may be the hardest part, 42.75 MHz being a frequency probably only used in TV test equipment.

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Wed 28, 2020 10:40 pm 
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Location: St. Louis, MO
Barry H Bennett wrote:
Put the bad crystal back in. It may work now lol.


I did that and it works now.



I hate electronics. I wasn't working anywhere near it when doing resistors!


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Wed 28, 2020 11:52 pm 
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It was probably just a cold solder joint. It happens in stuff this old. Makes it for experts only! (or the lucky)

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Thu 29, 2020 2:52 am 
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What about substituting my own cable for the output cable? It doesn't look like I'll need the 300 ohm output that often. Just use a BNC tee, a BNC 75 ohm terminator and BNC croc clips?


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Thu 29, 2020 8:14 pm 
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Have you checked with B&K for parts? They do support some old stuff.

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Fri 30, 2020 6:03 pm 
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Oh that's right, they are still around... Would I need to pay retail prices for 50 year old tech?


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Fri 30, 2020 11:12 pm 
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eh? you hope to actually FIND a part for 50 year old tech, and are concerned about the price? ummm. ;-)

Only way to find out, is to ask them. 8)

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Oct Sat 31, 2020 1:44 am 
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ZombieElvis wrote:
What about substituting my own cable for the output cable? It doesn't look like I'll need the 300 ohm output that often. Just use a BNC tee, a BNC 75 ohm terminator and BNC croc clips?

If you want to stay with 75 ohms, you may just use a BNC to F connector adapter instead of the clip leads.

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Mon 02, 2020 12:48 am 
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Notimetolooz wrote:
ZombieElvis wrote:
What about substituting my own cable for the output cable? It doesn't look like I'll need the 300 ohm output that often. Just use a BNC tee, a BNC 75 ohm terminator and BNC croc clips?

If you want to stay with 75 ohms, you may just use a BNC to F connector adapter instead of the clip leads.


They do make 75 ohm BNC terminators from what I've found. Well, I mean, terminator plugs with the 50 ohm BNC connector yet themselves are 75 ohm rated.


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Mon 02, 2020 1:21 am 
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There are two varieties of BNC, 50 and 75 Ohms. However, at these frequencies one needn't be concerned over the impedance of the connectors. It's necessary at VHF and above but not for ordinary radio work.


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Mon 02, 2020 1:38 am 
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Location: Dallas, TX
I meant that if you think you only need the 75 ohm, that implies that you will be only using the coaxial F antenna connector. The main purpose of the clip leads is to connect to the 300 ohm antenna connector.

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Thu 05, 2020 1:45 am 
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Location: St. Louis, MO
OOOOOOOOOOH, 300 ohm is the old dual screw connector! I haven't seen one of those since I was a kid!


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Thu 05, 2020 5:47 pm 
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Location: Dallas, TX
ZombieElvis wrote:
OOOOOOOOOOH, 300 ohm is the old dual screw connector! I haven't seen one of those since I was a kid!

The 300 ohm connection is for twinlead antenna wiring of course. Even when the VHF channel input has a 75 ohm F connector, the UHF tuner connections were 300 ohm for a bit. i guess the changeover happened after 1970, maybe 1980. FM radio still uses the 300 ohm.

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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Fri 06, 2020 12:30 am 
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Location: St. Louis, MO
I got an NTE108 in for that bad transistor and the crystal for 42.75 is behaving itself now... I'm ready to resume alignment.

What about the demodulator probe? Apparently Steps 12.4.1 through 12.4.3 require it. Does anyone know how to improvise one?


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Sun 08, 2020 8:35 pm 
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Does anyone know about the demodulator probe?


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 Post subject: Re: B&K 415 Sweep/Marker Generator
PostPosted: Nov Sun 08, 2020 10:25 pm 
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you can google how to build a simple demodulator (aka detector) probe. There are many all over the internet.

I have not been able to find a schematic for the one that B&K provided however. Assumed to be just a run of the mill demod probe, should not be critical.

Try one. ;-)

It may be easier and ultimately cheaper to just find a donor unit. They usually go pretty cheaply....shipping is the killer. By cheap I mean $25 or less.

_________________
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Last edited by Barry H Bennett on Jan Tue 05, 2021 7:19 pm, edited 1 time in total.

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