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 Post subject: B&K 415 Repair
PostPosted: Dec Sat 09, 2017 2:53 am 
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Joined: Jan Sun 18, 2015 5:01 am
Posts: 1459
Location: Waxahachie Texas
A generous member of the other Forum gave me a BK 415 and a 1077B and most all the cables. A big thank you to you know who you are.

When I got home and finished the Zenith 19" Porthole chassis I did the first IF alignment using my Knight KG-687 sweep generator. Then I remembered the 415!

I cleaned it up and changed the mic connectors for BNC and made up the missing cable. All very easy. I remembered that Bob Andersen had a video on using one of these. After searching his videos I found where he got one and turned it on for the first time and did a preliminary check of his 415. so I did the same thing. Uh, oh. mine wasn't working very well at all. Looks like no 44mc sweep!


Attachments:
File comment: I DL the manual and printed out the pages I thought I'd need. The first picture shows what to check when there is no IF sweep. Cool.
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File comment: So out came the MM and I did some voltage checks per this page:
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File comment: And this page:
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100_8566a.JPG [ 64.66 KiB | Viewed 1125 times ]

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 Post subject: Re: B&K 415 Repair
PostPosted: Dec Sat 09, 2017 2:56 am 
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Joined: Jan Sun 18, 2015 5:01 am
Posts: 1459
Location: Waxahachie Texas
I also checked the 10.7mc section of the same board just as a comparison. Looks like the 44mc sweep transistor Q-201 doesn't match the voltage chart. Might need a new transistor. It is marked as a MRF 502. I checked the other components near it and didn't see anything out of order.

Then I noticed the manual has a list of a bunch of voltage "TP" spots. I then check out each one. Then I noticed a resistor that looked like it was burnt. Not a char burn but a light color dusting on it. It is R56 on the Waveform Generator. It's a 100 Ohm 1/2w resistor. I replaced it with a 100 Ohm 1W resistor. I turned the 415 back on. In a few seconds it got hot also. Hmm...something must be shorted. I looked on the schematic and saw that a tantalum capacitor C52 goes between the 100 Ohm resistor and ground. I lifted the ground lead and turn the 415 back on. R56 didn't get hot! I didn't have a 10uf 35V tantalum but I did have a 6.8uf 25V one and used that for now. I've ordered some caps and a few MRF 502 transistors.


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 Post subject: Re: B&K 415 Repair
PostPosted: Dec Sat 09, 2017 2:58 am 
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Joined: Jan Sun 18, 2015 5:01 am
Posts: 1459
Location: Waxahachie Texas
The manual has a page that showed some wave forms and what their voltages should be. I then used my scope to check them. All voltages looked reasonable with the exception of the base voltage on both Q-12 and Q-13. The voltage measured was about 1/2 of what they should be. 2V vs 1V and 1V vs .5V. I haven't had a chance to look into it any further.

Here is a picture of the 44mc and 10.7 sweep board and its schematic just for reference.


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 Post subject: Re: B&K 415 Repair
PostPosted: Dec Sat 09, 2017 2:58 am 
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Joined: Jan Sun 18, 2015 5:01 am
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Location: Waxahachie Texas
I noticed these caps across the line input. Should I replace these with safety caps


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 Post subject: Re: B&K 415 Repair
PostPosted: Dec Sat 09, 2017 5:19 am 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
Crist Rigotti wrote:
I noticed these caps across the line input. Should I replace these with safety caps

The ceramic caps are very reliable. I have replaced them with safety caps in other equipment however. i guess it is sort of a coin toss.
By the way, this type of line input filter can cause problems, especially if the cap values are on the high side.
The caps go from each side of the power line to the chassis (and cabinet) ground. Because the caps do pass some 60 Hz current the caps form a voltage divider and would tend to put the ground at half the 120 V power voltage. If you have another piece of equipment that has the case grounded thru a three prong plug, there can be 60 V or so between the cases. It is current limited by the cap values so not much fireworks, but you might feel it. My philosophy is if it has a metal case it should be grounded through a three prong plug. If you install a three prong plug here it will short across one of the input caps, neutral to ground. That's not really a problem however.

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 Post subject: Re: B&K 415 Repair
PostPosted: Dec Sat 09, 2017 3:14 pm 
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Joined: May Wed 18, 2011 2:40 am
Posts: 4576
Location: Littleton, MA
Crist Rigotti wrote:
I noticed these caps across the line input. Should I replace these with safety caps

They are already AC rated, though not marked as UL-approved safety caps. I couldn't find a spec sheet on them as AVX no longer manufactures that type of capacitor.

I'd leave them.

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 Post subject: Re: B&K 415 Repair
PostPosted: Dec Wed 13, 2017 7:08 pm 
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Joined: Jan Sun 18, 2015 5:01 am
Posts: 1459
Location: Waxahachie Texas
OK, I left them.

I replaced the resistor, Tantalum cap, and the transistor Q-201 and all is well!

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