Thread: BK 415 Repair
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Old 12-08-2017, 07:04 PM
Crist Rigott Crist Rigott is offline
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Join Date: Oct 2015
Location: Waxahachie, TX
Posts: 1,506
BK 415 Repair

A generous member of this Forum gave me a BK 415 and a 1077B abd 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!

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.



So out came the MM and I did some voltage checks per this page:



and this page:



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.







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.




Last edited by Crist Rigott; 12-08-2017 at 08:01 PM.
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