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 Post subject: Schematic for BK Precision 1655
PostPosted: Jan Tue 15, 2008 7:37 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
Posts: 1735
Location: Fargo, North Dakota
Anyone have the schematic for the BK Precision 1655 variable AC power supply / isolation transformer? I got a used one yesterday, and the metering is screwed up. The meter grossly underestimates the output voltage and current (it was reading 60V with the voltage knob turned fully clockwise; checking the voltage at the isolated outputs with an external DMM shows that the correct voltage is being output). I opened up the unit and did not find any obvious cause except that one of the diodes appears to have been replaced (actually the new diode is just soldered across the old diode). I need the schematic to see what the component values are for the metering circuit so I can check to see if any components are out of spec. If anybody has the schematic I'd greatly appreciate it if you could share it with me or point me to a link /site with the schematic. I did google for it and came up with zilch.

Don


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 Post subject:
PostPosted: Jan Tue 15, 2008 7:50 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Mpls, Minnesota
I have both manuals, 1655 and 1655A

Dave


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 Post subject:
PostPosted: Jan Tue 15, 2008 8:08 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
Posts: 1735
Location: Fargo, North Dakota
Dave, I have the manuals for 1655 and 1655A which I dowloaded from the net, but neither one has the schematic. I know the schematic is available as an addendum but it was not part of the files I downloaded. Do you have the schematic in your manuals? If you do I would greatly appreciate it if you could email me a copy of it. (I need just the schematic, not the whole manual). Thanks!

Don


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 Post subject:
PostPosted: Jan Tue 15, 2008 8:14 pm 
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Joined: Jun Thu 15, 2006 1:21 am
Posts: 4683
Location: NE Ohio
I have the 1655 schematic and can scan & email same to whom ever needs it.

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Bruce


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 Post subject:
PostPosted: Jan Tue 15, 2008 9:13 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
Posts: 1735
Location: Fargo, North Dakota
Bruce Hagen wrote:
I have the 1655 schematic and can scan & email same to whom ever needs it.

Bruce, PM sent with my email address. Thank you very much!
Don


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 Post subject:
PostPosted: Jan Wed 16, 2008 1:37 am 
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Joined: Jun Thu 15, 2006 1:21 am
Posts: 4683
Location: NE Ohio
Schematic sent,

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Bruce


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 Post subject:
PostPosted: Jan Wed 16, 2008 10:20 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
Posts: 1735
Location: Fargo, North Dakota
Thanks to Bruce, I have the schematic and having been poring over it since yesterday. Unfortunately, I have not been able to figure out what is wrong. Obviously the meter is not getting the amount of current it needs to get to display the correct voltages (or current in the amps setting). Looking under the hood, diode D6 (an 1N67 germanium diode) has been bridged with an unknown diode by the previous owner (in appearance, the diode looks like diodes D4,5 7 and 8 in the schematic which are 1A 600V silicon diodes). I snipped one of the leads of this diode and the meter movement goes dead - shows 0 with the voltage control knob fully turned to max. I tried bridging D6 with various diodes I have (don't have a 1N67 though). The meter works with any of the replacement diodes but again grossly underestimates the output voltage (for e.g., reading 60V when the output voltage is actually 130v) Checked the diodes D2,3,6 and 9 with a DMM and they seem to be OK. So I am at a loss as to why the meter is showing a low reading. When I get home from work, I will try changing the values of resistors R8, R9 and R15 to see if it will make any difference. Bruce or easyrider8, or anyone with the BK 1655, any ideas on troubleshooting?

Don


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 Post subject:
PostPosted: Jan Thu 17, 2008 2:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12549
Location: Mpls, Minnesota
Don

Check D2,D3,D6,D9 These are the bridge, 1N4007's should be fine for replacement, if it won't calibrate properly you will have to go back to germaniums. Check R9, R8, R14, R15. R14, R15 and R9 are the meter shunt resistors. D7 and D8 are meter protection diodes and can be replaced with In4007 also, If you don't find any bad resistors I would suspect the bridge diodes. Check your voltage across C4, this should read your output voltage. I will post a simplified schematic of this circuit as soon as I get it scanned.

Dave

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 Post subject:
PostPosted: Jan Thu 17, 2008 5:10 am 
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Joined: Dec Sun 02, 2007 3:20 pm
Posts: 1735
Location: Fargo, North Dakota
As far as I can tell, diodes D2, 3, 6and 9 are OK (resistance in one direction is much higher than in the other although I don't remember the exact values). R8 measures 1.003K (I guess this is within the acceptable tolerance range), R9 is 32.5 ohms (again, OK). R14 and R15 both measure 60.6 ohms with the trim pot turned all the way in one direction and 2.0 ohms with the trim pot turned all the way in the other direction. Since R14 and R15 are connected in parallel, I guess that's the best I can do to measure them without disconnecting them from the circuit. By my calculation the combined parallel resistance value of R14 and R15 should be 68.75 ohms with R14 at its max value of 100 ohms. The actual value is 60.6 as stated above. I don't know if the difference is sufficient to cause the problem I am experiencing with the meter. Please keep the suggestions and ideas coming... I'd like to get to the bottom of this!

Don


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Jan Sat 28, 2012 4:47 am 
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Joined: Jan Sun 15, 2012 4:20 pm
Posts: 609
Location: Lawton Oklahoma
I realize this is a very old post but for those out there like me with a B&K 1655 with voltage metering problems like the one posted in this thread, every tiny bit of help finding an answer will help a great deal. The primary problem is schematics for these things are like top secret government documents to B&K I suppose as you cant find them anywhere other than maybe paying $25.00 to B&K for a copy. Fortunately for me I have two of these, so I opened them up and made a point to point comparison of resistance readings and diode tests.

What I found was the problem for my IDENTICAL issue as the OP in this thread, was that the 4 germanium 1N67A diodes, those glass colored ones on D2, D3, D6, and D9 were shot. I had one ell of a time finding 1N67A diodes though as they are obsolete for the most part. Some places were selling a few they had for 25 to 50+ dollars each!! Yikes! I did a deep search and found some place that had 9 of them for about 4 bucks each shipped and I bought them. I replaced the 4 germanium 1N67A diodes and it came to full life at the meter, and I was able to fully calibrate this fine machine.

I was fortunate to have the other 1655 to test with since the schematics and parts list are near impossible to find and I hope this information may help some other poor soul doing the deep search for an answer to this particular issue with thier B&K 1655. One day I would like to find someone with a 1655A and have them tell me what they use in the newer versions in place of the 1N67A as I couldnt find a published replacement. I sure would hate to think that one day Ill have to pay 50 bucks a diode to keep these otherwise fantastic AC power supply Variacs going.

I hope this helps someone out there and again I appologise for having to bump this very old thread, but there just was no reply from the OP to verify what he found to fix his. Heck, maybe I even purchased his all these years later for all I know.. LOL


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Jan Sun 29, 2012 8:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3426
Location: Olympia WA USA
NTE website shows a NTE109 as a cross for the 1N67A


nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search

Data sheet also available there.
IIRC, these could also be ordered as matched pairs. NTE109MP should be the part number, as that the old ECG part # was ECG 109MP.
Used to replace these a lot in some stereos.


These should be under $2.00 each, IIRC on the retail pricing.
HTH
FrankB

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WB7ELC
"The average family has 2.5 children. Where is the other .5 child?"


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Jan Mon 30, 2012 11:29 pm 
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Joined: Jan Sun 15, 2012 4:20 pm
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Location: Lawton Oklahoma
Super! Thanks for the info Frank!


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Jan Tue 31, 2012 1:15 am 
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Joined: Jun Thu 15, 2006 1:21 am
Posts: 4683
Location: NE Ohio
To repeat myself I have the manual & schematic in PDF form. Instructions above. You will find, by the way, B & K happy to supply parts at very good pricing.

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Bruce


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Jan Tue 31, 2012 2:35 am 
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Joined: Jan Sun 15, 2012 4:20 pm
Posts: 609
Location: Lawton Oklahoma
Thanks Bruce, I saw that post but it had been 4 years and I didnt feel right about PMing you for a copy all these years later. I would very much appreciate a copy.. PM on the way.

BTW I have contacted them for a price on a probe once and it took a few days to get a reply but they did and the price seemed fair on that part.


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Feb Fri 15, 2013 7:00 am 
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Joined: Feb Fri 15, 2013 6:48 am
Posts: 3
Bruce, I'm so happy to see your post here and I'll greatly appreciate it if you can send me a copy of the schematic for the BK Precision 1655. Thanks!


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Feb Sat 16, 2013 9:58 am 
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Joined: Feb Fri 15, 2013 6:48 am
Posts: 3
Thank you Bruce for emailing me the schematic, you saved my day.


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Feb Sat 23, 2013 8:31 am 
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Joined: Feb Fri 15, 2013 6:48 am
Posts: 3
I bought the 1N67A diode from Ebay and I just installed them today and this has solved the calibration issue. Now the unit is spot on.

PMs me if you need the seller info on Ebay.


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Nov Sun 10, 2013 4:12 pm 
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Joined: Nov Sun 10, 2013 3:50 pm
Posts: 1
Just a quick "bump" to this thread in hopes that it continues to help people that use this great piece of equipment.

The BK1655 is one of the most utilized and important pieces of equipment on my test bench. When the meter failed on it last month I was really upset, as I work on audio gear for a living, and rarely have time to work on my own equipment. Like the other posters here, I had great difficulty finding any technical information on this wonderful instrument. Fortunately I found The Antique Radios forum and this thread on the 1655. It had not occurred to me that the bridge diodes would be germanium, and this was causing me great grief in getting the meter working again. I tried 1N4007s, and a number of other low voltage diodes to no avail. The meter would work, but could not be calibrated on the Amperage / Current settings. After reading this thread, I looked at one of the original diodes I had removed from the unit with a jeweler's loop - it was a 1N270 germanium. I found that these are easily found on Ebay and other resellers. Many people like to build guitar "stomp boxes" utilizing these diodes, because germanium produces a signature "sound" that Silicon diodes can not produce. Result - these parts are in demand, and they seem to be available. I purchased a bag of 10 diodes for about $2.75 from an Ebay seller. I installed the parts and the meter came to life and was easily calibrated. I checked the meter accuracy against my Fluke 179 DMM and was pleasantly surprised that the old 1655 was nearly spot-on with the digital meter.

Thank you to this forum for the clues in getting this rig going again. I believe that nearly any low voltage germanium diode will perform correctly in this circuit.


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Jun Thu 19, 2014 3:14 am 
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Joined: Mar Mon 10, 2014 6:53 am
Posts: 56
Location: Bremen,Georgia
Just a quick jump in as i am a new member. My name is Harold and i have two of these units and would be very grateful if i could get the manuals and schematics for these. This forum is the greatest for info. Thanks for time guys..HR


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 Post subject: Re: Schematic for BK Precision 1655
PostPosted: Jun Thu 19, 2014 4:32 pm 
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Joined: Jun Thu 15, 2006 1:21 am
Posts: 4683
Location: NE Ohio
Anyone still needing the manual just PM me with your email address.

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Bruce


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