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B&K 1655 With Issue: Incorrect Dial Readings

M

MFRD

AK Subscriber
Subscriber
Hi all, I just found this B&K 1655 isolation transformer / variac in a (self-identified) junk shop:

img1.jpg

I was told it works and it does, but the analog dial reads way off. If there's 120V at the output per DMM, the dial will only indicate 55V or so. If there's 40V at the output the dial will read maybe 16V, and so on. The meter reacts to input changes, and the indicated voltage is proportional to the output voltage, so the meter itself seems OK.

I looked around and found this thread and this video (best watch it at 2x) that looked promising. There is also a helpful circuit description in the manual, but no schematics unfortunately.

The meter control board has a small bridge rectifier to make DC for the control circuitry and the meter itself. It uses germanium diodes, apparently 1N67A. The gist of it seems to be that when these diodes go bad the dial starts displaying wonky numbers like I'm seeing. Same thing happens if the germaniums are replaced with general-purpose silicon diodes.

Looking inside:

img2.jpg

Looks like D9, D3, D2 and D6 make the rectifier. Sure enough D9 and D6 are not original. Apparently whomever performed the repair was obsessive enough to tape the dead D9 to the board for posterity (left), yet didn't see fit to pull out the PCB and do a clean repair.

I can't make out what type diode they replaced D9 with, but it's open anyway. The other three read OK in circuit; D3 and D2 = 2.5V, D6 = 5.7V. So I figured I had best replace all four.

1N67A is unobtainium. NTE109 is a replacement, according to NTE, but it's unobtainium too. CDSH270 is also mentioned as a (newer) replacement, but that's a silicon Schottky and that seems a bridge too far.

So I figured I'd take a chance on the theory that any small voltage germanium will work. I found reasonably priced 1N270 from a US ebay seller (advertised as Micro Semi) and I ordered 10 pieces, so we'll see!

Wondering if anyone had the same issue with that unit? Also, if you happen to have schematics, please do share. Thanks! :)
 
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mrfredo said:
I can't make out what type diode they replaced D9 with, but it's open anyway. The other three read OK in circuit; D3 and D2 = 2.5V, D6 = 5.7V. So I figured I had best replace all four.

1N67A is unobtainium. NTE109 is a replacement, according to NTE, but it's unobtainium too. CDSH270 is also mentioned as a (newer) replacement, but that's a silicon Schottky and that seems a bridge too far.

So I figured I'd take a chance on the theory that any small voltage germanium will work. I found reasonably priced 1N270 from a US ebay seller (advertised as Micro Semi) and I ordered 10 pieces, so we'll see!

Wondering if anyone had the same issue with that unit? Also, if you happen to have schematics, please feel free to share. Thanks! :)
Click to expand...
Surplus sales in Nebraska has lots of obsolete parts. I use them a lot for weird items.

www.surplussales.com

Mixer, Germanium Diode

1N67A Mixer, Germanium Diode Available at Surplus Sales of Nebraska. In-stock and ready to ship today!
www.surplussales.com www.surplussales.com

Athanasios
 
Hi Athanasios, I did visit that page before. I could swear it said minimum order $100 per line item, and I thought that was ridiculous... Looks like it's $1.00 not $100, I just missed the period somehow. Thanks for pointing it out

I did order those 1N270 so let's see what happens with them first
 
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I see yours didn't come with the leakage probe.

I need to read up on what that actually does.

Athanasios
 
Nashou66 said:
I see yours didn't come with the leakage probe.
Click to expand...
The probe comes hardwired into the unit from the factory. Makes it easy to get ripped out, which is probably what happened to mine.

The probe is used to touch various parts of a device under test (faceplate, knobs, etc) to measure the leakage current a user might be exposed to, which should be 500uA or less under US OSHA rules. It'd be nice to have it, but for me it's not a must-have.

Also missing are the nice turned-aluminum knob inserts...
 
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Here is mine .

Not as nice condition as yours .

Someday I want to repaint the top and sides.

55180578523_4004b164ee_b.jpg
 
Quick update... I started pulling out and testing all components that didn't test right in-circuit. To be honest I'm probably just looking for excuses to fire up the new-to-me Keithley 2000

The 4 diodes in the 2nd bridge rectifier tested fine. But D7 and D8 each had one lead cut and soldered back together on the component side of the PCB. It was done cleanly, and on one lead only. The other lead was properly soldered on the solder side and the joint looked fine, so factory bodge? Weird.
 
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My 1N270 germanium diodes came in so I replaced the four 1N67A. I didn't like the bodged leads on two of the other four diodes so I replaced the lot with 1N4005 I had on hand.

Switched the unit on... the dial works!

1000000046.jpg

I zeroed out the needle and calibrated the meter against my DMM and the two are in sync:

1000000047.jpg

They're spot on between 95-125V but they drift apart a bit outside that range. I ran my Brymen DMM in parallel just for giggles, and none of the three agreed with each other. The two DMMs are spot on at 5V, but by 20-25V they're 300mV apart and stay there all the way to 160V.

Measuring current, the analog dial and the DMM were within 50mA.

What surprised me is the PSU's output voltage dropped from 120V to 115V under a 1A load. @Nashou66 do you see the same voltage drop on yours?

I gave it a little clean and it doesn't look too bad! Wish it still had its spun aluminum knob centers though

1000000061.jpg

Also, I tested the "Solder temp" function, and it worked fine. It sets the voltage at the rear outlet between 80V and 120V using the knob on the front panel, letting the operator adjust their iron's temperature to their liking. I thought it was a pretty useless feature nowadays, until it dawned on me that it can use it to power my JDM 100V Hakko desoldering gun!
 
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Not sure , I dont think it does. I have it powering my DBT that has a voltage meter on it and that does not fluctuate .

This is the DBT and its meter .

52259604061_fbe1b9c089_b.jpg
 
I played around a little with the voltage drop.

Under a 1A load (120W incandescent bulb),, the 3A-rated B&K's output dropped 5V to 115V
Under a 2A load (2x 120W) it dropped to 110V
My 20A-rated Chinese red variac dropped 2.3V under the same 2A load.

An unregulated device probably cannot be expected to produce a stable output voltage.

In any event, the main takeaway of this thread is that 1N270 diodes will work in place of the original 1N67A, and that 1N67A diodes are available as of this writing thanks to Nashou's tip (ref. post #2).

All in all I'm pretty pleased with my new instrument. If only it still had its spun aluminum knob centers :)
 
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