View Full Version : B&K 470 CRT tester problems.


Eric H
02-28-2012, 01:11 AM
Hi all, picked up this tester fairly cheap but with some issues.

First try it didn't work at all, you could do all the set up except the G-2 was dead, found and replaced an open Red G-2 pot with an exact replacement part.

Now you can do the setup but when you switch it to the "Emissions" setting the meter swings past the Good scale and smacks the stop, you can turn the G-2 control down and bring the needle down but then of course the G-2 setup is off.

I am using my 5AXP4 test tube for this, primarily because it's like new, using an older 10BP4 I can't set the G-2 at all even though I can with my other 466 tester.

I suspect the G-2 is more than it should be, the meter is supposed to deflect one small division when you adjust the G-2 but at that setting the meter pegs when reading emission.

I've checked all the resistors and Diodes, cleaned the switches (which were clean to start with) and replaced one 4.7mf Cap.
There are two 47@450 caps still not replaced but they seem to be OK but what else is left?

There are no adjustments inside except a Heater Calibration pot, I think that just adjusts the meter to match the voltage.

Kevin Kuehn
02-28-2012, 02:07 AM
Maybe compare the voltages around the G2 controls with those of your 466. They must use basically the same circuit.

Eric H
02-28-2012, 11:38 PM
Hi Kevin, it's probably similar,

I checked the G-2 at the CRT, it goes from zero to about 500 volts, if I adjust it so the CRT tests "Good" without slamming the meter it's around 95 volts, that seems reasonable since your just trying to nudge the meter up one notch for this setting, however at that voltage the meter doesn't move at all when in the "Set G-2" position.

Someone has spliced another test cable on to this at some time, it seems to be the correct B&K cable, all the colors match up and are hooked correctly as far as I can tell.

Maybe I should replace the two 47mf Caps and see what that does.

There are no internal adjustments (other than heater) so it's pretty hard to see how it can't work if everything checks good, the Caps are the only thing I can't be sure of.

Kevin Kuehn
02-29-2012, 01:52 AM
It looks like the two 47mf caps are used in a voltage doubler circuit to produce the 500V supply that feeds the G2 controls. That G2 setting would be very sensitive to any leakage current in the wiring going out to the CRT socket. I assume that G1 sets up correctly for 45V? The G2 setting is dependent on G1 being set correct.

Kevin Kuehn
02-29-2012, 01:52 AM
deleted double post.

Eric H
02-29-2012, 08:36 AM
The G-1 sets up OK, it's actually 50 volts on this model.
I measured about 57 at the tube with it set so it may be slightly off.

Kevin Kuehn
03-01-2012, 01:48 PM
Eric, Have any success yet?

For the heck of it I tried measuring G1 and G2 at the base of a 5AXP4 using my BK 466. My Fluke DVM completely messed up the G2 reading. Apparently you need to use a VTVM or 10x scope probe. It's way more sensitive to loading than I suspected.

Eric H
03-01-2012, 08:42 PM
I haven't had a chance to mess with it any more, perhaps this weekend.

I noticed some weird readings trying to check the heater voltage with my Digital meter.