B&K Model 400 CRT Tester

[Kawooa]

I have bought a b&k model 400 crt tester. I just went through the parts testing resistors to determine what needed to be replaced. The schematic lists no capacitor in parallel with the meter. But on my tester there is a capacitor in parallel. It just so happens to be 250uf 6vdc. Should I remove I or replace with a new one? Everything else looked fine aside from some resistors being off. The big power resistor of 10w is off. The 40kohm side measures 14kohms. Both 10kohm sides measure 8.5kohms. Should I remove from circuit and test again or just replace with new ones? I am going to see about ordering parts soon to get it working. Then I'll test my 17bp4(listed in TV cabinet, nothing on tube) tube. Will this tester cover it and what if anything needs to be done to ensure I won't blow my tube? Or tester. The tester over all is in good shape with some small to medium patches of rust. Two of my three ne-48 bulbs are broken. Don't know how to test the third. Aside from normal cleaning which I finished like a hour ago and standard replacement of off spec parts, what else should be done to ensure safety and something in calibration? If possible with this thing. Sorry to be asking so much. I'm young(18) and only into trying to repair vintage electronics for not even a year. Only been collecting vintage technology for 2? years. Well I got a lot to learn. Thanks for any help.

[Kawooa]

Heres a picture showing the inside. At the top is the capacitor in parallel with the meter.

[SwizzyMan]

Hmmm that is definitely odd that cap is there. Maybe it is indeed original out of the factory but it just isn't listed in the schematic because it may be a possible revised model. Or maybe the meter wasn't working properly so a repairman threw it in there. Either way, it is a cap that commonly goes bad. You should replace it because the meter may not function properly without it and then you couldn't rely on the unit. So yes, replace that one and all other ones that look similar to that one e.g the paper caps and the electrolytic caps if they are there. Also looks like you are missing a tube from what I see in the photo. Did you take it out for the photo? One more thing i'd replace those 10k resistors just in case they have started to drift and they can get worse just with time and with use. Better safe than sorry.

[Kawooa]

Originally Posted by SwizzyMan

Hmmm that is definitely odd that cap is there. Maybe it is indeed original out of the factory but it just isn't listed in the schematic because it may be a possible revised model. Or maybe the meter wasn't working properly so a repairman threw it in there. Either way, it is a cap that commonly goes bad. You should replace it because the meter may not function properly without it and then you couldn't rely on the unit. So yes, replace that one and all other ones that look similar to that one e.g the paper caps and the electrolytic caps if they are there. Also looks like you are missing a tube from what I see in the photo. Did you take it out for the photo? One more thing i'd replace those 10k resistors just in case they have started to drift and they can get worse just with time and with use. Better safe than sorry.

I pulled the tube so it was out of my way when doing stuff to it. When I first opened this there were two more tubes taped together inside it! What a bonus! I will order that capacitor and after replacing everything see how it works. If the meter seems odd I could try adding the replacement in. If that seems to fix it then I guess it stays. It could have been some person "repaired" it because other more pricey parts were failing. If you can add a $1 part for 5 minutes time why replace 5 parts for an hours time? Still charge the same but make more. I think that is what someone must have been thinking.

[SwizzyMan]

Originally Posted by Kawooa

I pulled the tube so it was out of my way when doing stuff to it. When I first opened this there were two more tubes taped together inside it! What a bonus! I will order that capacitor and after replacing everything see how it works. If the meter seems odd I could try adding the replacement in. If that seems to fix it then I guess it stays. It could have been some person "repaired" it because other more pricey parts were failing. If you can add a $1 part for 5 minutes time why replace 5 parts for an hours time? Still charge the same but make more. I think that is what someone must have been thinking.

Yup repairmen back then were pretty lazy and most used crude methods of repair. At least that is what I have noticed.

[Kawooa]

So I was looking at the crt end plug. There were two screws in it. I removed them and it appers you can move the wires to different pins. I guess that can make it do different crts if you can adjust the wires to the correct pins. But what crt models can this test. Like a list. So in the future I know what I can test without problems. I barely have any experience with crts other then they look amazing. Bright, crisp and heavy! Much better then crappy lcds from China. My commodore 1701 monitor is so much better looking on DVDs then any other screen I've ever seen.

[Electronic M]

Too bad you are doing this now...I had a junker just like that with the manuals and adapters that I sold early in June. Mine would have been a good parts unit for that.

[old_tv_nut]

Testing a resistor while it is still in the circuit will always give a lower reading than the actual resistance. How much lower depends on what the rest of the circuit is. So, don't jump to the conclusion that a resistor that measures low is bad until you have disconnected it and measured it disconnected.

On the other hand, a resistor that measures too high while still connected to the circuit is definitely bad.

[earlyfilm]

Originally Posted by Kawooa

I have bought a b&k model 400 crt tester. . . . . ordering parts soon to get it working. Then I'll test my 17bp4(listed in TV cabinet, nothing on tube) tube. Will this tester cover it

This and the electrically identical B&K 350 will test any 6.3 volt filament CRT that will fit the test socket. The tester has a socket for the Small-Shell Duodecal 7-pin and the 17BP4 family has a Small-Shell Duodecal 5-pin base, so it will fit and it will test.

This base style came into use in 1946 and went out of favor in the mid-1950's.

I would not advise making adapters for more modern tubes, nor changing the wiring of the tube socket.

I also would not advise using the rejuvenate function, if the tube just tests weak until you have tried the tube in a working set. Sometimes just letting the filament run for a half hour with no load on the tube works better than rejuvenation and is almost entirely without risk to the tube. Rejuvenation is only used for extremely bad cases. I've seen more usable tubes ruined by rejuvenation than weak tubes brought back and lasting more than a few hours.

And if you are wondering where the normally blue wire from the CRT socket is supposed to be connected, it is just for looks and it goes no where. That is the difference between the 7-pin and 5-pin base, however on some CRTs this pin is sometimes connected to a grid. On tubes of this era, that pin is not critical to testing.

Originally Posted by Kawooa

The schematic lists no capacitor in parallel with the meter. But on my tester there is a capacitor in parallel. It just so happens to be 250uf 6vdc..

My guess this was added by the former owner. I suspect that since the meter has no capacitor and many meters reading DC where is a RF carrier use a small non-polarized capacitor to bypass the RF, he assumed that this meter needs one, too. (This meter reads a poorly filtered 60 cycle DC current.)

Another theory is he added this as a shock absorber for those spikes from static electricity.

As far as I know, this was never a factory option. When working properly, one sometimes gets a meter kick when switching from switching between short test and emission test, due to a static charge on the tube elements. This will not harm the meter.

A third theory is someone in the past used a drum-tuner cleaner (common during the era of this set) that had graphite in it and this soaked into the wafers on the switch and caused a little leakage and this caused a major kick when the rejuvenate button was pushed. To clean these switches, use a small amount of non-oxy type (the residue from the oxy's collect dust) and after working the switch a few times, put a fan on it to dry it out, before it soaks into the wafer.
If this meter is stored in a damp area (like a basement floor)moisture will penetrate the wafers and that will also cause the rejuvenate function to give the meter a real kick.

If it were me, I'd leave the cap off and only reinstall if needed, and frankly if it is needed there is probably something wrong with the meter.


Oh, by the way, please check the 11 pin terminal strip. In your photo, it looks like a solder bridge between two terminals.

Originally Posted by Kawooa

. Two of my three ne-48 bulbs are broken. Don't know how to test the third.

To test them, switch the suspected lamp with a known good one.

Those lamps are inexpensive, I'd replace all three and then you probably will have a spare.

Originally Posted by Kawooa

Aside from normal cleaning which I finished like a hour ago and standard replacement of off spec parts, what else should be done to ensure safety and something in calibration?

Despite this circuit having a power transformer, there is still a connection to the AC line, so always unplug the set before testing. If the set is connected to a grounded cable, disconnect that also.

Always warm up the CRT for five minutes before the test. Test for shorts first and if none, gently thump the neck to see if any appear. Never emission test a CRT that shows shorts, or you may fry the tester.

And, the best reason I have this tester is that it is damn near idiot-proof !

Good luck !

James

[Kawooa]

Originally Posted by earlyfilm

This and the electrically identical B&K 350 will test any 6.3 volt filament CRT that will fit the test socket. The tester has a socket for the Small-Shell Duodecal 7-pin and the 17BP4 family has a Small-Shell Duodecal 5-pin base, so it will fit and it will test.

This base style came into use in 1946 and went out of favor in the mid-1950's.

I would not advise making adapters for more modern tubes, nor changing the wiring of the tube socket.

I also would not advise using the rejuvenate function, if the tube just tests weak until you have tried the tube in a working set. Sometimes just letting the filament run for a half hour with no load on the tube works better than rejuvenation and is almost entirely without risk to the tube. Rejuvenation is only used for extremely bad cases. I've seen more usable tubes ruined by rejuvenation than weak tubes brought back and lasting more than a few hours.

And if you are wondering where the normally blue wire from the CRT socket is supposed to be connected, it is just for looks and it goes no where. That is the difference between the 7-pin and 5-pin base, however on some CRTs this pin is sometimes connected to a grid. On tubes of this era, that pin is not critical to testing.



My guess this was added by the former owner. I suspect that since the meter has no capacitor and many meters reading DC where is a RF carrier use a small non-polarized capacitor to bypass the RF, he assumed that since this meter needs one, too. (This meter reads a poorly filtered 60 cycle DC current.)

Another theory is he added this as a shock absorber for those spikes from static electricity.

As far as I know, this was never a factory option. When working properly, one sometimes gets a meter kick when switching from switching between short test and emission test, due to a static charge on the tube elements. This will not harm the meter.

A third theory is someone in the past used a drum-tuner cleaner (common during the era of this set) that had graphite in it and this soaked into the wafers on the switch and caused a little leakage and this caused a major kick when the rejuvenate button was pushed. To clean these switches, use a small amount of non-oxy type (the residue from the oxy's collect dust) and after working the switch a few times, put a fan on it to dry it out, before it soaks into the wafer.
If this meter is stored in a damp area (like a basement floor)moisture will penetrate the wafers and that will also cause the rejuvenate function to give the meter a real kick.

If it were me, I'd leave the cap off and only reinstall if needed, and frankly if it is needed there is probably something wrong with the meter.


Oh, by the way, please check the 11 pin terminal strip. In your photo, it looks like a solder bridge between two terminals.




To test them, switch the suspected lamp with a known good one.

Those lamps are inexpensive, I'd replace all three and then you probably will have a spare.




Despite this circuit having a power transformer, there is still a connection to the AC line, so always unplug the set before testing. If the set is connected to a grounded cable, disconnect that also.

Always warm up the CRT for five minutes before the test. Test for shorts first and if none, gently thump the neck to see if any appear. Never emission test a CRT that shows shorts, or you may fry the tester.

And, the best reason I have this tester is that it is damn near idiot-proof !

Good luck !

James

So would it be recommended to replace the selenium recifier with a full bridge recifier? And not just replacing that because it can fail. Then maybe a 200ish uf electrolyic. I will check the terminal strip for a solder bridge. But I don't remember seeing one. But it seems like this tester is going to be easy to use. Idiot proof shouldnt be needed. I'm not THAT dumb. Just kidding. Don't think I'll fry anything.