Hunts CRB3

[neikeel]

Many thanks
I will try that method, yes, suck it and see appears to be the method, and interpretation of the results is interesting too!

[Chris55000]

Hi!

Once more the magic eye iris changed colour to a gentle green (with central bluish pupil and a grey internal ring (arcus).

That sounds suspiciously like your EM34 "magic–eye" balance indicator may be needing replacement due to the presence of gas in the envelope, causing a bluish glow from under the cathode/heater system light shield cap.

The price of these indicator valves is rocketing alarmingly every year because of their scarcity, but you can get a much cheaper Soviet–made 6E5S type that will do the job adequately enough without impairing the appearance of the instrument.

EM34 characteristics:–

Heater 6.3V 0.2A, Target Anode pin 5, 250V, Anode Resistors, pins 3 and 6, 1M, Grid volts for shadow–closure, –5V (1) & –16V (2).

Pin 1:– i.c., pin 2:– h, pin 3:– a', d', pin 4:– g, pin 5:– t, pin 6:– a", d", pin 7:– h, pin 8:– k, g'

6E5S characteristics:–

Heater 6.3V 0.3A, Target Anode Pin 6, 250V, Anode Resistor, 1M, pin 3, Grid Volts for shadow–closure, pin 4, –22V.

Pin 1:– Not used, pin 2:– h, pin 3:– a, d, Pin 4:– not used, pin 5:– g, pin 6:– t, pin 7:– h, pin 8:– k, g'

To rewire an EM34 holder to suit a Soviet 6E5S, disconnect the 1 megohm resistor between pins 5 and 6 and discard it.

Disconnect the h.t. lead from pin 5 and reconnect it to pin 6, and also disconnect the 1 megohm resistor connected to pin 3 from pin 5 at it's other end, and reconnect the free end to pin 6 along with the h.t. lead.

Also transfer the 4M7 resistor and 0u01 capacitor from pin 4 to pin 5, or simply connect pins 4 & 5 with a short piece of wire.

No other wiring alterations are needed.

Chris Williams

[Chris55000]

Hi!

It's normal for the eye balance indicators on bridges to open fully at one end of the balance (and sometimes the "power factor" control) this is because, with the usual wiring arrangements, (see my circuit diagram and others in this thread) the wiper of the balance control is directly earthed along with the cathode of the indicator, so when you turn the wiper towards the end connected to the grid circuit of the indicator, you're short–circuiting it's input and therefore the eye will open fully!

Chris Williams

[Chrispy57]

Hi Chris Williams,
that's a really helpful description of how to substitute the cheaper Russian magic-eye tube for an EM34 - I shall save it for future reference.

I wonder if you have seen the adaptors which claim to fulfill this task without any circuit alterations to the original equipment? I have attached a photo of a Chinese adaptor from EBay as a random example of the species. There may be some issues in finding the extra inch or so of space where the tube is a tight fit in the original equipment.

Cheers
Chris King

[neikeel]

When I get chance I will post a picture of the magic eye, which, after looking at more images of how it should look seems to be ok.
It is more the neon with has splodges of colour inside the halo rather than the clean ring in the photographs above.
My suspicion is that the rectifer may not be up to snuff so I will pull the chassis and check the voltages are as I expect form the very helpful diagram V4.
Cheers

[Chrispy57]

Hi - in case you haven't already figured things out, the method I used to test the neon was short out the capacitor terminals, set the selector to "Ins" or "Leak" and increase the right hand switch through the fixed voltage levels. The neon should illuminate sequentially more quadrants as the volts go up.

Of course it's possible the neon may have been replaced with a less sofisticated type and just light up when it's strike voltage is reached.Good luck with it.

Cheers
Chris

[Alistair D]

More than one person has commented that the schematic is difficult to understand and I agree. To help understand what is going on I have prepared some simplified diagrams for the basic functions of the instrument.

As a result of seeing the diagram of the insulation and leakage functions I am now wondering what setting users use for reforming electrolytics.

Al

[Alistair D]

Originally Posted by OldTechFan96

Here is a capacitor location chart. More to come.

Does anybody else have a 5Pf capacitor between the leak terminals?

I collected my CRB3 yesterday. Thanks again Norman.

The first thing I noticed after opening it up is that it also has a 5pF 600volt capacitor across the leak terminals. The serial number on my unit is 246 so it may be that this cap was deleted on later production.

I am going to do one last update to the manual so I will include this component.

Al

[HamishBoxer]

Joy of joy it is a Hunts! Must check mine and see serial number.

[Alistair D]

There is an easy way to measure the leakage in that capacitor or any externally connected cap.

This is a draft of an entry in an updated manual I am slowly compiling. It explains the method

CAPACITOR REFORMING

The CRB3 can also be used to reform electrolytic capacitors.
Select the voltage you want to start the reforming process at and INS. As the reforming progresses the neon should flash less and less often. Repeat the process at higher voltages until the capacitor’s maximum voltage is achieved. Due to there being a neon in the current path the actual voltage applied to the capacitor will be less than selected. As a final stage of reforming select LEAK or LEAK HIGH CAP for a short period of time. Connecting a high impedance voltmeter between the CAPACITY negative and EARTH terminals will give an indication of the leakage current. In the LEAK setting 1 volt equates to approximately 15uA leakage current. In LEAK HIGH CAP 1 volt equates to approximately 30uA.
Stubborn capacitors may benefit from short spells on the LEAK or LEAK HIGH CAP settings at the start of reforming. Figure 16 shows the active circuitry for these processes.

Al

[JayBee66]

Hello Alistair

I saw your updated manual on http://golbornevintageradio.co.uk/ posted 13 Jun 2020. Will you be updating this further?

Thanks for your hard work.

Originally Posted by Alistair D

There is an easy way to measure the leakage in that capacitor or any externally connected cap.

This is a draft of an entry in an updated manual I am slowly compiling. It explains the method

CAPACITOR REFORMING

The CRB3 can also be used to reform electrolytic capacitors.
Select the voltage you want to start the reforming process at and INS. As the reforming progresses the neon should flash less and less often. Repeat the process at higher voltages until the capacitor’s maximum voltage is achieved. Due to there being a neon in the current path the actual voltage applied to the capacitor will be less than selected. As a final stage of reforming select LEAK or LEAK HIGH CAP for a short period of time. Connecting a high impedance voltmeter between the CAPACITY negative and EARTH terminals will give an indication of the leakage current. In the LEAK setting 1 volt equates to approximately 15uA leakage current. In LEAK HIGH CAP 1 volt equates to approximately 30uA.
Stubborn capacitors may benefit from short spells on the LEAK or LEAK HIGH CAP settings at the start of reforming. Figure 16 shows the active circuitry for these processes.

Al

[Alistair D]

I do have quite a bit more in the next version of the manual. Currently the whole document is in draft form.

When I do get round to tidying up and completing it I will post a copy.

Al

[Alistair D]

Alistair D has reported a post.

Reason:

Mods, can you re-open this thread. I have now completed the manual and it is ready to post

TIA
Al

Post: Hunts CRB3
Forum: Vintage Test Gear and Workshop Equipment
Assigned Moderators: N/A

Posted by: Alistair D
Original Content:

I do have quite a bit more in the next version of the manual. Currently the whole document is in draft form.

When I do get round to tidying up and completing it I will post a copy.

Al

[Cobaltblue]

Opened as requested.

Cheers

Mike T

[Alistair D]

Well it took long enough but I have now managed to complete the full version of the manual.

A big thank you to everyone who provided information, pictures etc.

Al