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Old 4th May 2020, 9:21 pm   #21
Chris55000
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Default Re: Hunts CRB3 parts list

PS!

I recommend no more than 425V across C3 and the chain of 6k8 resistors – any more than that and the target–anode of the m.e. will be supplied with more than is good for it!

If we assume the max h.t. is 420V, for example, each resistor in the chain will drop 84V, 3 x 84V = 252V, allow about 10–12V extra dropped in the other (top) two from the target–anode current of the EM34, this will give about 240V target–anode voltage, about the absolute max. you want to supply end–viewed m.e. indicators with – most other makes, e.g. the Heathkit C3–U and the Taylor 110, use much less!

The circuit of the EM34–based "AC Bridge" in Scroggie's "Radio and TV Laboratory Handbook" and the one in the September 1966 issue of P.T., both suggested a mains transformer with a 150V h.t. secondary!

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Old 4th May 2020, 9:46 pm   #22
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Default Re: Hunts CRB3 parts list

PPS!

I don't how well Hunt's RC bridge mains transformers last up to 65 years of non–use, but the instructions for my Heathkit C3–U have got invoices for no less than three replacement mains–transformers taped to 'em!

Me thinks I could well be better off using a 6.3V heater supply winding and an EF91 amplifier stage in my Heathkit bridge rather than trying to get yet another 50V transformer!

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Old 4th May 2020, 9:47 pm   #23
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Default Re: Hunts CRB3 parts list

I've taken some nice 4K photographs of my CRB3. I'll sort out a parts location guide soon.
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Old 4th May 2020, 11:33 pm   #24
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Default Re: Hunts CRB3 parts list

Quote:
Originally Posted by Chris55000 View Post
Hi!
If you fit 5 X 68k resistors on the large tag board, you'll not get the magic–eye to light if they're R14 to R18 as I suspect them to be!
Chris Williams
D'oh... can't proof read my own work...

Thanks for the correction, you are quite right. They are 6k8 and that's what I have fitted. There's only one 68k and it is on the other board.
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Old 4th May 2020, 11:47 pm   #25
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Default Re: Hunts CRB3 parts list

My C3U still has its original transformer. I've just looked at my notes from its rebuild. Using a 1N5408 rectifier plus series 4k7 in place of the old rectifier, I measured 170V for the M.E. supply* and -460 for the polarisation supply. Then I tweaked the values in the resistor chain (with parallel resistors) to set the selected test voltages to their nominal values. Maybe I will do something similar with this CRB3 when it's finished.

These testers all seem to use much the same circuit, simple but effective.

* rather more than the 140V in the book, but measured with a modern hi-z voltmeter.

Last edited by m0cemdave; 4th May 2020 at 11:52 pm.
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Old 5th May 2020, 6:09 am   #26
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Default Re: Hunts CRB3 parts list

H!

Quote:
My C3U still has its original transformer. I've just looked at my notes from its rebuild. Using a 1N5408 rectifier plus series 4k7 in place of the old rectifier, I measured 170V for the M.E. supply* and -460 for the polarisation supply. Then I tweaked the values in the resistor chain (with parallel resistors) to set the selected test voltages to their nominal values. Maybe I will do something similar with this CRB3 when it's finished.
I can't remember if it was S.W. Amos or M.G. Scroggie that published the Wireless World F.M. tuner design of 1954/1955, but I do remember the author discussing the circuit for the tuning indicator featured in it in a bit of detail, and I remember he recommended the indicator target–anode was operated at 170V for a reasonable trade–off between brightness and useful life, so 170V in your Heathkit C3–U is about right!

Nearly all end–viewed indicators, apart from the rare Y62 and Y64, and the amplifier–less stubby American 6AF6/6AD6 which is rated 375V max, are rated for operation at 250V but many other Members, including myself, have commented that they don't last very long at that voltage – it didn't stop Ferguson pushing the boat out with a 275V main h.t. line, also feeding an EM34 direct, in one early 1950s R.G. chassis tho!

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Old 5th May 2020, 11:25 am   #27
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Default Re: Hunts CRB3 parts list

That CRB3 manual shows only 3 terminals on the front, the CRB3 in the photos and elsewhere on the web show 4 terminals, there seems to be a difference between the circuit in the manual and the circuit in the photo's relating to one of the test terminals, C1, C2 and R8's connection to R1......So far as I can make out.

There also appears to be an extra resistor fitted in the photo's posted and also in the other photo's on the web.

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Old 5th May 2020, 1:14 pm   #28
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Default Re: Hunts CRB3 parts list

Might be some errors but might do for a start, component values from photo's submitted to the forum so far and other web photo's:

R? 1.2k
R1 150 Ohm
R2 1.2k
R3 500 Ohm
R4 5k
R5 500k
R6 5k
R7 50 Ohm
R8 1k
R9 33k
R10 68k
R11 1 Meg.
R12 1 Meg.
R13 4.7 Meg.
R14 10k
R15 6.8k
R16 6.8k
R17 6.8k
R18 6.8k
R19 6.8k
R20 2.2k
R21 27k

C1 200pF, C2 0.02uF, C3 2uF, C4 0.01uF, C5 16uF, C6 16uF, C7 0.5uF.

Lawrence.

Last edited by ms660; 5th May 2020 at 1:34 pm. Reason: question marks
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Old 5th May 2020, 2:10 pm   #29
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Default Re: Hunts CRB3 parts list

Lawrence, that pretty well ties up with what I have. One thing does not look quite right as far as the schematic is concerned, I would have expected R14 to be 6k8 and R19 to be 10k. Can you double check this.

Thanks

Al
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Old 5th May 2020, 2:13 pm   #30
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Default Re: Hunts CRB3 parts list

Hi!

Thanks for your list Lawrence – I've got almost enough info to complete my diagram now, apart from a point I make in the last paragraph below!

The picture shown on the Hunt's book shows three terminals but the circuit diagram correctly shows four as in all the pictures of those I've seen!

The third terminal left blank on the circuit diagram is labelled ,"Resistance" on all the pictures of these instruments I've seen, so all I need to do is to place it on my diagram in the right place!

What isn't clear from Hunt's scrawl tho', is where is the large main Colvern balance control is on the circuit itself – I presume it's R4 – but why two tappings?

I presume the wiper is connected to earth as is customary in this type of circuit, but what about the other "tap" – have Hunt's run two separate components into one symbol, if so, what resistor is connected to one end of it? – it's not over clear from the photos!

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Old 5th May 2020, 2:47 pm   #31
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Default Re: Hunts CRB3 parts list

Quote:
Originally Posted by Alistair D View Post
Lawrence, that pretty well ties up with what I have. One thing does not look quite right as far as the schematic is concerned, I would have expected R14 to be 6k8 and R19 to be 10k. Can you double check this.
That did have me puzzled with the limited info I had from the photo's so I took a guess, but now looking at it again from a voltage selector point of view 6.8k at the bottom of the chain would make sense.

Lawrence.
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Old 5th May 2020, 3:32 pm   #32
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Default Re: Hunts CRB3 parts list

Well here it is. Please let me know if you find any mistakes.

I am still working on a parts list. After that I hope to sort a component location diagram.

Chris, No need to create another schematic.

Al
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File Type: pdf CRB3schematic.pdf (9.6 KB, 91 views)
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Old 5th May 2020, 4:31 pm   #33
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Default Re: Hunts CRB3 parts list

In daves photo the wiring pertaining to R1 is different to that shown in the schematic, the photo's on radiomuseum appear to be different from the schematic as well, would that be a production change ?

https://www.vintage-radio.net/forum/...3&d=1588589218

Lawrence.
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Old 5th May 2020, 4:53 pm   #34
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Default Re: Hunts CRB3 parts list

Looking at the picture I notice that the centre of the group of three nuts has the centre tag of the tagstrip attached to it. I wonder if that is the tap rather than the wiper and the wiper is the connection further round the body.

One question I forgot to ask previously. Is the tap on the pot a centre tap or is it offset towards one end of the track?

Al

Mods, this thread should probably be moved to the test equipment section.
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Old 5th May 2020, 5:54 pm   #35
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Default Re: Hunts CRB3

The slider of the large pot is connected to the fourth, i.e. right-hand, front panel terminal (marked "Earth") and to an earth tag.
The hi-stab 500R goes to the fixed tap on the pot.
The tap looks like it is offset from the centre. It's a while since I had the back off the pot for cleaning and I didn't take a picture.
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Old 5th May 2020, 6:33 pm   #36
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Default Re: Hunts CRB3

Here is a capacitor location chart. More to come.

Does anybody else have a 5Pf capacitor between the leak terminals?
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Old 5th May 2020, 6:46 pm   #37
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Does anybody else have a 5Pf capacitor between the leak terminals?
Not on my one.

And is that an extra (68 ohm?) resistor wired to the main pot? It's a wire link on mine.
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Old 5th May 2020, 7:19 pm   #38
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Default Re: Hunts CRB3

Blue, black, gold.

It is in between what I think is R2 and R3. I don't think it is on Hunts own circuit diagram.
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Old 5th May 2020, 7:23 pm   #39
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Default Re: Hunts CRB3 parts list

Quote:
Originally Posted by ms660 View Post
In daves photo the wiring pertaining to R1 is different to that shown in the schematic, the photo's on radiomuseum appear to be different from the schematic as well, would that be a production change ?
The one on the Radiomuseum https://www.radiomuseum.org/r/hunt_c...idge_crb3.html is mine and I will be going up to Poole later so if there is anything specific anyone wants a picture of I can take them tomorrow.

I have been watching this thread with interest it seems there may be some differences during production.

Cheers

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Old 5th May 2020, 8:27 pm   #40
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Default Re: Hunts CRB3

My thanks to all who have contributed to this thread. I have one of these test bridges, I paid £8.00 for it about 12 years ago. It was listed on eBay as a"Ship to shore radio". It is untouched as far as I remember, EM34 in good order. I have recently restored my father's AVO bridge, the CRB3 will be next. The instruction manual is most useful.

Les
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