4th May 2020, 9:21 pm | #21 |
Nonode
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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! Chris Williams
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4th May 2020, 9:46 pm | #22 |
Nonode
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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! Chris Williams
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4th May 2020, 9:47 pm | #23 |
Octode
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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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4th May 2020, 11:33 pm | #24 | |
Octode
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Re: Hunts CRB3 parts list
Quote:
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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4th May 2020, 11:47 pm | #25 |
Octode
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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. |
5th May 2020, 6:09 am | #26 | |
Nonode
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Re: Hunts CRB3 parts list
H!
Quote:
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! Chris Williams
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5th May 2020, 11:25 am | #27 |
Dekatron
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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. Lawrence. |
5th May 2020, 1:14 pm | #28 |
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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 |
5th May 2020, 2:10 pm | #29 |
Nonode
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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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5th May 2020, 2:13 pm | #30 |
Nonode
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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! Chris Williams
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5th May 2020, 2:47 pm | #31 | |
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Re: Hunts CRB3 parts list
Quote:
Lawrence. |
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5th May 2020, 3:32 pm | #32 |
Nonode
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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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5th May 2020, 4:31 pm | #33 |
Dekatron
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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. |
5th May 2020, 4:53 pm | #34 |
Nonode
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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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5th May 2020, 5:54 pm | #35 |
Octode
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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. |
5th May 2020, 6:33 pm | #36 |
Octode
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Re: Hunts CRB3
Here is a capacitor location chart. More to come.
Does anybody else have a 5Pf capacitor between the leak terminals? |
5th May 2020, 6:46 pm | #37 |
Octode
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Re: Hunts CRB3
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5th May 2020, 7:19 pm | #38 |
Octode
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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. |
5th May 2020, 7:23 pm | #39 | |
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Re: Hunts CRB3 parts list
Quote:
I have been watching this thread with interest it seems there may be some differences during production. Cheers Mike T
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5th May 2020, 8:27 pm | #40 |
Hexode
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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 |