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Old 15th February 2004, 05:45 PM   #1
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Question Robets Cadet III PU Adapter

Hi

I've a Roberts Cadet III preamplifier with a module plugged onto the side of it called a PU Adapter. I think its some kind of impedance matching thingie for a particular type of cartridge. Its silver, and inside there is (for each channel) an electrloytic capacitor, colour coded GREEN BLACK VIOLET YELLOW - 50pF? wired in parallel with a 330k resitor. The phono input appears to connect to one end of this and the output goes via a 1M resistor to - I think - the grid of an ECC807 valve, having passed through the input selector switch.

Does this make sense? I'm a total newbie to all this (although I do have some electronic experience - obviously not enough 'cause I can't read the value of the capacitor even after researching it on the net..). The amp is working at line levels and a signal can be heard through the phono stage but it is very weak.

I'm wondering if it's the 1M resistors that are causing this. I might have the adapter for a crystal cartride. Is it worth switching these resistors for - say - a 500K resistor and seeing if it improves things? I don't want to damage the valves as I understand ECC807s are hard to get hold of these days.
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Old 16th February 2004, 12:19 AM   #2
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Hi,

That adapter is probably for a crystal or magnetic cartridge.
As I don't have a diagram for it I can't tell for sure.

I believe however the Rogers Cadet is similar to the Brimar VP10.

Those ECC807s are hard to come by indeed...

Maybe our fellow member 316A can sort this out for you as I believe he knows quite a bit about the Rogers range.

Sorry I can't be of more help to you,
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Old 16th February 2004, 12:24 PM   #3
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Just what I thought. I assume you get a much higher strength output from a crystal cartridge...?
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Old 16th February 2004, 08:53 PM   #4
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Default Well whaddya know....?

I've shorted out the 2x1M resistors and now have a very good gain indeed. Bit too good, really; the volume goes about halfway before distorting, so I guess 500k is the value I need (or 470k I suppose).

However, its a bit bass light compared to the line input and I wonder if I've accidentally spoilt the RIAA bias circuit's workings.

On the other hand, I am using a Marantz CD 63 Ki-Sig for a comparison and that player is known for a good bass reproduction. The only way to compare and know for sure is probably to get a line out from my tranny amp and play the turntable through that.

If anyone can advise I'd be glad to hear from you.

Thanks

Jon Bradbury
(PS : Next up : a slightly burned out Audio Innovations series 500)
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Old 16th February 2004, 10:55 PM   #5
sreten is offline sreten  United Kingdom
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I had a Cadet IIII as as a student and I'm sure the MM adaptor was Gold.

I also happen to have a Cadet III pre-amp in my cupboard.

Gold coloured disc again.

Its its wired symmetrically , pins 1 to 4 away from the earth (0).

resistor : blue - grey- orange - gold : pins 1 and 3. (68K 5%)
resistor : voilet - red - yellow - gold : pins 2 and 3. (720K 5%)
capacitor : polystyrene - 680P - 5% - 350V : pins 2 and 3.
capacitor : polystyrene - 2500 - 5 - 400V : pins 2 and 4.

I assume the 1st capacitor is 680pF.

I think (and the size agrees) for the second capacitor the last
digit is a power of 10 multiplier for PF = x1 = 250pF.
The 5 I assume is 5%.

sreten.
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Old 17th February 2004, 07:30 AM   #6
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Hi Jon,

I'm pretty sure I still have the schematics for the PU adaptors somewhere. I'm away for a couple of days, I'll look when I get back.

Paul
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Old 17th February 2004, 12:47 PM   #7
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You're in luck. Found them, photocopied them, send me a postal address and I'll send them on when I get back. Got these 19 years ago from one of Roger's retired engineers so the ink has faded a bit. Also copied my service manual if that helps.

Have fun

Paul
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Old 22nd October 2006, 04:46 PM   #8
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I realise this thread is quite old, but I also have a Rogers amp with a missing gold PU adapter. If anyone still has a wiring diagram I would be very grateful for a copy.

Thanks

Paul
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Old 23rd October 2006, 05:20 AM   #9
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Page 2 of this thread has the preamp schematic and shows the PU adapter socket (power amp schematic is on page 1).

Rogers Cadet III Schematics

Note: with the selector switch in the DISC position then:
pin 4 of the adapter is hooked to the input socket (hot)
pin 2 of the adapter is hooked to the grid of the input triode (for conection of feedback and input)
pin 1 is connected to the cathode of the 2nd triode via 40uF (C3). this is a low impedance drive of the feedback signal.
pin 3 has no external connection

The GOLD adapter is the Magnetic Cartridge Adapter.
The SILVER adapter is the Ceramic Cartridge Adapter

For the GOLD adapter (from posts above) feedback network is 68K to 720K in parallel with a cap which you thought was 680pF.

These should be doing the 50Hz (3180us) and 500Hz (318us) timeconstants for the RIAA response. If that were in fact the case then the cap should be about 4700pF??? Hummm -I'm lost too.

Hey SuppersReady - any chance of posting the adapter schemes here??

My Cadet III has the GOLD adapter ONLY.

Cheers,
Ian
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Old 28th October 2006, 12:25 PM   #10
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Ian - I don't think we are going to get a reply now. Will the top come off yours? If so could you possibly check the values of the components and the pins they are connected to?

Many thanks

Paul
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