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Old 8th June 2012, 11:52 PM   #1
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Default Nytech ca252 help request!

Hi, I'm new to the forum this week and hoping one of you experts out there can help diagnose a problem with my Nytech. I acquired it from the school I work at as it was not functioning for them.

So upon getting it home I discovered it works wonderfully with a pre-amp plugged in to the pre-amp in DIN socket. This bypasses the balance and volume faders and sounds amazing.

However the problem is - there is barely any audible signal using any of the other inputs. I would of course like to use it with phono in mainly. If the volume is up high, a very faint signal can be heard using tape in or phono in.

I opened the case to see if there was any obvious problem, and found it fairly busy with additional processing cards - a little different to the ca202.

It all looks clean and there were no obvious defective components / wires etc. It's very clean inside.

I'm at a bit of a loss as how to approach this issue. Your help would be much appreciated. Here's a photo:
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File Type: jpg Nytech c252.jpg (463.6 KB, 86 views)
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Old 9th June 2012, 07:22 AM   #2
sbrads is offline sbrads  United Kingdom
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If both channels are u/s then it ought to be something common to both input sections. Pre-amp power rails?
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Old 9th June 2012, 07:50 AM   #3
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Thank you for your reply. Would the power rails be separate for each input? And how would I identify this component?

Apologies for my "noviceness". Cheers for your assistance.
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Old 9th June 2012, 10:31 AM   #4
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The schematic is on the Internet. It is an unusual design with preamp running off +28V and power off +48V singe rails. The power amp is ac coupled in and out, so ecaps may have gone. The 28V is regulated from the 48V.
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Old 9th June 2012, 12:05 PM   #5
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Thank you David. I've found the schematic and will diagnose shortly - I'll let you know how it goes. Why is it do you think that there are the two different voltages?
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Old 9th June 2012, 01:11 PM   #6
sbrads is offline sbrads  United Kingdom
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48v would be needed to obtain the rated output power. 28v is ample for a pre-amp, and there are sound quality benefits to having a separate rail.

I thought of possibly one other common component and that could be an op-amp IC, I've seen them shared on both L/R channels before - not very likely though.
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Old 9th June 2012, 02:27 PM   #7
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The 48V is a simple bridge psu, so plenty of ripple. The derived 28V should be stable and clean. Single rail amplifiers are always more sensitive to power supply noise.
There is a 82R series resistor on the 28V rail forming an extra RC filter for the input stage power supply of both channels. Check the voltages on this part
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Old 9th June 2012, 03:02 PM   #8
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Thanks all for you help. I'll be at home tomorrow and sure to test it as you recommend.

I'm very impressed by the response from everyone. Thank you.

The long story is that I was so pleased by the sound of the ca252 that I bought a poor condition ca202 in order to swap out the working internals into the perfect case from the 252. But it transpired that they were both a little different inside and each have pro's and cons (i.e the 202 has headphone out that I love, and phono pre works, but the case is awful - badly resprayed and missing fader).

So my quandry is I have two great amps and can't bear to dismantle one and leave it unused! I think 202 will be a dedicated headphone amp in a restored case. And the repaired 252 will be my main system amp.

...I wish I could have a remote control fader for the volume!
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Old 10th June 2012, 02:19 AM   #9
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Good luck and choose your speakers carefully. This is not an amplifier for inefficient speakers or those with wild impedance swings. A pair of vintage Mission 752s like mine would be ideal
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Old 11th June 2012, 10:30 PM   #10
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Hi there,

I've cracked open the Amp again wielding a multimeter in it's general direction for a while tonight. I could not find any 82r resistors to check on any of the boards.

However I have noted a couple of happenings - there is no L or R voltage to the main pre amp board (fixed to the underside of the lid) leading from the phono/aux switch or the resistors next in line.

There is also another separate smaller module with 34V input (down to stable 28). This has outputs to both the volume fader and back to the Pre/Amp socket - none of these outputs show a voltage.

So I am wondering now if the switch is defective.

However, after vigorous testing well into the night I stupidly popped one of the little glass fuses - 250mAV so now nothing works! I'll get to the electrical wholesaler soon...
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