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Old 9th April 2012, 02:10 PM   #3471
johnm is offline johnm  United Kingdom
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Well it measures 24V unloaded and nothing is over-heating. No fluctuation in voltage, but I'll give it a go anyway.

My gut feeling still says it's the B1 with that continuity reading across the input, or the pads where C1 was.

Have to leave it for a few hours now, but will hopefully get on that this evening.

Many thanks!

- John
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Old 9th April 2012, 02:17 PM   #3472
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PSUs can be misleading.

Try loading it as suggested just to make sure it is OK.

Even that is not the acid test.
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Old 9th April 2012, 02:19 PM   #3473
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I've got the redundant LM317T PSU out of my B1 built before I installed the Salas Shunt Reg. I can let you have that at cost if you need it.
It includes 30VA Traf so is ready to go.

I've just put under SWAPMEET for £30 if you are interested.

Last edited by KatieandDad; 9th April 2012 at 02:31 PM.
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Old 9th April 2012, 04:30 PM   #3474
johnm is offline johnm  United Kingdom
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K&D: MANY thanks for the kind offer of the PSU, but I have more than enough components here to make several, plus am also using a 30VA 0-18V transformer.

Anyways, cooking finished, back to the testing! On a whim I had a rummage around in the garage and found a 12V power brick/wall wart. Cut the end off that and connected it up to the B1, and the LED lit up! Great start. Measured the voltage at the input and no shorting 'beep' - just a reading of 12V.

Great! So it's not the B1 board afterall So for some reason the design registers a 'short' if one puts probes to the input with no PSU connected, but if the correct voltage is applied that "short" is no-more, and it registers the correct voltage.

Mad!

Anyways rather than building one from scratch (which I could do) I need to work out what's wrong with the existing PSU. As I said it's one of Peter Daniels 'universal' PSU boards. Haven't a picture to hand, but one identical can be seen on 6L6s rather tasty build here:

B1 preamp build thread

Mine's the same BUT instead of using an adjustable pot I'm using a resistor combination of R1:220R R2:4K to get 23.9v from the LM317T (manufactured by 'ST').

Just measured it again now (unloaded) and I get exactly that. When it's attached to B1 however something obviously isn't right :s What could that be?

There's a 1000uF cap before, and after the reg. In addition I've added a 1uF tantalum cap at the input, and output legs for stability, as detailed in the ref. notes for this regulator. Have also added a 10uF tantalum cap across R2, which again is mentioned in the notes as desirable. Triple checked all wiring/soldering and all appears to be fine.

I do recall reading that the resistor combo. used to get the voltage on the LM317T is actually not ideal. Instead of a 240R (I'm using 220R as I had it to hand) for R1 as most sites mention, apparently it's better to use 120R:

New Tool: LM317 Calculator

Perhaps this is the issue here, at least with B1?

Or perhaps I just have an iffy LM317T that measure OK unloaded, but just doesn't perform well?

- John

Last edited by johnm; 9th April 2012 at 04:43 PM.
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Old 9th April 2012, 04:51 PM   #3475
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Test your reg PSU by adding some resistors across the output and measuring what the new output voltage is.

10k will draw ~2.4mA and dissipate ~60mW
3k3 will draw ~7.2mA and dissipate ~170mW starting to get warm.

Add on extra 3k3 on thereabout, until you see the output voltage drop below 23.9Vdc.

What is the total current drawn through all the resistors?
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Old 9th April 2012, 05:06 PM   #3476
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Quote:
Originally Posted by AndrewT View Post
Test your reg PSU by adding some resistors across the output and measuring what the new output voltage is.

10k will draw ~2.4mA and dissipate ~60mW
3k3 will draw ~7.2mA and dissipate ~170mW starting to get warm.

Add on extra 3k3 on thereabout, until you see the output voltage drop below 23.9Vdc.

What is the total current drawn through all the resistors?
Thanks Andrew - KatieandDad mentioned loading it up too so will take a trip to Maplins tomorrow and get some 3K3s. Think I have some 10Ks.

Did occur to me to check the diodes I used from the parts bin, and they're 4xMSRF860 (8A/600V). They're really designed for switch-mode PSUs, so wondering if they're part of the problem perhaps?

Also occured to me that perhaps I need to set it for a lower voltage (20-22v). I'm using a 0-18V transformer, and perhaps that's not offering enough regulation when set for 24V output with the LM317T...

Tired now so will continue tomorrow - don't want to make any silly mistakes due to lack of concentration.

Thanks for all your help today K&D and Andrew.

Cheers,

- John

Last edited by johnm; 9th April 2012 at 05:24 PM.
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Old 9th April 2012, 10:16 PM   #3477
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Quote:
Originally Posted by intrator View Post
use the diodes as well, or 10 ohm resistor. MegOhm is only useful in parallel with something of much lower impedance like the diodes.
OK, I will run diodes, each with a MegOhm resistor in parallel, to ground. I see where you are floating your differential ground above the chassis (unbalanced) ground. It seems difficult to maintain that. If I understand what you’ve done, a lot of care must be taken so as not to short the differential ground to chassis.

Quote:
Originally Posted by AndrewT View Post
Does the transformer can (wht) get connected to the enclosing chassis?
In my basic implementation, unbalanced, the external PS ground carries through to the chassis, the circuit board ground and the RCA connector’s low side. So the chassis and signal ground are really the same.

Ground Practices

I admit that I don’t quite understand why the 2 transformer grounds [WHITE/BLACK] are being connected to different places in my setup. I think I understand why in Tom’s setup. I hope that what I am doing will be sufficient to provide isolation.

[FYI, I will eventually use transformers in my B1 Hypno (unbuilt yet) perhaps to provide a balanced input, but I won’t go as far as Tom did with a differential setup. This “test rig” is an SSP preamp which uses a +/- 15v supply.]

Looks like I will need a trip to RS to pick up some diodes.
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Old 10th April 2012, 05:25 AM   #3478
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Quote:
Originally Posted by AndrewT View Post
Does the transformer can (wht) get connected to the enclosing chassis?
yes, but use anti parallel diodes in parallel plus another resistor in parallel with those
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Old 10th April 2012, 09:56 AM   #3479
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Hi, I completed my B1 build and I was wondering if it's normal to get fluctuating DC at the output (measured across R105 and R205) with music playing through it? With no signal running through my B1 the offset is steady around 5mV, but if I hook up a CDP and play music through it the DC fluctuates to around 40. Thanks in advance
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Old 10th April 2012, 10:17 AM   #3480
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Quote:
Originally Posted by johnm View Post
Thanks Andrew - KatieandDad mentioned loading it up too so will take a trip to Maplins tomorrow and get some 3K3s. Think I have some 10Ks.

Did occur to me to check the diodes I used from the parts bin, and they're 4xMSRF860 (8A/600V). They're really designed for switch-mode PSUs, so wondering if they're part of the problem perhaps?

Also occured to me that perhaps I need to set it for a lower voltage (20-22v). I'm using a 0-18V transformer, and perhaps that's not offering enough regulation when set for 24V output with the LM317T...

Tired now so will continue tomorrow - don't want to make any silly mistakes due to lack of concentration.

Thanks for all your help today K&D and Andrew.

Cheers,

- John

Funny I did suspect the PSU earlier

#3438

What kind of PSU are you using ?

Some will not like the high inrush current of C1.

Either try substituting C1 with a smaller say 1000uF cap or try the series resistor as I have mentioned.

C2 is not such a problem because is has R2 in series with it.


There probably isn't anything wrong with the PSU, that particular one just doesn't like a highly capacitive load.

Andy
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