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Old 19th May 2021, 5:52 pm   #1
yestertech
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Default Where next for this Lecson Preamp noise problem ?

I am trying to find the source of excessive noise on a Lecson AC1 preamp.
It's of the 'rustling and thumping' type rather than a gentle white noise.
I am leaning towards it being a problem with one of the FET's which controls input selection.
Here is my logic ( see circuit extract )

With no inputs selected at all the noise is present, but only on the LH channel.

As expected, the noise increases when either phono input is selected as FET switching adjusts the feedback loop to increase gain and apply the RIAA equalisation.

To try and isolate the input amplifier itself ( used for all inputs including phono ) I have grounded the junction of R84 and R86, when the noise disappears.
This leads me to think that it is generated by one of the FET input switching transistors.
Unfortunately, all the output of the devices are summed to provide the input to R84.
The following readings seem incorrect:
At the point marked 'T9' on the RH channel the voltage is 0v as stated. ( within a few mV )
same point on the LH channel measures - 1.05v approx.
For any input not selected, the voltage at the T1, T2, T3 etc. points is -15V EXCEPT that for the PU 1 input its only around -8v. This voltage is common to both channels so it's hard to see whether this is relevant, but it seems to be the only one that's incorrect, other than the -1.05 stated above.
I can't see an easy way to determine which of the FET input switches may be noisy, ie TR1 through 13 ( LH channel numbers only ) assuming that this actually the cause.
I am also hampered by a complete lack of any PCB layout information, so pairing circuit references with the physical component is a nightmare. I'm rather stuck !!

Any thoughts welcome.....

Andy
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Old 19th May 2021, 6:40 pm   #2
Craig Sawyers
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Default Re: Where next for this Lecson Preamp noise problem ?

Do you have any test gear? Like an oscilloscope?

There are dotted lines that suggest that the input switching is on a different board to the gain stages. How is that connection made? Is it soldered, or via a plug and socket arrangement?

Craig
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Old 19th May 2021, 6:46 pm   #3
yestertech
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Default Re: Where next for this Lecson Preamp noise problem ?

Thanks
The whole preamp is constructed on a single PCB ,with tracks on both sides and some through board connections . It's also extremely densely packed so tracing and measuring anything is very trying.
Yes, I do possess a scope.

Andy
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Old 19th May 2021, 6:51 pm   #4
Craig Sawyers
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Default Re: Where next for this Lecson Preamp noise problem ?

Peering at some poor quality schematics, two of the options for the switching FETs are BF244 and UC734 - both of which are RF jfets. Not sure why they chose RF devices, other that they might have had low on resistance, and hence good for switching.

But such devices are somewhat sensitive to damage, and to self oscillation.

Craig
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Old 19th May 2021, 7:15 pm   #5
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Default Re: Where next for this Lecson Preamp noise problem ?

The careful application of heat or cold to the suspect devices may well give a clue. A dab of meths is more easily localised to one component than freezer spray and maybe enough. A hot soldering iron, briefly applied is a good heat source. I have had resistors, caps and active devices all cause such problems so try them all !
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Old 24th May 2021, 6:03 pm   #6
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Default Re: Where next for this Lecson Preamp noise problem ?

These are not good even when working properly IME.... I recall measuring one to be about 3dB down by as low as 14KHz!
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Old 24th May 2021, 8:11 pm   #7
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Default Re: Where next for this Lecson Preamp noise problem ?

I use that sort of FET switch in a breadboarded preamp here. It hasn't been very reliable. The very high value resistors to bias the gates and not load the bootstrap circuits (10 megohms) seem to keep going high and need a new set about every 10 years.

I used 2N4391 30 Ohm channel resistance JFETs

14kHz, -3dB?

But I thought this was one of the earliest Boothroyd-Stewart designs, and I would have expected something so poor to have been noticed. There's something odd, somewhere.

David
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Old 24th May 2021, 8:18 pm   #8
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Default Re: Where next for this Lecson Preamp noise problem ?

That's an expensive FET David! They make good CCS when you need several mA from a floating CCS.
I was shocked at that poor performance also yes. It was about 15 years back but I recall ruling out any fault and that it seemed to be a design issue...
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Old 24th May 2021, 11:44 pm   #9
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Default Re: Where next for this Lecson Preamp noise problem ?

They were the standard analogue switch FET at HP. I can still remember the part number 1855-0020. the 2N4391/2/3 were a family of progressively higher Rdson devices (reducing Idss) They were about 20 cents then.

Only years later did I learn that Marconi used them as RF amplifiers at HF in their monumental H2900 receiver (aka 'Sosin's Folly')

They make a very good low distortion audio switch with that trick of bootstrapping the gate to the input, and feeding DC bias to the gate via a big resistor. It was a circuit Bill Miller showed me.

David
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Old 25th May 2021, 4:59 pm   #10
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Default Re: Where next for this Lecson Preamp noise problem ?

Quote:
Originally Posted by Radio Wrangler View Post
They were the standard analogue switch FET at HP. I can still remember the part number 1855-0020. the 2N4391/2/3 were a family of progressively higher Rdson devices (reducing Idss) They were about 20 cents then.

Only years later did I learn that Marconi used them as RF amplifiers at HF in their monumental H2900 receiver (aka 'Sosin's Folly')

They make a very good low distortion audio switch with that trick of bootstrapping the gate to the input, and feeding DC bias to the gate via a big resistor. It was a circuit Bill Miller showed me.

David
£32 each for sot23 version from RS now!! I have a few dozen of the 2N4392 in TO18 on boards where they were indeed used as AF switches and have harvested them as required.

Interesting that they were used as RF amp.

I've used the linearised FET idea a few times as voltage controlled attenuators for compressors or VOGAD.
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Old 26th May 2021, 4:56 pm   #11
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Default Re: Where next for this Lecson Preamp noise problem ?

Looking at the manual this jumped out:-

Quote:
AC1s up to serial number 1300 (approx.) used a double-sided p.c.b. with griplet or pin connections between the two sides of the board. The griplet and/or pin system proved to be somewhat unreliable, typical symptoms are:

(i) Crackling and/or intermittent contact between sides of the board.
(ii) Channel fading or low-frequency disturbances.
(iii) Microphony.

Amplifiers of serial number from 1300 approx. onwards use a through-hole plated board which is much more reliable.
https://manualzilla.com/doc/6026917/...--ac1-and-ap1-

If these are present I'd see them as a primary suspect.

Cheers
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Old 26th May 2021, 5:33 pm   #12
yestertech
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Default Re: Where next for this Lecson Preamp noise problem ?

Useful thoughts thanks
The noise is more of a rustling or ‘thunder-like’ noise than a crackling
Very akin to the din a noisy EF86 can make
I’m still desperate to obtain a PCB overlay . Someone somewhere surely has one ?
According to the circuit diagram I have, as well as a multitude of series FET switches on the inputs , there are also 4 additional ones across the two disc inputs which would be my first suspects, but I Can’t locate them anywhere on the PCB. All The FETs fitted are type ‘LECSON 6’ and have a green/white package and gold leads so easy to identify.
I’ll refer back to my original post here fir a sanity check :
‘To try and isolate the input amplifier itself ( used for all inputs including phono ) I have grounded the junction of R84 and R86, when the noise disappears’
Would the consensus be that this eliminates the input amplifier as the source of noise ?

Andy
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Old 20th Jun 2021, 12:31 pm   #13
Craig Sawyers
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Default Re: Where next for this Lecson Preamp noise problem ?

Quote:
Originally Posted by Jez1234 View Post
£32 each for sot23 version from RS now!! I have a few dozen of the 2N4392 in TO18 on boards where they were indeed used as AF switches and have harvested them as required.
TO18 versions of that series are made by Central Semiconductor, from Mouser £2.43 each. Thousands in stock.

Craig
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