2254C build/support thread

[Igor]

calibrated both channels and all seems good except the right channel seems to be phase reversed. im using carnhill transformers and i built both channels up side by side and have checked over all components over and over and still cant find the cause. i've also tried using the threshold and ratio boards from the left channel to try and eliminate sections of the build but the signal still comes out phase reversed. any help would be great

.

If you can sync the scope with audio oscillator's "sync" output, and see stage by stage where
the phase goes wrong, it is easy way.

If no, possible to try sum signals from l and r channels thru 2x10k resistors after the interstage
transformer and see if there's no phase reverse.
This way, we can know where phase is reversed.

If you can post some pictures with bypass relays, will be good.

 

[gg85]

ok so the bypass problem is now sorted....wrong output wiring so i was shorting pin 2 to ground...still have the problem of inverted phase. when the unit is in bypass the phase is fine, but when comp is in it gets reversed. my transformer wiring is like this

T1 vtb9046
2 pri +
3-4 shorted
5 pri -
6 ground
7 sec +
8-9 shorted
10 sec -

T2 vtb9045
2 pri +
3-4 shorted
5 pri -
6 ground
7 sec +
8-9 shorted
10 sec -

they are wired the same on both channels so i very much doubt there is a problem here otherwise i would be seeing the same fault on the other channel

thanks for the help so far

 

[gg85]

an update...using a scope i found that the phase is reversed after T2 (vtb9045) secondaries. the sec + is out. i swapped the transformer from the left channel (working channel) and the signal is now in phase. they are wired exactly the same so i have no idea why this is happening but i just reversed the secondaries and the signal is now in phase and the unit seems to be working fine.

thanks again for the help

george

 

[haima]

strange! perhaps someone was napping when they wired up that transformer...

good job on the troubleshooting 

 

[Igor]

Does this means you had transformer with wrong marked secondary's?
They should have this kinda QC at the factory....strange!

 

[gg85]

Yeah you'd think that wouldn't you

so the unit is now working fine although I'm having trouble calibrating it. Firstly should I be sending it 1kHz at .775v or 1.228? I'd like to cal it for +4 but everytime I try it gets to the rv5 trim pot and I can't seem to get it to show me a voltage reading for -1. Also once it is calibrated (I've just used .775v at the moment) even with threshold at 0 it is still compressing just the slightest bit. Is this normal or am I calibrating wrong? I've biased the 183 with 4.7v across the 47r resistor, where should all my other trim pots be at the start of the calibration, ie cw or ccw or does it not matter? I'm sure I'm missing some simple logic here but some help would be great.

Thanks

 

[Igor]

Yeah you'd think that wouldn't you Smiley

Yep. Anyway, you can take the trafo off circuit and check if the phase reverse was caused by trunny.

so the unit is now working fine although I'm having trouble calibrating it. Firstly should I be sending it 1kHz at .775v or 1.228?

I'd say, from original cal proc., it is 1.228 VAC, but...God knows what they meant 0dbU

I'd like to cal it for +4 but everytime I try it gets to the rv5 trim pot and I can't seem to get it to show me a voltage reading for -1.

You mean, you can't cal it for higher than 0dbu threshold?
IMHO, they meant 0, not +4 at test proc. Simply try to use 100k at place of RV5 trimpot to higher the threshold.

It is not Neve'ish, but may work. Originals (if I don't miss here a point calibrated for 0dbu, not +4.

Also once it is calibrated (I've just used .775v at the moment) even with threshold at 0 it is still compressing just the slightest bit. Is this normal or am I calibrating wrong?

Yes, it starts to compress bit before actual threshold.

I've biased the 183 with 4.7v across the 47r resistor, where should all my other trim pots be at the start of the calibration, ie cw or ccw or does it not matter? I'm sure I'm missing some simple logic here but some help would be great.

183 part should be adjusted for symmetric distortion at 600 Ohm load at out.trafo secondary.
Logic depends on component's tolerances, i.e. as it is random-there's no logic.
Just stick to test procedure.

 

[gg85]

this might be a stupid question but how to i measure the symmetric distortion? i don't have a distortion meter but i do have a scope.

 

[Igor]

I meant, symmetric clipping at 600 Ohm load.

 

[haima]

gg85 - an oscilloscope makes it easy, raise the input level of the test tone to clipping point and look for equal clipping of the top and bottom of the waveform.

if you don't have access to a scope you might be able to do it using your computer DAW - if you have a way of passively padding/attenuating the output (otherwise your computer will probably clip before the output of the neve) - by recording passes of test tone, zooming in on the waveform and adjusting the trimmer, then recording another pass. repeat until it's symmetrical.

i imagine it doesn't need to be PERFECT - a little bit of asymmetrical clipping will just tweak the odd/even harmonic ratio of the clipping (?). but you don't want it to be WAY off...

 

 

[Igor]

Thanx Haima!
gg85: google BA283 adjust
http://www.auroraaudio.net/dcforum/DCForumID1/621.html
http://recording.org/tech-talk/43476-neve-ba283-output-stage.html
etc

Really, people, please search before asking!

 

[haima]

one more thing, use a triangle waveform test signal - they are the easiest to see clipping on a scope.

and don't forget to load the output transformer with 600 ohms (a 620ohm resistor will do) - as igor mentioned above.

 

[Igor]

BTW. This can be helpfull.

Question:

C4 on the BOM is listed as 470-680uF, 25volt
C'4 and C"4 on the board show a value of 80uF.
Are all of these the same component (C4, C'4, C"4), and what value should it be?

I sticked to numbering at BA283 board, and due to this can be some mess.

The values are correct. Please check the PCB silkscreen.

C4" (470...680uF): power supply bypass capacitor at B183.112

c"4: 80(100)uF: capacitor in feedback path of B183.110

 

[tongenerator]

i am interessted in builidung one of those (2channel)

but i am not sure about the in/output transformer. which transormer do i have to use an does somebody knows where to get them, please?

 

[ultra-alex]

http://www.groupdiy.com/index.php?topic=32605.msg398047#msg398047

 

[tongenerator]

thanks

 

[king-rb]

does someone has a frontpannel file of the 2254 for me?
i want to use special meters so the standard pannel is nothing i can use.

 

[Igor]

Feel free to PM or mail for .cdr file.

 

[king-rb]

thank you igor, but i only have the schäfer frontpanneldesigner. so anyone got messurements or a file for this programm?

 

[Igor]

It is easy to convert .cdr to .plt which is acceptable by Shaeffner.
Draw 10x10 cm rectangle in Corel, convert it to .plt, open in fpd-
you can check if conversion is right, maybe to play with 1000/40/1016 plt units....
I rare do drawings with measurement: as I have front template and
pcb dimensions exported directly from pcb software, and conversion is right,
the rest is only right conversion to g-code for engraving/milling...