Repairing a Soundcraft Delta Theatre..

[DanSteely]

I have just worked out the pinouts on the old transistor as (flat side up) BCE and the replacement is CBE. So do I just have to spin legs 1&2?

[boatman]

I have just worked out the pinouts on the old transistor as (flat side up) BCE and the replacement is CBE. So do I just have to spin legs 1&2?

 

I don't know which way you are looking at them, but one of those is wrong. Look at this diagram and you'll see that you have to rotate the BC557 clockwise by 120 degrees to make the legs match up to the PCB.

 

http://www.mydesk.myzen.co.uk/_Useful/2SB737_BC557.gif

[DanSteely]

Hi,

 

I'm sure you're right and I'm wrong.. I'm new to this as you can tell.... Below should be a collection of pictures:

 

http://i1217.photobucket.com/albums/dd400/nevillethannhauser/BC557Spec_zps905d7483.jpg

http://i1217.photobucket.com/albums/dd400/nevillethannhauser/DeltaMicPCBPic-02_zps559730e3.jpg

http://i1217.photobucket.com/albums/dd400/nevillethannhauser/DeltaMicInputStage_zps1f822859.jpg

 

Let me know your thoughts...

 

Hi,

I've had another look at the PCB and can confirm the pin outs on the 2SB737 and drawn it compaired to the BC557:

 

http://i1277.photobucket.com/albums/y496/dansteely123/2SB737vsBC557Transistors_zps9f96da4c.jpg

 

Or have I got the wrong end of the stick?

 

Many thanks for your help in this...

[boatman]

Sorry you're still wrong with the BC557! Historically, data sheets for transistors with legs have always shown the pinout looking towards the legs and not from the top. With ICs and SMD components (including transistors!!) the pinout is always shown from the top. It's just something you have to get used to.

 

Look at that data sheet again and you'll see the pin numbering for the BC557 is from the bottom and that you have collector and emitter reversed on your drawing.

 

I've been designing & building electronics for 45+ years and still have to think about it sometimes.

[dbuckley]

Here in New Zealand RS do free delivery on any order regardless of value, and they have no minimum order value, pay with credit card, no account needed. Can't remember last time I ordered from Farnell. Is NZ unusual, or is that normal RS practice these days?

[DanSteely]

Post Script:

 

The bits arrived this morning. 3 broken mic channels sorted by 5 oclock... BTW: It was the dual opamp in all cases. Also opened and cleaned all the Alps faders and lubed with Servisol Super 10..

 

Thanks to everyone for their help on this..

 

Happy days...

[timd]

Here in New Zealand RS do free delivery on any order regardless of value, and they have no minimum order value, pay with credit card, no account needed. Can't remember last time I ordered from Farnell. Is NZ unusual, or is that normal RS practice these days?

 

Used to be the case in the UK, introduced a £20 IIRC free delivery threshold recently.

[boatman]

Post Script:

 

The bits arrived this morning. 3 broken mic channels sorted by 5 oclock... BTW: It was the dual opamp in all cases. Also opened and cleaned all the Alps faders and lubed with Servisol Super 10..

 

Thanks to everyone for their help on this..

 

Happy days...

 

Well done. We'll make a service engineer out of you yet!

[DanSteely]

Thanks Boatman :-))

 

 

Here's another one for you.... The PSU supplies +17V, -17V and +48V. I'm looking at adding a couple of BNC gooseneck LED lights for the desk. The cut-outs for the male BNC's are already there and the current draw from the LED's will be minimal. I would though need to drop the 17V to 12V and wondered if you can recommend a suitable DC-DC regulator...

 

This is the kind of gooseneck I'm looking at: http://www.audiomate.co.uk/proddetail.php?prod=L100CA There's no detail on the current draw so I guess I could get a couple and decide on the reg once I've got them..

 

What are your thoughts on this?

[Mixermend]

Most mixing desks use a simple resistor dropper to feed external goose neck lamps - but I would prefer to use a 7812 voltage regulator IC, which would be perfect for this application. It would also have the benefit of being short circuit protected - always an advantage when the mixer audio power supplies are 'exposed to the outside world' - (a BNC connector!).

[boatman]

Personally I wouldn't go near BNC gooseneck lamps as they have a tendency to rotate just when you don't want them to! If you have space, enlarge the connector hole, fit a 4-pin XLR socket, wire it up to some standard and buy a gooseneck to suit. Many sound and lighting desks have 4-pin XLR connectors for lamps (the Zero 88 Frog for example) so you can get a standard pinout from the user manual. If you don't have the space for an XLR then you'll need to cable-tie the BNC gooseneck to something to stop it flopping at the most inopportune moment which it will certainly do!

 

For the supply, a 12V fixed output regulator is what you need. If the lamp current is less than 100mA then a 78LS12 (TO92 case) will do the job, otherwise you'll have to go to a 7812 1A version (TO220 case). Both will need a 100nF cap on input and output to stop oscillation and, as you have a constant load, you can probably get away without smoothing caps. Mount everything on a small piece of stripboard and you're done.

 

Be aware that the desk power supply may not be specified with enough capacity for a couple of lamps so you should check first. You'll probably get away with 100mA or so but there may not be 1A to spare.

[DanSteely]

Thanks Mixermend & Boatman. I agree about the BNC vs 4-pin XLR and have experience with both. In this case, there is not the room on the top surface for an XLR connector.... Anyway, the bits are on order.... I'll try to post a picture when the jobs done..

 

Thanks again.

[boatman]

Thanks Mixermend & Boatman. I agree about the BNC vs 4-pin XLR and have experience with both. In this case, there is not the room on the top surface for an XLR connector.... Anyway, the bits are on order.... I'll try to post a picture when the jobs done..

 

Thanks again.

 

BNC connectore were the bane of my life for years. There are two types of BNC connector with 50Ω & 75Ω characteristic impedance. The diameters of the central pins were different and, although they would interconnect, if you mixed the types the connection would either be intermittent or significant damage could be done to the socket. Luckily, some wise committee somewhere decided to rationalise the situation and both types now have the same pin diameter, just different dielectric configurations.

[alistermorton]

BNC connectore were the bane of my life for years. There are two types of BNC connector with 50Ω & 75Ω characteristic impedance. The diameters of the central pins were different and, although they would interconnect, if you mixed the types the connection would either be intermittent or significant damage could be done to the socket. Luckily, some wise committee somewhere decided to rationalise the situation and both types now have the same pin diameter, just different dielectric configurations.

 

I remember the days when you got intermittent problems after someone had connected network cables and video cables together,enlarging one set of connectors and rattling around in the other.

 

These days, I can' actually remember which had the bigger centre pin - 75 ohm?

 

 

 

[Mixermend]

There is an IEC standard for BNC connectors which states that 50 and 75 ohm types should be mechanically compatible and mate together in a non destructive way - which makes sense, as it would be terrible if damage were to be caused surely?