Echobase transistors critical?

[newfish]

Hi All,

Am most of the way through building an Echobase, and am wondering how critical the transistor part numbers are.

My uneducated hunch says that the 2N5089 is simply there to switch - and any one of many NPN devices would be a suitable alternative (BC108, BC109 etc...)

The BC560 has me stumped, as I don't truly understand its function in the echobase.

http://www.eskimo.plus.com/fxstuff/echobase.png

I have BC560C devices to hand, but am wondering if the higher gain 'C' suffix will make a huge amount of difference.

FWIW, I've made an EBase before (using BC560C in place of BC560), and the LFO only sounded for the first second or so after engaging the LFO switch.

Thanks for your help.

[slacker]

Your hunch is correct, virtually any NPN transistor will work in place of the 2n5089.

The BC560c will work fine for the PNP, the problem you had with your other one won't have been caused by the transistor unless it was faulty. People have also used 2N3906 without any problems.
The BC560 acts as a voltage controlled resistor to vary the delay time, as the voltage from the LFO goes up the resistance increases, making the delay time longer, and as it goes down it decreases, making the delay time shorter.
The mod depth pot controls how much it varies by.
Basically the same as if you grabbed the delay time pot and turned it backwards and forwards.

[newfish]

Excellent.  Thanks for explaining this!

I hope to have my EBase up and running by the weekend.

Cheers!

[newfish]

<bump>

I've removed the offending 1N4001 diode.

Have taken time to re-flow all joints (with ICs out of their sockets), and now have a delay.
Tails switch in and out correctly, and there is full, clean signal in bypass.

Same as before though as far as LFO switching in.
The LFO switches in briefly when the switch is activated - and then stops.

I've tested the switch with my DMM - and it is not faulty.

I've also checked the datasheet for BC560, and may well have mine in backwards, despite following the white layout of this component.

Has anyone else come across this situation where manufacturer 'a' makes the same component as manufacturer 'b', but changes the pins round?

Knowing that the BC560 is acting as a voltage controlled resistor, does its polarity matter in this instance?
I'm presuming here that the voltages applied to its Emitter and Collector will, in turn,  each be higher than the other at some point during the up / down sweep.
<VCR configuration is new to me - so if this seems like a 'newbie question, it is...>
*Edit - the polarity *has* to matter, in order for the device to conduct at all.*

I did not socket the transistors during my build, so would like to take some advice before comitting to a double-sided de-solder if anyone has any to offer.

Many thanks.

[newfish]

Whilst de-bugging an Echobase, I discovered the following 'quirk'...

On an otherwise perfectly functioning EBase, the LFO section works for as long as I 'pinch' the two end lugs of the LFO Depth pot together.

I have tried two different pots, with fresh wiring to them, and each time the results are the same - no LFO until manual intervention.

Moving the 'LFO depth' wires at the PCB end makes no difference at all.

Am somewhat stumped.

Has anyone come across anything like this before?

[slacker]

weird never come across that, have you got all 3 lugs of the depth pot connected? I know a couple of people missed the connection to ground.
About the BC560, in my experiments it worked both ways round, but it was much better the way round on the schematic.

[newfish]

Thanks Ian,

After all the looking and thinking and head-scratching, it works.

It works because I tried it with a 9v adapter. 

That said, I think I understand why the LFO was patching in then out with the old (weak) battery.
The voltage available was sufficient to provide the initial 'hit' when the LFO was taken out of bypass, but could not sustain the supply after this initial surge - hence the LFO was only a one-shot deal.

Because everything else was working, I assumed (there's the dangerous word) that the supply voltage was enough for the whole circuit to be well fed.  Oops.

Feel slightly smarter, and lots more embarrassed at such a simple mistake.

The case is drilled and drying - and this will be a 'keeper' on my effects board.

Feel slightly smarter, and lots more embarrassed at my mistake.

Thanks to Taylor and Slacker for their brains and PCB skills.

[jacobyjd]

Interesting. I think you may have solved my debugging issue as well!

[slacker]

Ahh yes, the LFO will stop working with a weak battery.  

This is either a flaw in the design or a low battery indicator depending on you point of view