Vox Repeat Percussion - Effect fades out

[guidoilieff]

Hi. I built the vrp  but I cant find the mpsa18 or bc109/108. I used some 2n2222 and c549 and the effect lasts for a few seconds with the c549 and maybe a minute with the 2222, then it shuts down. The rate LED wont turn on. I replaced the plastic version of the 2n2646.

I used a ceramic capacitors for the 47 and 4,7uf, a 563 for the "68k" capacitor and a 25k instead of the 50k pot. Then I hooked up everithing without the 3pdt.


I know I used a lot of replacements but its the only thing I can find in Argentina and I can not import components from other countrys (except the 50k pot, I forgot to buy that).



so... any idea where I should start? Is it just the capacitors "saturating"? (whatever that may imply)

[guidoilieff]

Sorry, this is the translated project https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.pisotones.com%2FVRP%2FVRP.htm&edit-text=&act=url

[guidoilieff]

Voltages (with 2n2222)

T1 - B1 0.04
       B2 7.36
       E 3.4 / 4

T2 - E 0
       B 0.53
       C 0

T3 - E 0.4
       B 1
       C 4.64

Cap 1 - 8.5
Cap 2 - 3.4 / 4

[PRR]

> Voltages (with 2n2222)
> T1 - B1 0.04
>        B2 7.36
>        E 3.4 / 4

http://www.pisotones.com/VRP/imgs/RepeatPercussionEsquemaYMateriales.jpg

T1 appears to be a Uni-Junction transistor.

This is totally different from an ordinary transistor (2N2222 etc).

And true old-type Uni-Junctions have become hard to get. (en español: https://es.wikipedia.org/wiki/Transistor_uniuni%C3%B3n )

The key trick is that a unijunction is (was!) a very simple way to get a ramp-wave oscillator. The ramp appears on the E pin. Other ramp-wave oscillators can be made to work. But they may need re-biasing into T2.

If anybody is wondering: T2 is working "inverted". It is shorting a very small AC signal. Inverted connection is sometimes better for this.

[guidoilieff]

Hi. Thanks for the article.


I used the plastic version of the 2n2646. I found the metal case ones for 2.63usd (40 pesos), but the plastic version was half that price.


Just tried with a pair of c549 and the problem that I had is gone, but I think there is some volume drop.

[R.G.]

There is a volume drop using the attenuator style of volume modulator - which is what this circuit uses. The second transistor is a gain stage to make this back up. If the second transistor stage is not providing enough gain, you get a perceived volume drop.

You can try to improve the volume a little by changing R12 (3.3K) to a lower value, perhaps 2.2K. Might work, depending on the actual value of the gain of the transistor. MPSA18 is one of if not the highest gain bipolar transistors that are commonly available. The C540 (BC549) is a good device, but probably lower gain.

[guidoilieff]

There is a volume drop using the attenuator style of volume modulator - which is what this circuit uses. The second transistor is a gain stage to make this back up. If the second transistor stage is not providing enough gain, you get a perceived volume drop.

You can try to improve the volume a little by changing R12 (3.3K) to a lower value, perhaps 2.2K. Might work, depending on the actual value of the gain of the transistor. MPSA18 is one of if not the highest gain bipolar transistors that are commonly available. The C540 (BC549) is a good device, but probably lower gain.

Cool, thanks. I told that to the guy in the electronics store and gave me a few transistors to try.
I think I've got this.