Sadelta Echomaster Plus - Echo Fading Out

[Warf135]

Mic Make and Model: Sadelta echomaster plus
PCB Number: EMP-2

Symptoms:
Microphone will echo for 3-5 seconds then echo will fade out. audio remains perfect, just with no echo. If mic is left for a while it will again echo for 3-5 secs.

What is working:
Microphone is working properly other than this fault (and the beep isn't quite right too)

What happened:
When I bought this (well used) microphone and it had no echo at all. I resoldered loads of bad joints on the board and replaced a broken 56 picofarad capacitor with a 47p and the echo came back but with this fadeout problem.

Anything else that may be of help?:
The beep isn't working right in this mic - it may or may not be related to the same problem. I have and can post schematic if required.

Thanks,

Norm.

[The Defpom]

Please post the diagram for the mic.

[Warf135]

Here is Echomaster plus schematic - courtesy of Rick Jackson -euro radio co.

[Warf135]

ok, I've now changed all electrolytic caps as a matter of course. This has had no effect on either the echo or the beep problems.
both sliders and the 50kpot for repeat speed test fine on the meter.

am i right in thinking that the MN3008 and MN 3101 chips are to do with echo? and the LM324N is preamp and 4011 is for beep, or have i read schematic wrong?

if i'm right, then i need to concentrate around the two echo chips to locate this fault.

[The Defpom]

Check the voltages across the IC's, specifically pin 1 and 3 of MN3101 and see if it drops off when the echo does.

Also check pin 3 and ground of the LM324, in case of a problem with the 100K resistors.

Does the TX LED light up and stay lit up the whole time, even when the echo fails?

[Warf135]

Hi Scott,

Thanks for reply. The TX led remains lit at all times, and the audio works fine, but just looses echo.

The voltages:

pins 1+3 MN3101 = 8.00v that stays when echo gone
pin 3 + GND LM324 = 4.20v that stays when echo gone
pins 1+5 MN3008 = 8.60v that stays when echo gone

this has got me puzzled

[The Defpom]

I would say to check all the resistors and capacitors, if they are OK try replacing the LM324, it is a common part, and will rule that out, failing that it could be one of the echo IC's themselves.

[The Defpom]

If you have a scope there, it may be an idea to check pin 2 and pin 4 of the MN3101, it generates the clock frequency for the MN3008, check to see if the clock frequency drops off on those two pins when the echo fails, if it stays OK, then the fault is probably with the MN3008

[Warf135]

Thanks for great info.

I have replaced LM324 as a spare was to hand, but no change.

My scope isn't working at the moment, but I checked pins 2 and 4 of MN3101 with high impedance frequency counter and read approx 9khz from each pin to gnd, so assume that that chip is ok.

So far all resistors and caps that I have tested seem fine, but I'm now I've decided to work through the schematic and test each component and track (as some of the tracks were bad when I first got the mic) Individually. This will take a while as I'll have to takeout, test and refit each one, but I'll be sure to find the fault eventually.

I'll post my findings, when I finally trace the fault.

[Warf135]

Update: I've now tested all components in the echo chain and have come to the conclusion that the MN3008 chip is kaput. Since a new chip is about £15-20 and I would hardly ever use echo anyway, I have decided not to bother and have grounded the echo fader to kill echo altogether. Beep has now also been permenantly disabled.

The mic is wired to a Lincoln and I'm getting excelent reports on the audio from everyone - so sod the echo and beep

UPDATE: I found out that my Sadelta Microphone was a fake! this was due to the fact that genuine Sadalta Mics use genuine Panasonic echo IC's (with panasonic written on them) and my mic doesn't. This was also drawn out by the fact that the beep wasn't working properly (too fast).

I removed all beep components from the board and replaced them following the Genuine Sadelta schematic. The beep now sounds correct, however all the parts have gone back in compleatly different locations from where they started!!!