Amek TAC Bullet : switches causing raise in THD ?

[ricothetroll]

Hi,

I'm currently working on an Amek Tac Bullet. After having done some cleaning, resoldering and removing the AFV section, I measured every channel's response using HOLM Impulse and Visual analyser software. This gives the frequency response, phase, THD/freq and impulse response.

I was surprised to see that,  when most channels have a 0.00x % THD at 1kHz, some others show one as high as 0.x %, 100x higher. The most suprising thing with it is that this high THD sometimes disappears, when you check the measure again after having the channel moved from the motherboard...

I finally found out that the action causing this THD rise and fall is messing with the switches. Keeping the THD monitored in real time, pushing some switches back and forth sometimes lead to this raise in THD (quite hard to hit the "sweet spot" where the THD rises actually). This occurs not only on some particular switch but in any switch that's in the signal path, like PAD, MIC/LINE, PHASE, EQ IN/OUT... What's also surprising is that those are not noisy audiowise, that's why I didn't suspect them in the first place !

I guess there could be some stray resistance caused by dust or corrosion, that could overload th output drive capability of some active components.

Have you ever had this kind of issue ? Is there a known issue with that console's switches ? Would a shot of contact cleaning be enough or is it bette to replace those switches ? (that's a LOT of switches to replace...)

Thanx in advance for sharing your experience.

Best regards.

Eric

 

[ricothetroll]

BTW, this is a picture of one of the incrimated switches. The one showed is an internal one, for photo shooting purpose.

There's a "P" logo on it, and "916U".

I've noticed that on the stereo modules, that seem more recent (1995 written on it instead of 1989 on the mono input modules), the switches are different, the brand is ITT. Could that be because they have noticed some issues with the other series ?

 

[ruffrecords]

The trouble with making THD measurements in the 0.00x% region is that you are operating at the limits of the test equipment - especially if that equipment is a PC soundcard (wither internal or external). For example, 0.001% is -100dB which is very close to the noise floor of even the best sound cards. Unless your measurement software is extremely sophisticated, measuring THD also measures noise - the measurement is actually THD + noise.

I therefore suspect that what you are seeing when you operate switches is in fact small changes in noise level rather than distortion.

Cheers

Ian

 

[ricothetroll]

Hi Ian,
You're making a good point. I will compare the spectrum between "low THD" and "High THD", to see if there's a significant raise of harmonic distortion !
The interface I'm using is actually an RME Multiface, quite far from a dedicated measurement system indeed.
I'll post some updates...
Thanx a lot for answering me !
Best regards.
Eric

 

[ricothetroll]

Ian, you were right, the raise of THD is caused by noise and not harmonic distortion !

 

[henk]

Did do some changes on a Bullet..
Did not like that Cmos switch inside every channel , so replaced that by relay on some channels.... to hear the difference.... but that is up to you
The schematics should be on the internet.... if not have a look on the meta schematics