82E01 Channel Amplifier Issue

[horizonsound]

Hi all,

I have had an issue with the one 82E01 Channel Amplifier card I have (the type with the transformer). When the line gain crosses unity on the pot, it produces white noise.
So far I have ruled out the pot, as it happens regardless of which channel it is installed in, and I have changed out the pot.
Measuring the resistance over pins 1 and 3 of the pot gives 22kohms, but when installed on the channel and with the 82E01 card removed, it measures 16kohms, and appears non-linear (5k and 11k at centre position). Adjusting the "Line Trim" trimmer on the channel board seems to affect the reading by a few ohms.

I have almost completely rebuilt the 82E01 card itself, so this is really strange.

Was there any modification needed to the channel itself to get these cards to function? I note that the 82E149 Transformerless Channel Amp, does not use the Line Trim as per the schematic?

Any clues would be appreciated.

Anthony.

[radardoug2]

Can you put a scope on it? Sounds like it might be starting to oscillate.

[horizonsound]

Yeah, I scoped around T3 which seems to be where the issue is.
The output on pin 6 is low, leading in to unity gain, then jumps considerable (x5 times), once the pot is swept past the white noise point.

I've recapped the board, excepting the feedback resistors (R43, R61) around T3, but they measured generally within tolerance.

[amillar]

Hi,

I used to see this on brand new build 611s sometimes, as radardoug says it is bursting into oscillation. 5534s are really intended for higher gains, so were never ideal in this circuit (but in those days there was nothing else with noise performance that good).

First step I'd suggest is trying a different 5534, perhaps swap it with T1 or T9 from the mic stage.

Check the CT capacitor (should be 22pF, not 22uF, that was a circuit diagram error) is not damaged. (If anyone's thought the circuit diagram was correct and has replaced the original capacitor with a 22uF that would definitely make it behave very oddly!!)

If none of that works, check the wiring to the front panel control looks original (usual short wires straight down to the 611 motherboard). If these wires are significantly too long then that will do it. But this is very unlikely to be the problem - we had long wires on the '291 line amp and that worked fine.

Don't connect a 'scope to pin 6 while you are testing it. The capacitance if the 'scope probe is likely to make it oscillate itself. 

Let us know how you get on! I really like '01s, ok the transformer is really too small (because of the size of the 611) so the bass response isn't great, but they are nice simple robust cards. And of course they are the "original" channel amp design (hence the number!), everything else was tweaks to the '01.

Good luck,

Andy

[radardoug2]

Wow, thats a pretty weird looking circuit! Who designed that bit?

[horizonsound]

Hey Andy,

Thanks for the advice. I have changed out the 22pF capacitor (on T3), and changed out all the 5534s on the board. The wires to the 22k pot are original and in good condition.

On looking closer, although the board is marked as an 82E01, the component labels are all for a 241 mic preamp, and all component values match that (i.e. R43 is 2k0, and R41 is 2k21 - all measuring correctly). I also changed out the two 100k resistors without luck (R42, and R61).

I'm thinking now it may be an issue with the first stage preamp T2, so will have a look around that area. If I recall, the output on pin 6 of T2 was quite low when I measured it, but assumed T3 then picks it up.


241 schematic

[amillar]

Wow, thats a pretty weird looking circuit! Who designed that bit?

I assume Colin did. If anyone's got the original "hand drawn" circuit diagram that was in the early manuals it's probably got someone's initials on. Once you take out all the AC coupling components (R42 & R61, C5 & C37) it's just a normal inverting op-amp. The gain trim pot (5.1k pot + 5k resistor) is only there to let you adjust the gain when the gain pot is at the centre position.

R76 was added by Trevor Stride in the 241 (at least, I'm 90% certain the 241 was Trevor's), it's part of changing the gain of this stage (see below), it also keeps the pot wiper connection connected to something when the pot gets dirty and starts having bad connections when moved.
I think I've got this next bit right (after several attempts!) perhaps someone could confirm the 'T3' gains!!
The '01 has T2 with gain of 0dB and T3 with gain of -4dB (with pot at centre), the '241 has T2 at -4dB and T3 at 0dB so the overall gain is the same for both at -4db, but with 4dB more headroom in the '241 at low gains. That's the difference (and advantage) of the '241.

R42 & R61, C5 & C37 are there because the inputs of 5534s draw a lot (relatively) of dc current. If you connect them directly to pots then this causes a dc voltage to appear on the pot, then as you move it the slightest amount of dirt on the pot causes horrible dc "jumps" which comes out as crackles and bangs. So all over the 4k designs you'll see lots of extra electrolytics, then you need extra resistors (R42 & R61 here) so the op-amp can draw its input current from somewhere i.e. dc bias resistors. R42 is there so that the two inputs have the same bias voltage (i.e. same current through the same resistance) which helps keep the dc output voltage as near zero as you can get it. These resistors have to be quite large otherwise you mess up the gain in the rest of the circuit. C5 just keeps the noise down.

On the '291 inputs we sorted this out much better with a "dc servo" circuit e.g. IC14/C19/R39 (I say "we", that was entirely Paul F, I'd never come across them before), and then in the 9k channels I used dc servos everywhere. 

Cheers,

Andy

[amillar]

Hey Andy,

Thanks for the advice. I have changed out the 22pF capacitor (on T3), and changed out all the 5534s on the board. The wires to the 22k pot are original and in good condition.

On looking closer, although the board is marked as an 82E01, the component labels are all for a 241 mic preamp, and all component values match that (i.e. R43 is 2k0, and R41 is 2k21 - all measuring correctly). I also changed out the two 100k resistors without luck (R42, and R61).

I'm thinking now it may be an issue with the first stage preamp T2, so will have a look around that area. If I recall, the output on pin 6 of T2 was quite low when I measured it, but assumed T3 then picks it up.

Hi,

The way to tell is: if R74 and R76 are there it's a 241, if they aren't it's an 01.

T2 is unity gain in a '01, and a gain of -4db or about 0.6 for a '241 - as described above, that's the difference between them. Once again, sticking a 'scope on the output of T2 is likely to cause it to oscillate, but using a test meter will be fine.

Cheers,

Andy

[horizonsound]

It seems I have solved the issues, but by pure chance, and in an unusual way.

The line pot has a centre tap on it (see pic below - on opposite site to the main connections), that is connected to Pin 5 (Even input) of the LF, RF, LB, RB Routing Switch Card. I have no idea why this is the case.
Anyway, I disconnected it in this instance, and the 82E01 Channel amp is operating correctly, without any oscillation.

Can any one put some light on to the purpose of this centre tap?

Anthony.


Line Trim Pot Centre Tap.

[amillar]

Oh dear, this is another of those cases where I have a very, very faint memory nagging away at me - I'm sure I have seen this before but I can't remember why.  I'm really hoping someone answers this one because I'm fascinated to know what it was for!