TC Electronic 2240HS

[Pentium]

Hello I have TC 2240HS eq, I guess the eq was made in the early 90's, it has some scratchy pots and TC have said they will not release schematic as it is their intellectual property. : (    The issue I have is that the pots are labelled as follows
Freq = 100K C W7 (dual gang 6 x connectors  to pcb)
Q = 22K B V8 (3 connectors straight to pcb)
gain = 22K A W9 (3 connectors straight to pcb)

Based around a single TL074 per band, now as I understand it, pots labelling got confusing when the A / B got changed, now I am not sure when the "old code" was in place and I have not got a clue wither they are lin or log and need to find out to buy replacements. Any assistance appreciated greatly. (I am thinking the labelling is "old" as there is a C on one of the pots but would like a verification of when the new convention came in.)There is also a capacitor which i am not sure is 100pF or 1,000PF, I think it says 1n0 on it but am not 100pct sure that it is not 100 with a dodgy 0 in the middle (i.e. mis printed in the first place), it's a ceramic and in the highest band which sweeps from 200Hz to 20kHz if thats any help (once I have the boards out I can verify this I guess as there are 2 identical boards) It actually sounds very nice except for the scratchy pots. Thanks for any advice that can be given. I had a read here

http://sound.westhost.com/pots.htm

Thank you.

 

[okgb]

Is it only scratchy when you turn the pot ?
and o.k. when it is left alone ?  
and you have tried to clean them right ?

if it's a matter of dc getting on the pot
then you need to look at the capcitors

 

[pucho812]

Is it only scratchy when you turn the pot ?
and o.k. when it is left alone ?  
and you have tried to clean them right ?

if it's a matter of dc getting on the pot
then you need to look at the capcitors

+1 buy some craig detox spray and spray inside then exercise the pot.

 

[QUEEF BAG]

if the pots do not make noise without program going thru...
probably not DC.

on pots, IF i'm gonna spray goo inside 'em,  the Caig product i would
go for is the Cal-lube moving contact cleaner in the blue can.

clean the out side of the pot first, squirt some of that goo in and exercise it.
after that i use a paper towel to "wick" out as much of the fluid as possible,
to pull out any crap that has worked loose, instead of leaving it there to cause
problems again.  possibly repeating, especially if i see smeg being wicked out.
then i take some canned air and blow the excess goo out of the pot.
...don't like that stuff in there.
distilled water is the only thing i will clean faders with.  if i could get inside the
pot to the wiper & track... that's what i'd use.  see penny & giles  fader maint. info)

kinda my way of doing switches too, but with deoxit or electrolube, or caig gold in some cases

 

[MHanson]

Sending the schematic of the TC 1140 (mono 2240) to the GroupDIY email acct.

I hope it helps.

Michael

 

[Pentium]

Hello again and thanks everyone.Embarrassingly I also broke one of the pots by over pushing a push fit knob on, the back tinplate came off and now tracks badly, D shaft knob onto round shaft won't go... durr.... live and learn novice mistake, so at least 1 broke. The scratchy pots are actually the master L output R output and R input. Thanks for tips of cleaning first. I was thinking plastic pots as an upgrade but maybe this is not required now. Many thanks for schematic as I hope it helps with the pot values. Edit the schematic confirms the ceramic on the high band as 1nf / 1000pf /1n0, so thank you and all pot tapers are confirmed thank you ! I can swap the broken pot for one of the EQ match ones as I doubt I will be needing it. 4mm shaft pots are difficult to find it seems. Pots only crackle when being swept and then quieten down soon after so I suspect some decent pot cleaner fluid may do the trick.(thanks for recommendations)

So as it stands:

F= 100K log
BW = 22K log
Gain = 22K lin

much obliged.

 

[MHanson]

For what it's worth...

TC sold me some new pots and knobs years ago for a more than fair price.  If you think you need to replace any, call them up.

 

[Harpo]

..So as it stands:

F= 100K log

F= dual 100k NLog
not that easy to source

 

[gyraf]

..These 2x100k neg-log pots for the TC1140/2240 were for many years the ONLY neg-log pots available anywhere around here. They have them as spare parts.

I would replace all electrolytics in the unit - clears up the sound a lot.

Also replacing the higher band's opamps with MC33079 does good to the sound.

Jakob E.

 

[Pentium]

thanks for further insights the electrolytic capacitors quite old now would
Panasonic capacitors be ok, cheers

 

[gyraf]

Yes, Panasonic FC are nice - available from e.g. RS components.

Take care when soldering the PCB's - the copper tracks does not stick too good to the base material..

Jakob E.

 

[kenny]

First time poster - long time reader

Summary question: After upgrading the electrolytic caps and op amps in a TC-2240, is it possible the 32ma fuse may require a higher rating?

The story so far...

New caps: Panasonic FC, voltages stepped up 1 notch, i.e. 10v is now 16v
Replaced 074's on the eq boards with socketed MC33079PG's.
Powered up, tested audio worked on both channels then after 2 hours the fuse blew.
It has a 240volt - 20v | 0v | 20v output transformer which powers both channels via their individual recitifier/filter circuits which are located on the main board of each channel.

On inspection, 3 of the 4 new secondary power supply filter caps (2 on each board) had bulged. I checked all my soldering, looked for a shorts, cleaned the boards etc but couldn't find anything obvious. I figured there must have been a cold solder on one of the caps... possibly the one that didn't bulge.

I replaced all 4 secondary caps.
It was then I found that the blown fuse was rated at 50ma, however the specs on the back of the unit specified 32ma. So I ordered some 32ma's in.

I then tested with just the left channel powered up, no problem.
Same with right channel.
Fuse did not blow.
However when both channels are powered, after a 1-2 hours the fuse blows - but at least the caps didn't bulge.

Thanks to this thread and gyraf, I have the circuit diagram for the TC-1140 and have checked the voltages.  Interesting that the power jumper wires are after the 1st 2 recitifying diodes. I guess this has to do with the 2240 being 2x 1140's. This is not shown on the TC-1140 schematic so I've added those connections to the pic.

Latest measurements - see pic.
There is a difference of about 1v on the output +/-15v rails. No biggie?
Transformer AC measures 24v both sides. As they should be?
But the big stand out was that the +/-20v rails measure +/-29v on both boards, in either mode -  individally powered up and measured - or when both boards are powered.

Summing up, at present, indiviually both channels work with out blowing the fuse
Connecting both to AC blows the fuse after a while. I've racked it back up with one channel and the mod has certainly cleaned up the sound - thumbs up!

Question: Is it possible I had a cold solder which is now repaired AND the fuse may require a higher rating due to the new op amps and higher voltage caps?
What about the +/-20v rails which measure +/-29v?

What I've learnt on this little project so far...
1) Always take V readings everywhere BEFORE removing any componants. Then you'll have a reference point if things go bad.
2) When modding multiple channel equipment, do 1 channel at a time.


Any help is most appreciated, cheers.

kenny.

 

[usekgb]

Your new op amps could be oscillating, causing extra power draw.  Do you have an oscilloscope to check for this?

 

[kenny]

Thanks usekgb.
I do have an oscilloscope, nice old dual trace.
It's obviously time I learn't how to use it rather than just look at the pretty waveforms .
I'll also look up oscillating op-amps and current draw.

Plus I'll test it with the old 074's.

Cheers for the help.

 

[gyraf]

Summary question: After upgrading the electrolytic caps and op amps in ..

The story so far...

New caps: Panasonic FC, voltages stepped up 1 notch, i.e. 10v is now 16v

Hopefully at least 35V types for the power supply lines where there's 30v?

Jakob E.

 

[kenny]

Hi gyraf, thanks for the input.

C1 + C2 are 50v 1000uf
C3 + C4 are 35v 100uf

 

[kenny]

usekgb re: op-amp oscillation - most interesting.

The pic shows about 120mV of noise coming from the MC33079PG's.
When EQ is bypassed - no noise.
When 074's are installed, no noise - eq bypassed or not.

However, no matter which set of op amps are installed, if both channels are powered up the fuse blows after a while.

Using the scope is fun and pretty simple. Knowing where to probe and what to look for will take some time.
Thanks again usekgb. Your simple 1 line post has opened a good can of worms.

 

[gyraf]

Some 220KHz?

 

[kenny]

At those settings on the scope, I need to feed it 2200KHz to get the same waveform.
T is 1uS x 10 (switch pulled out).
I bought an old freq counter. Upon arrival, I'll reload the TC-2240 with MC33079PG's, do some cross referencing and report back in.

Still, it blows fuses when either chip set are installed - so I must have introduced a fault when re-capping.
I figure this could be the cause of the oscillation.
I'll remove and test the new caps when time permits.

Thanks again for your help gyraf.

kenny

 

[kenny]

Time flies!

Currently, the TC-2240 still blows the 32mA fuse after a few minutes when both sides are powered up, however the oscillation problem has been resolved.

With 4xMC33079 EQ chips installed, oscillation occurs.
Replacing any one of the 4 MC33079's with a TL074 equals no oscillation.

Reviewing this thread:
"Also replacing the higher band's opamps with MC33079 does good to the sound."
Jakob E.

In my rush to do good to the sound I over looked - "higher band's".

2x074's in the 2 lower band's and 2xMC33079's in the 2 hi band's equals no oscillation. Woo Hoo.