[ACMP investiupgradifications] All things PREAMP

I was able to see them by adding .jpg and using the address bar, but it didn't work in the forum. So I put them on imageshack for ya.








[EDIT] Changed schematic per Zmix's request.

[quote author="tarnationsauce"]I was abl;e to see them by adding .jpg and using the address bar...[/quote]

Maybe that's why I can see them in the forum... I pasted the URL (with the .jpg addendum) first before I created the post with the IMG tags. The photos were probably already loaded in my cache.

Regardless, there they are.

Thanks!! Could you please post this schematic instead? It has the same part designations as the PC board photo:

http://recforums.prosoundweb.com/index.php/fa/10264/5734/


[quote author="tarnationsauce"]I was abl;e to see them by adding .jpg and using the address bar, but it didn't work in the forum. So I put them on imageshack for ya.
[/quote]

[quote author="zmix"]Thanks!! Could you please post this schematic instead? It has the same part designations as the PC board photo:

http://recforums.prosoundweb.com/index.php/fa/10264/5734/


[quote author="tarnationsauce"]I was abl;e to see them by adding .jpg and using the address bar, but it didn't work in the forum. So I put them on imageshack for ya.
[/quote][/quote]
Done!

Fantastic!!

Now that we can all see the schematic and the layout... any thoughts?

I removed the boards from the chassis yesterday and it was RF city.. I have never heard more simultaneous radio stations... it's also important to note that even when it's in the chassis, simply touching the top of Q4 qnd Q5 will allow you to hear local radio stations.
I've got some BC441 - 6 and BC461 - 6s and heatsinks on their way to me now. I hope that this will clear up the RF induced buzz.

I subbed in some output transistors with the proper ratings and voila..! NO RF... Therefor no HUM .!.!.!

http://recforums.prosoundweb.com/index.php/fa/10273/5734/

[quote author="zmix"]I subbed in some output transistors with the proper ratings and voila..! NO RF... Therefor no HUM .!.!.![/quote]

Super-duper. That's great.

Shame for the many people who will kind of be a bit donald-ducked, but it's better than having a flawed PCB design.

Roddy

Well,, Chuck is our hero. Here is what he found


"
The TnC 81 hum problem was exactly as I had theorized:

The amplifier circuits in the EQ sections were oscillating, leading to RF induced hum and overheating.

Chance sent me some NTE substitutions for the for the parts I requested, I swapped them out and everything is running absolutely perfectly.

I do NOT recommend that these parts (NTE-186 and NTE-187) be used under any circumstance except with an expert repair person installing them because the pinouts do NOT match the actual part and it requires some pretty dicey crossing of the leads to fit them into the board... not a good bet for reliability.


To Sum up:

NOT powersupply related
NOT grounding related
NOT layout related
NOT induced EMI related
NOT wiring related

Also, NOT manufacturer error... the part in the schematic is the part used, and it is simply the wrong part for the circuit. the fix will be to order the correct parts for each of the 4 EQ boards ( Q4 must be a BC461 - 6 and Q5 must be BC441 - 6). The input board and HP / LP boards are fine as they are..



Let me know if I can be of any further assistance in sorting this out."

A few other TO-39 candidates came to mind as well, which will broaden the options and ease of finding: I expect that the BC161 & BC141 (or even BC160 & BC140) would be well suited as well. So:


Suggested replacements (to be confirmed by actually soldering in & testing):

Q4/PNP BC461-6 --> BC161-16
Q5/NPN BC441-6 --> BC141-16

Ratings, current-gain, min-fT are identical, as are the pinouts.

H*ck, I guess that for even more ease of finding you could even slam in the 'usual' BD13x-16 types, most likely the same dies, but different packaging.

BTW, various people use these BC-devices also in their 2520's (although I don't recall the current-gain range for now)

And last but not least, thanks guys for fixing this problem :thumb:

Bye,

Peter

[quote author="maxwall"]Will these original Q (BC)461/441 parts need to be selected for gain, matching , or performance or just plug in and solder up and its all done ?[/quote]
Note that a range of current-gain is already specified: '-6'.
More matching wouldn't hurt but I've never toyed around with these actual circuits. I do think though that it wouldn't really be required - escpecially for this circuit since it's not the 'usual' push-pull but (simply said) Q5 is a current source for Q4.

Will the BC 441/461 need to be heatsinked like original BA type amp boards ?

Click to expand...

We'd like to run then at the same voltages & current, same TO-39 package (identical thermal resistance), so the same heatsinking-need I'd say. But I have no idea how big the need for heatsinking was in the originals - maybe just there because it was a more crowded DOA. The TNC-version seems to have ample room for it though, and there are some nice types out there so it'd look cool as well :wink:

Alternatives/ Subs below ?

BC441 = 2N3053 or 2N5320
BC461 = 2N4037 or 2N5322

Click to expand...

FWIW, I just posted some other candidates one post up.


Bye,

Peter

Chance sent me some NTE substitutions.

I do not recommend that these parts (NTE-186 and NTE-187) be used under any circumstance except with an expert repair person installing them because the pinouts do not match the actual part and it requires some pretty dicey crossing of the leads to fit them into the board... not a good bet for reliability.


http://recforums.prosoundweb.com/index.php/fa/10273/0/5734/

I wanted to post an update about the weird negative cycle distortion I was experiencing with one of my 81s when the EQ was in circuit.


Here is a screen shot, and this was only present when the input signal was above -36dB:


http://recforums.prosoundweb.com/index.php/fa/10246/0/

After I cured the oscillation issue, I was dismayed to hear this same distortion in that 81 when the EQ was engaged.

I manually bypassed each EQ section with a jumper and discovered that this distortion was coming from the HP / LP Filter section, and specifically from the first ampllifier stage.
I traced the source of the distortion to Q2 (a BC557) and replaced this defective transistor with one that I had previously removed from the other EQ amps and am very happy to report that the distortion is now gone.

Lucky for me these two unrelated problems were in the same unit...!

A list of test voltages and test points should be established so each person can check their own units for any defective transistors or other components in other sections of the circuit besides the known ones from the Factory, just in case.

Please Note:

If your 81 has a HUM rather than a BUZZ... it may be a simple fix...

The difference in hum is very important. If the hum has a raspy, chainsaw like quality, then this may indicate an oscillation in the circuit. In addition, if the signal sounds distorted when the EQ is engaged, then the unit will require a different approach than the one I will outline here.

If the hum is a mellow, sine wave + overtones type of hum, then try this solution.

You will need a #1 and a #2 phillips screwdriver and an 8mm nutdriver, and a pair of rubber gloves.


Step 1:

Unplug the power cable.

Step 2

Remove the lid of the preamp using the #1 Phillips screwdriver..
There are two screws on each side, two in the back and two screws on the top.

Step 3

The power supply transformer is located in the rear right corner of the preamp as you face it from the front. It is fastened to the bottom of the chassis by a large phillips head screw and an 8mm nut. Put the nutdriver on that nut and the #2 phillips screwdriver on the screw on the bottom of the chassis. Gently loosen the screw, do not remove it, just loosen it by a quarter turn or so. so that the power transformer can be moved without too much force.

Step 4

prepare the unit as follows:

Input Gain: Minimum
EQ: IN
LF: 330hz, gain fully up (clockwise)
LMF: 220hz HiQ IN Gain fully up
HMF: 1.5khz Hi Q IN Gain fully up.

Output fader fully up.

Connect the output of the preamp to a suitable monitoring device (console, etc).

Power up the module and connect it to the monitoring device.

Step 5

Turn up the gain until you can hear the HUM plainly.

Put on your rubber gloves.

Grip the power transformer and begin to rotate it. You will hear the timbre of the HUM change as you rotate the transformer.

Rotate the transformer to the point where the HUM is at it's minimum level.
Put the cover on temporarily and check the hum level. The cover will influence the magnetic field of the transformer, so this may take a few trials.

Step 6

Tighten the center bolt and replace the top cover.


Done!!

I just tried Mouser and Digikey and neither have the -6's

You can also use Mu Metal to surround the transformers to prevent hum. You can find Mu Metal pretty easily and it is affordable. Just mold by hand or cut pieces and hot glue...the place it over the transformers and glue it to the PC board but that will not correct the wrong components used.

Rotating the transformer is something each and everyone of you should do no matter if you have noise or not, as it can make a difference in the noise.

Alan Hyatt
PMI Audio Group
Joemeek - Toft Audio Designs - Trident Audio Developments - Valley People - Studio Projects - CLM Dynamics
1845 W. 169th Street
Gardena, CA 90247
toll free: 877-563-6335 fax: 310-323-9051
url: www.pmiaudio.com
email: [email protected]

I think that I have discovered the LAST of the TnC 81 issues..


Here goes:

The Line input of the TnC 81 is in antiphase ( ø ) with the input signal... the mic input is not.

I tested all of the other modules I have... the 84 and the 73 check out fine. Both their mic and line inputs maintain polarity.

The rear connector PC board is traced and labled correctly so the inversion must be happening at the preamp board.. which is odd because that mic pre / line amp board is the same part as the 84 and 73 use.


The solution I used was to de-pin red and white wires from the wire connector that plugs into "BCZ2" on the input / output jack PCB and swap them.

So, I have a few NTE-186 and NTE-187's in my drawer, but I can't figure out what substitutes what? what is Q4 supposed to be and what is Q6 supposed to be?

I can't find any BC461 - 6 and BC441 - 6's either so I figured these NTE"s would would according to Chance's post.

would would they?

bump for new forum software....

Just received my ACMP81's

New problem discovered

replaced 4Q4, 4Q5 on the Upper EQ Board which fixed buzzing oscillation issue on this board.
Payed careful attention to pin orientation between the BC547,557 and 441,461 when soldering back
into the PCB. After power up , I noticed a buzz oscillation coming from Lower EQ board. It also has a
output complimentary NPN/PNP pair so I replaced them ( 5Q4,5Q5 ) like the upper eq pcb board but this did not
cure its oscillation problem. So as it stands there is a possible oscillation problem on the lower eq pcb board
that is evident when engaging the rotary switch from 220 -1200 hz followed by its associated 10K
sweep fader. when the rotary switch is dis-engaged to the off position the oscillation is gone - eq amp out of path.

Looks like I'll have to replace the comp transistor pairs for the following boards below, which I have not
done yet. It may be possible that the oscillation is being passed by another eq amp , not sure.

3Q4,3Q5
6Q4,6Q5

Any ideas on this being another transistor issue on the 220 -1200 hz lower eq board ?

I have some test equipment handy but don't know where to start dropping the scope probe ?
maybe I'll start checking transistor emitters for broken waveforms.

The noise I'm getting is not a ground hum or 60 cycle low pitch hum , its sounds very close to the oscillation
that I heard on the upper eq board before I fixed it but a different pitch.

By the way, the BC461,441 ( TO-5, 39 ) cans don't fit very nicely on the board since their bigger than the T0-92 types.
Its a tight sqeeze in some spots and the legs stand higher than I care for just to make room. In fact the legs should be close to the pcb board to prevent possible oscillation or EMI interference as I understand it.