8 Channel, 3U NEVE 1073 completed

[MicDaddy]

Yea I have an older Tektronix/Sony 2channel portable which doesn't help as when the suspect channel is working it works and exhibits no abnormalities.  Only then out of nowhere it will start this spitting fit and crap out...spit some more and come back, I cannot reproduce the fault with any consistency. 

I suppose I grab a book and throw it on the bench and camp out for a bit.  When it craps out I'll try to see where the audio goes wonky, any other suggestions?

 

[MatthisD]

Regarding the static, have you used polystyrene caps? if you have then something to try is to blow on each of those caps with the unit on to see if this alters the level of noise. I had this same intermittent problem recently in a mic circuit and was able to clean the cap which solved the issue.
Matt

 

[atticmike]

It's not a delicate matter at 22vac...as the powerstation will take 15 to 30 vac....but output 24(dc).

It's a matter of power...and for 4 channels, the amveco is fine at 22vac - 50va.

Do you really live in an attic?>

As a matter of a fact, considering how much time I spend in a studio environment, I do

I'll be harnessing this spool with 22vac since I kind of hinge on european 230 voltage.

 

[MicDaddy]

Upon visual inspection, I find 2 of my R7 47R 1W cooked.

These photos were taken from the same chassis, 3 different channels.

Channel 6 looks unrecognizable and I'm sure it's only a matter of time before it looks like Channel 8.

Channel 7 Looks perfect, like it hasn't seen much heat.

Channel 8 is cooked to pieces, this is the same channel with my intermittent spitting/dropout.

Channel 5 is not shown but looks exactly like Channel 7, in mint condition.

While I could replace these and hope for the best, I'd prefer to get down to the 'root cause' of the failure.  Obviously these are dissipating much more heat than they are wanting to, why?

It's time to dig into the 'hows it work' in the schematic, and I'm not sure exactly what this guy is doing but hanging off of the emitter of the 3055? 

Edit: I should add that all channels function as what I consider normally, only when channel 8 started spitting did I open the top to find these resistors burnt.  I've been using the pres off and on for however long it's been since they've been completed with no problems.  Lucky me I was just playing around when I noticed the spitting/dropout.

I'm going to open up my other chassis to inspect R7 on those 4 units. 

See photos below
In order Channel 6-7-8

 

[MicDaddy]

quick update,

I poked at some transistors and found some funky voltages on channel 8

TR1:
E .1V
B .6V
C 13.7V

TR2:
E 13.1V
B 13.7V
C 18.8V

Played some with the 5K trimmer, now TR voltages are as follows:

TR1:
E .33V
B .9V
C 4.5V

TR2:
E 3.9V
B 4.5V
C 22.7

The burnt up R7 reads 58Ohms in circuit.  Since adjusting the bias trimmer, no dropouts or static from channel 8 (~24hr burn in, recording via DURec and monitoring waveform)

I'm almost to the point to swap out R7 and hope for the best, unless someone cares to drop some knowledge.  Did I simply forget to bias this channel?  I honestly cannot say 100%, if not properly biased is it possible to dissipate too much heat at R7? 

Channel 6  IS biased properly so why does it also show stress from heat?  Bunk resistors?  ( R7 is unrecognizable from heat)

additional info:

Channel 8 TR3 collector was showing 22.7 for over 24hrs after adjusting the trimmer.  I went ahead and swapped R7.  It immediately burnt up though not as bad as the previous.

TR3 collector is now showing some 15.x volts

I've done some rough comparisons with the trimmer, it appears to be functional though this time it's having no influence over the circuit. 

R7 (all blackened up once again) reads 45Ohms in circuit and has 15V across it    whereas other channels only 2.xV.

 

[Siegfried Meier]

Hey guys,

Any reason why the BA183AM section wouldn't be seeing proper voltage?  My TR1 (2N3055), TR2(BC184C) and TR3(BC184C) aren't getting the proper voltage - neither are the caps surrounding this section.  All else seems to check out perfectly.  The PSU is 100% as it's been used on several other units with no problems.  Can't see any shorts or reversed caps/parts etc.  There are 2 channels and they're both doing the exact same thing, which leads me to believe I've done something identically dumb to both units.  Could a pair of bad 2N3055's cause this issue?

Thanks for any advice,
Sig

 

[MicDaddy]

;D  Siegfried Meier you too?  I only kid, out of curiosity what are your voltages at those designations?  Which make 2N3055 did you use?

On my end, I've ruled out the output tx.  My faulty channel is pulling some 340mA while the other 3 in the same chassis combined pull some 200mA.  It's all being dumped into the 47R and I haven't nailed it yet.

Went ahead and pulled 2N3055 and did a diode test.  (Base+ Collector-), (Base+ Emitter-), both read out at some half a volt or so, no conduction in any other configuration.  Could it still have failed and read out ok?

    Gonna run to RadioShack tomorrow for a TR3 and toss it in blind.

 

[Siegfried Meier]

How are you determining how many mA you're pulling from each channel?  What device are you using?

I'm using an ST Electronics 2N3055 but 2 of these were sitting in parts shelves in my shop for over a year, not in anti-static bags, which is making me wonder if they're faulty.  Especially since both channels are exhibiting the same behaviour.

I ended up buying 500 BC184's since I got a notification that they were now end of line.  Wanted to have some for the future just in case they get discontinued and nothing replaced them properly.  I doubt those are the problem though.

Can a faulty 2N3055 cause this entire section to be wonky?

Thanks!
Sig

 

[MicDaddy]

I'm using my bench PSU for +24V and watching the current draw on the meter.  Some maths tells me that 15V across R7 (47R)  E/R=I  15/47=.319

Can you measure what your V drop is across R7 and let me know? 

There's not a lot that can fail in that BA183AM section, I am still suspecting my TR3. 

Are 2N3055 ESD sensitive? 

Can a faulty 2N3055 cause this entire section to be wonky?

Just a guess, but I'm thinking yes.

Today I'll try a ratshack replacement at TR3, and if that doesn't work, back to the books to try to understand this topology.

Didn't know about BC184c going end of life, Jim Williams has recommended a few replacements that he claims spec much better but I've not tried (but now I might since I've got this thing on my bench already).

 

[MicDaddy]

;D update,

I found the error, C3 was reading a dead short in circuit.  I pulled one leg and the 4700pF(polystyrene) reads ok out of circuit, I measured between C3 pads and dmm reads open.  I looked and looked under magnification both sides of the pcb both prior to and after pulling one leg of the cap and observed no short.  I cannot say why it was shorting in circuit, but after installing, this channel is now operating properly..... wtf.... 

 

[Siegfried Meier]

Really eh??  Very odd.

Well, the fact that both of mine behave identically makes me think my problem is elsewhere.  All voltages are exact from one unit to the next, and each transistor that's dead on one is dead on the other.  Very strange...

 

[MicDaddy]

With no B+ applied, use a DMM and check your capacitors for shorts.  This is how I found my issue... I'm still trying to wrap my head around the way this circuit works.

 

[MatthisD]

I cannot say why it was shorting in circuit, but after installing, this channel is now operating properly..... wtf....

Do you mean you replaced C3 or just re-soldered the same one?

 

[MicDaddy]

Do you mean you replaced C3 or just re-soldered the same one?

Re-soldered the same one...

C3 in circuit measured dead short.  No visible short on pcb either side.  Lifted one leg, measured and found no short at the capacitor, and no short between C3 pads (scratching my head) soldered leg back into circuit, measured no shorts....circuit now showing proper voltages.  Chalking it up to Pure F***ing Magic.

 

[sr1200]

could have had a cold solder that you couldnt see...

 

[MicDaddy]

could have had a cold solder that you couldnt see...

Sure, but that wouldn't cause a cap to show as shorting, would it?  Did I cook the polystyrene to the point of an intermittent short?  Dunno, I'm really dumbfounded with this one, happy it's at least working for now.  I'll monitor it and report any further issue.

 

[MatthisD]

That is strange, as mentioned in the assembly guide polystyrene caps are sensitive to high temp so need to be soldered carefully or with a clip to absorb some of the heat so I don't understand how re-installing the cap, applying more heat would help. If you handled it? to pop out one leg for testing maybe your fingers somehow removed some residue or something from the body although that would seem to have more affect in high impedance areas like the output of a mic capsule like in my situation I mentioned earlier.
Glad its working for you whatever the reason.

 

[danjpiscina]

hi guys. what's the best way to get a trim pot on one of these 1290s?

i have 4 pairs and i want one pair to have output trim. are there any wiring guides for this?

thanks!

 

[sr1200]

Its on page 1.  I believe its a separate pdf file with direction on how to do this.

 

[erikb1971]

HI All.. Iam back... damn!

To celebrate the fact I bought these really AWESOME looking red winged neve knobs from ChuckD (http://www.groupdiy.com/index.php?topic=45767.0) I decided to call in an expert to calibrate my dual channel 1290s.... just to find out that there are some things wrong....

On my second channel the input transformer is dead... Colin is helping me out on that one. 

On the first channel, I still have a serious amount of distortion in the audio signal. Plus that adjusting the small trimmer does nothing to adjust the audio signal. The trimmer is working though, adjusting it does affect the v I can measure on its legs.

these are my cap values (first how they should be, second line what I measure.. in bold anything with more than 0.5v difference):
C   B         E
1 3.6   0.9 0.4
2.1   0.9 0.3

2 22.6   3.6 3.1
23.6   2.1 1.5

3 22.6   3.1 2.5
24.1   1.7 1.1

4 3.8   2.4 1.8
3.8   2.4 1.8

5 12.4   3.8 3.2
13.1   3.9 3.3

6 20.6   12.4 11.8
22.2   13.4 12.8

To make things more complicated, last night I measured perfect values for 24v on the powerinput on the board and the 48 phantom, now they are up to 30v and 60v!

so....

Anyone?