Very Perplexing Langevin AM-16 Problem

[tommytones]

Schematic


So I finally decided to re-rack my AM-16's into a nice case, new PSU, face plate, etc.. only to run into a very bizare problem. These units have been working perfectly for years and have never been altered or abused. They have been on the shelf for a few months but well stored...once again not damaged.

I powered up the box and proceeded to pass signal. CH-1 is working just as it should. I plug into CH-2 and low and behold, oscilation and mega noise! I probe around to find that Q7 has died. I change it...oscilation and mega noise still. All the DC voltages are fine...they match up very close to CH-1 and the values in the Langevin manual Voltages.

Suspect that another transistor is funky, I changed all of them. Oscilation and mega noise! Here's the bizare thing. After futzing with it for a while, I discover that if I touch my finger or my dmm probe or a screw driver to any point between the base and collector of Q2 & Q4 (left of R7 on the schem) everything settles down and the unit works perfectly :shock: :?: The only other way I can get the unit to work is if I load the input with some 600 ohm source. By this I mean if I plug my cheapo dynamic mic with a trafo or if I load the output of my test set (via impedence selector on the test set itself) the unit works perfect. But if I feed it any other lower impedence source, I get the oscilation and mega noise thing. I've tried loading at the XLR with a resistor but all it does is drop the level a few db...the oscilation and noise stay. Now, after several hours of playing with things, I'm beggining to see the same symptons on CH-1 arise.

I'm quite stumped on this one. What the heck might be happening here?? It's doubly frustrating since these units were working just fine :twisted:

 

[NewYorkDave]

Are points "J" and "K" connected together back at the PSU, and only at that one point?

 

[CJ]

Those circuits seem to be a might touchy over here too.
Try the balancing pot. Thats what fixed mine after racking.
Did you remove the transformers or are they in the same place as before?
Try messing with any tranformer grounds/shields also.

 

[tommytones]

Are points "J" and "K" connected together back at the PSU, and only at that one point?

Yup. I've tried just about every alternate grounding scheme as well.

I played around with the pot but with little success although I'm wondering if it's not defective due to the fact that it is pretty crunchy. This has to be the next step I suppose...trying to swap out for a new pot and see what happens.

I was suspect of the transformers; whether the shield broke off or something. I checked all the wiring at the connector and all is good. I don't see how anything iternally could have been damaged. I retouched all the solder points on the pcb as well just to be thorough. The units are still in the original metal chassis. I didn't remove the trafos.

 

[tommytones]

Okay, so I finally had a few minutes to get back to these suckers. I'm still having the same issue. I pulled the units out of the 19" case and put them on the bench. I hooked them up directly using clips (no connectors...straight to the pins). The 1st channel works perfectly. No noise, proper gain/bandwidth, etc... The 2nd channel is passing signal but I'm still getting the oscillation and noise. Again, if I touch anywhere on the trace connecting the base and collector of Q2 & Q4 with my probe (or anything, even my finger) everything settles down and works perfectly.

I've tried messing with the trim pot but it doesn't seem to have any effect on the problem. Should I still try changing it out? I'm pretty stumped with this one :evil: :evil:

 

[MikoKensington]

I'm having the same problems with 3 of mine. All to varying degrees of oscillation madness. I can't figure it out, either. I'm tempted to take mine to Vintage King, (right by my girlfriend's place BTW) and tell them to fix them right.

 

[tommytones]

I'm sure it's something small and easy to fix. I'm wondering if I shouldn't try removing all the old solder completely and redoing the board top to bottom.

I'm almost ready to tape a piece wire to the trace and be done with it! :green:

 

[CJ]

The balance seems real touchy on those.
Check all your resistor values against the chassis that works, just in case.
Do a side by side ohms check.
Gound neg clip to both and do point A vs Point A on the bad unit.
Good thing is no caps to mess with.
Do they all have the same type transistors?

Play with the transformer taps.
Play with the ground wiring.
How far away is the pwr supply?
Maybe a bypass cap right on the B+ might help.

Interesting to note how Mr. John Hall went from the tube model to the transistor circuit.
There are siilarities.
Look at the feedback and balance control on the 5116.

 

[tommytones]

Alrighty...finally figured this one out with some help from our head tech. The problems I was having were due to drifting/age of the transistor pairs. They really need to be paired up well in order to work properly. Both units I have are now spot on with the data from the manual. I did have to change 1 of the trim pots as it was really fried.

Thanks to y'all who helped!