ALTEC 9475A pre

:shock:

HI, wondering if anyone can shed some light on these symptoms

I recently hooked up a discrete Altec Mic pre (quite old!) , and it loves to burn up a particular resistor (22ohm). The voltage around the circuit is quite high in some parts compared to the schematic (but never exceeds 23 volts) its 24volt powered. IN parts where it should read 3.5 volts etc its way up at 23! IM using a regulated 24volt dc PS from the electronics store.

All the resistors and transisitors seem in spec and I changed the electrolytic caps.

The resistor that burns seems to be linked to the negative end of the power supply (which Im assuming is straight dc as opposed to bi polar) but the voltage to the resistors measures quite low (its after a diode). I replaced with a much higher value resistor but still all I get out of the mic amp is a buzzing sound and there seems to be no input signal. ONe thing I haven't done is try a higher watt 22ohm resistor.

Im hoping its not (but thinking it is) a bad output transformer (do these cause wild current in a circuit?) but Im a bit unsure how to test the transformer. Im also wondering whether a functioning full circuit should automatically show continuity between the input and output pins (cause this one dosen't!)

Sorry for the convaluted speil but I have to get it out of my head and in to the realm of someone who knows what they are doing properly!

Id love to have this up and running, Ive heard good things, it is all discrete class A (if that helps) and very big transformers either end so it looks good anyway! I think these were the first solid state altec pre's (or among them) and are similar in construction to langevin AM 16's.

Any tips would be very much appreciated as Im liable to burn myself on the soldering iron the longer I stare at it.


thanks

seems like a look just upstream frmon the 22 ohm resistor. if its like the langevin i think its a current limiting resistor possible a bad junction in a transistor if you have 23 v where you should have 3,5 that means that you arn't getting your voltage drops in the right places. if you use good old ohms law you should be able to calculate you currents and voltage drops . I know that not what you wanted to hear can you post the schematic? Wil

Wilebee

Apologies Wilebee for the delayed response!

I actually thought Id posted a reply a week ago but musn't have uploaded, thanks for your help.

Unfortunately don't have access to a public server to post the schematic otherwise Id just attach it but if I can work it out I will.

Fingers crossed what you say about the transistors is the case, Im going to replace two of them (there did seem to be a slight leak and the damage seems to be around there). Ill let you know how I go.

Ohms law, is something Ill have to learn a bit more about,
but for starters Ill replace the transistors.

If anyone can shed light on the simplest way to test transformers that would be grand as Ive managed to confuse myself looking for continuity I think in the wrong places.

Anyway thanks again much appreciated!

Tim

:thumb:

Hi,
This is a really old thread, but I have the same problem in 2 Altec 9475 units.

They both burn the R15 (22ohms) resistor.

Does anyone know if this is a common problem in this units?

maybe replace them with higher wattage resistors?

thanks

It's common if something else is wrong.  I've owned a 100 channels of units that worked perfectly without restorations or fixes.  They are a real pain in the ass if they are behaving badly. 

Oh yeah, not in the manual, but on the hang tag on a new unit. 

MAKE CERTAIN ALL GROUNDS ARE CONNECTED, AS RF ENERGY WILL BURN UP THE AMPLIFIER.

I had one incorrectly grounded once and measured 1.5A current draw for a microsecond.  Pulled the plug and saved it, somehow. 

count all the various grounds.  note they are all brought out separately, and must be tied together. 

be sure the case tie point is still connected, reflow the solder. 



then when that doesn't fix it, assume an output transistor pair is bad.  they draw the most current by a large margin.