Altec 9475A clone PCBs High Noise- Anyone seen these?

[amplexus]

I was handed a bunch of these cards (fully assembled) for racking. I have been having trouble finding documentation on them (other than the original 9475A schematics/manual). I know the 9475's were prone to RF killing them if all the grounding wasn't perfect. From what little I've seen these cards are supposed to avoid that... but whoooo boy are they noisy as hell. Staticy whistling like crazy.

All proper grounding procedures on my case have been done- I have RF/RMI filtering on the input xlrs, chassis and 0V are isolated and tied only at the external power supply, Pin 1's all tied right to chassis at input. On the cards, tacking in a cap from 0V to chassis on each card (which was recommended by the seller of the PCB's so says the person who handed them to me) seems to reduce the noise somewhat, but not nearly enough. I've verified this guy assembled them correctly and all parts are correct values etc... and i've isolated with a bench power supply with no change.

Just wondering if anyone has seen these cards or knows anything about them, or if they need some special way to be implemented to work quietly.

 

[radardoug]

You have transformers on input and output, why would you not just earth the 0 volt line? What is the nature of the noise? Hum or hiss?

 

[amplexus]

You have transformers on input and output, why would you not just earth the 0 volt line? What is the nature of the noise? Hum or hiss?

Neither. It’s a randomly modulated rising and falling ‘whine’ with some white noise underneath. It’s a HF oscillation basically- but given what i’ve heard about the circuit in general my brain went to RF issues first.

Strapping 0V to chassis ground at the PCB makes no change to the noise, and is louder than with the cap across.

I just feel like the layout is bad, but my guy tells me the dude he got these from built them to replace bad modules in his altec console and did so with the assistance of John Hall…. So if that’s true I have to assume they’ve been made to work effectively for his situation..

 

[beatnik]

Do you also have noise when the cards are in an earthed metal chassis ? Sometimes even having the top lid open makes the noise worse as crap from lights and other equipment can be picked up.

 

[amplexus]

Do you also have noise when the cards are in an earthed metal chassis ? Sometimes even having the top lid open makes the noise worse as crap from lights and other equipment can be picked up.

Yup. I even ground the paint off at panel contact points to get solid, clean, metal to metal contact. Sadly, no difference.

 

[radardoug]

Do you have the right grounding on the input and output transformers? Do you have a scope and proper tools?

 

[amplexus]

Do you have the right grounding on the input and output transformers? Do you have a scope and proper tools?

Yes. And yes, my bench is quite fully equipped.

I've gotten some more insight and dug around the layout a bit more and I'm not convinced these ever worked as advertised. I think the person who did these was- not as skilled as was required.

 

[freshtapescent]

Although as you said you may be in trouble layout-wise, in my experience the Altec 9470/9475 amps need really well-matched transistors. In high RF environments I’ve often had to leash them above 40kHz or so with input caps. Best of luck.

 

[amplexus]

Although as you said you may be in trouble layout-wise, in my experience the Altec 9470/9475 amps need really well-matched transistors. In high RF environments I’ve often had to leash them above 40kHz or so with input caps. Best of luck.

I had begun wondering about this myself after perusing the original schematic. Also Since I didn’t assemble the boards. Your comment makes me even more curious.
I’m gonna grab some spares this morning and get a few tightly matched pairs in there and see if that changes the landscape at all.

 

[mushy]

Keep us posted on your progress. I was looking at those at one point.

 

[amplexus]

Ok. It looks like i've got it solved...

Layout seems to not be the issue- but MATCHING also wasn't the issue. Nor grounding, or input RF filtering- My racking and power supply configuration seem to be spot on.

Turns out- at least as much as I can figure- the person who 'designed' these cards chose the wrong transistor sub for the first two stages. The boards were designed for metal can 2N2484's which look like they work on paper, but even when i went back and got VERY tightly matched pairs the 'white noise' component got significantly worse, while the oscillation 'whine' went away completely.

So i decided on a whim to double check the modern cross reference for the 2N3900A, and between some datasheets and a few recommendations i grabbed a pile of 2N5088's. Matched up a bunch of pairs and installed them into a card- and it was so quiet i thought it wasn't turned on.

I swear I said "No way, this must be broken"- but I plugged in a mic and god damn if there wasn't a beautiful amount of quiet gain coming out the ass end of these.

Apparently the 5088 is made with a similar/same process as the 3900A's so it seems like a natural sub. And it was damn easy to find close matches in a batch of 50. The 2484's were much wider in tolerance.

I'm not calling it case closed until I burn this in for a few hours- but it certainly seems to be resolved at the moment!

 

[iampoor1]

I wonder, are the 2n2484s you have counterfeit/bad? Hard to compare the noise figure between the 2n5088 and 2n2484 as they are specified at different collector currents in datasheet (atleast the ones Im looking at hehe)

 

[amplexus]

I wonder, are the 2n2484s you have counterfeit/bad? Hard to compare the noise figure between the 2n5088 and 2n2484 as they are specified at different collector currents in datasheet (atleast the ones Im looking at hehe)

Have two different batches- but at least the one batch I have is from a proper dealer. The other is from a local electronics surplus. Same result
With both batches.

 

[iampoor1]

Have two different batches- but at least the one batch I have is from a proper dealer. The other is from a local electronics surplus. Same result
With both batches.

Interesting! I wonder if maybe the collector current/source impedance isnt a good match for the 2n2484s.

Always great to find out that more common parts that are readily avaliable work better. I have loads of 2n5088s and have been curious to try out building some of these

 

[Whoops]

Just wondering if anyone has seen these cards or knows anything about them, or if they need some special way to be implemented to work quietly.

Those PCB's were offered/sold for the last years in Ebay, there was also the Langevin AM16 version.
I know the person who did (and sold) those PCB's and I have his contact.
PM me in case you want to contact him.

I checked my docs and I don't have any docs on those boards besides the "Parts List".
But I have plenty of Altec 9475 documents in case you need them.
Altec 470A, 475A, 9470A and 9475A are all the same circuit.

 

[amplexus]

Ok so i did a bit more experimenting and I'm just convinced that the 2484's aren't the right sub in this circuit.

I tried multiple terminating impedances on the input side, strapping the transformer for 1:5 vs 1:10... nada difference to the noise. I've tried two sets of well matched 2484's, same result. I verified the rest of the circuit values match the original...

5088's? Dead silent in each card. As long as these pass burn-in, I'm calling it solved.

 

[Whoops]

On the Altec units the 4x input transistors are 2N3900A an the 2x output transistors are 2N3053

 

[amplexus]

On the Altec units the 4x input transistors are 2N3900A an the 2x output transistors are 2N3053

Yes, I know, I have all the original docs/schems. Which is how I came about to the 5088's for inputs- very similar process and specs to the 3900

 

[radardoug]

Good spotting!

 

[emrr]

I've got pair of originals in for service, and both have had later transistor replacements/subs made. I can hear and watch different oscillations slowly sweeping up and down in both. Response measures great, so it's not the output pair, when those are off balance the low end goes away quickly.

In my other 9470/9475 threads I've mentioned having trouble with modern replacements having too high a gain. I could get matches, but if gain was too high they'd be in instant oscillation meltdown. If I could get matches with gain in the specified ranges, all was good. One example was NTE199's (3900A sub) and they all measured beta 500+ rather than 100+.

From my other post:
The 1st issue of these amps uses an Altec part # for Q1-4 which are paired 2N2716's. Polarity is reversed on this amps as well.