Help with AMPEX 350 noise issue

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jwhmca

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This noise is a low level "popping - crackling" noise.

I have subbed all the tube.
The noise stays the same regardless of input or output gain.
It is intermittent.

I have heard that resistors can cause noise like this. I would like to try the "Grid/plate resistor subs.

Red circles 1-2 are plate resistors? Where is the plate resistor for the V407 output tube? in green.

The blue circle, is that the "grid resistor"? could it cause a popping noise? It looks stressed.

Is the purple circle the plate resistor?

Any other thoughts?

Thanks!
 

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I had the same problem on a 350, i thought it might be the playback/record switch being a bit crusty. Then one of the primaries on the output transformer went open circuit! Got it rewound and the problem has gone. Hopefully your one might just be the resistors though.

Resistor 1 is only in the playback path for the tape, are you using that?

R447 is the screen grid resistor, no harm replacing that if it's looking a bit tired. That shouldn't be contributing any noise if C428 is still ok. Has it been recapped? R443 is feedback res, could possibly be noisy. The plate res for V407 is the output transformer. Hmm....could be the same problem

Transformer looks a lot like this once you get the potting compound off. The elastic band isn't original...
 

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Thanks for the response guys,

I'm using it as a FAAAAAAT tube amp mic pre... I haven't removed anything. But, someone has already recapped it. I have a recording of the noise if there was someplace to post it or send it... maybe it doesn't really help. I'm really JUST getting into this tube stuff, so pretty much a complete tube... err I mean noob.

*edit*

I'm not using it as a playback unit, just micpre.
It is intermittent, but constant if that makes sense...
 
Assuming no major mods have been done....

What position is the "METER AND OUTPUT SWITCH" (S405)?  Unmodded and used as a mic preamp, that switch should be set at "RECORD"  "LEVEL".

Correct?  If so, then follow-up questions/ideas....

Bri
 
jwhmca said:
This noise is a low level "popping - crackling" noise.

I have subbed all the tube.
The noise stays the same regardless of input or output gain.
It is intermittent.

I have heard that resistors can cause noise like this. I would like to try the "Grid/plate resistor subs.

Red circles 1-2 are plate resistors? Where is the plate resistor for the V407 output tube? in green.

The blue circle, is that the "grid resistor"? could it cause a popping noise? It looks stressed.

Is the purple circle the plate resistor?

Any other thoughts?

Thanks!
Click to expand...

Yes, red circles are plate resistors.
The blue circle is the screen grid of the output tube.
The purple resistor is the negative feedback resistor to the cathode of the previous stage.
The output stage does not have a plate resistor. Its plate load is the output transformer.

If you do not have a 'scope to trace the source of the noise then substitution is your best approach. The fact that the noise does not change with the position of input or output pots implies the problem lies either in the output stage or the power supply. If you have not already done so, it would be wise to replace all the electrolytic caps.

Cheers

Ian
 
i reckon youve got a slight stuffed output trafo. the windings are just thin wire coated with shellac. soo with time the shellac can turn to dust and the windings start to short out a bit and can cause the popping crackling noises.
 
I do have a scope... where should I scope to check the flow of the noise? anyway to bypass the output transformer to test it that way?
 
> noise stays the same regardless of ....output gain.

Then it is in V406 V407 stages. Or power supply. Maybe jack.
 
PRR said:
> noise stays the same regardless of ....output gain.

Then it is in V406 V407 stages. Or power supply. Maybe jack.
Click to expand...

I ran it without V406 installed and the problem was still there.

It looks like the "Monitor AMP" out bypasses the transformer? Can I run that to eliminate the transformer as a cause?
 
jwhmca said:
I do have a scope... where should I scope to check the flow of the noise? anyway to bypass the output transformer to test it that way?
Click to expand...

You need to look at the points where it might be being injected. So look at the screen grid of the output tube and also at the HT supply itself. BEFORE you do this it would be wise to check if you scope can withstand the 320V dc HT supply. It probably can't and even if it can you should tale great care. I would be inclined to make up a little test rig consisting of a  series 100nF 450V capacitor and a 1Meg resistor to ground:

Code: 
in   --------| |---------  out
            100nF   |
                   | | 1Meg
                   | |
                    |
0V---------------------- 0V

Make the measurements through this rig with the scope set to ac and you will be able to see quite low levels of noise.

Cheers

Ian
 
Ground v407 grid, listen. 

Monitor out won't help, noise from an intermittent output transformer will still be present. 

If you are down to the output trans, you can divert B+ through an appropriate resistor or choke to take the load off the output transformer and see if noise changes or leaves.  Probably need cap isolation from plate to trans as well.  I have found output transformers that no longer liked current flow, and made successful repair changing to parallel feed with a choke, amp still running fine years later. 
 
Ruff,

I have a TEK 2246A. and a TEK 2236

Emrr,

Ground pin 5 of v407 with the tube still installed?

I'm not sure I follow you on the "Divert B+... need cap" etc... could you whip up a drawing or something of what you are talking about?

Thanks guys!
 
if the noise is still there after gounding v407 pin 5 - then the noise is definitely coming from that output stage.
and

diverting b+ is all about effectely removing the output trafo and just listening to the v407 stage as a voltage amp without the trafo in the circuit.

replace the output trafo with say 20K 1 watt resistor - and listen to the signal via a .1 uf  600v cap feeding from the v407 annode. just like is was a normal voltage amp.

if the noise is still there - then the tube or stage components or tube socket is faulty.
if the noise is gone - then the trafo is the cause.
 
Grounding V407 pin5 did NOT eliminate the noise. So onto trying the output transformer bypass.

Does grounding pin 5 also eliminate all other components around V407 as issues? Like R447 C428 etc...? leaving only the transformer as the culprit...
 
r447  or the 150K divider resistor (cant see the #) or c448 or r445 or c427 could still be faulty.

my money is on the electrolytics - which popp and squeak when old.
OR the trafo - which definitely cause a pooping scratching intermittent noise when old.
good luck. u are close.

 
jwhmca said:
Ruff,

I have a TEK 2246A. and a TEK 2236
Click to expand...

The 2246A manual says is has a basic max input of 5V per division so with 10 divisions that's 50V. With a 10:1 probe that becomes 500V but frankly I would not trust a 10:1 probe with 500V on it. So you definitely need to use the little jig I drew.

Cheers

Ian
 
I got my parts in to do the transformer bypass... I have connected a 20k resistor across wires (pins) 7 & 10. I'm wondering if I can just listen through the Monitor Output now instead of rigging up the .1uf 600v cap?
 
I would suggest connected just one half of the transformer primary; disconnect circuit from pin 10 and connect the circuit to pin 8/9. If nothing changes then try the other half; disconnect pin 7 and connect pin 8/9 to plate v407. Frequency response might be poor but it will at least tell you if only one of the windings has a fault. I presume you can measure the resistance of both windings and that they are near enough the same?
 
Matthis,

Would I need to keep a resister in series with the winding to makeup for the loss of load?
 

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