Some Vintage Test Gear Madness

[mayfly]

or better - learn from other people's mistakes

 

[mayfly]

Just got a package from AES for the signal generator re-build.  Check out those NOS tubes, with the exception of the JJ 12AT7.  Interestingly, the old tubes were under $10.00 each.  Cheaper than the JJ!!

For the signal caps I splurged on Solen.  The can caps are the shockingly expensive CE Manufacturing caps.  Everything else is good old IC.

 

[Cagey]

Wow. Really gonna make that baby sing, eh?

 

[mayfly]

Wow. Really gonna make that baby sing, eh?

Sure - but only one note at a time 

 

[mayfly]

Starting on the decoupling caps in the power supply.

Old vs New

ok - I sure hope I remember how this all goes back together!!

A strange ass cap for power supply bootstrapping.  No replacement available - so I'll build something using regular caps (I've already a plan in hand)

 

[Cagey]

I vote for alligator clips on the end of a long-ish household extension cord, and take those old caps outside for some vintage-tech pyrotechnics  :icon_biggrin:

 

[mayfly]

Here's my plan for that bootstrap can cap - a couple of 47uf caps on terminal strips.  The third cap was another decoupling cap that the original design just had hanging in space.  ok, there was some *tape* residue on the chassis left over from holding the cap in place - but I figure this is much safer 

I fired it all backup after doing these mods and the power supply is working fine <phew!>.  Unit is still working as it was as well.  That is, working good on anything higher that 2KHz and not so much on anything less 

I have a theory that the caps in the feedback section of the tuned oscillator are leaking which is screwing up things.  We'll see if things improve as I replace them.

 

[Logrinn]

I'm extremely impressed by your eletrical knowledge/know-how.  ccasion14:

This is very enjoyable to follow.

 

[Cagey]

It may be just the angle of view, but it seems like the high voltage side of those caps is awfully close to the mounting lugs for those terminal strips, which are tied to the chassis, which I assume is also ground. I wonder if it would be worth it to find a way to isolate those mounts?

Otherwise, looking good so far.

 

[rgand]

I'm extremely impressed by your eletrical knowledge/know-how.  ccasion14:

This is very enjoyable to follow.

You can count me in on this. It's an area I never learned anything about and it's fascinating.

 

[mayfly]

It may be just the angle of view, but it seems like the high voltage side of those caps is awfully close to the mounting lugs for those terminal strips, which are tied to the chassis, which I assume is also ground. I wonder if it would be worth it to find a way to isolate those mounts?

Otherwise, looking good so far.

The terminal strips that I mounted?  It's just the angle - the terminals are completely different from the mounting points and have good isolation.

 

[mayfly]

I'm extremely impressed by your eletrical knowledge/know-how.  ccasion14:

This is very enjoyable to follow.

You can count me in on this. It's an area I never learned anything about and it's fascinating.

Thanks gentlemen!  But all I'm really doing is replacing worn out parts.

 

[mayfly]

All new caps in this section!  Still does not go below 2KHz though.  Seems to run about the same as it did.  Let's keep going shall we?

 

[mayfly]

ok, all caps have been swapped.  Here's a little sample of the dead solders:

Note that each has the tell-tale bump that says "I'm dried out and about to spray my internals all over the inside of this thing".  I'm glad I replaced them.

Here's the thing though:  The unit still runs like it did before.  No change to the waveform at all.  Time for Occam's Razor:  The simplest explanation is likely the correct one:  Tubes!  Tubes in old tube equipment might bad.  I'm going to do a hail mary and replace them all with the NOS units that I bought.  Here goes.

 

[mayfly]

Well Crap!  That was easy!

Unit is now running low down.  Here it is at 500Hz:  Waveforms look nice and clean!

Here's the original tubes that were in there, along with the boxes of the NOS replacements.

Here's how things look running up to daylight (well, 1MHz in our case  )

I think it's time to do another calibration run then declare victory!

 

[Logrinn]

Looks mighty good!

 

[Cagey]

Cool! I'm surprised a tube change brought back the lower frequencies. My money was on the range switch. You'd think if one of the tubes were bad, you have an all-or-nothing situation.

 

[mayfly]

Yea - I cleaned the range switch pretty early on and measured all the resistor values on it - and resistance of the contacts!  It was OK - but it was on top of mind for me as well.

 

[mayfly]

Here's the old girl with the unit she's replacing!  In almost every way the stanford unit is superior - except the user interface. 

 

[Cagey]

Kind of an AxeFx vs. Fender Princeton thing.