UTC 3AX restoration

[tardishead]

I want to access the EQ filter circuit in order to match components on 2 units
The filters are encapsulated in what looks like a kind of wax.
Parafin wax?
I need to heat it up and drain it off.
Will it be toxic? Any ideas of how to heat - I presume a domestic oven would not be a great idea. Fumes etc

 

[emrr]

I'd probably try a heat gun on low setting first. 

 

[tardishead]

I've tried a hair dryer style paint stripper gun on the outside. Only melts the wax in small areas.
I haven't used it blowing direct onto the wax because I thought it might be toxic and I should find out more info first.
I guess if that was ok I could melt bits at a time and scoop them out. Can I damage components possibly by blowing hot air directly on them? Caps and inductors?

 

[emrr]

Doubtful you'd damage inductors, and you'll be replacing caps anyway if it's a match you are after. 

Everybody would already be dead if it were that toxic.  CJ's still with us, right?  The nasty stuff is usually black, and I would just avoid bathing in it. 

An oven at 225F is the best thing if you have a way, most people don't. 

 

[tardishead]

By heat gun do you mean propane/butane torch??

 

[mjrippe]

The wax is not toxic.  Use your "hair dryer style paint stripper gun" to heat the metal can.  It will take a while, but keep moving it around to heat the whole can evenly.  Eventually it will be hot enough to melt the outermost layer of wax all around and you can pull it off.  Or heat the top layer of wax, pour it off, and repeat.

 

[Dr Gris]

By heat gun do you mean propane/butane torch??

My guess is it means the same type used for tube shrinking...

 

[tardishead]

Filled a 100M2 warehouse space full of noxious fumes
Still here!

So I started with the HF filters of both units. As I suspected they were quite different - totally different types of caps.
I think in stead of measuring the coils and capacitors individually I will set up a spectrum analyser and see what needs tweaking and in which direction.
Some of the caps look like the Sangamo silver mica style you see in Ampex 350 preamps
The biggest caps look totally hand made with no manufacturer logo or packaging and have no leads even. The wires are soldered direct to the end of the body.
Not sure if anyone is interested in this.

 

[CJ]

here is that old link,


www.technicalaudio.com

 

[tardishead]

I used the hairdryer paint stripper in the end. Took about 30 minutes each filter.
I measured the caps in each unit but did not measure the coils because I did not want to mess with the wiring looms.
Unit1                          Unit2
7.6nf                          7.1nf
9.2nf                          9.6nf
0.3uf                          0.28uf
0.39uf                        0.447uf
They do not have matching/similar looking capacitors.
On my spectrum analyser plugin the 2 look almost identical with white noise but I can definitely hear differences especially with music going through them.
My SA is not precise enough so I measured the max output/resonant frequency by dialing in the frequency from an oscillator.
There are 5 frequency positions so obviously one frequency uses parallel capacitors
Unit 1                        Unit 2
13.79khz                  14.57khz
9.59khz                      9.73khz
7.61khz                      7.76khz
5.69khz                      5.80khz
3.76khz                      3.99khz
So its obvious all things being equal that the coils are not matched because if they were Unit 2's lowest frequencies would be lower not higher like they are. Which makes sense because I don't think stereo even existed when these things were manufactured.
So unfortunately anything I modify at this stage will be a compromise.
At this point I wanted to hear differences between these resonant frequencies so I dialled them in to a digital frequency generator. Turns out quite a bit. Definitely audible when listening to the pure tones.

Hard to know what to do
I guess I should measure the inductance of the coils and check with the impedance nomograph  to see if theres anything I can do.

 

[emrr]

You can stretch the cap values to get matching centers, but the overall shapes will be different because of it.  It may depend on how far you typically boost as to whether it's audible.    What is max boost again?  In fact, I wonder if it's the frequency difference you are hearing, or if it's a shape difference.  The frequency differences look rather small to me, and I'm not sure that several modern kit EQ's I've built have been substantially tighter in matching the centers.  I might be tempted to decide on a singe recap value, and see if anything changes from that alone before pondering more.  I have definitely seen calculated centers prove different in practice. 

 

[tardishead]

What capacitors should I use
I think the highest 2 caps are silver mica because they look very similar to sangamo silver mica in Ampex stuff.
Square encapsulated in plastic
I was thinking russian k40-y caps for the bottom 2 and maybe polystyrene for the top 2?
Yes it might be a shape difference
I think I am hearing a wider bandwidth on one of the eqs
I was thinking the only way to be sure is unsolder the coil connections and measure the inductance a and Dc resistance.
Looks a bigger job than I envisaged

 

[emrr]

Can you not pull one side of each cap loose, marking the broken connection, and get L measurements? 

 

[tardishead]

Yeh I think I'm definitely going to have to get those L measurements.
And draw a proper schematic out of the mound of spaghetti.
My plan is something like this;
Find out the inductance of each coil
Work out the ideal resonant frequency for each unit each band and calculate a new resonant frequency as an average
But I guess I need those L measurements before I can even think that far ahead

 

[tardishead]

I really want to use these as a pair
They sound so good

 

[emrr]

I have loose HC-19X and HC-118 filters that could probably benefit from your drawing.  Somewhere I've written down L and C measurements.  I think they cover some of the same frequencies, if not all. 

 

[tardishead]

Traced out the schematic
Not sure if this is right - does it look right?
Unfortunately I forgot that my multimeter with inductance had perished and I only had another one without. I need help with understanding this - I understand the basic resonant circuit at the top but not the bottom half. Isn't it strange that the lower DCR coils are paired with the larger caps? Help

 

[emrr]

I might have to redraw that to get it, will have to come back when I have more time.  Is this the whole thing or just the high side? 

Each Daven is a dual ganged assembly?  Can you tell if they are T's or ladders?  I would assume T's, with one of the sections being a reverse throw to keep attenuation constant.  'A' the lows, and 'B' the highs?  Or the gangs? 

 

[tardishead]

It is just the high frequency band
Is the lower half possibly a compensation pad as it says in the UTC brochure?

 

[emrr]

Yes, one would be reverse throw from the other to keep attenuation constant.  One is in the EQ circuit, the other is just to counteract the gain movement. 

Here's a quick redraw I did assuming T's, haven't thought about it or checked for errors, probably closer to truth in at least some ways.  Maybe it helps.  If you have an Audio Cyclopedia or any other old text that addresses LC equalizers, you should find examples that show typical construction.