The Secret Society of Lathe Trolls

Self-lather · Joined: 27 Jun 2007 Posts: 48 Location: Atlanta, Ga

Hey CutterCollector, I was going to send a private message, but decided to post to the forum because I figured anyone seeking info on the Recordette could benefit from this.

So I sent in my recording head to Gib at West-Tech to have it rebuilt. I also payed for a new micro-groove stylus so I can cut 33s and 45s records properly. I haven't gotten these back yet, but as advised, I would like to check the capacitor behind the cutting head to make sure it is not leaking DC voltage, and destroying my new cutting head.

I have a multi-meter, and know the basics of it. Do you have any suggestions on the best way to measure this before I plug in my new head? Thanks for any help!

-thomas

cuttercollector · Joined: 11 Jun 2006 Posts: 285 Location: San Jose, CA

First, let me say that you need to be careful as there are potentially lethal high voltages involved with tube equipment! Don't touch the wires with your fingers, clip the leads to them.

The best way of testing capacitors out of circuit is with an old fashioned capacitor checker bridge made by Eico, Heathkit and others back in the days of tube equipment. This is because new capacitor checkers don't necessarily check them at the higher voltages used in tube gear where they are more likely to break down and leak voltage. If you plan on restoring lots of old tube equipment one of these might be a good investment. They will read out the actual value of capacitance and then allow you to measure leakage as you increase voltage to near the rating of the cap. They will do this for little paper coupling caps and for big electrolytic poweer supply caps.
You can get some idea of the capacitance with a modern cap checker like might be built into your digital volt/ohm meter.
You can sometimes see if a larger value cap is completly gone with an analog (or digital with bar graph) meter by "watching" it charge as the resistance value change causes the meter to "ramp up".

But what I would do in your case is to clip the meter leads of a digital (because analog ones typically have too low of an input impedance unless they are VTVMs) voltmeter set on a high range right across the leads where the cartridge/cutter goes in the recordette. Turn it on and put it in the "record disc" mode and see what DC you measure across the leads.
You can keep downranging (after the unit warms up a while) untill you see some voltage there. If the cap IS leaky you could see up to the entire B+ voltage of the amp there (a couple hundred volts, give or take). What you see with a good capacitor or what will not harm the head, I am not so sure of. Probably would need to see less that 10V I would guess, but the lower the better. If you then turn up the volume and feed a source, and switch your meter to AC voltage you might see 60-90V of audio information at peak levels but none with the volume down and no signal in.

An osilloscope can also be used to see these things. If switched to DC coupling, you will see the trace rise above the center line indicating DC leakage voltage if any is present. The AC audio voltage should look like a much higher voltage waveform that mirrors the audio you are feeding. Switching to AC coupling on the scope inserts a DC blocking coupling cap so you don't see any DC offset.

Self-lather · Joined: 27 Jun 2007 Posts: 48 Location: Atlanta, Ga

Excellent info as usual Cuttercollector! I will let you know how things come together when I get my new cutting head back in the mail. Thanks for all the help.

-Thomas

cuttercollector · Joined: 11 Jun 2006 Posts: 285 Location: San Jose, CA

I was thinking about this and considering your investment in the cutter rebuild and the fact you want it to actually work right, a new cap and the time to install it is a pretty cheap investment! Perhaps at least that one and the main coupling cap to the output tube.

Self-lather · Joined: 27 Jun 2007 Posts: 48 Location: Atlanta, Ga

Having built electronic projects from the ground up before, I feel confident that I could replace the caps. I haven't really thoroughly inspected the electronics of the Recordette yet, do you think it would be relatively easy to identify these two caps?

-Thomas

cuttercollector · Joined: 11 Jun 2006 Posts: 285 Location: San Jose, CA

Both of these probably are small paper type caps with values probably somewhere around .01 to .1 microfarad at probably 200-400V. They should be replaced with modern mylar types of the same value as each original or the next closest higher value and at least the same voltage rating, though this could be higher without hurting anything, it's just the max it will take before shorting out.
The coupling cap from the previous stage or perhaps the volume control, would be connected at one end to the grid of the 6AQ5 amplifier output tube, which is pin 1 or 7. The other connecting the cutter to the amp out is probably harder to figure out because of switching. In the record disc mode there should be a connection through this cap from, I believe, the plate of this same tube which should be pin 5, but probably through switching so the connection is only made complete in the disc cutting mode. The idea is that somehow the output of the tube (amp) gets coupled to the cutter/cartridge in record mode. Because it's a high impedance device it does not just come off the transformer where the speaker would connect for playback. The confusing thing is that the cutter as a playback cartridge gets switched to the input of the amp (through the volume control) and the speaker (through the output transformer) gets connected to the output of the amp in normal playback (phonograph) mode. It is one of those multi-function rotary switches that does several things at once, so it is hard to sort out sometimes.
To tell you more, I would have to look myself or see a schematic, but I hope this much helps.

cuttercollector · Joined: 11 Jun 2006 Posts: 285 Location: San Jose, CA

After further checking in my own unit (if it is the same) the capacitor that couples the cutter to the amp in record is a .01 mfd. 500V ceramic (not disc) and not too likely to fail or leak. It is a little round black device with a brown and an orange dot on it, hooked to the plate of the output tube as I said. The coupling from the previous stage is something I have never seen before called a "capristor" which is a part which is both a .005 mfd. cap, voltage unknown but probably around 300-400V and a 470 Kohm resistor. It could be replaced with both of those parts seperatly, but it also looks to be of the type cap that would not be so likely to fail. If it does go bad you will start to hear severe amplifier distortion on all sources and the output tube plate will start to run away and glow dull red.
So back to my previous suggestion of checking the dc voltage across the cutter in the recording modes and verifying there isn't any leakage with your DC digital volt/ohm meter.

cuttercollector · Joined: 11 Jun 2006 Posts: 285 Location: San Jose, CA

Here is another of the same photo where I attempted to id the caps.
The .01 for the head is the black one and the white one with the additional wire is the .005 "capristor". they are toward the rear center of the chassis in this view, just to the left of the output and power transformers. The socket they run to in the back is the 6AQ5 output.

Self-lather · Joined: 27 Jun 2007 Posts: 48 Location: Atlanta, Ga

Excellent info and pics Cuttercollector! I will give this a try. I just got word from Gib that my rebuilt head and stylus are on the way back to me. I can't wait to start cutting.

I will continue to update...

-Thomas

Self-lather · Joined: 27 Jun 2007 Posts: 48 Location: Atlanta, Ga

Hey, my new cartridge and microgroove stylus arrived today. I was surprised that the stylus was actually made by Presto, but I guess that makes sense.

This may be a stupid question, but what is the cutting tip made of?

Wish me luck getting this thing going..

-Thomas

cuttercollector · Joined: 11 Jun 2006 Posts: 285 Location: San Jose, CA

If it is a jewel tip, probably saphire. I would use a steel non-microgroove one to play around with. One cut through with the arm too heavy to the aluminum base of a lacquer and you ruin the tip! Be very careful depending what you are trying to cut with letting the full weight of the arm down on the disc till you can examine how deep you are cutting with this particular stylus and material by examining a couple of turns of test groove under a microscope. The original weight was designed to cut laquer coated thin cardboard discs with a 78 wide groove steel stylus. Anything else may require...ahem... adjustments. Smile

Just ease it down with the table spinning but don't let the full weight rest on the stylus. You should start to see a nice chip come off and there should be little or no mechanical noise. Then see how deep you were cutting. If the grooves were realy shallow you might need to let the full weight down and check again. If already virging on too deep you may have to counterbalance somehow. There really isn't much of a cutting pressure adjustment on these. Also adjusting from all the way inserted to slightly less than that before the screw is tightened on the stylus might give you the slight change in angle needed if it tries to stutter, vibrate, hiss excessivly or screech. Remember, if you can't cut a silent groove with no audio, you are not going to get much of a recording as all that noise will be in the groove when you play it back. All this will have to be tweaked for each type of cutting material and stylus combination.
Others here have gone through this a lot more than I have, so now it is somebody else's turn.