Gyrophonic Projector Puzzle

[Regan]

I don't see any need to use the field coil setup for anything then. I would scrap it and just go with a dc power supply of up to 40v and the controller.
Regan

[Tillydog]

I don't see any need to use the field coil setup for anything then. I would scrap it and just go with a dc power supply of up to 40v and the controller.
Regan

?? The motor won't run without the field coil...

@hossy - why do you want to change the controller you have?

[Regan]

Whoops, my bad! I was thinking autotransformer not field coil. The whole setup still has me a bit confused and still working my head around it.
I'm not used to this style of dc motor and keeping the field coil excited all the time.
I'm learning just how much of this I have forgot because of dealing with the familiar all the time.
Tillydog- Hossy's friend wants a variable speed control on a footpedal, I think the easiest way to accomplish that is with a modern dc controller, and then using an expression pedal to control the speed pot.
If not needing a foot pedal, a small variac could be used instead of the autotransformer.
Regan

[hossymandias]

Regan, you are right about the foot pedal. I am pretty far from understanding the schematic myself. I see the four wires that go to the motor, two to energize the field coil (is this with AC or DC current?) and two that carry the variable current (this MUST be DC). I still can't figure out why it is only running on full speed.

Tillydog, I WOULD really like to get the power supply behaving as designed before I start modifying for the foot pedal or scrapping the original power supply. I'm starting to think the relays may be the problem.

Any thoughts?

[sluckey]

I WOULD really like to get the power supply behaving as designed before I start modifying for the foot pedal or scrapping the original power supply. I'm starting to think the relays may be the problem.

There may not be anything wrong with the electronics. You say you can get the relays to switch. So, measure the dc voltage between pins 1 and 3 of the motor plug. This is the field. How much? Now connect a dc voltmeter to pins 5 and 7 of the motor plug. Switch the relays thru their 4 modes, ie, both off, RL10 on, RL 11 on, and both on. Does the voltage change for each position? (Both relays off will be no voltage). If the armature voltage changes, the control unit is probably OK.

I'm thinking the problem may be mechanical. Too much friction in the belt drive system. Try this. Remove the drive belt to unload the motor. Does the motor shaft turn freely by hand? Will the shaft turn for all speeds now? Are there any oil ports for the motor shaft bearings? If so, give them a generous helping of machine oil. While you are at it, look for oil ports on the pillow blocks that support the pulley shaft. I see what appears to be an oil port on one pillow block.

Just basically get everything oiled and moving as freely as can be and maybe that's all you need. It's possible that you may need to replace the motor bearings or pillow blocks. That should be fairly straightforward.

[hossymandias]

Great suggestion, sluckey. I will try this when I get home and post the results.

[hossymandias]

Okay, I remember now, I have measured the voltage going to the field and the armature.

With power on, the field is energized with 116vdc.

With both relays engaged, the armature gets about 72vdc.

With either relay engaged by itself, the armature gets 0vdc.

I'm hoping the bridge rectifier is the culprit. It's a big fat selenium monster. Same with the diode in the schematic, it's actually a selenium rectifier as well.

Please don't tell me you think it's the transformer...

[Tillydog]

I am pretty far from understanding the schematic myself. I see the four wires that go to the motor, two to energize the field coil (is this with AC or DC current?) and two that carry the variable current (this MUST be DC). I still can't figure out why it is only running on full speed.

Can you see where the AC comes in on the left? (AC input, which also supplies an AC socket directly 'For Amplifier')

The AC neutral (probably) goes across the middle of the schematic as far as the junction between CR20 & CR21. I'll call this 'ground' for convenience, but it's not the AC ground, or anything.

The AC 'hot' from the input splits 2 ways:

One leg goes up the page, through the fuse, through R86 and is half wave rectified by CR19, smoothed by C37 and goes to the field coil of the motor through pins 1 & 3 of the connector - this is DC and just provides a steady magnetic field for the motor, and is on all the time. (Slightly confusing because it's rectified to provide a negative voltage with respect to ground - I can't see that this is significant, though).

The other leg goes down the page, through a fuse (above the word 'GYRO') and into the 115V tap of the auto-transformer (I'll call it AT to save typing!). The other end of the AT is connected to ground.

ATs work just like regular transformers, except the primary and secondary share some turns of the windings, so you have 110V, 115V and 120V "primaries" to match the mains AC voltage, and 85V, 75V, 60V, 27V and 16V "secondaries".

One of the "secondaries" is selected by the relay network and passed to the full wave bridge rectifier made up of CR20..CR23. The rectified voltage is smoothed by C38 and passed to the armature (the rotating part) of the motor through pins 5 & 7 of the motor connector. This is also DC

I'm guessing that the big resistor (R87) is there for braking the motor - it is connected directly across the armature. When the armature is switched off, the motor will act like a generator and feed current back into the circuit. Letting this current shunt through R87 will slow the rotor down more quickly, and may also help avoid high voltages being generated as the rotor runs down. (It seems a bit wasteful that it is permanently connected!)

It appears that the relay coils are powered separately by a low voltage supply (the GND/+14VAC input), and that the foot switch bridges pins 4 & 3 of connector P10 to activate RL10, an/or pins 2 &4 to activate RL11.

The worst case scenario where you will get full speed, regardless of relay switching, and that is if the connection from the AT to ground is lost (the COM terminal), or the "secondary" winding becomes open circuit. The AT then appears to be in series between "primary" and "secondary" taps.

To check this: With power off, and the unit unplugged, verify that you have got continuity from what I've called 'ground' (AC neutral) and the COM terminal on the AT.

Also verify that you have continuity from the com terminal of the AT to all of the other taps. The resistance should increase gradually as you go to higher voltage taps (maybe a few hundres ohms, tops).

Otherwise, your issue of only one speed probably centres on the relay switching:

Can you hear each of the relays click when you activate the foot switches?

If not, with power off, unplug the foot switch and use a continuity meter on the lead from the foot switch to check that you can get continuity between pin 4 and pin 2 and/or pin 3, depending on which switch is pressed.

I think that this is the most likely scenario, as if one of the relays had a bad contact (for example) you would end up with three 'off' positions and only one 'on' position, which doesn't sound like it is the case.

N.B. The AT will not give any isolation from the AC mains, to everything (except possibly the relay switching supply) is connected to the socket in the wall. Please exercise caution if you decide to probe around with the unit live. Ideally you would use an isolation transformer while working on it.

Good luck!

[Tillydog]

Okay, I remember now, I have measured the voltage going to the field and the armature.

With power on, the field is energized with 116vdc.

With both relays engaged, the armature gets about 72vdc.

With either relay engaged by itself, the armature gets 0vdc.

I'm hoping the bridge rectifier is the culprit. It's a big fat selenium monster. Same with the diode in the schematic, it's actually a selenium rectifier as well.

Please don't tell me you think it's the transformer...

Just seen this -

Probably not the rectifier - if you have '3 offs' and one 'on', then I think you have a relay with a bad contact - RL10, if I were guessing.

If you were very careful, you could check the AC voltage at the 'common' of both poles of RL10 (the dots on the schematic) when it switches. The top one should flip between 16V and 85V, the bottom one between 0V and 27V.

BUT please heed the warnings about the lack of isolation in this circuit.

[hossymandias]

That was the right stuff, Tillydog! The relay contacts were dirty.

I polished them up and now it works perfectly with all three speeds. I can't thank you all enough for all your help!

So now to figure out what to do with it...

I'd still love to do a few modifications if its possible:

1. A foot pedal to control the speed (preferably with an empty wah enclosure and a pot). I've been interested in those DC speed controllers mentioned earlier by Regan. However, I can't find any documentation or explanation on how they work. This one, for example. Does it take an input voltage of say 80vdc and allow one to vary that from 0-80vdc?

If that is the case, this could work really well. I'd just have to replace the pot with a wah pot...

2. I have the parts to replace the selenium rectifiers. How do I calculate the difference in voltage drop? I've heard I will need to add resistors in series with my modern diode and bridge rectifier. Also, where would I put the resistors in relation to the bridge rectifier?

3. The tube amp is an Allen 75. I've read these are desirable to audiophiles. I'm not necessarily a hi-fi obsessive...there's just not enough room in my basement. But because this is a "clean" sounding tube amp, is there any modifications I can do to make it more of a guitar amp, or should I just modify the sound using pedals?

4. I'm putting some **** casters on it.

You guys are great. I'll post some pictures of it when I get back from Texas next week.

[Regan]

Glad its working!!
Thats exactly how that controller should work, good find!
Only thing that might be an issue is it may introduce noise being that it only modulates at 15k, you might have to shield wires and put it in a metal enclosure. Other than that use a jack, pick up a 20 dollar expression pedal and change the pot and your done- or use your old wah shell
Regan

[Tillydog]

... now it works perfectly with all three speeds. I can't thank you all enough for all your help!

Great stuff!

The Amazon controller works as you describe - you would need to provide a decent DC supply for it - I would strongly recommend that you ditch the AT and use a conventional transformer for that if you will be having trailing leads to a foot-control. You'd also need to retain the field control supply, and possibly R87. (Try and find out if the controller has motor braking - ideally you want one without, as it may not cope with trying to bring that behemouth of a rotor to a rapid halt.)

2. I have the parts to replace the selenium rectifiers. How do I calculate the difference in voltage drop? I've heard I will need to add resistors in series with my modern diode and bridge rectifier. Also, where would I put the resistors in relation to the bridge rectifier?

Have a look at this document (pdf file): LINK

The resistor could either go directly before the bridge (AC) or directly after (DC) - between the bridge and the smoothing capacitor.

Note also the comment about replacing the electrolytic capacitors!

Have fun

[Edit to fix link]

[pompeiisneaks]

You could try a rheostat made for 120v applications, like a ceiling fan control. Just don't use a light bulb dimmer, it's not heavy duty enough.

Ceiling fans do that.

It seems odd to mention that ceiling fans do what someone said ceiling fans do, to a 5 year old thread?

Edit: I looked up the link, it's on a malware site, time to ban someone.

~Phil