View Full Version : Moto VT71: a better ballast?


polyphase
11-26-2010, 02:33 PM
Here is a home-brew ballast that pin for pin is a direct substitute for the 17A485459 ballast in my VT71. It uses capacitors in the filament strings instead of resistors. The result is 80% less heat dissipated and elimination of the current surge at turn-on. The tubes instead enjoy a slow, gentle life extending warmup. A pic of this contraption, (along side its predecessor) and a schematic are shown below.

The math, in spread sheet format, is as follows:

reactance X of cap =10^6/(2*PI()*F*C)
for F=60Hz and C=10uF, X=265ohms

resistance R of HOT filament string =260ohms.

X in series with R results in impedance Z:
Z=SQRT(X^2+R^2)=370ohms, so current at 110Vac line voltage is 110/370=.3amp

The COLD resistance of the filament string is about 50ohms.
Substituting 50 for R in the above formula, the result is about .4amp, +30% of nominal. This is trivial compared to the .71amp (+150%) surge that occurs at turn-on with the original 105ohm resistor ballast.

note: The caps shown are Panasonic ECQ-E2106KF, avail. from Digikey
168718168719

bandersen
11-26-2010, 07:03 PM
Very clever! I've seen talk use of using a series diode + resistor to improve efficiency, but never a capacitor.

I wonder if adding a thermistor like a CL-90 would completely eliminate the surge ?

kvflyer
11-26-2010, 08:03 PM
Very nice. I am a sucker for the VT-71. I have five of them and one is restored so far. I am lucky to have ballasts for them but I like your idea.

Phil Nelson
11-26-2010, 09:31 PM
Tidy construction. Did you reuse an original base & ventilated can?

Phil Nelson

Red Raster
11-26-2010, 10:33 PM
Necessity can be the the father of invention and in this case, bastardization

http://www.pbase.com/redraster/image/130659585.jpg

Pretty ugly
http://www.pbase.com/redraster/image/130659601.jpg

Can you beat this?
http://www.pbase.com/redraster/image/130659575.jpg

Like Frankenstien's monster it is ugly but it works! :scratch2:
http://www.pbase.com/redraster/image/130659573.jpg
Note! this not of my doing, just another fully restored set on ebay

polyphase
11-27-2010, 08:48 AM
Tidy construction. Did you reuse an original base & ventilated can?

Phil Nelson

Yes I did. A very tight squeeze..

John Folsom
11-27-2010, 09:16 AM
Very elegant. Any issue with dissipation in the film capacitors?

polyphase
11-27-2010, 09:44 AM
Very elegant. Any issue with dissipation in the film capacitors?

None whatsoever. The 2 resistors in the circuit together dissipate about 5 watts. If there is any dissipation in the capacitors, it would be measured in milliwatts. The overall power consumption of the set is reduced by 20 Watts although VA stays the same at about 113.

polyphase
11-27-2010, 11:02 AM
Very clever! I've seen talk use of using a series diode + resistor to improve efficiency, but never a capacitor.

I wonder if adding a thermistor like a CL-90 would completely eliminate the surge ?

I looked at the specs for the CL-90, and you may very well be right.
Have you tried it with a conventional ballast? Shall I?

kx250rider
11-27-2010, 11:45 AM
This thread is why I feel inadequate for never having taken engineering courses...

I like the "soft start" idea on the filament string; especially since 7JP4s are now so scarce (with good emission), and I've seen plenty of them with open filaments, which this should prevent from happening.

And aesthetically, it is the best I've seen! I think it's a MUST for any VT-7x which will be more than just a display piece, and a MUST for anyone who is repairing sets for other collectors, and doesn't want to explain to an annoyed customer or friend, why a 7JP4 was "good" before, and now isn't...

Charles

bandersen
11-27-2010, 05:32 PM
I looked at the specs for the CL-90, and you may very well be right.
Have you tried it with a conventional ballast? Shall I?

I used one in my Hallicrafters series strung set. It definitely provided a soft start. It does get a little hot during operation though so mount appropriately.

Eric H
11-27-2010, 05:43 PM
Very nice work, thank you for posting it.

I'll have to try it out on my VT-71.

jshorva65
04-13-2012, 09:07 AM
I've been working with Big Dave since 2009 toward developing commercial replacements for the still-available Amperite 17A485459 and the Unobtainium 7A470303 into which we plan to incorporate a few advanced features. The goal we have set for ourselves is to achieve the ability to offer rebuilding of customer-supplied "dud" ballasts of either version by the end of this year. We have four proposed circuits (two for each VT-71 Ballast type) undergoing evaluation for use in the final version. We plan to include some pretty slick features in our final version: inrush limiting; overload protection; visual fault warning; and provisions for servicing instead of outright replacement in the event of a Ballast failure. The technologies we are evaluating for performance-to-cost ratio are Reactance versus Rectification, however, field trials of both technologies are still in progress.

Since most of the work we do is for set owners who are not experienced at post-Restoration troubleshooting, we hope to make subsequent Tech Support as simple as possible and eliminate as much Shipping as possible. Hopefully, developing the ability to determine by phone or email whether post-Restoration servicing (during or after warranty) will require the entire set or only the Ballast Tube to be returned for servicing. With a bit of creativity, we hope to make it easy for the customer to provide meaningful feedback based upon Ballast appearance to facilitate quick troubleshooting of a variety of Fault conditions for which some manner of visual indicator has been built right into the Ballast Tube.

Reece
04-13-2012, 03:23 PM
Use of capacitors for filament string voltage dropping does result in a very soft start. I have seen about twenty seconds before voltage came from an initial approximately 60 up to just below line voltage on a string, so in such cases a CL90 wouldn't be needed. CL90's are good where you don't need to drop voltage (although they do drop a couple of volts) but want to just give a soft start. It's also a good idea to put a high value resistor across the dropper capacitors, around 100K, to quickly discharge them at shutdown. A quick turn off/turn on could send a surge through the string so a quick discharge helps.

jshorva65
04-13-2012, 10:11 PM
Use of capacitors for filament string voltage dropping does result in a very soft start. I have seen about twenty seconds before voltage came from an initial approximately 60 up to just below line voltage on a string, so in such cases a CL90 wouldn't be needed. CL90's are good where you don't need to drop voltage (although they do drop a couple of volts) but want to just give a soft start. It's also a good idea to put a high value resistor across the dropper capacitors, around 100K, to quickly discharge them at shutdown. A quick turn off/turn on could send a surge through the string so a quick discharge helps.

It seems like there are some really good ideas evolving in this thread. Our developmental circuits appear to be very similar to those being discussed here, but we're using thermistor types other than the CL-XX series for inrush limiting along with a combination of an internal master fuse plus fusible-link devices on each of the three main current branches. The master fuse inside the Ballast Tube has a slightly larger rating than and is in series with the under-chassis fuse we include in standard Restoration work, intended to be a tamper-resistant backup for the standard fuse. The fusible-link devices are designed to show failure visibly and are positioned to allow inspection under a strong light source through the ventilated metal shell of the Ballast Tube.

Our standard Restoration procedure includes pigtail fuses under the chassis for AC Line and B+ branch circuits or a Custom option substituting a fuse block mounted in an inconspicuous location under the chassis or inside the HV cage making the fuses user-replaceable like tubes upon special request. Therefore, we designed our Ballast Tube substitute in such a manner as to utilize its internal protective devices as tamper-resistant emergency backup devices.

bgadow
04-13-2012, 10:54 PM
Mine looks something like the one Red Raster pictured-ugly, but working. I'd very much like to try this upgrade.

jshorva65
04-13-2012, 11:02 PM
Ultimately, we hope to be able to offer our version whose internal circuitry will be technician-serviceable at the component level at a price reasonably competitive with that of the one-shot disposable Amperite units currently available as NOS items as direct replacements for the 17A485459 and a similarly-priced counterpart as a direct replacement for the 7A470303 whose one-shot disposable Amperite replacement appears to still suffer from Unobtainium status.

jshorva65
04-14-2012, 12:56 AM
Mine looks something like the one Red Raster pictured-ugly, but working. I'd very much like to try this upgrade.

We're going for the maximum ease of maintenance and neatest appearance we can achieve, experimenting with mounting the fusible-link array using turret pins on perfboard and selecting the resistive elements in our design with a precisely-determined safety margin to provide cool and reliable operation while conserving space inside the enclosure. The Rectification version has proven to be the most compact, but the Reactance version only requires a single thermistor to provide adequate inrush limiting. Although we have found that the Reactance circuit paired with thermistors resulted in startup current barely rising above normal operating current, the additional space consumed by two additional thermistors resulted in an excessively tight fit of the assembly within the enclosure. The reduction in startup current derived from the Reactance approach alone seems quite adequate.

The fusible-link protection also minimizes the potential for damage to the set as a result of excessively leaky heater bypass capacitor or severe H-K leakage in one of the tubes near the upper end of one of the heater branch circuits. Basically, an open fusible-link on one of the heater branches is intended to function as a guide in troubleshooting which would indicate the strong possibility of some underlying cause existing within the circuitry of the set itself.

compucat
04-14-2012, 08:29 AM
If this ballast solution becomes available, I would be interested in it as I had a metal ballast fail and am now using an Amperite glass one.

polyphase
04-14-2012, 01:01 PM
I have an extra unit like the one shown in my original post, and I have parts for a few more. Send a PM if any interest.

If anyone is or is thinking of building one of these, the value of the 30ohm 5W resistor has been changed to 20ohms 5W. 30ohm is fine, but it was running hot and I didn't have room for a higher power one in the can. It seems the only purpose of this resistor was to limit charging currents through the original selenium rectifiers. With silicon diodes, it is unnecessary, but I'm more comfortable having something there.

Incidently, the reactance of the 10uf capacitor in this design limits current to less than .5A per string, even under short circuit conditions. Adding fuses or thermistors in this circuit would be pointless.

Phil Nelson
04-14-2012, 04:17 PM
I would be interested in it as I had a metal ballast fail and am now using an Amperite glass one.What's the objection to using the glass Amperite ballast? Appearance?

Just curious.

Phil Nelson

Big Dave
04-14-2012, 10:06 PM
The Amperite ballast requires moving the connection from pin 5 to pin 6 (removing the B- connection from pin 6). It also requires the addition of a 22 ohm 5 watt (minimum) resistor from pin 8 to B-. They also get very hot, possibly hot enough to light up a smoke. I'll try that one tomorrow.

bandersen
04-14-2012, 10:15 PM
You don't need to modify the later TS-4J chassis. They were designed to use the 17A485459.

Why is the 22 ohm resistor required ? I didn't add it to my sets and they work fine.

Big Dave
04-15-2012, 01:06 AM
The earliest ones had a dropping resistor between the CRT heater and (I think, I'll need to check) one of the 12SN7's.

bandersen
04-15-2012, 01:13 AM
Ah right. The '303 has that extra resistive element.

polyphase
04-15-2012, 08:16 AM
Early sets had a 6SQ7 audio amp. Its filament was bypassed with the '303 element and placed in series with the CRT filament. Later versions used half a 12SN7 for this, which did not require the bypass. Oddly, the TS4H (maybe others) does not have the 6SQ7, but has the '303.

compucat
04-15-2012, 11:11 AM
What's the objection to using the glass Amperite ballast? Appearance?

Just curious.

Phil Nelson

I have no objection to the Amperite glass ballast, in fact I prefer it over the metal can type with its glowing nichrome wires. I realize that at some point these ballasts are going to keep a lot of otherwise restorable Motorolas from working because of lack of availability. These modern alternatives are appealing for that reason plus they sound like they run cooler. The ballast tube is the hottest running component in my 9VT1.

Phil Nelson
04-15-2012, 02:30 PM
Yes, I do like the modern alternative, which is compact and cool running. If I still owned a Motorola, I might replace the funky sub that I fashioned using big power resistors.

Phil Nelson

jshorva65
05-01-2012, 01:45 PM
I have an extra unit like the one shown in my original post, and I have parts for a few more. Send a PM if any interest.

If anyone is or is thinking of building one of these, the value of the 30ohm 5W resistor has been changed to 20ohms 5W. 30ohm is fine, but it was running hot and I didn't have room for a higher power one in the can. It seems the only purpose of this resistor was to limit charging currents through the original selenium rectifiers. With silicon diodes, it is unnecessary, but I'm more comfortable having something there.

Incidently, the reactance of the 10uf capacitor in this design limits current to less than .5A per string, even under short circuit conditions. Adding fuses or thermistors in this circuit would be pointless.

The reason for adding a fuse is to account for the eventual possibility of a substantial shunting of the reactive element by an unexpected resistive (leakage) element, which could change the impedance vector substantially from the desired angle of approximately 45 degrees and allow considerably more than 0.5A to flow in such a fault-compromised circuit. Replacing the original selenium rectifiers which had a forward voltage drop on the order of 5V with 1N4007 silicon diodes having a forward drop of only 0.7V and the fact that modern household power specifications are 120V instead of the 115V of 60 years ago will mean that the B+ could be as much as 15V above OEM specifications with the original 37-ohm resistor and almost 30V too high with that element of the Ballast omitted. The design we have been testing and are in the process of miniaturizing as much as possible to fit inside a container of size comparable to the Motorola original Ballast Tube was specifically designed to address such a "worst-case" scenario as an eventual component failure inside the Ballast itself.

polyphase
05-03-2012, 06:41 PM
The only thing that could shunt the 'reactive element' (capacitor) is a failure of the capacitor itself. For a modern film capacitor, this possibility is so remote that I stand by my above comment.

I agree that omitting the other ballast element would not be a great idea. I chose 20 ohms for my ballast so I could get away with a 5W device. This choice had the added benefit of boosting the B+ voltage by about 5%. IMO this is a good thing, as in my view the VT71 chassis is somewhat 'voltage starved'. Performance improved, particularly the sync lock. However, I appreciate that meeting OEM specs is more often than not the goal of a restoration. To each his own.

bandersen
08-04-2012, 11:24 PM
Any update on the better ballast project ?

Big Dave
08-04-2012, 11:31 PM
We sold a prototype to a VT71 restoration client. So far, it works as designed. I will build new ballasts as we get orders. I'm not sure about pricing at this point. Email John for more information.

compucat
08-05-2012, 10:43 AM
We sold a prototype to a VT71 restoration client. So far, it works as designed. I will build new ballasts as we get orders. I'm not sure about pricing at this point. Email John for more information.

Please keep us posted on this. My Motorola 9VT1 is in daily service and a ballast replacement with cool running components is desired. I hope to do more Motorola electrostatic set restorations in the future and it would be good to know I can get ballasts.

polyphase
08-28-2012, 08:58 PM
I looked at the schematic for the 9VT1, and saw that the ballast is 17A485459, same as the later VT71. All you need to do is replace the pair of 105ohm filament resistors with 10uf caps as specified in post #1 to get cool.