Moto VT71: a better ballast?

[bgadow]

Mine looks something like the one Red Raster pictured-ugly, but working. I'd very much like to try this upgrade.

[jshorva65]

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]

Originally Posted by bgadow

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]

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]

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]

Originally Posted by compucat

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]

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]

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]

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]

Ah right. The '303 has that extra resistive element.

[polyphase]

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]

Originally Posted by Phil Nelson

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]

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]

Originally Posted by polyphase

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]

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.