MTI SVT Fan Replacement

[Old Fart]

I plan to place an order for some electronics stuff. Various projects. I likely will not find all I need at one supplier. I am not even an electronics guy, but I do like to do things myself, even if I have limited knowledge. One of the projects involves replacing two bias supply caps in a Dynakit ST70. Yes, I know how to respect circuits and avoid getting killed. Anyhoo......

The fan in my MTI SVT sounds like a cross between a jet engine, a locomotive and a tornado. Some have said this or that fan will not fit, but I notice it is mounted against a metal screen with holes, and some of the screws are drilled where there is no hole. I think any fan with a flat output surface could fit. I have drill bits. Would you agree? Somebody (Beans On Toast?) gave the name of the plug between this fan an the amp, which can be sourced and just replaced with new. What is the name of that?

What fan specs should i look for? CFM? Low noise. Recommend me a common product you think might work. Any helpful tips would be....helpful. Below are two pics showing the existing fan location and installation.

 

[agedhorse]

You need to look at the old fan for the specs. Placing a fan that close to the screen will increase the noise level due to turbulence of the air sheared through the holes.

 

[Old Fart]

Thank you. Would you recommend using some grommets to get the fan a little further from the screen?

 

[agedhorse]

Maybe. I believe you may have a 115Vac fan which will greatly limit your choices.

 

[ThisBass]

Yeph. The smallish holes of the metal plate generate way more vent noise than the fan itself.
if you want to reduce the noise "effectively" You'd have to saw a big hole into the plate and then mount a typical grill.
Ampeg did it this way with the VR series svt

 

[Old Fart]

I can make a big hole in this thing, but I will consider all the facts before proceeding. Here is a photo of the label on the fan, and here is a link to the fan, which is still available. It appears this fan is not original. 2007? I see it moves 72 CFM. Is this the appropriate spec? Or should I call Westinghouse and see if they will sell me a used jet engine?

https://datasheet.octopart.com/027119-Comair-Rotron-datasheet-11768176.pdf

 

[Old Fart]

The fan starts out louder, then quiets down somewhat. The fan has axial play (in the same direction of the direction of the shaft...I hope I am using that term properly). It has not sideways play, but it has perhaps a coupe of mm of axial play.

 

[ThisBass]

Thank you. Would you recommend using some grommets to get the fan a little further from the screen?

nope, not really. It may help to reduce the noise but also would help (as drawback) to let the fan work as kinda "circulating whisk" inside the chamber.
Less noise, less effective CFM through the chamber

 

[ThisBass]

The fan starts out louder, then quiets down somewhat. The fan has axial play (in the same direction of the direction of the shaft...I hope I am using that term properly). It has not sideways play, but it has perhaps a coupe of mm of axial play.

some axial play is fine

 

[agedhorse]

some axial play is fine

and in fact is necessary.

As I suspected, this is a 115V 120mm fan, it's very hard to find slow speed versions due to the number of poles required. Noise level was not much of a consideration with these fans due to the majority of applications being commercial/industrial.

Here's a fan that I would consider suitable, especially if you were to eliminate the perforations in the fan's path and use a standard wire grill. This way the static pressure spec will be zero and there should be plenty of CFM. This is a deeper fan (38mm rather than 25mm) which contributes to the lower noise/higher volume per static pressure unit. A deeper fan can operate at a slower speed for the same airflow.

Invalid Link Removed

 

[Old Fart]

That fan is the same depth (1.5" / 38mm), but it does operate at a slower speed. It moves less air, but a cutout would help with flow and noise.

Then there is the problem of the screen to cover the hole.

 

[beans-on-toast]

Don’t cut out a hole in the grille. Do try fan isolation mounts. I would start by removing the fan and cleaning it. Sometimes removing the dust can help. Don’t apply oil unless there is an oil port, the older Rotrons had a removable rubber oil cap.

The fan wire connectors that I posted part numbers for were made by Molex. The ones that I spec’ed were for the vintage SVT, I don’t recall what is in the MTI amp. They may be different so check.

 

[agedhorse]

The problem with the perforated holes is that they generate noise from the shearing action of the air at the surface of the vent.

If the hole can be safely cut cleanly and smoothly, that would be ideal BUT it’s essential that a finger guard be used.

 

[beans-on-toast]

Pre and post MTI revisions did have a fan hole cut in the grille and a plastic finger guard. I prefer the metal ones. Cutting a hole may be an issue if the amp is sold which prompted my cause for concern.

Using a vibration mount can help mitigate the noise, adding a bit of space between the grille and the fan frame also helps with airflow noise.

 

[Wasnex]

I have an 87 Skunk. The fan is mounted the same as an MTI and blows out through the grating. I went through the trouble of reversing the fan, and it made a much more obnoxious high frequency turbine sound. My assumption is forcing the air through the grating creates small, high velocity jets that make noise when they hit the fan blades. So first thing to do is check to see which way the fan is blowing.

I also have 69 which has a hole cut in the grating and air is sucked in through the hole. I bought a replacement fan hoping for a quieter operation, with no perceivable difference in SPLs, so I reinstalled the old fan and the new fan sits in the box dreaming of future glory.

Here are the Orion P/Ns I used if you need them:
Fan: OA125AP-11-2TB (120x120x25mm, 60CFM, 37dBa)
Guard: G109-15AB
Cord: C45-12 (45 degree plug with 12" lead)

The reason I had hoped this P/N would be a bit quieter is because Mouser incorrectly lists the SPL as 27dBa instead of 37dBa. I think the P/N listed above is pretty much equivalent to the OEM fan.

The designer of the SVT has stated it will pass UL listing without a fan. I don't recommend running the amp without a fan as I think the extra heat will shorten the life span of various components. But you could probably derate the fan a bit without major concerns.

The OA125AP-11-3TB drops air flow down to 42CFM and noise down to 34dBa. It's the quietest fan in the series. 3dB is not much of a difference, but this model costs $17 plus shipping from Mouser if you are interested.

The nice thing about these Orion fans is they use the same plugs as the fans in my 69 and 87.
This is the plug on the C45-12 listed above and it inserts right into the side of the fan.

It's probably not necessary to use a fan with this connector as most SVTs have a two pin Molex to disconnect the fan from the umbilical. You should be able to salvage the pins in the Molex, or worse case scenario you can order new pins or a new Molex connector if necessary.

The 38mm fan @agedhorse horse recommended (OA109AP-11-3TB) may be a good choice, but you need to check clearance between the fan, umbilical, and transformer to make sure. I have a hard time getting the rear panel on one of my SVT without either the umbilical or fan wires hitting the fan blades, and I am using 25mm depth fans.

 

[Wasnex]

Then there is the problem of the screen to cover the hole.

This is the P/N for Orion 120mmx120mm fans.
Guard: G109-15AB (black) or G109-15A (anodized nickel)

I am not necessarily recommending you cut a hole in the grating though. I bought this for my 69, but haven't installed it.

With the air blowing out through the grating, my 87 is not really any louder than my 69 which has a hole cut through the grating. I do think the grating hinders air flow a bit, but I don't really think it's enough to worry about.

 

[Wasnex]

Don’t cut out a hole in the grille. Do try fan isolation mounts. I would start by removing the fan and cleaning it. Sometimes removing the dust can help. Don’t apply oil unless there is an oil port, the older Rotrons had a removable rubber oil cap.

The fan wire connectors that I posted part numbers for were made by Molex. The ones that I spec’ed were for the vintage SVT, I don’t recall what is in the MTI amp. They may be different so check.

I believe this is an MTI. Looks like the same (white) Molex connectors for the fan and umbilical as any other vintage SVT I have seen.

Both my 69 and 87 have one of these wired to the 2-pin quick connect.

 

[beans-on-toast]

I believe this is an MTI. Looks like the same (white) Molex connectors for the fan and umbilical as any other vintage SVT I have seen. View attachment 4017588

Both my 69 and 87 have one of these wired to the 2-pin quick connect.
View attachment 4017589

It is the same Molex connector then. Depending on the fan, some have fixed wires, others have a connector on the fan like you illustrated. The two pin molex connector attaches to this.

There are quieter fans than the Orion but they usually cost more. In SVT’s I use:

fan: 4530Z ebm-papst | Mouser
fan guard: LZ30 ebm-papst | Mouser
power cable: LZ126 ebm-papst | Mouser

The 4530Z is an older model, they may have better product available now. EBM-Papst products are excellent.

 

[Aidenn7]

keep the old back, make a new one.. trace it on thin wood or metal.. drill out the holes.. and paint it with heat resistant black spray plaint

 

[Old Fart]

Hm. I am forming the impression there may be little I can do here without significant modifications. FWIW, here is a picture of the 3-conductor plug. We think this is the twelfth MTI SVT made. It is more like a Magnavox-era SVT than a MTI. Except for the outward appearance.