Sunday, August 1, 2021

Possible Explanation of "Too Fast" Recordings.


 OK, so after pouring over the Service Manual for the TEAC, I see no mention of any "Speed Adjust", which makes sense as all three of the motors are AC motors, meaning they're pretty much "locked" to the frequency of the incoming AC power.

Everything turns freely, and recordings I made using the Pioneer on FM for a source sound great. The recordings I made at 3-3/4ips sound good, while the ones made at 7-1/2ips sound even better.

I'm shopping now for a "Test Tape", aka "Calibration Tape", which will confirm that the recorder is running at the correct speed. Until that gets here, I'm going to run 10 seconds of tape, and measure the length. Should be pretty close to 45" of tape at 3-3/4ips, and 90" of tape at 7-1/2ips. I know this isn't "exact", but it'll be a pretty good ballpark guesstimate, and will have to do until I can get a real test tape.


BUT.....what could have caused the deck to run slower than "normal" when these old recordings were made?

The only explanation I have is that the AC power at U-Tapao where these tapes were made was 50Hz power, and not 60Hz power. 220V/50Hz could be stepped-down to 110V/50Hz, making the Voltage OK, but not the frequency. Since I know little to nothing about the base infrastructure back then, or where they got their AC power from, this is a "First Order SWAG", and I might be 100% wrong.

Anybody know anything about the AC power used in SEA back then? I'm pretty sure if the were running on US generators they'd have 220/110 60Hz, but what if the "Host Country" supplied the power? The wikkipedia entry indicates that Thailand currently (no pun) uses 50Hz power, so I may have found the "problem" with the tapes running fast.

14 comments:

  1. Heh. Sounds like you need a variable frequency drive like people use on machine tools. Dial up or down the frequency to get the speed you need.

    Hmmm... I wonder if that would actually work? Hmmm...

    ReplyDelete
  2. I am echoing Beans, I don't see why it wouldn't work. I used to use a variable freq unit to test prosthetic implants because they sometimes used different freqs at same or different voltages.

    ReplyDelete
  3. On board ship, clock radios would not keep correct time. Variations in engine speed or engine speed set incorrectly would vary the frequency or in the latter case have a different frequency because of an rpm difference. Maybe.

    ReplyDelete
    Replies
    1. This was used on a large air base, with fairly stable power.

      I did a simple test, mark a tape, run it 10 seconds, and then mark it where it stopped when I hit stop. The reel brakes work great on this thing, and when you hit stop, it stops.

      I expected 37.5" on 3.75ips, and got 45" of tape.

      I expected 75" of tape, and got almost 90".

      This would indicate it's running about 20% too fast, or the difference between 50Hz and 60Hz.

      I'll do final analysis and report what I found, as this is an oddball unit that was only sold to US Military personnel via the BX/PX on their base.

      Delete
  4. I agree with both of you, and one of the commenters at a forum said the same thing. The ironic thing is I used to design that stuff before I moved to Kommiefornia.

    ReplyDelete
  5. Sounds like you're getting close to solving the frequency issue. Now the question is how do you re-master the "Alvin" tapes to play at normal speed?

    ReplyDelete
    Replies
    1. I may not have to. See my reply to SiG below.

      If I *do* have to correct them, Audacity will do that. Since I use Audacity for my recording, it's easy to do once I have it on the PC.

      Delete
  6. I know in the 70s when I was there, Utapao was on 50Hz power for anything other that communications and ops. Those were on big Cat generators.

    ReplyDelete
  7. Do you have a way to measure the RPMs of the drive? That should be something that will tell you right away if it's a 50 Hz system running on 60, although it sounds like that.

    I expected 37.5" on 3.75ips, and got 45" of tape.
    37.5 * 6/5 is 45

    I expected 75" of tape, and got almost 90".
    75 * 6/5 is 90.

    ReplyDelete
    Replies
    1. WELL......Talking to some people that really know this deck well, there's a 99.9% chance I have the drive belt on the wrong pulley step. The smaller pulley step in under the larger one, and I'm told it's a common rookie error to get the belt on the wrong pulley, as it's hard to see the smaller step lurking under the large one.

      I'll pull the front panel after my PT appointment today and see if that fixes the problem.

      Delete
  8. I spent time at Camp Friendship in Korat Thailand. Out power was 50hz which caused some strange problems without radio equipment on occasion.

    ReplyDelete
  9. Replies
    1. Since I'm not sure where The Colonel used this deck, I don't know if he ran the deck on 50Hz or 60Hz. Since at least half the tapes he gave me are labeled as "Background Music", it was probably used in an office area, which means 50Hz. He may have had the belt position changed. Been too busy to get the front off the deck, but I'll get to it tonight.

      Delete

Keep it civil, please....

Saturday Night Music

 One of my favorite songs, from one of my favorite bands, featuring one of my favorite guitar players. It was also written specifically for ...