Hello again everyone.
Long time no hear
Anyway, after nearly a year of use my fantastic RT-707 was needing a little more TLC. Electronically the machine is perfect, my overhaul documented in this thread has the machine in outstanding condition. The recording and playback performance is just spectacular, especially when using my dbx-224X noise reduction system. I have stunned more than a few skeptics when they hear the dynamic range and recording quality of the RT-707 when used in conjunction with the dbx gear.
However, recently I began to notice a couple of odd problems.
First to appear was what appeared to be a small amount of dropout in the left channel at the very beginning of a tape. The volume of the left channel would drop and appear to fluctuate slightly. Once some quantity of tape had been wound onto the take-up reel this problem seemed to go away. I found that by applying the slightest bit of pressure to the supply reel with my finger, thereby effectively increasing tape tension, the left channel would return to normal. This got me thinking that perhaps the back tension on the supply reel was
too low, despite having previously set it using an Extech 475040 load gauge.
The second issue only occurred when the amount of tape on the supply reel was getting low, within the last 10 minutes or so at 7.5ips. The speed of the tape began to vary, and this was very audible as the music began to warble and suffer from what sounded like chronic WOW. After some research on this I found many suggestions that this can be caused by back tension on the supply reel that is
too high.
Now, it seemed unlikely that the back tension was going to be both too low, and too high. But something was clearly not quite right.
Originally when I performed the various tension adjustments I used the Extech load gauge 'hand held'.
I got to wondering just how accurate this would actually have been. The Extech is very sensitive and any movement of my hands would affect the reading.
So instead I decided to have another go at doing the various tension adjustments using a much more stable mount for the load gauge.
So here is my machine ready for some service.
And with the outer case removed.
And for those that have not done this adjustment. The two large green coil wound resistors with the movable taps is how the tensions are set.
The adjustments are clearly defined in the Service Manual.
This is how I decided to verify these measurements. The Extech has threaded mount holes on its reverse.
I found a fine thread screw that fitted and mounted the Extech on my camera tripod. This allowed me to carefully position the Extech directly above the supply and take-up reels so that the string pulled straight on the gauge.
I soon discovered that my original measurements were not at all accurate, and were generally too high. (Despite what the first image above might suggest)
It's almost as though the settings have drifted somehow?
Anyway, by using a good firm mounting for the load gauge I was able to get good repeatable settings for the tensions on both reels.
Here is the Forward Playback back tension on the supply reel. The spec's say between 70 and 80 grams.
And here is the Reverse Play tension on the supply reel. The specs say between 140 and 160 grams.
And now about to perform the tests on the take up reel.
The measurement shown is the Forward Playback tension on the take-up reel. The specs say between 125 and 135 grams.
Just a tip for the unwary. When making any adjustment to the taps on the big resistors, be sure to power down the machine AND pull the plug from the mains supply.
There are some BIG voltages available on the circuit boards around those resistors!
So, I then moved on to test the machine. To my horror both problems I described above were still evident!! Say What!!
Clearly something else must be going on. After some more thinking and research I began to wonder if the Capstan Pinch Roller was doing it's job properly. I had replaced the Pinch Roller rubber tyre during the overhaul with a new one I had obtained from a European seller. These new replacements were not pure rubber, but rather some more modern NBR rubber based material which are supposedly resistant to "most of chemicals, oil products, lubricants and most solvents and also weather conditions, including UV radiation resistance additive". These items are just the replacement tyre which you need to fit to an original Pinch Roller wheel.
So, I took off the Pinch Roller and inspected it closely. To my surprise the rubber had developed a pronounced indentation where the tape ran! As a test I then simply put back on my original Pioneer Pinch Roller, which was OK but a bit old. To my amazement the speed and dropout issues disappeared. So it appears that the problem was simply that the tape was not being pulled evenly by the Capstan at the beginning of a tape, whilst at the end of the tape the load was too great allowing the tape to slip slightly causing the speed variations.
Here are some images of the 'new' Pinch Roller after a year of light use.
Here is a crop from the last image above. You can just make out the channel that has formed where the tape has been running
Since discovering this I have sourced an alternative supply of quality Pinch Rollers and their rubber tyres. These new units are from Germany and include new bearings, beautifully machined alloy Roller with quality rubber tyres that are very accurately fitted and are very round (most important). They are not inexpensive, but are very well made. I have been using these 'newer' new Pinch Rollers for a little while now with no sign of any issues (Yet).
I'm hopeful that I have resolved these couple of problems, and have also now set the tension adjustment more accurately. All in all, another educational exercise.
Best Regards to all,
Trevor