RS-1700 RTR questions

[leopardus2]

Hi there!

The RTR fever bite me and afer buying an Akai GX636 I have now acquired *two* Technics RS-1700 decks! Why two? I bought a very low mileage one on ebay and while waiting for it to arrive, I found another one on a local ad for 100 bucks in unknown condition, and since I have read parts are difficult to find, I thought for 100 bucks it was a good buy anyway!

OK now to questions...
First, these decks have two three-position switches for EQ and BIAS. The user manual is evasive at best about what the different positions mean... I am using RMGI LPR-35 tape, any advice on setting of these switches is welcome!

Second question - this may sound odd, but while recording at 3 ips and 3/4 I noticed that I have *way* more hf than the source! I mean, when I switch from source to tape I have an HF increase. THis does not seem to happen at 7 and 1/2.... question is, how is that possible that I have more HF than source?

Pardon my newbie questions. I hope some of the knowledgeable people of this forum can help me shed some light :-)

Thanks in advance,
Rick

[dr*audio]

I'll answer your question about the speed first. You get more highs because the bias is not properly adjusted for that tape at that speed. There are separate internal bias adjustments and EQ adjsutments for each speed and all 3 types of tape. Do not use 3 3/4 IPS anyway, because you will not ever get very good performance. Use 7 1/2 IPS or 15.
I am familiar with the RS1500 but not the 1700. I can't remember what the EQ settings were on the 1500 anyway :scratch2:. In general, it should be marked something like I II and III, or Normal, Low Noise, and Low Noise Hi output. I am not familiar with the tape you are using. My advice is to properly adjust the bias and EQ on the deck for that tape. You need at minimum an audio oscillator and a DMM that has flat frequency response on AC up to at least 20KHz. If you don't have those, you will have to take it to a qualified tech. If you have the equipment, you will find that you can't get flat frequency response at the wrong tape setting. When you find the setting that works, it will be obvious that this is the correct setting to use for your tape.

[leopardus2]

I'll answer your question about the speed first. You get more highs because the bias is not properly adjusted for that tape at that speed. There are separate internal bias adjustments and EQ adjsutments for each speed and all 3 types of tape. Do not use 3 3/4 IPS anyway, because you will not ever get very good performance. Use 7 1/2 IPS or 15.
.

Thank you for your explanation. Of course I don't have the instruments to calibrate the bias myself. I have made some more tests after posting. I noticed that I get some distortion on HF as well (and I am way below 0 db). This happens in forward mode only! not when playing in reverse mode. Can this be a head alignment related problem? Because although you are right saying there is internal bias calbration for each speed, I am not sure there is a separate setting for each direction... I have the service manual, will try and see if I am wrong..
But. based on what you explained, looks like the previious owner modified the internal bias settings from the default. What do I need to calibrate the internal bias to the original settings? Assuming that is possibile, of course...

An unrelated question: the deck #2 does not record the left channel. It plays fine what I record on deck #1 (the fully working one). This happens on both directions... what should I investigate?

Thanks again
Rick

[leopardus2]

. You need at minimum an audio oscillator and a DMM that has flat frequency response on AC up to at least 20KHz. If you don't have those, you will have to take it to a qualified tech.

Warren, I have a DMM which has a 0.08% of accuracy in the 40Hz-20KHz
(it is a professional Fluke model). I do not have the audio oscillator, but could eventually borrow one. What would the calibration procedure be? Do you think
I could re-set internal bias to my tape's specs? Is it hard to do it?

Thanks again

Rick

[dr*audio]

It's easy to do, just get the service manual and follow the procedure in there. You should really buy an alignment tape and set up the playback and head azimuth first. You need an oscilloscope to do that.

[leopardus2]

It's easy to do, just get the service manual and follow the procedure in there. You should really buy an alignment tape and set up the playback and head azimuth first. You need an oscilloscope to do that.

I got the service manual already.
Assuming heads are OK (I guess they are since it records and plays just fine at 7 and 1/2) I could try and calibrate the bias. I have verified there are 12 variable resistors, i.e. one pair for each speed and each direction. This actually makes sense, since I hear the HF increase and distortion in one direction only. So my assumption is that the previous owner played with the internal settings and did not care to carefully calibrate it for lower speed of 3 and 3/4 (which I still use to get six hours of music out of a single tape - yes, I know quality is not the best but hey you can't have it all...)

Back to the service manual,
I have a "Record Head overall frequency response" procedure (the 12 VR's,
two for each direction and speed)

It is something like: input 400Hz at -44db , adjust VR's for maximum line output,
then change frequency to 10 KHz, get same level of output, etc

So in order to just do this kind of calibration, I need an audio oscillator (which should allow me to select frequency and also volume in db) and a DMM which I already have, so is it correct that I should connect the DMM to the line out plug, switch the source/tape to tape and adjust the VR's to read the highest voltage on the DMM?

Thanks for your patience,
Rick

[SaSi]

If you have a PC/Laptop and CoolEdit Pro or Audition (I believe you can download trial versions from Adobe) then you practically have a signal generator.

The programs have functions to generate tones (sine, square,triangle for those who care) and any decent sound card gives a flat frequency response to it's output.

I have used that to generate tones for calibrating one of my machines that was way off.

[dr*audio]

You have to adjust the bias first. Basically it's a matter of inputting a 400 Hz or 1Kz signal at some level which they tell you, then adjusting the bias for the maximum reading on the DMM (in Tape Monitor). Then you are supposed to turn the bias cap in the same direction until the output falls a specified amount, usually 1dB. You do this first for all speeds and directions, then you do the record EQ, which is the procedure you described. You will adjust it so that a 400Hz and 10KHz tone recorded at -20dB play back at equal levels (not necessarily -20dB, because you haven't adjusted the record gain yet. You do the EQ cal for all speeds and directions, then you set the record gain do that a 400Hz signal recorded at 0dB plays back at 0dB, monitoring the tape.

[leopardus2]

You have to adjust the bias first. Basically it's a matter of inputting a 400 Hz or 1Kz signal at some level which they tell you, then adjusting the bias for the maximum reading on the DMM (in Tape Monitor). Then you are supposed to turn the bias cap in the same direction until the output falls a specified amount, usually 1dB. You do this first for all speeds and directions, then you do the record EQ, which is the procedure you described. You will adjust it so that a 400Hz and 10KHz tone recorded at -20dB play back at equal levels (not necessarily -20dB, because you haven't adjusted the record gain yet. You do the EQ cal for all speeds and directions, then you set the record gain do that a 400Hz signal recorded at 0dB plays back at 0dB, monitoring the tape.

Hmm I am afraid this is for trained people :-(
I have even difficulties understanding how can I read -2db on a DMM set to AC
voltage...
From the "overall frequency response adjustment" section:

1) 400 Hz, -44db of input in Line in
- this is easy
2) Adjust VR503 (L-CH) and VR-509 (R-CH) for maximum line output
- I guess I should measure AC volts on DMM and find the max value, easy
3) Change the input frequency to 10 KHz and again adjust the same VR's so that the Line output becomes the same value as for 400 Hz in step 2
- easy
4) Again change input freq to 25 KHz and adjust L101 and L202 so that the line output becomes 2db lower than that of step 2 above
- ??? if I am measuring volts here how can I find out the dB reading? Is there a formula?

I think it is better to leave it as it is... :-(

Thanks
Rick

[dr*audio]

INV LOG(-2dB/20) = .794 You would multiply .794 x your max reading to get the value that is -2dB. It's a pain in the ass to do. You could get a Keithley 197 DMM. It has dB and relative functions, so all you have to do is set it to dB, adjust for the max reading and hit relative. Now the meter will read 0dB and you adjust the coil until it reads -2dB.
You can get an analog AC voltmeter, they read in dB. You could buy a HP333A or 334A distortion analyzer, they can be used as an AC voltmeter, and they read dB. Then you'd have a distortion analyzer, too, a very useful tool. It all depends on how committed you are to the hobby, and to learning. Calibrating and restoring a reel to reel deck is a useful skill and then you could charge people to do the work and recoup your investment in the equipment. It's going to cost you as much to have someone else do this work as the equipment. Probably more. You can get the HP333A for $100 or less. The Keithley will be between $100 and $200, I think. Heck of a nice meter; auto ranging, 4 lead ohms measurement, 6 1/2 digit.

[leopardus2]

INV LOG(-2dB/20) = .794 You would multiply .794 x your max reading to get the value that is -2dB. It's a pain in the ass to do. You could get a Keithley 197 DMM. It has dB and relative functions, so all you have to do is set it to dB, adjust for the max reading and hit relative. Now the meter will read 0dB and you adjust the coil until it reads -2dB.
You can get an analog AC voltmeter, they read in dB. You could buy a HP333A or 334A distortion analyzer, they can be used as an AC voltmeter, and they read dB. Then you'd have a distortion analyzer, too, a very useful tool. It all depends on how committed you are to the hobby, and to learning. Calibrating and restoring a reel to reel deck is a useful skill and then you could charge people to do the work and recoup your investment in the equipment. It's going to cost you as much to have someone else do this work as the equipment. Probably more. You can get the HP333A for $100 or less. The Keithley will be between $100 and $200, I think. Heck of a nice meter; auto ranging, 4 lead ohms measurement, 6 1/2 digit.

Warren, thank you very m uch, you have been very detailed and explicative. RTR is not my only hobby, but I could certainly afford to spend $100-$200 in the Keithley, I will look for one. Learning this kind of things is my passion so no problem there :-)

Thanks again
Rick

[cabinover]

Wow, you are the second person I've ever heard of having an RS-1700. The other would be my Dad. He bought his new for the princely sum of $1000.00 in Germany when they first came out.

As for the lower tape speed wouldn't help sonically, why don't you simply buy an ipod for extended listening? I'm not being sarcastic or anything, just asking. I try for the best sound out of my GX747 and Tandberg.

Heck, that's why I'm in this crazy audio hobby in the first place although my wife thinks it's just to spend money :lmao:

Good luck with the adjustments!

[leopardus2]

Wow, you are the second person I've ever heard of having an RS-1700. The other would be my Dad. He bought his new for the princely sum of $1000.00 in Germany when they first came out.

As for the lower tape speed wouldn't help sonically, why don't you simply buy an ipod for extended listening? I'm not being sarcastic or anything, just asking. I try for the best sound out of my GX747 and Tandberg.

Heck, that's why I'm in this crazy audio hobby in the first place although my wife thinks it's just to spend money :lmao:

Good luck with the adjustments!

An IPOD? You must be kidding. As far as I know, the IPOD doesn't support 10" reels :-)

Rick