TECHNICS 1500/1506 again!

[mossfred]

I seem to be having a headroom problem on the  playback of my RS1500, as soon as i am over 0db on the meters i start to run into clipping. If i reduce the output gain from its 0db position of 8 (3 o/clock) then it is ok.  Is this normal with this machine or is it maladjusted?  I have confirmed that  0db is  185nWb /m with one of my test tapes.
Fred

[ironbut]

As I recall, I never used the output levels over six. Eight is awful high and just a little under full output. What you're hearing may even be a combination of the machine's amp clipping along with the input device of your preamp being driven past the normal voltage swing.
If the level of your machine doesn't seem crazy loud at that level, there may be a problem with your tape or maybe something isn't correctly set or adjusted on your machine.

[Ki Choi]

Fred:

One of the most confusing issues for playing tapes is to understand the tape recorder's normal operating level.  Tell us more about the calibration tape you had used to determine that your machine is set for 0dB.

Ki

[mossfred]

As I recall, I never used the output levels over six. Eight is awful high and just a little under full output. What you're hearing may even be a combination of the machine's amp clipping along with the input device of your preamp being driven past the normal voltage swing.
If the level of your machine doesn't seem crazy loud at that level, there may be a problem with your tape or maybe something isn't correctly set or adjusted on your machine.

Hi Steve,
Its clipping on bass notes thru the headphone socket on the deck and the phono outs. The level is not high at all in fact i would say a little low! There is just no headroom whatsoever, my thinking is that it is somehow in the way the machine has  been calibrated?? I can put a much higher level onto the tape (over 0db on the meter) without distortion but to play back i would have to turn the output level down.

[mossfred]

Fred:

One of the most confusing issues for playing tapes is to understand the tape recorder's normal operating level.  Tell us more about the calibration tape you had used to determine that your machine is set for 0dB.

Ki

Ki,
Its an old reference tape that i have, but no details. When used on my Otari shows 0db on the low flux position.

Fred

[ironbut]

Hey Fred,
What you describe could be a lot of things (unfortunately).
The first thing I'd do is play a pre-recorded tape (or one that you've recorded on the Otari) to rule out a problem on the record side.
If you have a 1/2 and 1/4 track tape to listen to, you may be able to rule out one of the heads. It would be a good idea to examine all the heads to check for corrosion too.
If the sound seem bad on both formats, try adding some tension by holding a finger lightly against the supply reel as it's playing. If the sound cleans up and the volume increases significantly, the tension may need to be adjusted.
Before you do that, watch the tape as it moves through the head nest. Is it making full contact with all the guides and heads? Sometimes it's hard to tell but taking a q tip and pushing the tape in before and after all the elements can sometimes make seeing this a little easier.
Also, a common issue with these machines is dirty contacts on the head switch. Try switching between the 1/2 and 1/4 track heads about ten times and see if this helps.

Past those things, if you give us an idea of the level of your knowledge of electrical circuits you have and tools (voltmeter?) we can proceed.

[mossfred]

Hi Steve,

The deck records perfectly even going over 0db as long as playback is reduced to below 0db. Its not the heads, i have 3 headstacks, 2 and 4 track including a brand new one and apart from being slightly out for level the problem is the same. It sounds like i am clipping the input to the playback amp and reducing the output level solves the problem, but i know it shouldn't be like that, it should have perhaps 10db headroom before clipping?
My electronic knowledge is basic and i have tools, voltmeter and a scope! And i know where the rec/play amp is!!
Regards
Fred

[ironbut]

Hey Fred,
If you don't already have a service manual (both volumes) try and get one. You'll need to know the test points and voltages for troubleshooting.
It's always possible that someone has fooled around with the eq or gain adjustments so don't do anything radical till you can confirm these with the manual.
Just to be sure, work the monitor switch up and down a number of times and rotate the output knob while listening. If either has a scratchy sound, it may just need to be cleaned.

Past that, unplug the power, and remove the back and top.
Facing the back of the machine, the power supply is on your right (vertical board).
BTW, be sure that the switch above the power cord inlet isn't on battery power (I don't think that the machine will work at all if it is but I've never tried it).

There's a set of wires that leads from the bottom of the power supply board to a multi pin connector on the main control board beneath (large horizontal board behind the VU meters).
Try pulling that connector (use a little flat head screw driver and pry up a little at a time) and reconnect it (if you have some contact cleaner you might as well give the pins a little cleaning).
There's also a smaller connector immediately behind that one. Pull it and reconnect. Those connectors are the power pathways for the main board that's at the very bottom of the machine.
Try the machine again.
If that didn't help, your troubleshooting should probably focus on the electrolytic caps on the power supply board. Look for leaky caps. If the power supply is ok, you should wait to get the manual and go through the checks for the playback amp.

[mossfred]

Hi Steve,

Button received yesterday, thank you.
The issue of clipping on the replay amp is not resolved but only occurs when in the recording mode, so when in normal playback as opposed to monitoring a recording is doesn't clip at all even past plus 6db.

Regards
Fred

[ironbut]

Hi Fred,
Glad you got the button.

So, the only time the bass clipping occurs is when you're monitoring while recording.
But when the recording is done, the recorded tape sounds OK?

[mossfred]

Hi Steve,

Yes thats correct!!

Fred

[ironbut]

It sounds to me like when both the record and playback amps are processing signal at the same time, the playback amp is running short of current/voltage.
It could be something like a bad cap or regulator that's choking off the juice going to the amp/record board and the record amp is hogging most of it so the semiconductors on the playback board are under biased.
Just a WAG of course but I do know that both amps are powered from the same rail (maybe the tape bias too).
So, measure the voltage from the power supply to the amp board at the test points and connectors. There's a pretty good chance that you have a voltage drop that shouldn't be there.

[mossfred]

Problem solved. I was monitoring through my headphones on the deck. The output into my pre amp does not clip at all, so the headroom problem is in the headphone department!
Fred

[ironbut]

Hey Fred,

Good deal.
Those headphone outputs are usually pretty awful anyway. If you should decide that you want to use headphones to monitor during recording I'd suggest getting a decent standalone headphone amp.
I know a feller around here that sells a great headphone amp kit for around $229 (Bottlehead Crack amp).