CT-939 MkII Recording problem?

[QSilver]

I've recently been right through a CT-F850, all new capacitors and then carried out a calibration with a home made calibration tape that I verified with a number of other tape machines. I used a CT-F1000 to make the tape.

So far, everything has been great, and it sounds and makes great recordings which also playback just fine on other decks as you'd expect.

Until now... I got hold of a CT-939MkII. Now, it plays back my tapes just perfectly and I've been using it to play back tapes for a few weeks now. BUT.... When I record with it, weird things start happening...

I record something with the CT-939 and the instant playback sounds great. You play the tape again, sounds just fine. Now put it into any other tape player or for example my CT-F850, and there's a big drop in high end...

Maybe its misaligned? But how could it be if the CT-939 plays back tapes I've made on the CT-F850 just fine AND tapes it has recorded just fine too??

I tried passing some tones through the 939 as I was recording... 1kHz @ 0dB and 10kHz @ -10dB.

They played back okay in the 939 but in the CT-F850, 1kHz is on 0dB but 10kHz is at -15dB on left and -20dB on right....

So what's going on? Just need to calibrate the CT-939? Or am I missing something here?

All the checks were made with Dolby B on.

 

[hdrobien]

If tapes recorded on the F850 play fine on the F850 I would suspect an bad azimut or head position on either the 939 or both F850 and F1000, probably on the 939.

 

[QSilver]

I have played these tapes successfully on a number of other decks I own too. The 939 is the only one that stands out.

What confuses me is that the 939 plays both back fine and as I understand it, both the 939 and the F850 have REC/PLAY combination heads so the azimuth adjustment affects both REC and play heads?

I could be wrong.... I'm just a little cautious of touching the 939 azimuth in case it is correct?

 

[hdrobien]

To be sure, can you make the same tests with Dolby off ?

 

[QSilver]

Same tests with Dolby off, same results.

I recorded two tones too. 1kHz @ 0dB and 10kHz @ -10 dB

I set the tones levels based on what the CT-F850 fluroscan said which I have also calibrated.

Funnily enough when I fed the 1kHz tone into the 939 at 0dB, the same 10kHz tone at -10dB only showed up on its VU as -15dB....

Then play back the recording of 10dB from the 939 onto the CT-F850, and it barely registers -20dB....

Playing back the tones recorded on the F850 on the 939 yields 1kHz @ -2dB and 10kHz @ -15dB left and -20dB right...

 

[QSilver]

I ran my calibration tape through both decks now and on playback, they match and output the correct levels. 0dB @ 333Hz and -20dB at 10kHz, 333Hz.

So perhaps my recording calibrations are wrong somewhere? Or could the azimuth be out for recording even though playback is fine AND the playback and record heads are on one block with one azimuth adjustment??

 

[QSilver]

I might have found the problem, badly adjusted tape guide on the feed side pinch roller. It is adjustable and I think it was adjusted incorrectly along with the azimuth...

This tape guide seems prone to slipping. The nut securing it is often nylon...this one however is a brass nut...

 

[hdrobien]

Indeed that could be the reason.

You need a mirror cassette or a gauge to adjust its position.

If not, you can use a cassette with a transparent shell and remove the plastic around the tape path.

As a first step lets assume the azimut is correct and only fiddle with the tape guide nut.

Memorize initial position then turn it 1/4 turn clockwise. Check record and play on both 939 and F850.

 

[QSilver]

I made this from a small old tape. First picture is the 939 which to me looks fine. Next picture is the F850 after some adjustment of the guide. The tape wasn't creasing but it wasn't running parallel with the head. I think I have about got it now?