AG-7350 linear audio with high pitch noise?

[BartoloniDavy]

Hi, i have a Panasonic AG7350 (120 hours of life) .. i replaced all caps on servo board to have a nice and clean image (also counter was not working before).. but still having an issue woth linear audio ( the HIFI audio is popping out randomly and the quality is very bad)... i removed, cleaned and replaced the audio+signaltracking head, and i litteraly worn out the screws, with no changes to this high frequency hiss ( a lot more compared with first WAV posted on this topic)
if i can ask.. i have some questions (to you, or to who can explain me)

linear CH1 and CH2 are someting like LEFT/RIGHT? because i adjust the head making signals on CH1 and CH2 at the same level (this to be sure it's perfectly aligned)... but when i do this.. the HIFI output is nearly impossible to obtain, when i have a bouncy HIFI output, looking atthe linear output, one channel is highter that the other and the sound is loud...

i saw this post: https://www.digitalfaq.com/forum/vcr...distorted.html (that is 3 years old)

and the user TimTape was posting images about a external regulation for audio head..
not sure where the long screw is going .. on what hole? or is connected to the entire head assembly?

Tim if you are around, please

[latreche34]

You've just screwed up the factory alignment, never remove or change anything based on assumption before consulting or asking questions first, especially if the red/blue seal is still intact. Try to find a test tape and follow the service manual procedure, Using home recorded tapes would take forever and you will neve get it perfect assuming the problem is alignment and not failing electronics, Lastly never attempt to align a normal audio head stack with a tape contain HiFi audio.

[BartoloniDavy]

i bought this unit as faulty and there was a previous repair attempt (made using manual regulation) , solved (now) replacing caps, i was not worried touching these screws beacuse them are already "moved"...
BTW... i also used an HIFI tape to set best settings to "activate" the HI-FI.. and when i have a stable HIFI, the linear audio is not good.

[latreche34]

No, that's not how it works, Linear audio is adjusted separately and should not have anything to do with HiFi, Linear audio is adjusted with a tape that contain linear audio only. Never set the VCR to HiFi and tweak the linear head stack, this is wrong. It's like driving a car forward with reverse mirrors only.

[timtape]

With the 7350 you can be switch between linear and HiFi using the switch at the front under the removeable panel. So you dont need to keep swapping HiFi and non HiFi test tapes. Just use one test tape containing both linear and HiFi tracks.

As latreche says the head should be set up as per the service manual. As you show in your photo, there are many screws on the A/C head baseplate. Before proceeding we need to know what each screw does. The head "tilt" angle is difficult to get right once it goes out. To get the tilt right usually we need a special alignment tool. Once all these angles are set, they dont normally need altering ... unless like me you care about getting the best out of the linear audio. Then only one screw normally needs adjusting to tapes where adjusting will improve the linear audio and only the linear audio. It's the "azimuth" screw. That fine adjustment should not affect other parameters.

Because I regularly adjust azimuth to the linear audio on different tapes, using the custom fitted adjuster I made, the screw thread gets much more work than normal. Normally these screw threads are not lubricated in the factory. So I've lubricated the thread with grease. Even after much azimuth screw adjusting I've never had a problem with a stripped screw thread.

As I recall, one of the baseplate screws is a locking screw. To alter one of the other screws the locking screw needs to be unlocked first. Failure to understand this may have been the reason you damaged the threads on other screws.

You asked me which screw is the one I fitted my custom azimuth adjuster to. If you had understood which screw did what task you would not need to ask me that. Do you understand what I am saying?

Best wishes,

Tim

[BartoloniDavy]

very thank you for the infos, these was very helpful
right now i have a decent volume on CH1 (ch2 is mute in this situation) .. talking about of linear audio, the strange thing is that the HIFI audio is still very difficult to obtain, maybe i can have perfectly working on a tape and completely not present on another one (hifi tapes) can be the tape position (on cassette) able to make HIFI not working?

can i have an idea of the position of the nut (the largest one used to remove the audio head assembly) ?
mine right now is at the same height of the screw thread

[BartoloniDavy]

today i re-checked all caps on the servo board and i found that there are 2 C40 capacitor... ( !!! )

an high quality picture is here: http://www.wcn.it/c40.jpg

never seens something like that maybe there are different board revisions? thi is the PAL one

[hodgey]

It's how panasonic labeled things - the boards are sub-divided into sections with 4 digit numbers, and the silkscreen around the components themselves just show the last 2 numbers so the actual numbers would be like C6040 and C2040 or similar.

If hi-fi audio is troublesome maybe the tape path alignment was also messed with or affected by tweaking the linear head. That will affect hi-fi unlike adjusting the A/C head. Also keep in mind there is no auto-tracking on these so to get hi-fi audio to be good you have to manually adjust tracking to each tape.

ALso for adjusting linear audio, keep in mind this VCR has a split stereo linear audio head unlike most vcrs which has a single mono one so trying to align after a tape with a mono audio track from a standard vcr might not be optimal or give audio more on one side than the other, you ideally want to use a commercial tape which typically has linear stereo audio and set audio track output to linear.

[timtape]

Yes good advice. The nut labelled 5 can be used to carefully raise or lower the AC head without affecting other adjustments. But if for example the head is tilting a little back, CH 1 may be weak even when the head height is correct. Head tilt is usually the most difficult angle to align.

[BartoloniDavy]

i readed everything from you guys.. and today i tried doing some tests... and seems that adding a ground to main mechanical tape adjustment (or audio head... or support of erase head... anything related to tape rolling) make some of the noise disappear...


here an image of spectrum (GOUNDED.jpg) ... and an didea on how i placed the ground wire. (ground.jpg)

and i think that the 16khz nosie is the hiss still present.

[BartoloniDavy]

and this is a comparison, a cheaper JVC...and the AG7350

[latreche34]

Doesn't your video drum missing a ground brush in the center? Most VCR's with the stator on the bottom have the ground brush on the top.

[timtape]

My AG 1750 has no grounding brush on the top. I think this is normal for this model.

[BartoloniDavy]

yep, there is not... ground drum is used when the motor is in bottom part.. and i think is necessary to discharge electrostatically the heads...

[latreche34]

Timtape said it is not suppose to be there, But by all means you can give it a try, just use a jumper wire and gently touch the center shaft and see what it does.

[BartoloniDavy]

i will do today.

[hodgey]

Quote:

Originally Posted by timtape

My AG 1750 has no grounding brush on the top. I think this is normal for this model.

Yeah the head drum carbon brush sits below the drum on these rather than on top.

[BartoloniDavy]

no chnages grounding the drum... right now i placed (aligning the linear audio tape) the maximum volume sound on CH1... i increased the amplification using trimmers on A2 board... and using the DOLBY i have a sound without any high frequencies hiss.. but (the DOLBY ON) a little bit "muffled" maybe when i have time i will recap all the A1 and A2 boards.

[BartoloniDavy]

on the ZIP there are 10 seconds of every attempt

AG7350 rollers not grounded
AG7350 rollers grounded
AG7350 grounded with Dolby and amplificated
CHEAPER JVC

[timtape]

There is always some background hiss on analog audio tape although Dolby reduces it somewhat. But if you want to demonstrate the background hiss or other noises give us a quiet section on the tape, preferably with no music or any other sound.