Sony U-matic VO5850 tracking and/or sync track issue - UK Vintage Radio Repair and Restoration Discussion Forum

collisr · 25th May 2021, 3:23 pm

Hello everyone.

Not seen this before on a U-matic deck. This machine I acquired has a play fault as shown in the video.

https://youtu.be/bcozuO1p0H8

I have cleaned the sync-head. Replaced the reel-drive belt, cleaned the capstan. The "servo" light on the machine remains lit. The tracking control does not seem to make any difference to the problem

Any ideas on what this might be? I have a junker machine to swap boards but do you think this would be the servo board or the video board (assuming I have eliminated obvious mechanical issues) ?

peterpixel · 25th May 2021, 4:15 pm

Hi Collisr, it looks like capstan speed is off slightly with the drop in rf off tape as the heads pass over the guard bands. I would trace the path from the control head through the capstan servo. The loss of tracking control is connected I think.Peter.

dj_fivos_sak · 25th May 2021, 6:01 pm

Hi there. If the control track pulses are present, you would have to look into the SV-31A board. Looking at the symptom, the capstan free-run speed might be a little bit off.

Fivos

collisr · 25th May 2021, 7:03 pm

Great advice guys. I'll have a look at that over the weekend and update you all.

Semir_DE · 26th May 2021, 1:10 pm

Hi collisr,

This looks like a loss of drum vs tape motion synchronicity. This indicates that the tracking servo is not operating. There has to be a fixed relationship between the head drum position and the helical tracks laid down on the tape at any given time. If this is not the case the video heads will not scan the tracks correctly. The tracking control knob changes the phase relationship i.e. momentary position of the video heads with respect to the helical video tracks. If the tracking servo is not functioning then this control will have no effect as in your case.

As has been pointed out already I would also suggest to check the signal path of the control pulses. There is a small magnet on the head drum. This generates a pulse for every revolution of the drum. These pulses let the servos know what the drum position is at any given time. I would also check for those pulses and follow their signal path. There is a good chance that e.g. the pulse from the head drum is missing.

To give more detailed advice I would have to look into the service manual to see how the capstan and drum servos operate in this machine. But checking the signals of the control pulses and those from the drum and capstan is a good starting point.

Good Luck!

collisr · 3rd Jun 2021, 10:51 am

Quick update for everyone:

Started the further diagnosing by following the capstan free speed adjustment supplied by Fivos. The total lack of wave at this test point implied there were no sync/ctrl pulses being received to the servo board.

I thought it unlikely the audio-ctrl head was at fault, so I replaced the entire servo board from a known good machine and the ctrl pulses into IC7 were all present and correct.

However, with this "borrowed" board, I am seeing a new problem. The tracking/RF-level meter continues to drop to zero every 10 seconds or so but the video output on the monitor now scrolls continually.

There are clearly multiple faults going on here and I'm going to put this machine back in the "tricky" pile for now.

Thanks for all your help though.

Rob

dj_fivos_sak · 3rd Jun 2021, 7:30 pm

Rob

If there's constant vertical jitter on playback, the head switching postition might need adjustment given that the entire board was replaced. Head switching should occur 6.5 horizontal lines before vertical sync. If switching occurs too early, you will have switching noise on the screen. If it occurs too late, you will lose vertical sync stability.

Fivos

collisr · 7th Jun 2021, 7:50 am

Ooh. Thanks Fivos. I did wonder whether I would need some alignments done if I swapped the servo board over.

I couldn't see a procedure for this. Is this documented in the service manual?

dj_fivos_sak · 8th Jun 2021, 1:56 pm

Yes it is documented. You can download it here:
http://freeservicemanuals.info/en/se...servicemanual/

Fivos

toprepairman · 8th Jun 2021, 2:47 pm

Hi Rob.

Do make sure the tape is travelling correctly across the audio/control head, if it's not doing so then you could have a pinch roller issue.

25th May 2021, 3:23 pm	#1
collisr Pentode Join Date: Oct 2016 Location: Ware, Herts, UK Posts: 171	Sony U-matic VO5850 tracking and/or sync track issue Hello everyone. Not seen this before on a U-matic deck. This machine I acquired has a play fault as shown in the video. https://youtu.be/bcozuO1p0H8 I have cleaned the sync-head. Replaced the reel-drive belt, cleaned the capstan. The "servo" light on the machine remains lit. The tracking control does not seem to make any difference to the problem Any ideas on what this might be? I have a junker machine to swap boards but do you think this would be the servo board or the video board (assuming I have eliminated obvious mechanical issues) ?

25th May 2021, 4:15 pm	#2
peterpixel Pentode Join Date: Jul 2013 Location: Wilmslow, Cheshire, UK. Posts: 210	Re: Sony U-matic VO5850 tracking and/or sync track issue Hi Collisr, it looks like capstan speed is off slightly with the drop in rf off tape as the heads pass over the guard bands. I would trace the path from the control head through the capstan servo. The loss of tracking control is connected I think.Peter.

25th May 2021, 6:01 pm	#3
dj_fivos_sak Heptode Join Date: Mar 2015 Location: Lefkada, Greece Posts: 969	Re: Sony U-matic VO5850 tracking and/or sync track issue Hi there. If the control track pulses are present, you would have to look into the SV-31A board. Looking at the symptom, the capstan free-run speed might be a little bit off. Fivos Attached Thumbnails

25th May 2021, 7:03 pm	#4
collisr Pentode Join Date: Oct 2016 Location: Ware, Herts, UK Posts: 171	Re: Sony U-matic VO5850 tracking and/or sync track issue Great advice guys. I'll have a look at that over the weekend and update you all.

26th May 2021, 1:10 pm	#5
Semir_DE Tetrode Join Date: Jan 2019 Location: Heusenstamm, (Near Frankfurt am Main), Germany. Posts: 59	Re: Sony U-matic VO5850 tracking and/or sync track issue Hi collisr, This looks like a loss of drum vs tape motion synchronicity. This indicates that the tracking servo is not operating. There has to be a fixed relationship between the head drum position and the helical tracks laid down on the tape at any given time. If this is not the case the video heads will not scan the tracks correctly. The tracking control knob changes the phase relationship i.e. momentary position of the video heads with respect to the helical video tracks. If the tracking servo is not functioning then this control will have no effect as in your case. As has been pointed out already I would also suggest to check the signal path of the control pulses. There is a small magnet on the head drum. This generates a pulse for every revolution of the drum. These pulses let the servos know what the drum position is at any given time. I would also check for those pulses and follow their signal path. There is a good chance that e.g. the pulse from the head drum is missing. To give more detailed advice I would have to look into the service manual to see how the capstan and drum servos operate in this machine. But checking the signals of the control pulses and those from the drum and capstan is a good starting point. Good Luck!

3rd Jun 2021, 10:51 am	#6
collisr Pentode Join Date: Oct 2016 Location: Ware, Herts, UK Posts: 171	Re: Sony U-matic VO5850 tracking and/or sync track issue Quick update for everyone: Started the further diagnosing by following the capstan free speed adjustment supplied by Fivos. The total lack of wave at this test point implied there were no sync/ctrl pulses being received to the servo board. I thought it unlikely the audio-ctrl head was at fault, so I replaced the entire servo board from a known good machine and the ctrl pulses into IC7 were all present and correct. However, with this "borrowed" board, I am seeing a new problem. The tracking/RF-level meter continues to drop to zero every 10 seconds or so but the video output on the monitor now scrolls continually. There are clearly multiple faults going on here and I'm going to put this machine back in the "tricky" pile for now. Thanks for all your help though. Rob

3rd Jun 2021, 7:30 pm	#7
dj_fivos_sak Heptode Join Date: Mar 2015 Location: Lefkada, Greece Posts: 969	Re: Sony U-matic VO5850 tracking and/or sync track issue Rob If there's constant vertical jitter on playback, the head switching postition might need adjustment given that the entire board was replaced. Head switching should occur 6.5 horizontal lines before vertical sync. If switching occurs too early, you will have switching noise on the screen. If it occurs too late, you will lose vertical sync stability. Fivos

7th Jun 2021, 7:50 am	#8
collisr Pentode Join Date: Oct 2016 Location: Ware, Herts, UK Posts: 171	Re: Sony U-matic VO5850 tracking and/or sync track issue Ooh. Thanks Fivos. I did wonder whether I would need some alignments done if I swapped the servo board over. I couldn't see a procedure for this. Is this documented in the service manual?

8th Jun 2021, 1:56 pm	#9
dj_fivos_sak Heptode Join Date: Mar 2015 Location: Lefkada, Greece Posts: 969	Re: Sony U-matic VO5850 tracking and/or sync track issue Yes it is documented. You can download it here: http://freeservicemanuals.info/en/se...servicemanual/ Fivos

8th Jun 2021, 2:47 pm	#10
toprepairman Tetrode Join Date: Jun 2021 Location: Leatherhead, Surrey, UK. Posts: 67	Re: Sony U-matic VO5850 tracking and/or sync track issue Hi Rob. Do make sure the tape is travelling correctly across the audio/control head, if it's not doing so then you could have a pinch roller issue.