JVC HM-DH30000U suddenly stopped playing DTheater?

samson7point1 · #1 07-07-2022, 07:35 AM

I followed a Videohelp thread about getting help from lordsmurf.

Last night I popped a DTheater tape into my DH30000u and while it played the image started to break up every couple of seconds like it was having trouble reading the tape. Since I had never played the tape before I thought the tape was bad, so I swapped in a different known-good DTheater tape and it seemed to play fine for a couple of minutes then it started breaking up as well. Standard VHS tapes seem to play perfectly well.

I thought perhaps the DTheater tape I tried first had fouled the heads so I used a Sony SVHS dry cleaning tape and tried again with the known good tape, but it didn’t help.

Next I took the cover off and used a sheet of heavy paper and IPA to clean the heads. I got some dark streaks on the paper so I was expecting that I just had some dirty heads, but when trying it out after cleaning the problem was even worse - I can barely get a frame or two before the image completely breaks down. Again, standard VHS plays with no trouble.

This happened too suddenly for this to seem like head wear, but I don’t have enough experience with these to know the next logical step.

lordsmurf · #2 07-07-2022, 11:09 AM

If you press too hard, you can damage heads. But it sounds as if the heads were pretty bad anyway.

Main choices =
- new deck
- new heads, but good luck there

The D-VHS decks are pretty crappy at VHS and S-VHS playback anyway, compared to the JVC S-VHS decks with line TBC.

samson7point1 · #3 07-07-2022, 11:27 AM

Thanks for the quick reply. I've been pecking at it all morning and I managed to get it back to a state where it will play DTheater tapes without issues. I neglected to mention something I didn't think was relevant, but I think it's actually everything.

I was reading through the service manual about cleaning the heads and there was a caution about making sure the heads were complete dry before trying them out. The IPA was evaporating so fast that I assumed the 10 minutes or so between reassembly and re-installation would be enough for it, but I couldn't be sure, so I took the cover off and re-cleaned the heads then dried them with one of those power duster machines. I noticed that I wasn't getting any visible dirt from the heads anymore but I was getting stuff off of the upper drum so I scrubbed the drum until the paper was clean.

When I tried it with the cover off and the VCR sitting on the floor a couple feet in front of the TV rack, it played perfectly. I figured that I just hadn't cleaned it properly the first time (despite the fact I'm sure I did).

I put the cover back on, re-installed it on the shelf and it started acting up again. The thing I neglected to mention originally was it was working perfectly but I relocated it a couple of months ago and this is the first time I've tried to play a Dtheater tape in this location. When I pulled it back away from the TV and the other components it started playing properly again. Based on this and some of the other stories I've come across on the Web about this and the DH40000U I have to think the real problem was RF interference messing with the DVHS heads. Whatever it is it isn't interfering with the standard VHS heads. I'm not sure what can be done other than just putting it back in the location where I had it before - I mean the whole cover is metal and it's got like three large contact points. I'll probably try putting a layer of copper tape on the cover just above the drum, but I don't hold a lot of hope that'll make a difference.

lordsmurf · #4 07-07-2022, 12:11 PM

Quote:

Originally Posted by samson7point1

there was a caution about making sure the heads were complete dry before trying them out. The IPA was evaporating so fast that I assumed the 10 minutes or so between reassembly and re-installation would be enough for it,

That was fine.

Quote:

then dried them with one of those power duster machines.

Not good!

Quote:

I noticed that I wasn't getting any visible dirt from the heads anymore

Good.

Quote:

but I was getting stuff off of the upper drum so I scrubbed the drum until the paper was clean.

Scrubbed?

Quote:

When I tried it with the cover off and the VCR sitting on the floor a couple feet in front of the TV rack, it played perfectly. I figured that I just hadn't cleaned it properly the first time

Probably.

Quote:

relocated it a couple of months ago and this is the first time I've tried to play a Dtheater tape in this location. When I pulled it back away from the TV and the other components it started playing properly again. Based on this and some of the other stories I've come across on the Web about this and the DH40000U I have to think the real problem was RF interference messing with the DVHS heads.

Interesting, that's new. Yep. All of those "entertainment centers" are crap for video gear, heat, interference, etc. This is why pros use racks.

latreche34 · #5 07-07-2022, 02:42 PM

If you have S-VHS and VHS playback with no noise, the heads are not the fault, Faulty heads exhibit consistent poor performance regardless how many times you clean them.

Keep in mind that digital tapes known to loose signal over time and exhibit drop outs just like any other digital tape format such as DV and D8, I had few D-Theater tapes that exhibit drop outs at certain places of the tape, some are worse than others.

samson7point1 · #6 07-07-2022, 06:57 PM

Quote:

Originally Posted by lordsmurf

That was fine.

Not good!

Good.

Scrubbed?

Probably.

Interesting, that's new. Yep. All of those "entertainment centers" are crap for video gear, heat, interference, etc. This is why pros use racks.

The irony is that I had it in a rack before I moved it there. I assume you think the duster was a bad idea because of ESD risk? FWIW it’s a DC powered device, I was grounded and didn’t let it come into contact with the VCR, though I see your point. Scrubbed was a poor choice of words. I just used paper and IPA just like with the heads.

I don’t think the extra cleaning made any difference as I can 100% recreate the failure by moving it into the jungle under the TV. I’m probably going to buy a second unit just to verify whether this is a design problem.

samson7point1 · #7 07-07-2022, 09:12 PM

Quote:

Originally Posted by latreche34

If you have S-VHS and VHS playback with no noise, the heads are not the fault, Faulty heads exhibit consistent poor performance regardless how many times you clean them.

Keep in mind that digital tapes known to loose signal over time and exhibit drop outs just like any other digital tape format such as DV and D8, I had few D-Theater tapes that exhibit drop outs at certain places of the tape, some are worse than others.

I don't understand the tech well enough to do more than speculate. I think whatever waveform is being read off of the tape is probably in a frequency range that either overlaps or with HDMI or WiFi and is picking up some leakage from another nearby component.

I ruled out the tape degradation (at least for the tapes that I own) because the interference did not occur consistently (it happened at different intervals during subsequent plays) and of course when I moved the deck out of the path of the interference it cleared up entirely. But yeah, it's a thing, it happens.

samson7point1 · #8 07-26-2022, 12:36 PM

Wanted to follow up because my speculation about the RF interference turned out to be completely wrong. It played D-Theater fine for a while after I moved it, then it started acting up again. Then I noticed that it would only play D-Theater tapes with the cover removed. Turned out the power supply was flagging.

My working theory now is that it was teetering on having enough power to drive the MPEG2 decoder and the back pressure on the fan was increasing current draw enough to starve the decoder. It didn't have a problem with standard VHS because that didn't require the MPEG2 decoder but it was probably just a matter of time. When I first moved it out from the entertainment center that relieved some of the backpressure from the fan, and later keeping the cover off did the same thing.

I replaced all the capacitors in the power supply and now it's working much better. I've tested it for several hours to make sure.

RobustReviews · #9 07-26-2022, 12:42 PM

Quote:

Originally Posted by samson7point1

Wanted to follow up because my speculation about the RF interference turned out to be completely wrong. It played D-Theater fine for a while after I moved it, then it started acting up again. Then I noticed that it would only play D-Theater tapes with the cover removed. Turned out the power supply was flagging.

My working theory now is that it was teetering on having enough power to drive the MPEG2 decoder and the back pressure on the fan was increasing current draw enough to starve the decoder. It didn't have a problem with standard VHS because that didn't require the MPEG2 decoder but it was probably just a matter of time. When I first moved it out from the entertainment center that relieved some of the backpressure from the fan, and later keeping the cover off did the same thing.

I replaced all the capacitors in the power supply and now it's working much better. I've tested it for several hours to make sure.

Good fix.

Your hypothesis sounds quite plausible, given the age of these devices, it's probable the digital video circuitry is quite demanding on available power. It's also plausible these circuits are more troublesome loads for a weakened PSU to supply: compared to a DC fan at least.

This is one of the few formats I've never owned, I'm glad to hear another machine survives!