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Old 26th September 2011, 01:21 PM   #431
SimontY is offline SimontY  United Kingdom
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Ok, reading around it seems probably any original CD should do for setting laser current, though setting focus offset ought to be done with the proper test CD (no idea where to procure one). I also read that the latter adjustment is not critical and if current adjustment doesn't cure a reading issue the laser is probably toast.
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Old 26th September 2011, 03:43 PM   #432
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I've just had a check of my laser current and focus offset voltages. The laser one read about 53mV so is within spec. I fettled the trimpot and couldn't get it closer than about 52mV but I don't think this is an issue...

The focus offset measured over 7V! It's meant to measure 400mV. I adjusted it down to 6.xxVDC and then ran out of adjustment. I think the 22K trimpot has gone faulty. With the player powered down I am seeing a low ohmic value between two pins and basically open (Megaohms) to the third. To me it does seem pretty broken. Please check my thinking someone: a pot should measure short->22K between any 2 pins, right?? There should be no meagohm values surely.

Once I've repaired all the point to point wiring from removing the audio board I'll have a listen and see if it's any better with respect to cutting out and skipping and being extremely sensitive to vibration.

Has anyone heard of these trimpots going bad?

Last edited by SimontY; 26th September 2011 at 03:53 PM.
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Old 26th September 2011, 03:50 PM   #433
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Anyone know the wattage rating of these trimpots?
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Old 26th September 2011, 09:03 PM   #434
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Well I've done some listening to a few CDs and it's sounding great and not skipping or dropping out, and doesn't overreact to physical knocks either. The adjustment seems to have worked well enough, for now.

In the picture the wire link with the probe on it shows where to test for focus offset voltage. The other probe goes to ground. You can also see the 4.7K (laser current adjustment) and 22K (focus offset adjustment) trimpots.

Simon
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Old 27th September 2011, 02:47 AM   #435
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Hi Simon,

The Rotel manual specifies a "Sony type 4 (YEDS 18)" test disc to use when checking laser and focus voltages. I haven't got one but used a normal music CD to reset my the Rotel''s voltages a while ago. The procedure is the same and test voltages are the same. The trimpots on Rotel are tin can type and adjust up through a hole in the board I have drilled holes in the bottom of the case to make this job easier (if I ever have to do it again). The pots look like 500mw type to me.

Did you change the trimpot on the focus offset? There shouldn't be any megaohm values there. For long term reliability I would change the pot, even though its a pain to get at.

Just a recent thought about doing mods under the transport without dismantling the player. I am going to try just removing the drawer from the mechanism and then taking out the whole swing arm and motor assembly and all its trays, just leaving the shell of the mechanism in place.

If the PCB of the Arcam is anything like the Rotel, it is a through hole plated type and the components, or their legs, can be desoldered and resoldered from the top of the board. I haven't tried it yet but will do so for the servo / driver regs.

My Rotel currently has 13 separate regulators with 4 transformers / PSU's, a new clock and big heatsinks, so I know what you mean about removing the PCB.

Joe
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Old 27th September 2011, 11:01 AM   #436
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Thanks for the response Joe, I will probably try to find a 500mW trim-pot when replacing the 22K one. I agree I should change it for long-term reliability.

I'd not fancy soldering things from the top personally, but maybe I'm just too clumsy. It's bad enough having about 6 wires soldered to the TDA's pins.

Simon
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Old 2nd October 2011, 03:21 PM   #437
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I installed a 7909 and 7809 regulator on each of the servo chips (TCA0372). It may be only wishful thinking, but it seems that the base has tightened up a little more and the presentation is just that little bit more dynamic. It also seems that instruments and voices are more clearly defined, or perhaps more 'separated' is a better description. The improvement is subtle, but my wife noticed it and told me it sounded a bit clearer than before.

The Rotel transport section is so similar to the Arcam and so it seems to me that the Arcam should show similar improvements.

I also added +ve and –ve 5V regs to the TDA8808 Photo diode signal processor (Arcam IC 6501) and the TDA8809 Radial error signal processor (Arcam IC 6503). However, I can't claim any perceptible difference in performance with this mod. It didn't hurt but it didn't help.

The only things left to do on my Rotel is to reclock the 68HC05C8 processor (Arcam IC 6530) and give it a separated +5V reg. Then I will rebuild the SPDIF out.

All of these mods are common between Arcam and Rotel.

I have included a photo of the Rotel, which has a total of 5 transformers, and now 21 regulators. The analog out is a FET stage based on a Nelson Pass paper and sounds better than any IC I have tried. The output caps are Mundorf bypassed with Russian teflons. Very nice.

By the way, here is a link to an interesting discrete I-V with onboard discrete regulators designed by Greg Ball here in Australia. It looks very good and has the benefit of having DC nulling so it is DC coupled and does away with capacitors. It's worth a read. I have used Greg's low-noise 'minregs' on my Rotel DAC and analog stage.
SKA sponsored Audio Forum - Discrete Stereo I-V with Onboard Discrete Regs

I'll let you know about the reclocking when I can get to it. Here is a link to AW DIY site with a nice little "miniclock" that should do the trick. AW DIY - miniCLOCK Version 2
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Old 19th October 2011, 10:39 PM   #438
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That's an interesting post Joe, your player must sound very nice now.

I've had a little time and inclination to look at my recently-stopped-functioning Alpha today. It stopped playing a few weeks ago and I thought it was due to the draw not closing right or something silly like that. Sadly it's not so simple.

It will occasionally read the TOC and once on the "test bench" it started to play a CD. This rules out broken PSU wires or badly broken signal connectors or tracks.

I removed the front panel to access test mode and got Error 2: No Track Loss signal at start-up. Not massively helpful.

I thought the laser was probably dying and I then remembered I had a CD52 and that had a CDM4. Sure enough it's even a CDM4/19 like in the Alpha, albeit visually subtly different in places. Anyway, the laser comes out with the sled and can be swapped so I did that... only to find the same fault. I then put my Alpha's laser in the CD52 and it works faultlessly! So my fault isn't the laser.

I'm now stripping down all my mods etc that are in the way of the "mono board", and I will remove that 22K trimpot that seems to be faulty. Maybe that is the cause of the failure, given that focus test point reading of 7 volts....

I just need to find a suitable trimpot to order now.

Simon
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Old 19th October 2011, 10:51 PM   #439
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Do I need 0.5W or will 0.15W do?

Looking at the service manual schematic I can't see why a good 10K pot won't do. It's just +5V at one end, -5V at the other and circuit feeds from the output somewhere between the two. Surely a good multi-turn pot will do an even better job, e.g. http://www.maplin.co.uk/square-22-tu...tiometers-2202

It may or may not have enough leg length to bend it over and lie it flattish if needs be.

Ordered a 10K multi-turn and a 5K to do the laser current pot if I fancy it.

Last edited by SimontY; 19th October 2011 at 11:14 PM.
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Old 20th October 2011, 01:00 AM   #440
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Hello Simon,

I think I would go for 0.5W as minimum and a single turn is sufficient. However, a multi turn will let you set the voltage more precisely.

A quick look at the RS catalog brings up part no 186-189, which is a 22k.
1 turn,linear, Cermet trimpot,16mm and 1Watt rated. It should fit staright in to your board, but you would need to check the pin spacing.

Neither RS nor Farnell seem to have the Bourns or Vishay .5W single turn types.

Anyway, see how you go with what you have ordered.

The other thing to keep in mid is how easy it will be to get at the pot with a screwdriver when you want to adjust it.

Joe
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