Oxford Crystal Reference restoration

[peter]

Here are some more pictures of the Prem-4 having removed the covers.



Power supply transformer, btw all the blue caps are for power supply smoothing and filtering, each is 1300uF 350V. They are wired in series pairs with 100K/2W resisters in parallel with each one. The plate voltage is 500V.



Output transformer, these are supposed to be special, don't know the details.



Labels are peeling off due to age.



Right, I have 30 years experience in electronics related fields, have a circuit diagram and I am still crapping myself at taking the covers of this thing. What I am about to do could kill.

Underneath now.

The PCB is double sided but not plated. No solder resist so hand soldered. No silkscreen. Not impressed, especially as I notice what might be a wiring error (white/red wires go to a green wire and not what appears to be the pin intended for connection).

Most of the wiring on the right is from the output transformers. The amplifiers is ultralinear so 5 wires per transformer.

the potentiomters are for setting the bias for each output tube. I think Kudos for using decent size pots for this and not presets.

First thing check the power supplies.

The iron items on the right are 320mH, 600mA DC chokes, these are wired between the 500V smoothing caps (2) and a seperate pair of filter caps for each channel. The "regulated" or rather "isolated" supplies come from the latter with each input tube having its own "isolated" output.

The isolated voltages are 20V higher than they should be. will need to look into this.

The heater supply is top left near the orange cap, This was not working correctly, half the rectifier has blown up (seems a common problem for me), replaced it on this side of the PCB. Now the input tubs glow nicely.



here is the dead rectifier



One of the channels had blown its plate fuse, most likely culprit is a cathode resistor which is 20R wirewound. By measuring the cathode to ground resistance with the tubes removed should find it, and sure enough we do. But to replace it means getting at the other side of the board and that means de-soldering some wires. I desoldered the left hand side ones after making lots of photos. This is quite scary. Before messing with this always check the big caps are discharged before approaching. Fortunately the resistors across the caps discharge them fast and the resistors are in good condition.



Crikey, theres a lot of expensive caps in here.



I am using an I-pod for source, it works well. It works fabulously well with my friends I-phone, but I don't want to blow that up.



Hopefully you can see the blown cathode resistor. near to it you can see why it went bang. Things have a habit of falling through the holes onto circuit boards in my house. This is one bad thing about circuit boards without a solder resist. Going to examine the rest of the board for FOD.



These presets are to match the gain in the phase splitter. No need to touch them yet.



Near the bottom is the circuit that allows for the bias setting. There is 1M between the cathode and the LF353 input and a 6V2 Zener to ground. I confess I don't really understand how this is supposed to work. Presumably the zener is there to prevent a big excursion taking out the op-amp.



Power supply isolator previously discussed. The high value brown resistors tend to drift, and these 560K resistors actually measure close to 700K. I will remove them and place them on the other side of the board for easy replacement later, maybe when this warms up the will be correct. The other resisters measure OK. This might explain the elevated voltages I read on the isolated supplies.




I think the 50uF caps are there to keep the input tube heaters alive when the power goes off long enough for the HT supply to discharge.



20R W/W resistors are also used to connect the screen grids to the ultralinear tappings.



Input valves go here



I have removed the Shakeproof washer and the blown resistor at this stage



Ok after re-assembly, I am using only 4 of the 8 output tubes. Each transformer tapping is driven by two tubes in parallel. We should be ok with just one tube so we have 50W instead of 100W, at least to check the amplifier is working.

At this stage I have not replaced the plate fuse (they are Very Expensive, only have 3 spare). The amp will actually work with a very small current (there is an LED across the fuse that tells you it is blown, the current through the LED is sufficient for the amplifier to give a very low output when it works. Of course the bias is then wrong so the LEDs are lit.



Plate fuse is in now, amp is running.



Lights out



Start to put some covers back on and audition, while thinking about getting new parts. Those W/W resisitors are outside spec and maybe should be replaced. The high value Allen Bradleys are easy enough to find.



It sounds very good, not sure replacing the Vishay W/W for KOA Film type is worth the risk, open to suggestions.

[peter]

Here are the Conrad-Johnson Premier four Schematics

[MartinT]

So what is the consensus on noise quality? IOW if you have to have noise, what is the most euphonic kind? (ie thermal vs shot vs 1/f etc).

Noise such as white noise can act like dither and be quite benign. In fact, I don't mind a little of this type of noise at all. What I cannot stand is hum, thankfully eliminated from my system since I swapped out my PS Audio GCPH phono preamp for a Whest.

In most cases, the S/N ratio is such that you shouldn't hear much noise when playing music. Very old CDs and some LPs are the exception.

[peter]

...So the subject of the Marantz CD-94 arose and I tried to photograph it, as I personally think it is rather nice looking. But none of my pics really brings out the stealthiness of it except maybe the third one. The wooden finish is very dark in the flesh.



It is deceptively simple having the toys under the flap. Unfortunately the phones socket and FF/REW is also there.



Here I am testing it in the AV rig. It has a nice tendancy to disappear.



The belts have turned to licorice, of course.



I have stolen some from dead VCRs so it just works for now, but, any idea if a belt kit (4 belts are needed) is available for reasonable money? I am pretty sure it is a Mk1 and FWIW has a vanilla TDA 1541 (no A).

peter.

[Rare Bird]

I have stolen some from dead VCRs so it just works for now, but, any idea if a belt kit (4 belts are needed) is available for reasonable money? I am pretty sure it is a Mk1 and FWIW has a vanilla TDA 1541 (no A).

PM

[DSJR]

The mk1 indeed - like my Micro Seiki is based on. Drawer belts the correct size are available from Cricklewood Electronics. While the belts are disconnected, make sure the drawer glides in and out very freely, as if the drawer assembly is bent slightly, as mine was, the friction becomes too high and defeats the loading belts I found.

[peter]

Cheers for the belt info chaps.

There is considerable leeway in size it seems, so I expect these from cricklewood should be fine:

2x RD038 (1.2x38mm) 1x RT040 (2x41mm) and 1x RT051 (2x51)

total about 2.10 per set + p&p.

I found the drawer was a little stiff due to dust. Cleaned and wiped on some WD40 and we're back in business.

The player is a very early version, after looking at various Philips and Meridian players and not liking them at all I bought the Marantz within an hour of seeing it. Now it just blows me away how solid it is, how fast it it is before it starts playing, and how it sounds.

In stock form it does not do badly against the 840C (that just went out the door), although I think the soundstage on the 840C is better, I prefer the warm tone of the Marantz. I am shocked actually as to how good it is.

First via the D/A in the Denon, It took 2-3 hours warm-up before it would image or soundstage or reveal any real detail though. Now warmed up and using its own D/A it sounds a hell of a lot more musical.

This warm-up time is baffling. Need to get the scope out and investigate it.

I have the Service Manual in case anyone needs it.

peter.

[Reid Malenfant]

First via the D/A in the Denon, It took 2-3 hours warm-up before it would image or soundstage or reveal any real detail though. Now warmed up and using its own D/A it sounds a hell of a lot more musical.

This warm-up time is baffling. Need to get the scope out and investigate it.

Interesting you say that Peter

My previous transport (Theta Data Basic) actually had the digital electronics powered up all the time, even when the unit was shut off Proof of this was that the "lock" LEDs on both DIP24/96 interface processor & DAC were always lit. Both the DIP & DAC can only be switched off at the rear so effectively they are on all the time also

My Krell DT10 appears to shut everything down as lock LEDS go off

Now i'm just wondering if it's worth modifying the Krell to keep those digital circuits running & at a set temperature? I have been thinking about this for a while

Nice looking player by the way

[Spectral Morn]

Cheers for the belt info chaps.

There is considerable leeway in size it seems, so I expect these from cricklewood should be fine:

2x RD038 (1.2x38mm) 1x RT040 (2x41mm) and 1x RT051 (2x51)

total about 2.10 per set + p&p.

I found the drawer was a little stiff due to dust. Cleaned and wiped on some WD40 and we're back in business.

The player is a very early version, after looking at various Philips and Meridian players and not liking them at all I bought the Marantz within an hour of seeing it. Now it just blows me away how solid it is, how fast it it is before it starts playing, and how it sounds.

In stock form it does not do badly against the 840C (that just went out the door), although I think the soundstage on the 840C is better, I prefer the warm tone of the Marantz. I am shocked actually as to how good it is.

First via the D/A in the Denon, It took 2-3 hours warm-up before it would image or soundstage or reveal any real detail though. Now warmed up and using its own D/A it sounds a hell of a lot more musical.

This warm-up time is baffling. Need to get the scope out and investigate it.

I have the Service Manual in case anyone needs it.

peter.

Its the TDA 1541 DACs they improve when fully warmed up. I have a CD94 mk2 it takes a week to reach optimum performance. Once up to full performance it sounds excellent in the bass and mid and its only in the treble where things are not quite so good. It can sound a bit gritty on cymbals etc.

So far mine keeps working drawer opening etc with no problem.....go figure.


Regards D S D L

[Ali Tait]

Cheers for the belt info chaps.

There is considerable leeway in size it seems, so I expect these from cricklewood should be fine:

2x RD038 (1.2x38mm) 1x RT040 (2x41mm) and 1x RT051 (2x51)

total about 2.10 per set + p&p.

I found the drawer was a little stiff due to dust. Cleaned and wiped on some WD40 and we're back in business.

The player is a very early version, after looking at various Philips and Meridian players and not liking them at all I bought the Marantz within an hour of seeing it. Now it just blows me away how solid it is, how fast it it is before it starts playing, and how it sounds.

In stock form it does not do badly against the 840C (that just went out the door), although I think the soundstage on the 840C is better, I prefer the warm tone of the Marantz. I am shocked actually as to how good it is.

First via the D/A in the Denon, It took 2-3 hours warm-up before it would image or soundstage or reveal any real detail though. Now warmed up and using its own D/A it sounds a hell of a lot more musical.

This warm-up time is baffling. Need to get the scope out and investigate it.

I have the Service Manual in case anyone needs it.

peter.

I wouldn't use WD40 for this,it dries out over time and the drawer will stick.