Thursday, 26 September 2019

The comprehensive Linn Valhalla repair guide.

John rang...

"Got a few Valhalla power supplies here that are defeating me, can you take a look?"

Yeah, why not.

Now these were the stalwart LP12 PSU once they'd decided that a crystal controlled supply was a good idea. Don't forget boys and girls; Linn ownership is all about upgrades, right?

Be aware all of this power supply is not isolated from mains. Use an isolation transformer during repair and fault finding. Make sure not to inadvertently earth the GND with your scope. You're no longer isolated if you do!
See the red bit in the disclaimer.

It a simple circuit. A 3.768MHz crystal is divided down to get a 50Hz square wave by IC3 (a 4060), and IC5 (4013) . This is switched through some control logic (IC4, another 4013), via the one preset on the board , to a square-wave to sine wave converter formed by IC2 (LM3902), which also provides some gain to drive Q1 & Q2 which drives the output stage formed by Q's 3 & 4. As the output is single ended it's DC component is removed by C18, and supplied to one phase of the motor. A phase shift capacitor is used to drive the other phase.

Most faults can be cleared by changing ALL the electrolytic capacitors, there are 7 in total.

C1,2 and 8 are 47uF 250V axial parts.
C3 is a 220uF 10V axial.
C8 & C15 is a 22uF 50V radial.
C14 is a 1uF 50V radial.

Use quality 105 degree parts. John had already done the caps ...

If that doesn't put it right, check for the presence of 320VDC on the positive end of C1 to GND. This is the HT supply. If that's missing, check the mains fuse (doh!). I've seen the bridge rectifier (BR1) fail a few times. If it's one of those odd looking square or rectangle things a Vishay W10G-E4/51 is an ideal replacement. R1 (47R) is also a frequent flyer!
A short circuit output stage will also cause violent fuse blowing!

Check for the presence of about 9VDC across C3. This is the LT supply. If it's missing, check R2 and R3, both 15K/5W for high or open. Change for a 7W part. They run damn hot. I always try and mount them a little off the board to allow for a bit of airflow. Some older boards can be quite scorched! IC1 can fail giving intermittent or no 9V. It can be replaced by MC3386/MC3346/LM3045/LM3046/LM3144 or an ECG912 (I don't think I've ever even seen one of those!) Z1, a 7.6V 0.25W zener diode, can also go short, open or drift. If you get an unstable or intermittent 9V, change this before IC1. If the 9V is being loaded down by a fault on elsewhere, generally an offending IC, which will reveal itself by getting damn hot! If you're really unlucky (and it has happened) I've seen instances where all the ICs have failed. Other symptoms of a poor 9V rail are inability to set the 85V up, and dim/no LED, even if the supply appears to be working.

Now the supplies are present, check for a 200Hz square wave output on pin 3 of IC3, if that's missing, the crystal or the IC itself may have thrown in the towel.

Square waves present? Good ... Now push the start button. 200Hz Square waves should be present on pin 12 of IC5. If they aren't there suspect the switch itself, or IC 4 or 5.

OK, you should now have a reduced amplitude 50Hz square wave on the end of R14 nearest the pot (there's a 4V DC offset here). If you haven't the pot has gone open (caught me out once!).

Scope the waveform on pin 14 of IC2, it should be a nice healthy 50Hz sine wave. If it's not, replace IC2.

Measure the voltage on pin 10 of IC2. Should be 1.9VDC with respect to GND. If it's not, check the value of R32 (560K). It often goes high, upsetting the bias of the output stage.

Still not working? Check Q1 (BC327, tends to fail open), Q2 (very rarely fails), Q3 and Q4 (either 2SC2501 or BUX84). The BUX84 has a nasty habit of going low gain, and causing issues. Measure it's HFe, should be over 20.

D2 very occasionally fails open, causing the motor to hum or not start. C19 causes the same issue.

The motor is often at fault on high mileage units! Check by substitution. Get a motor from Rega, it's the same and less money! Unlike the Axis, the motor will happily run all day without belt or platter.

Be careful when changing parts as the print quality varies from reasonable to awful. None as bad as the Axis!  There are many slight revisions to the boards, many have slightly different markings, a transistor can be marked Q,VT or TR. IC's U or IC.

Finally adjust the pot for 85V on the negative end of C18, or between the grey and red wires on the motor.

Now sit back, and enjoy some music.

Tuesday, 24 September 2019

LG GR-L206CPQA fridge freezer repair.

I'm sat in the lounge watching the news....

Ding-dong. Ding-dong. Ding-dong. Ding-dong.

Funny... that's the noise our fridge makes when someone pushes a button on the front panel.

I walk into the kitchen... it stops...

It appears our fridge is possessed (possibly by the curry I stuck in it's freezer compartment last night)

Ding-dong. Nope, it's altering it's fridge temperature settings on it's own...

I'll take a look before organising a priest....

It's an LG of some vintage. It wasn't new when we got it... ah, found the logo under some of the fridge magnets...

Let's get the front panel off... Grasp the panel and gently pull it off...

Unplug the PCB and retire to the workshop with the front...

The PCB is secured to the front panel with two philips screws...

... and removed shows the three tactile switches. One for freezer temp, one for fridge temp, and one switches the ice making thing into different modes.

Sure enough the fridge temp switch reads leaky, at about 40 kilohms. Checking the others show them to be better, but still a bit leaky at ~200 kilohms.

Sadly, I'm out of stock of suitable switches, so I order some. In the meantime, I give the switches a good dose of switch cleaner, which seems to fix it (at least temporarily, whilst the replacement switches arrive)

Another (and probably damned expensive thing to replace) saved from consignment to an expensive de-gassing and recycling scheme, if not landfill.