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Thursday, 24 October 2019

Hitachi HT-L33 turntable repairs.

I was given this by a kindly benefactor...


It's a Hitachi L33 turntable from about 1982.

"It plays too fast, I've tried to adjust the speed, 33 is just about OK, but 45 is wrong"

Great, this will do as a gift to my mate Alan, who's been on the lookout for a sensible small turntable for a while.

I check both speeds, which seem to be far too fast, and varying badly.

Here's the speed circuit from the manual...

The motor PWB (printed wiring board, don't ya know!) is housed inside the motor. It's a brushed DC motor, controlled by a small regulator on the board. Normally the speed is adjustable via a small hole, operating a pot inside the motor. In this case, that connection is made available outside the motor, via the orange wire.

The speed is first set to 33 RPM, whereby the FET Q01 is switched on by the micro processor, and is effectively short circuiting R42 (the 45RPM control) and R41. Once R40 (33 RPM control) is set, you can then adjust 45. Doing it the other way round won't work, as 33 RPM will also adjust 45! The whole motor and speed arrangement is fed by a regulated 9.9V supply from Q11. Q10 is there to rapidly bring the motor up to speed, under control from the micro.


OK, the pots are already set to minimum speed. Damn... why so fast ?

Incidentally, be careful to secure the platter in place with some tape (or remove it) before turning this upside down. If you don't the platter will hit the stylus and probably ruin your day.


Ah! The belt has ridden up the pulley! Effectively changing the gearing..

The belt is moved into the correct position and re-tried. It's now running too slow, and followed a few seconds later by too fast...

The belt had ridden back up the pulley.

Removing the motor shows that the motor mount hadn't been installed correctly.








Now it's running steady in the correct place, but too slowly.


















Using the cueing button, just moving the arm to the left starts the motor at 33, whilst we still have the lid up...

Adjust for 33.33333 RPM!

















Seems reasonable !
















and then press the speed select switch, and adjust for 45 RPM.

Close enough.















A quick look at the stylus under a microscope shows it's in good health, so off to Alan it goes!

An easy fix, and a stylish linear tracking deck. All Alan needs now is a phono pre-amp!

Another saved from landfill!

Sunday, 6 October 2019

Strathclyde STD305D power supply.

I'd popped down to Brighton for a few days, to see my good friend Edouard.

He's got a very nice looking Strathclyde STD305D ..




"Got it on eBay, doesn't work, and appears to have bits missing.. Fancy a look?"

Yeah, why not...

A super turntable, which uses a proper DC brushed motor, a tacho and proper servo control...

Those speed adjusting sliders you see there, are actually tiny thumbwheels, which move up and down the track, allowing precision adjustment of the speed. Nice!

There's a digital display (presumably) for speed, and soft touch controls.




Here's the control PCB...












And here's the motor, and what remains of the power supply....

Some bugger's had the transformer!

It looks like they had a hum pickup problem (or galloping paranoia) and moved it outboard. Edouard didn't have it...




Thankfully someone had carefully marked all the voltages on the board, the original transformer had a good few windings on it...

It was (allegedly) wound 18.5-9-1.5-0-1.5.-9-18.5 

This is not going to be so easy , and is likely going to need multiple transformers, as I did to get the Tascam Portastudio 488 going.

I quickly sketch out the supply schematic, and realise something odd's going on with the 1.5 volt side of things... it appears to supply AC off to the control board... The other rails are simply rectified, smoothed and regulated.

Remembering the Tascam again, I trace the AC from the 1.5-01.5 across the control board, and it supplies the filament for the Vacuum florescent display! Ah!

A couple of suitable transformers are ordered for the 18 and 9V supplies, and cobbled up in the usual lethal manner to prove the point. To supply the 1.5-0-1.5 , a 20-0-20 transformer has it's primary supplied by the 9V transformer.

The point isn't proved.. it's pulling the best part of 100W from the mains! 18V transformer has got damn warm.

Tracing the circuit for the 18V through shows whomever measured it in the first place got it very wrong. It's not 18-0-18 .. it's just 18 - 0 ... connected across the two 18V connections. It's not 0 at all.. it's a bridge rectifier circuit. Checking the 9V shows that's a bridge too, not a centre-tapped full-wave circuit as the note left on the board suggests. The other very odd thing is the four fuseholders on the board. They fuse each half of the bridge ... why?? The 1.5V is, however centre tapped.

Once this is sorted, consumption falls to a sensible 14W. Nothing gets warm!

Here's the diagram.

T1 and T2 supply the 9 & 18V supplies.
T3 is a 6VA 0-20 0-20 transformer, the primary of which is wired for 120V operation, but supplied from the 18V supply. It's output provides the 1.5-0.1.5 output for the filaments.

The output is taken to a 9-way D.





Excellent. Well, nearly. The touch selector isn't working. No matter what speed is selected. It's always 33 RPM. Disassembly of the front panel finds a 7400 which is rather warm, this is replaced and all is well :)
















 Touch sensors work perfectly, and the speed is stable


















Now to tidy up the rat's nest supply!















And done... :)

To sum up, a simple job which took far too long due to me trusting the incorrect work of two people... 1. The person who wrote out the label... 2. The person who laid out the power supply PCB incorrectly.

Nevermind...




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 12.8KHz 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.

Wednesday, 31 July 2019

Lindos LA102 test set repair.

Morning ... coffee ... switch the power to the bench on and contemplate the day's work..

... hang on, the ever-faithful Lindos LA102 Audio measuring test set fails to start up.

It's dead, nothing. Thankfully I have a spare.


A cursory check round shows the mains transformer to have failed open circuit...

Fair enough, it's been in there since the 80's...











A suitable replacement looks like RS 121-3839, and is duly ordered...









Now it physically fits in the hole, we'll need to switch the phases on one of the primary windings, and one of the secondaries.. The original transformer is wired 0-120 0-120, and 0-6 0-6, the replacement is 0-115 115-0 and 0-6 6-0.  I'm not worried about the 5V difference in the primary.

So let's deal with the secondary first ... This is a picture of beneath the transformer.

















Cut the tracks as shown.


















And connect two insulated wires to cross the tracks over...

You can now fit the transformer.

Next to deal with the primary. Cut the original link, and carefully remove the trace completely. Note I'm working on a 240V UK unit here... if you're in that funny bit of the world where you use 110 or 115V, you'll need to work it out so both primaries are in parallel.















Then add a link wire in to couple put the windings in series, and in the correct phase...

















... and reconnect the mains supply.


















Switching on, and it's clear something is not right. The transformer is humming and getting warm rapidly!  (It's never easy, is it??)

Turns out the bridge rectifier is short circuit as well... poor thing is cracked too...

It's replaced, and now the unit powers up, but it's still not right, as the power supply is still under some considerable  load...













... and the final piece of the puzzle is the ancient nicad battery is almost short circuit. I disconnect it, as I haven't ever needed it.
















Now, let's get on with some work!

Friday, 12 July 2019

Sharp RG-3915 In-car radio cassette repairs and restoration.

I bought this on eBay..



"Why?"  I hear you ask...













Because I've been spurred on by the warm summer evenings to finish off the green thing lurking in the garage... and it needs some suitable retro sounds. (Sorry if you're still waiting for the final Sony 9-306 YouTube video)

The RG-9305 is an auto-reverse radio cassette, released in 1983 (OK, it's a bit newer than my car, but hey..) It produced a trouser flapping 8 watts per channel (although some have appeared to have escaped.. read on!)

The seller assured me it was "working when removed" ... which I can only assume was sometime in 1985 ;) Thankfully I use the usual eBay translator for such. "Working when removed" means been sat in a damp shed for 30 years+ and the seller can't be ars bothered to test it. (For more eBay description translations check this out)

... and sure enough I'm not disappointed... the cassette deck doesn't work, and the radio is weak and distorted. The radio comes on for a few moments, then the volume fades away rapidly.

Let's tackle the tape first.

Take off the knobs and the securing nuts and remove the faceplate(s) ...













Remove the two screws holding the front panel. It should just pull away.. The remove the four screws to remove the top panel (two on the top in the label, and two on the front). Remove the top panel by gently prising it up with a small screwdriver.








The cassette mechanism is held in place with two screws on the front panel. Disconnect the motor/control plug adjacent to the motor, and the head wiring plug next to the volume pot.

Pop the eject button off (bottom left), and, although it's tight, you should now be able to extract the mechanism complete...

Excellent. Turn it over....















.. and remove the two screws holding down the capstan flywheel retaining plate.

Remove the two offending belts, find suitable replacements in the belt box, and replace them...









Yeah, they're shot ... ;)



















Here's an interesting manufacturing technique... real printed components...

It looks like R120, R62 et al are just printed blobs of some carbon type of material, not dissimilar to the "thick film" type of construction used in some TV's back in the day, but those were usually printed on ceramic, not ordinary PCB material. Even the tracks on this side of the board don't appear to be copper, but some form of conductive paint or ink. What if there's been some flexing and vibration going on and there's a microscopic track break? This fills me with dread ... let's see what happens.

On test and the output is low and distorted. There's about 3VDC appearing on the speaker outputs. Not good.

Reverse engineering the circuit shows the output amplifier is coupled to the speakers by two capacitors. They're leaking.











As there's a few others right close to the amplifier, I change the lot...












Ok... things are better, but there's still precious little output, and what there is, is still distorted and thin sounding. Tracing the circuit back from the amplifier, I get to a uPC1228 op amp. It's a dual amp, but in a single in line (SIL) package, and made from unobtainium.. The DC conditions around the amp are odd... There's a DC off-set on the input, which is building over the first second or so the unit is switched on. The audio is coupled in via two capacitors, and loaded by two of those printed resistors I mentioned earlier... and they prove to be open circuit.

An educated guess is made, and two 100K resistors are duly lashed up. One between pin 1 and gnd, and the other between 6 and gnd.

Audio is restored :)









The replacement resistors are properly mounted.



















Now the case has a chunk missing out of it, across the top where the cassette goes..













I used the remaining corner to bend a piece of tinned copper wire to get the outline shape. This is then flipped over, and warmed up with a soldering iron and melted into the plastic case...

It's rubbed down a bit, and covered with capton tape.










Some car body filler is mixed up, and used to fill against the tape. Once the filler has hardened, the tape's removed and the filler trimmed with a craft knife to the rough shape required.











A little more filler is added...













... and finally sanded down to get the final shape.


















A trip to a local paint supplier to get a matching colour ... a quick browse through the racks shows this Pastikote radiator paint looks a damn good match ...
















The front panel is carefully masked off...













... and painted. I'm rather pleased with the colour match and end result...
































The faceplate gets a coat too to get rid of a few scuff marks, and the unit mounted in an under-dash box ready for fitting


and I've got one of these bluetooth FM transmitter devices so I can play music from my phone! (£7.22 on eBay!)


Another saved from landfill :)