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!

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 :)