Tuesday, 22 December 2015

Panasonic SC-HTB10 repairs.

Ugh ... soundbars. No. Not for me. A decent amp and speakers, why not? But the thought of an amp and speakers placed in a tiny enclosure to put under your telly, to improve the (usually) woefully inadequate speakers in a flat screen ... anyway ... rant over.

This is the Panasonic SC-HTB10 sound bar. It belongs to a colleague.

"It just clicks. Can you have a look?"

Yeah , why not...

Dis-assembly involves removal of the myriad of screws from the back, there are 4 hidden under rubber bungs on the corners. The back fits really tightly, I needed to gently pry it free using a "spudger" (one of those cheap plastic tools you get for taking mobile phones apart).

 This is the only time I've ever had to take a photo of a piece of kit in panorama mode!

You'll need to disconnect the sub-woofer which is in the back...

Neat tuning port!

Unplug the other two speaker plugs (blue and white, just by the green circuit board)

You should now be able to lift the electronics clear from the case.

Now, I'll just draw your attention to the disclaimer up there on the right of the webpage. Yep, that one. I'm doing that, because this one caught me out a bit. Under the pressed steel shield is the power supply (actually two supplies). Because of the symptoms, my guess was it was failing to start up. I carefully removed the shield and removed the supply.

Now, we can see there are two switched mode supplies. There's a small switched mode transformer above the date code, and a larger one below.

The small power supply supplies the micro-processor and control electronics. It's always on, so the remote is kept powered. This appeared to be functioning correctly.

The larger power supply, and the components in the area marked "HOT" are used to supply the amplifiers, This is where the fault lay. Despite the fact that this supply had been disconnected for many minutes, the large electrolytic still held an impressive ~300V DC. Whilst I avoided getting a shock, I was surprised to find this charge still remained. Having safely discharged the cap with a watty 10K resistor, I started a few checks. See how dis-coloured the board is around the switched mode controller and chopper IC 5701? Thing's have been running HOT! (The HOT marking on the board is to warn you that this part of the power supply is connected to live mains, and is not isolated! Not the temperature of the electronics!) Poor old C5726 had suffered as a result. It's ESR had risen to about 25 ohms... far too much for this capacitor to be in any health. A replacement was fitted and restored operation!

Another piece of gear saved from landfill 

Thursday, 10 December 2015

Selmer "Futurama" Corvette repairs.

Float back to the 60's (man..) and Selmer was producing some really good "Truvoice" amps. Quite expensive though... then in the early 60's (around 1963, to match their Futurama guitars) they released some budget amps.

This has wandered in to the workshop...

It's a series two amp, dating from 1965. Schematic can be found here. It's a simple enough circuit, two ECC83's and EZ80 rectifier and an EL84 output valve. 

Cosmetically, it's in really good nick...

There's a little storage compartment in the back for the mains lead and tremolo foot switch.

This one's not so good electrically. Switching it on, it just hums loudly.

The amp is constructed on a small PCB, mounted on the rear of the front panel. The output transformer is on the left of the chassis, and the mains transformer on the bottom of the cabinet. It has a three core mains lead, and is nicely earthed, unlike the WEM copycat!

Removal of the chassis for service requires removal of the four screws attaching the front panel.

Investigation into the hum shows the main HT smoother cap to be totally unservicable. Once again the dreadnaught lets me down, you can't win 'em all....!

To preserve the look of this lovely amp, we'll re-stuff the original 32+32uF cap with modern equivalents...

First of all cut around the bottom of the can. I use a sharp craft knife, scoring round until the blade is able to pierce the aluminium can. Wear some rubber gloves.

Next, warm the can up with a hot air gun in a well ventilated area, and get a large screw, and screw it into the capacitor. (This is a concrete screw, they're really good for putting up shelves! No wall plug required!)

Grab the screw with a pair of pliers and gently pull to extract the insides...

Now make up two modern 33uF (450V in this case) capacitors, and solder them to the base. The can is the negative for both caps in this instance (it usually is, but best to check!)

If you can't get the leads through the original rivets, drill them out (or drill holes though them) so you can pass the leads through.

A lap of PVC tape to prevent any nasty shorts catching us out....

And get the whole thing together in a vice so we can re-attach the can to the base...

Here I've used a little Alusol (aluminium solder) to tack the can to the base, so there's an electrical connection there. It's a bit of a pig to do, and you'll definitely need some fume extraction, that Alusol gives off some really nasty fumes.

Finish off with some hot melt glue...  and tidy up with a craft knife.

Once this was replaced, I was rewarded with a hum-free amplifier... but that wasn't quite the end of the story. The tremelo didn't work (Why, oh why, am I always plagued with tremolo circuits?!?!)

A few checks round showed a 100K anode load resistor to one half of the ECC83 (V2, the tremelo low frequency oscillator) was very high in value, having risen to ~4 Megohm. Replacing this restored tremelo operation ... but I couldn't switch it off ! The wire to the foot switch proved to have a break in it somewhere.... replacement sorted that. A quick squirt of de-oxit sorted any noisy pots out, and that completed the repair.

Ferranti 146 Radio.

A little while back, a colleague bought this radio to me...

"Can you look at my late father's radio? I've found it in the attic. I think it stopped working in 1964. I'd love to hear it going again"

... and, as regular readers will know, I love a challenge, and hate seeing this stuff go to landfill....

It's a Ferranti 146, manufactured from around 1946. Electrically, it's almost identical to the 145, and the circuit can be found here. 

The case has a nasty crack across the bottom, and it's missing it's back and two knobs. The mains wiring is lethal, and there's a lot of rubber insulated wire that the rubber has broken down, and cracked, so that will require replacement, but it's complete....
Here's a really bad picture of the underside of the chassis...  There's plenty of horrible wax caps, one "traditional" electrolytic, and, (you can *just* make them out on the right hand side of the picture), some electrolytic caps, in wax covered cardboard boxes! , a quick check shows these to be really badly leaky. No matter how long the ole' dreadnaught is left connected to these, they're never going to be any good. Replacement is the only course of action. 

Warming the boxes up gently with a hot air gun (not too hot!) allows the wax to soften, and the old capacitor to be removed from the box. It's best to allow plenty of ventilation when doing this. I'd hate to think what the chemicals given off will do to you! Rubber gloves are another sensible precaution.   
One the old capacitor is removed it's simple enough to solder a new capacitor to the wires, and slide it back inside the box. This preserves the "look" of the set. 

It's a run-of-the-mill job to replace the other capacitors with vintage looking modern equivalents, changing any rotten rubber wiring as we go...

It's obvious that there have been a few repairs made in the radio's long past. There's a "Radiospares" capacitor in there that dates from the late 50's. There's also the anode resistor to the audio preamp, which has been replaced, soldered at one end... and "wire-wrapped" with a bit of fuse wire at the other! A quick check and resoldering sorts that!
The chassis top is cleaned up a little, it's just a bit grimy and dusty. It's not rusted or pitted, so a clean is all that's required...

Checking for obvious short circuits first, mains is applied via the variac...                                                                              ... and almost nothing ....

Tuning up and down the medium waveband produces one very weak local station. (Note: This set has no ferrite or internal aerial, the green wire you can see connected to a socket on the chassis is a long wire, draped around the workshop) Checks in the RF stages, and testing the valves shows nothing is amiss. Alignment is performed as shown in the service sheet, and... bingo! Dozens of stations on Medium wave, a couple on long wave and a few on Shortwave....

Worth noting that the early versions of this set (and it's earlier brother, the 145) use a 6V6G output valve. This set is fitted (from new) with an EL33B. There's some global negative feedback added in to tame it's output.

After a while of testing, it suddenly crackled and went silent... everything appeared to still be working except there was no audio. There was nothing present on the anode cap of the EBC33. Wiggling the wire temporarily restored operation. The wire is screened, so a new one was fabricated from RG59 co-ax and fitted. 

Next thing to sort out is the dial-cord or string. This is in a mess. The tension spring is rusted and has lost most of it's springy-ness. At some stage the string has broken, and been shortened, which has meant it's strung around the shaft which connects to the tuning knob the wrong way round. It slips because of this. New drive-cord is obtained from eBay here, and a suitable spring found in the junk box. Re-stringing is not an easy task, and involves a lot of cursing, but we got there! It now functions smoothly. The string connects the knob to the variable capacitor, and via 4 pulleys to the dial pointer.

Some thin super-glue is poured into the crack in the case from the inside and allowed to harden. As the crack is along the bottom edge, it's out of sight.

The case is cleaned up using some Servisol "Foam Cleanser 30" (other foam cleaners are available, although this stuff is good!) and 69 years worth of grime from coal-fires, fags and lord only knows what runs off in a brown slurry. I thought the case was a dark brown Bakelite, but it's jet black! Quite the handsome set! 

Now to fabricate a back. I obtain a piece of hardboard (the type that's shiny on both sides) and laid the case of the set on it. I draw round the outside, and work out I need to make it about ~6mm smaller so it fits inside the recess of the set. Now some measuring to get the various holes in sensible places, and cut it out with a craft knife. I screw the back to a piece of stout board to stop it warping, and paint it with two coats of 50-50 PVA/water mix to prime it, and once this is dry, I lightly spray the back with some left over black paint. The set is transformer isolated, so the chassis is not connected to mains, so it's safe to operate without knobs until something suitable turns up. 

The result is not so pretty, but functional!
Here's a video of it in action...