Rickenbacker / Rickenbacher model 59 Lap Steel Guitar Amp from 1939

Here's a delightful old timer. Norman (Sorry Norman, couldn't resist).  Anyway, he brought an amp over for a look...

It's a 1939 Rickenbacher (note pre-war spelling here, although there's no markings on it.), made in the US and designed for 110V 60Hz power. Norman had just finished restoring the lap-steel guitar that it came with, as they were sold as a kit.







The amplifier is a very simple design, featuring a 5Z4 full wave rectifier, a 6N7 double triode, one triode as an input amplifier, the other as a phase-splitter, and a pair of 6L6's in push-pull output.









The power supply deserves some mention. The speaker does not have a permanent magnet, so there's an extra set of windings in the back of the speaker, which forms an electromagnet. This winding is fed from the power supply, after a small smoothing inductor and capacitor. It's in series with the actual amplifier HT rail, so the current supplied to the amp, passes through, and energises, the electromagnet. Now, all things being equal, this is a stupid idea... but it's put to a clever use. There's some rippe on the HT supply, as the smoothing caps are quite small in value. Here's the clever part... the amplifier is busy humming away, but the field coil is so connected as to effectively cancel out the hum! So not only does it improve the filtering (as it's an inductor itself) but also cancels out a lot of hum. Once working correctly, this works fantastically!

So, how's it look? Well, those capacitors will be no good after 77 years. They're out.

A note about those resistors... they're dog bone resistors... and were about 20-25% accuracy when new. They don't have the normal striped colour code. There's an article on Radio Museum about them here.

All the resistors check out within 20%, and I'm surprised!

Main smoothing caps, "Beaver" brand!

The internal connecting wire is cloth-covered and in good condition. The mains lead is rubber, and is also in reasonable nick, but will have to go anyway.. read on!

Here's a close-up of the speaker. The tiny output transformer is mounted underneath it. On some pictures of this amplifier, I've seen the speaker the other way up.

After a recap, and a check of the integrity of the transformer insulation with a 500V Megger test, I gently power the amp up via the variac to 110VAC. Nothing, not a jot. There's heaters and HT, but no audio. There's no volume control (or indeed any controls of any kind!) but there is nothing coming from the speaker.

Checking around shows there's HT on the anodes, so the output transformer primary isn't open circuit, and the field-coil in the speaker is OK. A check on the secondary shows that's not open circuit. Damn, it's the voice coil....

The chances of getting another speaker are about a million to one, so I CAREFULLY disassembled it, lifting off the field coil, and gaining access to the voice coil. Sadly, I negated to take pictures of this process. I could see the break where the coil connects to the back of the cone. Carefully, I tinned the wire, and soldered it back to the remains of the wire on the cone. I check the resistance of the coil with the meter. 18 Ohms... Bingo! I painted the joint with a blob of lacquer to give it some insulation and mechanical support, and left it to dry before reassembling.

Powering up again, there was some very slight "blow" through the speaker. I plugged in a guitar and gave it a strum. Horrible distortion. It was clear that the voice coil was fouling on the field coil inside the speaker. I slacked off the 4 slotted screws holding the coil assembly onto the speaker basket, and tapped it slightly to one side. A couple of goes later, and it's working well. The level of white noise (or "blow" as it's called in guitar amps) is minimal. There's no hum whatsoever! That anti-phase field coil arrangement is working well!

Electrical safety, or lack thereof...

This amplifier has a metal cabinet, and metal chassis.... and a 2-core mains lead. It's transformer isolated, so I'm ok with that, but that metalwork needs to be connected to a safety electrical earth. I remove the rubber 2-core lead and fit a modern 3-core PVC lead. The owner has a 240-110V auto-transformer to supply power, so that mains is still "live" and not isolated. I fit a 3-pin american plug to the end, and, for peace of mind, perform another insulation test. It's all good. Sadly, although not unexpectedly, as the chassis carries the signal and HT grounds, adding a safety earth has introduced a small amount of hum pick-up. It's not significant though, and better to be safe and humming, than dead and not. As this amp was designed for 60Hz operation, there is some concern that the mains transformer could saturate when being run at 50Hz, and get hot, resulting in failure. I ran the amp for a few hours, and the transformer was barely warm. I pronounce it fit, clean up the years of dust, and Norman is delighted!

Arty valve shot (still getting the hang of the new camera!)

Norman has kindly supplied some pictures of the guitar.

Arduino 1.6.x compiling slowly ... and I mean S-L-O-W-L-Y

This has been really bugging me... the simplest of sketches taking upwards of 2 minutes to compile, seriously adding to the amount of coffee consumed, and the level of frustration in the workshop.

Googling provided a couple of pointers, non of which improved matters. 

The current Linux IDE doesn't have some of the nice features I like in the windows IDE, so that wasn't an option. I don't do Apple.

After much buggering around , it turns out my anti-virus is the cause of the delays.

I use Avast! Anti-virus, because it's free.

Here's how to fix it. 

Open the Avast! user interface, and click on the settings button (looks like a cog).

Click on Active protection.

Now click on Customise by File System Sheild.

Click on Exclusions and Add.

and add in , or browse to the Arduino directory, in my case C:\Program Files (x86)\Arduino\*

now Click OK, and enjoy the rapid compiling we were once used to. 

Coffee anyone?

Roberts RM33 Desktop Radio repair. Freezer spray on the cheap.

My good friend David pops by a few times a year to bring things. My wife dreads it ....

This time he brings me a small desktop Roberts radio. I like Roberts stuff, it's nicely made and sounds reasonable.

"Goes quiet on medium wave"

OK... let's take a look....

The first battle is to get the thing apart.
First, remove the two philips screws on the edge of the back, on the left and right. Then remove the slotted screw in the centre.

Once you've done this, slide the card back upward and it'll pop out at the bottom with some gentle persuasion. Disconnect the green aerial wire from the board. Now unscrew the five silver philips screws from the bottom. Remove the power supply first, followed by the chassis.

Having removed the chassis, I set about fault finding. I previously had run the set, and after about 5 minutes MW sensitivity drops off and the volume decreases dramatically. On higher frequencies, it faded to nothing. Most likely to be a thermal fault. I left the set running until it had faded. Now what we need is some method of cooling components individually to prove which is at fault.

Now, you can buy, at quite some expense, Freezer spray, which is designed for the job. But Doz has a cheaper solution!! Enter the "Lazer" Air duster aerosol. Available for £1 a can from Poundland! Bargain. Now this produces a blast of gas, ideal for blowing the dust etc from speakers, laptops etc... here's the trick. Attach the pipe, and invert the can. Now a stream of extremely cold liquid will emerge from the end!  There is a warning on the can not to do this. It's that cold it will probably give you a cold "burn". Use caution!

Gently push the nozzle and direct the liquid at the suspect component.

Be careful when using it around higher voltages, as water will condense on the cold component. Never spray it on a hot valve, you'll crack the glass.

I started by cooling the board in areas to narrow down the search. I used a hot air gun to warm it back up again to cause the fault to reappear.

I narrowed the fault down to two caps, or maybe the
BC548 transistor. I changed the caps and the fault remained clear! The replacements are pictured.

There was no physical sign of failure of the capacitors.

Another job done!

Kerr McCosh DS1 & CWA 10 amplifier restoration.

Oh, what a thing of beauty!

Plexiglass illuminated front panel. Switchable tone controls and EQ for phono pre-amp. Two CWA 10 amplifiers, each push-pull EL84's with ECC81 phase splitter, developing 10 watts.

Came in with a missing EZ81 rectifier, so one was duly ordered and fitted. The power amplifiers, despite missing a rectifier, has been overhauled not long ago, and were in reasonable shape, although the second amp was missing it's safety earth, relying on the earth from the pre-amp via the "Belling-Lee" interconnect. Easy to sort that though...

The pre-amp was in a shoddy state. A good few hunts capacitors in poor condition, as well as a few Wima caps, which had started to show signs of physical degradation.

Channels are laid out symmetrically, one above the other, easily accessible once the covers were off. All the valves (5x ECC83) bar the tone amplifier are mounted on a sub-chassis, isolated by rubber mounts.

 Some of the Wima coupling capacitors had little copper shields...

A nice touch to keep the signal path shielded.

When I extracted the capacitor from the shield, it had a distinct oily feel. Not good.

The individual phono EQ's are mounted on B9A bases, and are removable. Nice touch if you want to change filters. These filters are in-circuit regardless of input selection, there's a switch to bypass should you require.

Cathode by-pass capacitors have had it. You can see where the electrolyte is escaping from the one of the left, and corroded.

Recapping with quality Vishay parts, should ensure many years of trouble-free service to come.

An on initial tests there's a problem. There's a switch to eliminate the tone control. It's fine when set to Flat (bypass), but when the tone control is in circuit, one channel goes dead.

This has me scratching around for hours....

I sketched out a diagram, and went fault finding. On the working side, there's a nice 230V on the anode of the tone amp, 1.5V on the grid and 3.5V on the cathode. On the duff side there's 280V on the anode, and nothing on the grid or cathode. It's just like the thing isn't drawing any current. I try another ECC83. No change :(

I check the AC path. It checks out. So what else could cause this? I checked the valve base to see if the anode connection was damaged. It wasn't.
Now the ECC83 is a double triode. (AKA 12AX7 Click here for details). That's two triodes in one glass envelope. This is the only valve in the whole amplifier that shares left and right audio. One side works , one doesn't....

The ECC83 is suitable for 6.3V or 12.6 heater operation, as the heaters can be wired in series or parallel.... a quick look in the top of the valve when it's on shows only one heater lit!!! I check the valve base wiring, as the two are wired in parallel pins 4 & 5 should be shorted together, and they are. The valve base isn't very accessible. I check the wiring, and it shows pin 4 has never been soldered! Easy fix restores my sanity.

So, back to the power amps to sort out the lackadaisical 60's electrical safety issue of no earth, and restoring the mains switching operation to the pre-amp which has been previously by-passed. The amps had been disconnected for about 30 minutes at this point. I put my hand in to remove the poor mains wiring, and received a most unpleasant surprise! I uttered some words my mother would not approve of...  A cap was left charged up to about 350V, and I'd brushed against it. Someone, in times passed, had replaced one of the power supply caps, and not refitted it's bleeder resistor. The bleeder resistor is there to ensure the capacitor discharges in a few minutes to a safe level... to "bleed" the charge away. I usually work on about 1000 ohms per volt. I fitted two 390K 5W resistors across the capacitor. See the warning and disclaimer at the top of the website? Read it again. I had become complacent. Don't let yourself fall into this trap. Thankfully I've lived to tell the tale ....

On a brighter note, it's all reassembled, and sounding really good. It really does command a presence.

QED Mains Interference Suppressor 6/4C

This arrived in the workshop, smelling foul.

It's a mains filter. A simple LC network in line with a mains plug and socket. There's an X-class capacitor on the output side, which had obviously had enough. The stench was horrible. On with some gloves, and clean the thing up!

A new capacitor was soldered in, and the filter reassembled.

These caps have a hard life. It's difficult to date them, as the same Rifa capacitor is still available today.

Apologies for the appalling photo. I've got a new SLR camera on the way, which will hopefully improve matters. 

Video rack video modulator repair and warning! Model CCT811

Remember the video rack I built a while ago? It's details are here.

Well, it's developed a fault. The UHF modulator has been giving very poor pictures after a few minutes of operation. I decided to take it out, and repair it.

IF YOU HAVE ONE OF THESE UNITS, I WOULD HAVE SERIOUS RESERVATIONS ABOUT IT'S ELECTRICAL SAFETY. 

I purchased it ages ago from eBay, and it came fitted with an unfused euro plug, which I cut off, and fitted a standard UK Plug, and fused it at 1 amp. I'm glad I did... read on ...

It's a useful thing. Is stable (when working properly), is adjustable throughout European VHF allocations, as well as UHF, and has switchable FM sound between 5.5 MHz and 6 MHz sound sub-carriers....

Disassembling the unit reveals a few horrors....

Check the mains input to the diminutive switched-mode supply ..... No fuse is evident!

What you can't see by this picture is the bottom of the case, the black bit. It's metal. There's no earth, which would be fine if the thing met Double Insulated (Class II) standards, which I doubt it does. You can read about classes here.

There's also zero filtering on the mains input, so any noise from the switched mode makes it's merry way back down the mains lead and out onto our mains, spoiling our radio reception (and, ironically, our TV reception too!) if it radiates (which it will).

Grim. Glad I fitted a fused plug ....

Examination of the power supply shows it to be nothing more than a simple blocking oscillator.

There's a simple zener on the output attempting to provide a little regulation. You can see where the board is a little discoloured, as it's been running warm. There are two capacitors in the primary, the mains smoothing capacitor (4.7uF 400V) proved to be very low in capacity. I fitted a 10uF , as I had one to hand. I also changed the smaller cap (10uF 50v) whilst I was there.

 A quick check shows the unit to be working again.... but what to do about the safety issues?

Mounting it back in the rack, I've fitted an in line filter and fuse (100mA). I'm not so concerned about earthing the case, as the modulator is inaccessible when the rack is assembled, that doesn't mean you should be though!

Pity really ... "for a ha'p'orth of tar" an otherwise good unit is spoiled.... and possibly electrically unsafe and a fire risk. It carries a CE mark, which I've no doubt it doesn't deserve.

 This gives you some idea of the size of that supply transformer, that's a 1p piece!

The guilty parties. Caps. As usual!

Pye "Black Box" with Garrard 209

Just a quicky this one.

Derek called "My record players gone quiet"

No problem, bring it over...

A lovely Pye Black Box duly arrives..

These are a super player, much better (in my humble opinion) than the Dansette of the same era. This one dates from the early 60's, and is a rare one, as it sports the Garrard 209 deck.

A quick diagnosis, but a fiddly fix as one of the fine wires had broken from the cartridge to the amp, just at the pivot point of the arm.

The deck, for a change, wasn't gummed up with sticky grease, and worked fine.

The audio quality is very respectable indeed.

Here's a quick video.

Ferguson 3629 "Personal" Dual-standard monochome television receiver restoration.

It's nearly my birthday, so I thought the wife may turn a blind eye to a "new" TV. There's a sort of "gap" in the collection with regards to 60's dual-standard (405 line/ 625 line) monochrome sets. Then this turned up....

 It's a Ferguson 3629 "Personal" Television, using the Thorn 900 Chassis.

It's a compact set. The 900 chassis is often referred to as the "cool" chassis, designed to use less energy. It features no wasteful "dropper" resistor to obtain the correct voltages, but a mains auto transformer.

 This particular set dates from 1965, and has the early valve UHF Tuner. Later models had a much improved transistor UHF tuner. The VHF Turret tuner can be seen on the right, the UHF tuner on the left. The UHF tuner has the valves enclosed in small screw-on caps.

 Diminutive Mazda CME 1101 CRT

Red EHT rectifier tray. This is semi-conductor, and is mounted on top of the very reliable "jelly-pot" line output transformer.

The system switch can be seen running the whole length of the chassis. This is moved by a switch on the front panel, which moves a flexible cable, which in turn operates the system switch, and a switch on the VHF tuner.

Initial results are encouraging. Very poor frame linearity, and hold. Usually caused by a poor PCL805 or it's cathode decoupling. Not in this case, however! As usual, it was down to a capacitor on the secondary of the frame output transformer. Bob the bodger had previously been at work around the frame stage, and had fitted two replacement resistors (of the wrong value!) and a cap (of the wrong value!) After these were replaced, results on 405 lines were encouraging.

Initial results... poor linearity and frame lock.










UHF 625 operation was not good. Signals are available from my test rack at UHF channel 40, 50 and 60. In addition to signals on 36 and 68 from the house distribution system.

Reception on Ch 36 was OK, if a little grainy. Channel 60 was available strongly, although was extremely touchy to tune, and Channel 68 was good. Where are 40 and 50?

 UHF tuner removed...

 Valves inspected...

 Fine tuning mech cleaned and lubricated. Be very careful not to move any wiring!

 Apparently the small disc ceramic capacitor can crack, it's just to the left of the red coil... it's OK in this tuner.

It turns out that the PC88 is the guilty party!

 Good pictures on 405....

... and 625. After a few minutes some hum starts to disturb the sound and picture.... It's pretty obvious a re-cap of the set will be required...

Quite a few caps are in physically poor condition, or electrically faulty and are replaced, along with the PC88....








The set then functions well, so I re-install the chassis into the cabinet. The set then suffers with intermittent frame collapse (no frame drive), leaving a horizontal line on the centre of the CRT....

After much probing about with an insulated screwdriver, I spot something arcing on the scan coils...

... it's a thermistor (or rather was!). It's turned into a blob of something... I don't have a replacement (and I've got precious little chance of getting one), so I shorted it out, restoring operation.







And here's a video of the set working...