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...

1275GT - the rust continues (to be eliminated!)

Well, back in August the car came home (link here) , and it's been languishing back in the garage awaiting re-charging of funds.

I decided to bite the bullet and get a proper whole front panel. It wasn't cheap, but the front valance was shot. I was going to sped a few hours repairing it, but it had gone round the holes in the front badly, and that was going to be difficult to get right.

 Measurements are made, and spot welds drilled out.... there's some unpleasant bodgery around the headlamp surround. Thankfully the wings are sound.














... and the front comes off,

 The new one is fitted...











... took some cussing, and help from the wife....

... and is finally welded into place.











Now for paint ... (more funds required !!!)

Marantz PM6004 repair

A super chap called Richard from work collared me

"I've got a Marantz amp, intermittent low volume on the left hand channel, usually when changing between speakers A and B , and it only happens on one set of speakers..it's too good to throw out. Can you take a look?"

Yeah ... why not?

 Nice looking modern amp...

 ... and very nicely constructed.

 We're going to have to get the main board out, which is a bit of a struggle as usual, as all the sockets are connected straight to the main board. But before that, let's see if the amp will misbehave for us... Yep, one of the speaker protection relay's doesn't always make it across. They're only rated to 5 amps, which seems a little low for my liking, so some 10 amp relays are obtained, and fitted.

 Here are the guilty parties. Actually only one was faulty, but I changed both to be on the safe side. A check of the bias (it was fine) and a good soak test. An easy fix.








When I returned the amp, the guy said it had always done this since new, and had been returned under warranty twice. Each time it was returned stating "no fault found". I would have thought it was obvious, once the customer's speakers and wiring were eliminated from the equation...