Junque Box Boutique amplifier - making the cabinet

Having completed the electronics for the amplifier (see post here), a cabinet was needed.

I liked the idea of a simple box, similar to the type I built for the workshop video rack, so I sketched out a simple design, and asked my friend Julian to cut me some 18mm MDF to the sizes required. Now I'm no carpenter, and Julian has got just the right kit for cutting things square!








I marked out the exterior of the joins, 9mm in, also marking the positions of the screws.











I clamped up each side, using a piece of right-angled aluminium bracket to hold everything square whilst I drilled pilot holes, and screwed each piece together.









After the outer 4 sides were assembled, I test fitted the chassis. Everything is upside down at this stage. Looking good!











A quick test fitting of the speaker baffle board shows everything is nice and square.











I remove the baffle board, and mark it's centre. A screw is temporarily inserted in the centre, and I mark out the cut out for the speaker...











... and cut it out with a jigsaw. An important note here. There are some health hazards associated with MDF dust, so it's prudent to wear a dust-mask when doing any cutting.










... and, after a bit (quite a lot actually) of fettling, the speaker fits snugly in the hole...










Having marked out the position of the amplifier chassis, I mount up 4 small brackets to the sides to hold it in place...










Works nicely ...













Another trial fit of the parts, and it's coming together...











I make up a small back panel, so I've got something to mount a cover on to, to stop prying fingers finding the HT on top of the power supply, and to give somewhere to store the mains lead and footswitch. Once this is screwed into position, it's time to decorate!















MDF is a bit tricky to paint, as it's so absorbent. If you don't use the right primer first, it will suck the paint straight in, and you'll come back and wonder if you ever painted it at all!

I carefully remove any dust with a rag moistened in white spirit, and paint it with MDF sealer....













.... and put it in the garden to dry in the sun....

... and the sun promptly disappears for the weekend! Great British weather, eh?

A couple of coats will be required, especially on the edges.







Now, the edges aren't quite flush, despite by best efforts, so I smooth them over with some car body filler.....

... and rub it down ....

A quick coat of primer, followed by some nice gloss black....




and assemble the electronics...



.... and the final article! (Actually I'm thinking about making a proper front panel...)

1971 GEC Colour 2103 TV repairs. 2040 Chassis

My colleague Nathan came into work. He's just moved house.

"There's an old telly in the loft. Do you want it?"

Yeah, why not....

I take the set home, much to the delight of Mrs. Doz











The cabinet has seen much better days, and it looks like it's been damp at some stage in it's life...











No model number, as the sticker's fallen off, but some research shows it to be a 2103, dating from 1971. It's a single standard (625 line PAL) colour set.

... it looks to be complete...












... and un-molested...












Removal of the line output stage screening can, shows the anode lead to the line output valve's insulation has failed, leading to a bit of a burn-up











... and there's a resistor on the side of the line output transformer which falls into pieces as soon as I touch it. This is replaced, and the wiring made good.









The CRT is tested...

... and doesn't look good initially, but after a couple of hours wait, the meters rise into the green as the tube wakes from it's long sleep!











Some mains is gently applied to the set via the variac... nothing. The mains fuse is open circuit, so a new one is fitted... and as the valves warm up....

... a blurry green raster appears ...

.... and after some tweaking, some rugby players appear. The convergence is awful, and the picture isn't in colour, but it proves that the set is viable...









The tuner is seized solid as always, so it's removed...

... and cleaned up...












Being careful not to stress those rusted in plastic nuts!












The tuner is refitted once it's all moving freely, and I start fault finding on the lack of colour.

I prevent the colour from being removed from the picture (for when black and white programmes were broadcast) by disabling a circuit called the colour killer. There's still no colour, so it's likely the reference oscillator isn't running...  Just as I'm about to start looking at that fault, the sound and picture disappear! The set is still operating, but there's nothing coming out of the IF (Intermediate frequency) stages after the tuner. Damn.

There are 3 stages of combined IF amplifier. These amplify and filter both the sound and vision the signals together, coming from the tuner. The signals are then split off, and amplified further, the audio being separated and decoded from the video. As both sound and picture went off together, I suspect the fault lies in the combined amplifiers.... I check all three stages, all appears well. Perhaps the tuner has failed? A super chap called Gary has a spare, but it makes no difference....

After some further checks, largely out of desperation, I check through the vision amplfier, and find there's an emitter follower amplifier, and the transistor has failed. It's a BF194 HF bi-polar transistor. A replacement is fitted, and pictures are restored! Still no sound. I look at the first sound IF, and , sure enough there's another failed transistor. A BF194... still no sound!... I check the ratio detector, which is used to demodulate the FM sound, and.... it's another failed BF194! Sound a pictures restored, I look back to the colour decoder.

I check the reference oscillator, and it's running fine, producing a locked 4.43MHz signal. Good. I check through the stages, and find 3 faults. Two of the faults are transistors in the R-Y (Red minus lumenance) and B-Y (blue minus lumenance) ... guess what .... BF194's. Also the colour bust transistor is open circuit... you've guessed this next bit ... it's a BF194...

There have been questions raised on various forums in the past about the reliability of BFxxx and BCxxx lockfit type transistors. It's quite possible that that un-reliability (they are 40+ years old!) is what's been at work here. It's possible that flash over when the insulation failed on the anode lead in the line stage caused a spike which saw off the transistors, but the BF197's and BF196's in the IF stage survived, as did other transistors in the decoder...

After replacing this little lot, and a couple of suspect and corroded looking capacitors, things are shaping up. There's some hum evident on the power supply, and I find a very sad looking cap in the power supply. It's got a nasty bulge in it... It's changed out.







I replace the line output valve and boost rectifier (PL519 and PY500) and the width is better, set up the convergence on the tube, so the three colour guns are lining up correctly, and I'm rewarded with reasonable results.

Note quite sure what I'm going to do about the tatty cabinet though...

The guilty parties....

Little microphone preamp.

Now, I've not been very happy with the sound on my videos... you may have noticed...

What's needed is an external microphone, maybe a tie-clip or a lapel mic.

Now examining my new camera, it doesn't have a mic input. Damn... I google a few things, and, although there is a guy on a forum who's added a socket in to the camera, I didn't fancy taking my new camera apart. Other people with the same camera seem to prefer to record the sound separately, on a zoom recorder, or similar, and line it up in the video editing software. Good plan.

Now years ago I bought an Acer MP3-330 MP3 player, fitted with a mechanical 10GB hard disk... Now that has a record facility, but the in-built microphone isn't up to much. It does have a line-in socket, however. So we'll need to make a microphone pre-amplifier.

Left over from the Arduino beat detection and lighting controller project I had a number of small electret condenser microphones. One of these would work nicely as a discreet lapel mic.

The circuit is a simple op-amp, driven from a single rail 9V battery. R1 is used to supply the bias voltage to the microphone. It should be around 7.5 volts. C2 is used to block this DC. R2 and R3 put the incoming audio from the mic onto a half-rail supply, which is amplified by the op-amp, having it's gain set by R4 and R5 in the negative feedback loop. C2 gives us some DC stability, but ensuring that the DC gain of the amp is unity, otherwise any offset would be amplified also. C3 removes the amplified audio from the half-rail DC on the output. R5 is a level control. A snazzy blue led is driven at low current from the switched 9V supply.

I make the circuit up on a very small piece of veroboard and mount it, along with a couple of sockets for input and output into a small box, It works well.

I mount the microphone on a long length of screened cable, and enclose wiring side of it in heatshrink. A crocodile clip makes an excellent clip for it.

The finished unit. Watch this space for a demo!

Pye T19D radio from 1949, repairs and renovations.

A fine fellow called Derek called...

"I've got this Pye radio I've bought, and it doesn't work, it's totally dead.. Can you take a look at it?"

Why not ....

It's a very nice looking Pye T 19 D....

A cursory inspection shows a very clean chassis, but the usual rake of the dreaded Hunts capacitors, both wax type and electrolytics. Nothing to stop it powering up though...



.... so we power it up. Nothing. Not a sausage.

A quick check round with the meter show the mains switch is open circuit. The mains switch is unusual; it's mounted on the tone control/ mode switch. This switch is multi-funtion, and switches the set on, and provides a switched tone control, and selection of the "gram" input, for connection to your external gramophone...

Removal and inspection of the switch show it's stuck in the off position. It's a bit of a tin-pot affair, being paxolin construction and pressed together in a steel case. I managed to get it apart, and un-stick it, but it's not reliable and starts arcing over. Drat. A solution will need to be found....

The tone control/ mode switch without the mains switch sat on the end...

In the meantime, I hook up the main smoothing electrolytic capacitor to the Dreadnaught MK87 capacitor reformer, in the vain hope of it being OK...












...it is nicely dated APR 1949, but I don't hold much hope, the poor thing has been in there for 67 years....










 I start to re-cap the chassis, evicting the evil Hunts...

There was also some Sprague 1000v caps that read leaky; they had to go too to ensure reliable operation...















... and when this is complete, I check out the mains smoothing electrolytic, which has been gently reforming for a couple of days. It's OK! I'm amazed. 67 years in, and it's giving acceptable leakage, a reasonable ESR, and capacitance! ... I short out the mains switch and gently apply some power via the variac... and the set crackles into life! It's quickly apparent that's something's amiss with the tuning drive, which slips badly ... perhaps a re-string of the tuning drive is required? ....

Nope, the tuning capacitor is moving on it's rubber mounts as I move the tuning control...

I slacken off the bearings and give them a little lubrication.. Bingo! Smooth tuning and no sign of slippage on the drive string.. Good news!








The volume control appears to be open circuit for most of it's travel. It works at about 90%, but nothing below...

So, to re-cap (pun intended)... the remaining faults are:

An unserviceable on/off switch and volume control...

The volume control is a 1 megohm log pot. No problem there. These are easily available.
The mains switch? Made from purest unobtainium., no chance...

So what about getting a 1 meg log pot, with a switch, and have the on/ off switch on the pot? Seems a reasonable compromise.... but 1 meg LOG pots with switches are also unobtainium...

I go to the local Maplin, who stock a 1 meg linear pot with on/off switch.. and fit it. It's horrible as a volume control... the control is all "up one end" , as you'd expect.









Now a "log" pot isn't really a log at all, but an approximation of log, by damping one end of it with a resistor; so, after a bit of experimentation, I end up with a 47K resistor at the earthy end of the pot. It works well.

I did contemplate mounting the pot on the end of the tone/mode switch, and not bothering with the pot bit, but the mechanical resistance in the pot made the switch unusable, and worse, prone to arcing if it hadn't exactly sat in it's ident.

So this set now has a volume/on/off control. Not ideal, but it works nicely...

Here's the completed set in action...