The re-occuring Copland CSA515 fault.

Not the greatest of success stories, this one...

Many moons ago Gerald rang (his name's not really Gerald, but the names have been changed to protect the innocent)

"I've got this Copland Hybrid amp, it sounds thin and distorted on one channel, mainly in the mid-range, can you take a look?"

Yeah, why not? (in hindsight, there are now a good dozen reasons I can think of why not...)

It's superbly made in Denmark, and weighs a lot!













... the weight mainly due to the huge toroidial transformer, and gargantuan heatsinks!

Look at all that smoothing.








The problem quickly showed itself.... something in the back of my mind said "this sounds valvey to me" .... but swapping the two 12AU7's (that's ECC82 to you and me) made no difference. I eventually traced it down to a thermally poor bias transistor on the right channel... or so I thought.

The amplifier behaved faultlessly on test with my Rotel pre-amp driving it, so I sent it back....

About a week later the phone rang again ... it's Gerald

"It's doing it again" ...

Send it back....

It arrived. I put it on the bench, and listened to it all night, as I was working on other things... It performed faultlessly.

I removed my power amp from my hifi to give it a damn good test .... months it sat there, performing beautifully.

.... then ... it did it...

"I've got you now!" ....

Put it back on the bench, and it was fine! No amount of tapping or banging would get it to fault....

I removed the PCB (not an easy task in itself) and checked every soldered joint. Not one was poor.

I was about to reassemble it, and thought I'd just check the bias again... and sure enough the right channel had drifted a bit. I adjusted it, and put it back on test.... then about 10 mins later, it faulted again!!! I changed every cap in the right hand bias circuit.

Working again...

After a week of perfect operation, it was returned to Gerald...

... where it ran for a couple of days ... and came bouncing back once more....

On the bench, and the fault was immediately obvious. Bias on the right hand channel was miles out.

As I probed my way through the circuit, the fault was progressively putting itself right.... the DC bias to the bases of the output pairs was decreasing. I traced it all the way back to the drivers, when the fault just wasn't there anymore.... I switched the amp off and went for a nice cup of tea....

I came back, and switched the amp on. Excellent fault present for a few minutes, then the bias was decreasing to a sensible level once again. Out with some "freezer spray on the cheap". Output was connected to a dummy load, and monitored on the scope. Finally some progress. Freezing and cooling a transistor in the voltage amp would cause the fault to come and go...




Gotcha....


















... I hope

               ... and so does Gerald...

Sam's Bush VHF90

Sam popped by with her much beloved, but sadly non-working Bush VHF90 radio...

"It sometimes picks up Radio 2, but nothing else.. Can you take a look?"

Yeah , why not?

It's a dual-band MW and VHF (FM) set, dating from 1956. Now, back then, VHF was in it's infancy. There wasn't any commercial radio in the UK until the 70's, so it was all BBC only. The band extended from 88 - 100 MHz only, not up to the current band edge at 108 MHz.

Removal of the back, and the first thing I notice...













...the mains tap is set to 200-210 Volts .... aghh ! This is never a good sign. The mains voltage in the UK is 240V (listed as 230V since European "harmonisation", but nothing actually changed). As a set's valves aged, unscrupulous repairers (bodgers) would lower the tap setting, giving the warn out valves a bit more voltage in an attempt to eek a little more life out of them, but putting the rest of the set under undue stresses and strains..... It's a practice known as "tapping up the set". Bad news...



Putting the UL84 output valve on test shows the sorry state of things...

Every valve in the set tests bad, and will require replacement.

Thankfully, they're all U pre-fix (designed for 100mA series heaters) valves, and are reasonably inexpensive.




So, I've got a couple of valves in stock, and a couple sourced locally... I can get on with the rest of the chassis whilst the others turn up from the internet.

First off, let's check out the dropper resistor, as it's going to have a hard life, having had the wrong tap selected....

.... hang on a minute... the bodger's been here again! There's a section physically missing! It's the 240-250V section. The 220-230V section is also open circuit... both are replaced.










Whilst most of the wiring is PVC, there's some rubber insulated wire which has perished. It's replaced with PVC....








In a break with usual tradition, the whole chassis recapped wholesale...












After a couple of days, the new valves turned up, and the set was powered up... sure enough BBC Radio Two on 88.6 MHz is all that can be received... and with the tuning dial set to 100 MHz! There's absolutely nothing on Medium wave at all .... not even a crackle...

I start with the lack of medium wave. Much head-scratching, staring at the circuit diagram, drinking of tea, and measuring of components followed.... There's a positive voltage on the grid of the UCH81 mixer-oscillator which shouldn't be there. Tests proved the new valve wasn't the fault, so where was this voltage coming from? I disconnected the wire to the valve pin, and the voltage is on the wire... medium wave starts working (ish) but the voltage should be disconnected by the switch...

Then I found this ... http://www.vintage-radio.com/recent-repairs/bush-vhf90-1.html

The switch has become conductive and is leaking! I cleaned it up.. it didn't help. Thankfully the webpage describes a work-around...










... which is duly implemented, and works well...

Medium wave is restored, but it's really weak.

Re-aligning the set provides great improvements, the second IF amp being way off the mark, as was the front end... Mr Bodger again?

The cathode resistor to the UL84 output pentode was also found to be low in value!


Next to sort out the VHF....

The tuning drive cord had been replaced (bodged) with string at some stage in it's life, it was slipping badly... The pointer had been replaced with a bit of copper wire, something I've done in the past when the original is missing and unobtainable..

The whole drive was slipping badly. There should be a spring in there somewhere, to tension the cord around the drum and pulleys too. The drum is directly connected to the AM tuning capacitor. VHF tuning, is that piece of cord you can see disappearing into the grey VHF front end to the left. It pulls a spring-loaded ferrite core out of an inductor inside the can, thus changing the frequency. It looks like the whole lot had been assembled wrongly.

After more tea, a new drive cord, some more tea, and some considerable head scratching, it all works as it should.

After a quick re-alignment of the VHF stages, it happily tunes from 87.5 -102.5 MHz.


Now Sam was keen to do the cabinet herself... but we had a problem...

I won't let it leave my workshop with a gaping hole in the bottom, as the chassis is connected to the neutral, and there's all sorts of live bits accessible through that hole... Electric shock is not an option.






A small piece of glass-fibre matt is cut...













... and hardened with resin to make it safe.

Sam seems to think she knows where the bit that fell out is, so she can cut this out at a later date and refit.








The guilty parties ...












Here's a quick video description and the thing finally working on AM....

Radio Rentals Model 218 radio, repairs and restoration.

My good friend Derek, has once again dropped off a challenge.

"Always wanted a set with a magic eye. This one's got no mains lead. Can you sort it?"

Why not...

It's a well designed set, with long, medium, shortwave and FM bands. The FM band only covers 88-101 MHz, as the police used the upper bit to 108 MHz as late as the 1980's in the UK.

It's tone control isn't the conventional variable control, but a three position switch. I can't really see why this was done. It involves a rotary switch, and 3 caps and resistors, rather than a pot, and one cap. Strange... maybe a switch and caps were cheaper than a pot !?


This set is a "purchase model", rental sets were marked as "Property of Radio Rentals." It dates from about 1956/57

Nice un-cluttered chassis...


















First off to disconnect the mains smoothing cap and hook it up to the MK87 "Dreadnaught" capacitor reformer, and leave each section to reform.

It doesn't take long, the capacitor isn't original, and is in fine shape.

 Odd "dog bone" resistor (yellow body, purple end, red spot!) ... more the sort of thing you'd find in a 30's or 40's receiver forms part of the power supply filter... can't be original, can it?









A few caps are evicted as a matter of course, and the switches and pots are cleaned up.











Initial results on LW and MW are good, then... POP ... silence.... A new EABC80 valve restores audio. Faults are not going to be easy to find, as I have no service information on this set.
Shortwave works OK too, not much selectivity, but that's only to be expected really. FM is awful. Almost nothing.... it's unstable. A quick sweep with the signal generator shows the thing to massively high in frequency. It's tuning about 140-150 MHz! A long period of head scratching, and tracing signals with the 'scope shows there's a 68pF capacitor open circuit. That brings the tuning back into range. It's not very sensitive, but things improve with a re-alignment of the IF and front-end. One problem remains. It's not good at handling modern broadcasts. The deviation is too wide, and bass is horribly distorted. Not much I can do about that really.

I spoke to Derek, and he's not too fussed about the FM, so it's time to crack on... Derek would like a line-level audio in though, for MP3 use, which is easy on this set, as it's transformer isolated, and has a "gram" input we can use...

A 3.5mm jack is added to the gram input, and shunted with a 10K resistor to earth, and 20K in series. Works really well, but gives us an electrical safety issue, as if a fault were to develop, the chassic could become "live", as would the MP3 player! Thankfully as the chassis is isolated by a transformer, we can add a 3-core mains lead. Testing this out gives no issues :)

Sound quality from it's large Goodmans speaker and EL84 amplifier really are very good indeed.

Magic eye is a little dim, but works well.. The "wings" close up as the signal strength increases to show when the set is correctly tuned.

So to box it all up, and final test. MP3 input works well.... but nothing on the radio ! Back out with the chassis again. Poking about around the wavechange switch, and there's a wire broken! Duly soldered back on, and still nothing! Traced the signal back to the EACB80 again.... (?) Starting to doubt my sanity, when I find another broken wire on the valve base from the IF transformer! I wonder if that was the original problem with the valve, and changing it just caused the broken wire to meet up again? Yep, replacing the valve with the original and it's still good.

Soak test for a few hours, and it seems fine ..!

"Just a wire off" .... sheesh!


Cleaned up nicely too.

Grundig TK120 repair

Gareth (a fine chap from the motherland) walks into the workshop with a bin bag under his arm....

"My Father-in-law's dug this tape recorder out of the attic. It's got a recording of him on it, but he says there's no sound. Can you have a look?"

Yeah ... why not?

It's a Grundig TK120, in distressed condition. It looks like it's been left rather too close to the fire!

... and after removing the tape and the top cover, the immediate issue is all the rubber parts have disintegrated....










Now the belts are no real problem... but the drive idler tyre is going to be...

After some head scratching, and some looking about for suitable parts, an O-ring is super-glued to the remaining idler wheel....







Now to replace the belts. So I unscrewed the four screws with blobs of paint on, to remove the top plate. DO NOT DO THIS!

FATANG! All the spring loaded control levers fired out of their mounts.... Thankfully their positions are quite simple to work out, but very fiddly.... several hours later and the thing is back together....

Here's how it should have been done!..

Underneath the deck is a small plate, with two screws... undo those to gain access to the bottom of the capstan flywheel....


... and work the belts about the bottom of the flywheel, through the small hole. It's still fiddly, but much less agro than re-aligning the top mechanism!



















You'll need to remove the take-up reel and clutch to replace the take-up drive belt. The belt itself is straight forward. The clutch reassembly is a bit tricky...

This is the bottom of the deck, showing the new tape counter belt... Remove the little pulley and it's clip from the take up reel shaft. There's another clip underneath and an antifriction washer. Remove those.




Turn the machine up the right way, and remove the take up spool...











This will reveal the take up clutch, and it's little three pads... if they've fallen out, don't panic... they'll be there somewhere!

Fit the belt and reassemble the clutch. Some adjustment of the clip above the small pulley we removed earlier may be required to get the right amount of friction on those pads, but it's not difficult.

The motor was a little sticky on this machine. I stripped the lower bearing off and re-lubricated it. It now spins like a top....

... there's still no sound.


This is rapidly traced back to an open-circuit HT fuse, it's replaced and there's some audio briefly. The HT (and heaters) is provided from two extra windings on the motor. There's no transformer. Clever stuff. Garrard and BSR used a similar trick on their record changers in the 60's. The electronics are a simple design with just two valves, an ECC83 and an ECL86.

The main smoothing capacitor is getting warm. A sure sign that it's had it day. It's a two section 50uF+50uF 350V part. I replace it with two suitable caps.









Switching back on and there's audio again... but it rapidly fades.... A check round the other caps proves their innocence... I'm not liking the look of the getter flash on the top of the ECL86 triode-pentode valve though, it looks like it's had a hard life.... The getter flash should be sliver or black looking, and should have nice sharp edges...





This is sort of brown looking....













... and checks on the avo valve tester show the triode is very low at 0.6 mA/V (It should be 1.5 when new) and the pentode section won't read at all! When testing the pentode section, there's a blue glow inside the anode structure, a sure sign there's some gas in there. A replacement restores operation, and the dulcet tones of a young man playing the guitar once again issue forth from the speaker... recorded some 50 years ago!


Audio quality isn't fantastic. These machines were a bit of a domestic novelty really. After transcribing the contents of the tape, Gareth described the quality as "Like a walkman with flat batteries", which isn't a bad description at all...

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...)