The Workbench

Re: Master 39K

Dillen — Sun, 10 Oct 2010 02:38:16 GMT

Rudy,
Yes, it's very beautiful.
The Master Deluxe 38K is not in itself incredibly rare but finding one
in this condition definitely is.

Søren,
Please do.
I will add a couple more:

Martin

Re: Master 39K

RUDYV — Sat, 09 Oct 2010 17:25:48 GMT

WOW, what a beauty !!!

Unbelievable, a Master de luxe 38 K, NOS.

Still under waranty ?

I'm ready for it !!!!!!

Rudy

Re: Master 39K

Søren Mexico — Thu, 07 Oct 2010 19:18:08 GMT

its just beautiful, Martin may I copy the pic for keepsake ??

Re: Master 39K

Søren Mexico — Thu, 07 Oct 2010 19:16:09 GMT

What a beauty, forget the life beside and get on with it.

Dillen:

I haven't been updating this thread a lot recently. It doesn't mean that I haven't been
working on the set, merely that I haven't had time to do the writing.
The Master 39K is close to being ready as you can tell from my previous post.
That is, I still have the magic eye issue but I simply cannot see why it doesn't react;
The circuit matches the schematics, all voltages are spot on, all components and all
other valves are good and a factory new magic eye makes no difference.
I will have to deem this repair done but I will keep thinking about this issue until I
find a similar working set to compare with. The owner reads this post and I've also
email'ed him the status so he knows that I've been struggling with this.

When I haven't had a lot of time for writing it's basically because I have a nice
selection of other repairs here too at present. I also have a life besides this hobby thing
and just when I thought I couldn't get more busy, the Master 39K owner told me that he
bought another set.
I couldn't believe my eyes when I saw this ...

Martin

Re: Master 39K

Dillen — Thu, 07 Oct 2010 14:50:21 GMT

I haven't been updating this thread a lot recently. It doesn't mean that I haven't been
working on the set, merely that I haven't had time to do the writing.
The Master 39K is close to being ready as you can tell from my previous post.
That is, I still have the magic eye issue but I simply cannot see why it doesn't react;
The circuit matches the schematics, all voltages are spot on, all components and all
other valves are good and a factory new magic eye makes no difference.
I will have to deem this repair done but I will keep thinking about this issue until I
find a similar working set to compare with. The owner reads this post and I've also
email'ed him the status so he knows that I've been struggling with this.

When I haven't had a lot of time for writing it's basically because I have a nice
selection of other repairs here too at present. I also have a life besides this hobby thing
and just when I thought I couldn't get more busy, the Master 39K owner told me that he
bought another set.
I couldn't believe my eyes when I saw this ...

Martin

Re: Master 39K

Dillen — Tue, 05 Oct 2010 16:12:37 GMT

OK, time to realign this thing, then.
With an AM set like this, it's not very hard to reach a good alignment, even by ear.
Basically, what you do is tune in to a good clean station and simply adjust the IF coils
for maximum volume.
This set has two sets of coils to adjust.
It turned out, that even the coil(s) I adjusted earlier could be set better. That just
goes to show how things can sometimes be a bit "flimsy" and goofy with these old machines.
Anyway, the wavebands were now filled with stations. Particularly the Medium Wave
band as expected. Experience has it that short wave is pretty quiet in my neck
of the woods but still a few stations could be heard using a long antenna.
But no AFC and no reaction from the magic eye.

I then decided to set up my own little AM transmitter.
Programmed it to send at 850KHz, which is around the center of the MW range on this set.
The transmitted signal came through with good volume and great sound. Even the AFC did
something now.
The magic eye, however, remained frozen.

I scrutinized the circuit for the magic eye, even replaced a couple of resistors
but couldn't find anything wrong.
I found an old handwritten note in a service manual, telling something about changing
a resistor to a larger value for greater deflection on the eye. I tried it but it didn't
change anything.
I even tried a replacement magic eye with the same result.
Like a cathode ray tube (CRT "picture tube") the magic eye has a surface covered
by phosphor, that lights up when hit by an electron stream. This makes the eye glow green.
And just like a CRT, a magic eye can also have burn-in marks (remember screen savers?).
This particular magic eye has a well defined burn-in mark right at the exact same area
where it lights up now. It looks as if it was never lit any higher so I lean more and more
towards the theory that the previous owner, just like I, hadn't got an antenna strong
enough to give any deflection on the eye.
That could also explain the need for "greater deflection", as suggested by the
hand-written note.
Not having a similar known good set of the same type at hand, I am tempted to say that
this is how it's made and it will take a very good and strong, possibly directional,
antenna to give better results.
Whatever, I had plenty of international stations, even on just a meter of wire.
The tonecontrol proved to be a bit "muffled". A shot of contact cleaner and a little
exercise cured this.
A small amount of crackling when tuning. This comes from the large (open vanes) tuning
capacitor, it can be considered normal and will, no doubt, cure itself with use.
The set played all evening, good sound and good continuity. No drop-outs, cracks or
whistles. Just a slight modulation hum which is normal when I use the local
AM transmitter.

Martin

Re: Master 39K

Dillen — Fri, 17 Sep 2010 13:01:58 GMT

A replacement rectifier valve provided a voltage much nearer to that
mentioned in the service sheet but the set remained silent.
If the volume was turned way up, a "pleasant" humming could be heard but
nothing else on any waveband.
A buzzing sound could be heard if the control grid of the output valve was touched
with a screwdriver. This was a good sign and it told me that the output valve was
working.

This set has a gramophone input. It is connected to the EBC3 (pre-amplifer) valve
so makes for a nice amplifier test point.
A B&O TG7 signal generator got connected to the gramophone input and I was rewarded
with a nice sine sound, clean with no distortion and good sensitivity (volume).
This was also good news in that it told me that the preamplifier-valve and -circuit
was also working.
Next step backwards was the EF5 IF amplifier and CCH2 frontend heptode/triode.
Replacement valves were test-fitted but that brought no change.

Next step was voltage readings. Anode voltage(s) on a valve will tell a lot
about the condition and behavior of a valve and its circuit.
In this case, everything tallied pretty much with the noted voltages found in the
service manual. The CCH2 doubles as mixer and oscillator and both anode voltages
were fine. The anode voltage of the triode section was given as 90V and I had 86V
so the local oscillator was clearly running.
If it wasn't, the voltage here would typically have been considerably lower
at maybe 10-15 Volts or even less.

I then set up a signal generator, programmed it to the sets IF frequency
of 440KHz and injected a modulated 400Hz onto the control grids of the
frontend valve and IF amplifier valve respectively.
The sensitivity of the IF amplifier stage was given as 6-8mV and I had a nice and
clear signal at just about 3mV so that stage was doing fine.
The frontend, however, was a different matter; Sensitivity here was given
as 100-150uV (microvolts) but I had to inject about 2mV (millivolts) to get
an audible output.
The magic eye remained dimly lit but frozen.

Next step would be to check the adjustments of the first IF coils, the ones
working with the CCH2 valve.

With the signal generator connected, it was soon clear that the first IF section
operated at 444KHZ. Not the world off from the correct 440KHz but easily enough to
disturb proper operation and reception.
It was adjusted down to factory specs and the set was tried again.
A long antenna (about 5 meters of wire across the floor) and the set came up
talking swedish, I had struck a gardening programme.
What a nice surprise !
Well, it turned out to be the only station on the wavebands and it was a bit noisy
but it was a result.
The best so far and a sign that I was not too far from having a fully working
Master 39K on the bench.
The magic eye, however, was still frozen and I would expect a top notch set like this
to perform much better.

Martin

Re: Master 39K

Dillen — Fri, 17 Sep 2010 13:00:16 GMT

There can be many reasons for the high voltage to remain low.
- The output valve drawing a too high current is a typical fault, often caused
by a leaking grid coupling capacitor.
The capacitor in this set is a green Philips type. It measured fine when tested with
a meter and it is of a type that I usually find very reliable. It will, nonetheless,
be checked again. Components that measure fine with a multimeter at low voltages
can sometimes change their characteristics when a high voltage is applied.

The reason could also be the rectifier. A loss of emission (worn valve) will see a
low output from the valve.

So no worries, there's still some work to do.

Martin

Re: Master 39K

Dillen — Sun, 12 Sep 2010 02:06:32 GMT

Now, everything is, if not fine and working, then at least safe to power up.
Workbench cleared, voltmeter across the high-voltage rail for monitoring and
the mains connected to a B&O RT12 variac.
Switching on.
Increasing mains voltage slowly.
Watching as the mains current rises slowly.
Dial lamp starting to glow.
Up the voltage to about 100 Volts.
Dial lamp now brighter.
Up the voltage even further, now around 180V.
The high-voltage rail starting to show some voltage, climbing as the rectifier warms up.
Nice so far.
Around this time the amperemeter on the variac started to react a bit more as the
output valve also started to conduct current. That also caused an expected minor
drop in the high voltage.
Still good.
Up the mains to the full 220V.
The smell of warm dust coming now - to be expected.
The speaker now starting to emit a very faint hum - also to be expected.
High voltage now around 150 Volts. Where it stayed...
A bit low for a voltage that is given in the service manual as 200 Volts.
There may be other fault(s) that will eventually show but this voltage is wrong
and the reason will have to be investigated.
This low voltage will not allow for clean reception - actually I had no reception at
all in this case and it will not allow the magic eye to glow either.
The good thing is that the most basic functions seem to work and there were no flames,
smoke, bangs or other nasty surprises.

Martin

Re: Master 39K

Dillen — Tue, 07 Sep 2010 16:03:52 GMT

Chris is right;
- Absolutely not !
- Indeed !
- Long over due !

I've been busy with other things. Repairs from all over had started to pile up
here and I had to attend some non-B&O related things too but work on this wonderful
set has not ground to a halt. Not at all.

This set was born without a mains fuse (!) so I will have to add one.
I am perfectly aware that this will break away from the original design and inside
appearance but it is a matter of safety - for both the owner/user and the
set itself - so I will not compromise here and it should be a fairly easy task
to fit something below decks somewhere.
At 220-240V about 1,6A would seem fine. The running power consumption is never that high,
but we will have to take the warming up into account. The valve heater filaments and also
the dial lamp have very low resistances when cold so the rush-in current could be a
bit on the high side.
A DC reading showed apprx 200 Ohms when cold so that gives apprx 1.2 Amperes
at the moment of switching on.
The rush-in current could actually be even higher, namely in case the set is switched
on right after being switched off, i.e. while the valves are still warm, allowing
an instant current flow to rush in and fill up the drained filter and reservoir
capacitors through the still warm rectifier valve and further down through the also
still conducting output valve to ground.
I decided to go with a 1.6 Amps slow blow. Tests later on will (hopefully) show if the
theory holds water.

I found a good place under the chassis to fit a fuse holder for a
normal 5x20mm glass tube fuse.
Right next to the rectifier valve, an empty and already threaded hole was available.
Probably a hole that was originally used for holding the original output transformer
but with the external transformer sitting at the bottom of the wooden chassis, this
hole was now left unused.
It is fairly close to where the mains lead enters the chassis and there is exactly
room for the fuse holder underneath. Safe and fully reversible if need be.
Very nice.

Photos will follow when time permits.

Martin

Re: Master 39K

Medogsfat — Fri, 03 Sep 2010 16:41:13 GMT

Friedmett:

Has Dillen given up here?

I very much doubt it!

Or just to busy?

Quite probably.

Time for an update right?

Patience is a virtue

Re: Master 39K

Friedmett — Fri, 03 Sep 2010 13:11:14 GMT

Has Dillen given up here?

Or just to busy?

Time for an update right?

Re: Master 39K

Dillen — Thu, 19 Aug 2010 13:04:07 GMT

Work progresses slowly, but I have managed to do a little:
A close inspection revealed that there was nothing seriously wrong with the module,
it was simply the lubricating grease on the pushbuttons that had dried and become solid.
A few drops of acid-free oil was applied to the dry spots and the module
was left to soak in the oil overnight.
Next day, the whole mechanism functioned perfectly. An additional few drops of fresh oil
was added for good measure and all that's left now is to clean the whole thing, particularly
the buttons.
The tap that sticks up in this photo is connected to the release mechanism.
It is reachable from behind and pushing it sideways will release any button
depressed from the front.

Martin

Re: Master 39K

Dillen — Wed, 11 Aug 2010 08:58:14 GMT

The module is clearly stamped "Kø" for København (Copenhagen) which confirms that
this set was meant for use in the eastern parts of Denmark, near Copenhagen.

Martin

Re: Master 39K

Dillen — Wed, 11 Aug 2010 08:55:40 GMT

One thing is not smart with this set; It's not easy to test the chassis on the bench.
The output transformer and speaker sits in the cabinet - that could be solved with
a couple of long leads but the pushbutton station selector module is in there too
and the cable connecting it to the chassis is very short to prevent noise/oscillation etc. and should
not be extended for the very same reason(s).
I had to dismount the mechanical pushbutton station selector module from the
cabinet to allow for proper testing of the chassis on the bench.
Not a huge problem and since the pushbutton mechanism seems to have seized up badly
it had to come out anyways.
The top button is depressed and does not release when other buttons are pressed
which also prevents the others from latching in correctly.

Martin