Heater circuit in Kolster Brandes MP151

[Nuvistor]

Incorrectly labeled resistors, many of us have been caught in that trap, but only once. A few seconds of a job to test them before fitting.
You are in good company, has I say, most of us have been there.

[crackle]

Well done, you got there in the end, and another fine iconic radio bought back to life.
As I mentioned before I think the chassis you have is one of the earliest that KB made.
It has the correct tuning dial for the early models, but I just dont understand how it ended up with the plastic grilled front rather than the perforated gold anodised aluminium one.

Mike

[Mike. Watterson]

"preventing full gain as it leaked to 0V"
Usually 0V = full gain and -ve volts from detector reduce gain with increasing signal. Leaky caps on any screen grid though dramatically reduces gain.

"'That cap' was actually perfectly OK with only a couple of uA leak at 90V"
No, that's a failure. Um.. that's a lot, esp. if the grid resistor is 1M ohm. 1uA @ 1 M Ohm = 1V, a big change in bias and will only get worse.
Leakage on any cap needs to be so low that it doesn't register on an regular DMM, less than 1uA.

[brightsparkey]

The ac coupling cap - fair enough when you put it that way. It's been changed anyway (because to test involves removing it). In set it only runs about 25V and the grid -cathode voltage was only about -80mV, whereas I'd tested it at 90V. But there a nice LCR poly cap now.

The AGC voltage - the cap is to chassis, but the mixer cathode is at about +2.5V with no signal due to the directly heated cathodes. This means leakage turns off the mixer.

[brightsparkey]

Originally Posted by crackle

Well done, you got there in the end, and another fine iconic radio bought back to life.
As I mentioned before I think the chassis you have is one of the earliest that KB made.
It has the correct tuning dial for the early models, but I just dont understand how it ended up with the plastic grilled front rather than the perforated gold anodised aluminium one.

Mike

Yes I should put up some pictures for the benefit of other members of the forum. And thanks need to go to Crackle (Mike) for his initial assistance with identifying the correct schematic.

It has a few other minor variances from the original MP151 chassis:

• It has a siemens rectifier module rather than the finned stack in the original
• The mains filter cap C25 is in a different place
• Trimmer cap T2 has been replaced with a fixed value (committed as Mike put it)

So I suspect you observation that this is a transitional set to the later model is probably accurate.

[brightsparkey]

Well I thought I was home and dry with this radio, with good enough sensitivity on both bands on mains and battery, so I put it back in the case.

Turned it back on and got no stations. Checking with the sig-gen showed poor sensitivity on MW and LW. After checking I hadn't done anything silly getting it in the case, discovered that turning the chassis over made it stop working - at least decreasing sensitivity by a good 30dB.

Having been bitten before by valves with loose internals, I transferred them one by one into a working Roberts R22 and all were good with no tumble induced misbehavior. So I though there must be yet another bad joint to be fixed.

I was kind of right as after a little while the set went berserk, motor boating and behaving oddly in all orientations. This was (after a lot of confused head scratching) tracked down to a bad joint on the earth between parts of the oscillator coils which was then remedied. But sadly the orientation induced problem remained.

I then proceeded to go round the whole set tugging at every connection to try and find a loose joint or some such - to no avail whatsoever.

After a few days rest (always a good plan when foxed) I decided to be more scientific about it. Injected 470kHz AM signal into the IF transformer output - nice sine wave in both orientations, always consistent. Not the IF stage or the Audio then. Disconnected the antenna circuit and injected 1MHz in MW mode into the mixer grid - orientation sensitive still, but still demodulates, though with much reduced sensitivity - so Its in the mixer, not the Antenna and its switches ,tuning cap etc.

If the signal demodulates then the mixer LO must be running correctly, so the only other possible culprit is the 1st IF transformer. And with a sigh (after re-flowing all connections to no avail) I remove it from the set (no small undertaking) and un-clinch the can to see whats inside.

Answer includes about half a candle..

I attached a couple of pics - a bit blurry I'm afraid, but what I found was that one of the tunable cores was loose and if you tip the transformer up it drops out of the coil, de-tuning the transformer completely - well at least I'd found the culprit now.

It was initially impossilbe to see whats going on for all the wax. I've tried to remove the wax by gently heating it up and melting it out, but it's really quite viscous and has remained in all the intimate parts of the mechanism. I tipped the coil to the correct end whilst it was still hot, and the wax has bound it in place. Re-installing it into the radio, it works in so much as the sensitivity is consistent in both orientations, but the IF response is well off frequency. Adjusting the brass screws does nothing, so I suspect the mechanism is not correct and that bits that should move are jammed with the remaining wax.

So how to proceed - I really don't want to damage the transformer as replacing it is going to be tough, so I have not tried any solvents etc. yet.

Whats the best way to remove all the remaining wax, including that which is stuck in fine tubes by capillary action?

Looking at what I can see of the design of the adjustment, it seems to me that there are two plastic tubes that should be a friction fit, and a brass screw that is meant to fit tightly into the inner tube to allow adjustment of the ferrite slug which is inside the inner tube. Is that the way its meant to work? How do I return the proper function of the tuning screws?

Thanks for any advice. This has got to have been the hardest radio I've attempted yet! I'm getting stubborn with it

Kevin.

[crackle]

I have never seen inside one of these particular IF transformers.
However I have had ferrite cores come off the brass screws on some radios, not on one of these though.
If the ferrite and the brass adjuster are removed then you should be able to superglue it back on to the screw.
Is the internal body made of Bakelite? I would have thought that the wax would melt away long before any other part was damaged because of heat.

Mike

[Mike. Watterson]

Looks like a Philips design. I had a faulty one like that in a battery valve AM/FM set and replaced it with one from the same 'signal path location' that looked identical from a scrapped mains valve set. They are very hard to work on. I fix another radio where two of these had snapped wired just inside the case near the tag.

[brightsparkey]

I used a hot air gun at 150 degrees, plus a hot plate and the wax melts but still there is a meniscus of wax in all the fine crevices and in the tube which I describe.

I'm reluctant to go much hotter - at 200 degrees it was smoking a bit.

I'll take some better photos now that most of the wax has gone.

[crackle]

Use some absorbent material/blotting paper to suck up the remaining melted wax whilst still hot.

Mike

[TrevorG3VLF]

Wax will disolve in white spirit or paraffin. A few washes may do it.
Keep away from heat or have a fire extinguisher ready.

The ferrite will increase the inductance and the brass will reduce the inductance so the two must be attached correctly.

[brightsparkey]

So I managed to sort out the IFTs on this radio.

Started out with heat to remove most of the wax - my hot air gun is temperature controlled, and at 150 the wax melted fairly well without damaging anything.

Next step was to use lighter fuel (which is just paraffin?) as a solvent. Long and patient job to remove the wax from intimate spaces in the slug adjusters. But eventually they were clear enough that slug was free to move. Getting the last bits of wax out I resorted to using twirls of tissue-paper - the tough blue-roll stuff. Fill the tube with solvent then insert twirled 1cm wide strip into the tube. It soaks up the solvent and then you can rotate it to remove the last traces of wax. One can be seen in one of the attached pictures.

Next step is to remove the slugs in their tubes. The tubes are very fragile - slightly brittle with age. The design of the mechanism is interesting. The two plastic tube slide inside each other. The brass tuning slug is glued into the inside of the inner tube and then inserted into the outer tube. The ends of the outer tube are then crimped around the screw, presumable with some form of hot forming tool, to make the thread that the screw acts on. You can see the creases on the ends of the tube where they are formed.

Disassembly then involves pushing the inner tube out through the crimped end of the tube - and it was inevitable that the end would split. So I used the paper twists again, inserted into the other end carefully around the wires and as gently as possible pushed out the slug. Once out, the brass adjuster was re-glued onto the tube with a tiny amount of super-glue and once cured re-inserted into the outer tube. I was able to achieve enough friction in the thread for them to work OK.

Fortunately the Kolster manual tells you what to expect for the winding resistances, and although the numbers didn't match very well, I could at least tell primary from secondary which is just as well as they are symmetrical in construction. Once back in the radio I used our trusty HP3577 net-an to line up the 1st IF (I'm spoilt rotten, having access to this vintage marvel of technology). Then checked the 2nd IF and it was way off tune also, and has the same problem of having being filled with wax, so another two hours messing about required on that one too! Now the IF is nicely tuned and the sensitivity is back to normal. Minimal amounts of red nail varnish used to lock the screws this time!

Still something not working when on mains, but definitely getting there now. Have almost undone/re-made every solder joint in the set by now..

Pics showing the internal of the transformer sans wax attached for you interest.

[crackle]

Interesting, I have never seen inside of one of those types of transformer.
So are the 2 photos of the transformer each half of the single transformer.
Was the ferrite slug completely loose in the tube?

Mike

[brightsparkey]

Yes, the bakelite is symmetrical with one winding on each side. There are two ferrite rods to either side of each winding which are glued in place. The the adjustable core is the part you see in the third picture.

The adjustable part consists of a short ferrite rod which is glued into a thin plastic tube. The brass adjuster is the glued into the end of the tube with a few mm gap between it and the core.

This assembly then fits inside a second plastic tube which is the former for the coil. When I was having the orientation troubles the inner plastic tube had become disconnected from the brass adjuster and was free to slide up and down thus de-tuning the transformer if it was upside down..

The other item you see in the middle is the tubular capacitor that resonates with the coil.