Heater circuit in Kolster Brandes MP151

[brightsparkey]

I have a Kolster brandes radio MP151 - its a bit of an oddball, and it has an MP151 innards, but the casework is like an MP151/2 with the plastic open mesh grill, and some of the circuits are transitional between these two models.

Having repaired the oscillator transformer, which had a fractured wire, the radio basically works OK - none of the notorious Hunts caps are faulty and it has good sensitivitty when run off bench supplies as if on battery. However its very fussy about the heater voltage, and stops dead at 7V instead of the nominal 7.5V

Run off mains the heater supply is less than 7 volts and the set is dead as a doornail. This is even true if you set the heater rheostat current to the recommended 24mA.

This brings me to the thrust of my question. Here is the heater circuit diagram (from the MP151/1 trader sheet as the heater circuit is all in one place there - identical to the manufacturers manual)



R18 =390 R17=330 R16=120 R15=120

The user manual suggests running 24mA total in the heaters on mains and states that the voltage at V4+ is 6V - this allows for a paltry 1.2V per valve, and indeed that is the voltages marked on the manufacturers voltage table - exactly 1.2V per heater.

However, if that were so, then V3 would only have 10mA heater current, the rest going through R15. In actuality it gets much less, and the voltage across its heater is only 0.9V and it does not work, at all. This is because they have lower resistance when cold.

I don't understand the function of these resistors, or how it could be possible to get the right voltages as per the manual with them in place. And surely V4+ side gets overrun, stealing the volts from the others??

Can someone please explain?

Thanks.

[Edit] I should add that the valves are D type 1.4V Heater valves: DK96,DF96,DAF96,DL96

[Nuvistor]

With these being directly heated valves the resistors are there to pass the extra current that flows through the valves from their anodes/screens to the heater.
If the valve heaters had been in parallel fed from 1.4v one side of the heater would connect to chassis and the resistors wouldn’t be required.

[brightsparkey]

That makes sense then. The set HT current is 10mA, none of which flows through V3, hence 120R on each of V3,V2.

Nasty though - any fault in the set can unbalance the heater supply due to wrong DC currents..

In that case, maybe some of the capacitors are not so good - back to the bench with re-wired head - I shall report back.

I also realised that C29 could also cause trouble if leaky, so I shall test that too.

Once again, thanks for the proper insight - another trap for the unwary fallen into and helped out of again

Kevin.

[Nuvistor]

Good explanation here page 559
https://www.americanradiohistory.com...RC-1965-03.pdf

[crackle]

How do you know the "hunts" capacitors are not faulty. Have you tested the leakage at their rated voltage, or even 90 volts, I think you may be surprised. These old hunts capacitors including the brown and the red and the black mouldseal types, ALWAYS have a degree of leakage.
If each capacitor has a 1 or 2 mA of leakage this all adds up and puts additional load on the HT dropper and hence the voltage gets lower, this will also effects the heater supply.

Have you checked all the heater bypass resistors, sometimes the lower value resistors (say a couple of hundred ohms) can drop in value. If this has occurred then the lower resistance will reduce the voltage drop and current actually flowing through the heater.

Mike

[brightsparkey]

Thanks crackle - when powered from bench supplies, mostly the biases are correct. 'That cap' in particular seems to have the correct grid voltage at the output pentode. There are anomolies though - the af amplifier voltages are off, with screen and anode both around 25V instead of 4.

This set has the brown cylindrical caps.I hope there are not many duff ones, too as the likely candidates are in awkward places..

I checked the heater resistors - of the four 3 were high, out of spec (i.e. >10% high except one which was not toleranced i.e. 20%) and I therefore replaced all four. This made things worse, hence my resorting to the forum after much head scratching

Glad I did. Now it seems likely that something is leaky and unbalancing the heater supply. Isuspect the culprit will be found out now.

Thanks for the help.

Kevin.

[Mike. Watterson]

6.8V should be ideal mains LT.
It should work down to less than 6V on battery.

Note that a leaky paper dielectric on Oscillator will stop oscillation and on IF or AF control grids (g2) will kill gain.

The brown Hunts caps or similar are paper and will all need replaced. Only ceramic and mica caps in sets the old and older are good. Sometimes the Electrolytic caps reform.

The heater resistors are not actually that critical as it's only a percentage of the main filament current of 25mA @ approx 1.4 (1.3V minimum per filament on mains), or 50mA on older radios thus each filament is approximately 56 Ohms MAXIMUM when hot (nominal 50 Ohms) and about 20 Ohms or less cold.
The output valve has the most HT current, into filament, maybe an extra 6mA. The DK96 Mixer/Osc is next highest and the DAF96 preamp/detector has the lowest HT current, so has the most bypassed of the LT-

The valves will work at even 1.0V, though a worn osc/mixer may stop at 1.1V. The end point of a "7.5V" battery is about 5.2 to 5.5V depending on how good the mixer/osc is. The actual "technical" endpoint of a Zinc Carbon cell is 0.9V, for motor/flash-lamp use.

[paulsherwin]

These series filament battery sets are a bit of a nightmare, especially when run from a typical battery eliminator of the period.

[Mike. Watterson]

Usually the DAF9x is the last in the "chain" because it is the most sensitive to hum pickup in the filament, being the lowest HT current and the start of the audio amplification. So you check the voltage on it, the 1.35V was recommended by Philips / Mullard and RCA when on "mains", but as low as 1.2V is OK. If that's the case and the series current is according to service sheet (varies according to design), then the problem isn't the filament supply.

I've restored very many battery/mains series models and some parallel battery/mains. Only the 3rd party "battery eliminators" are a problem. All of the ones MEANT to work on mains have been fine, the only ones that are tricky are the parallel filament models as they use either a selenium rectifier as a SHUNT regulator to 1.35V or have a DEAC (an old kind of NiCd rechargeable cell) to regulate the LT.
If a series set with a good DK96/DK40/DK91/1L6 etc isn't running when series current is as per manufacturer's spec, then it's either the DK96/DK40/DK91/1L6 is worn or leaky capacitors. The Mixer/Osc should still go at 1.1V on it. Some series sets do have slightly higher (maybe 0.15V) on the DK96 than DAF96 which will still "go" even at 0.95V, as will the DL96. Note a parallel set will still go at reduced volume with 1/2 filament gone on the DL9x output. A Series set ought not to work with that.

Sort the capacitors first before ever applying power. Replace one at a time so as to be sure you are soldering in the right place. Just the paper ones (that can be in a can or moulded, not just card tubes), typically 1000pf (1nF, 0.001uF) to 0.25uF. Electrolytic caps are a separate issue and often reform unless the two part plastic case or lower voltage less than 8uF.
Selenium rectifiers, even if working, usually have too much AC leakage and fail in less than 50 hrs. They need replaced. A shunt selenium rectifier as "regulator" on the rare parallel filament battery/mains models will need the series resistor adjusted for 1.35V (+/- 0.03V) with 2 x 1N5408 from LT to 0V in series as "regulator".

[crackle]

Hi Kevin

Originally Posted by brightsparkey

There are anomolies though - the af amplifier voltages are off, with screen and anode both around 25V instead of 4.

I dont understand that. screen and anode at 4 volts?

Originally Posted by brightsparkey

This set has the brown cylindrical caps.I hope there are not many duff ones, too as the likely candidates are in awkward places..
Kevin.

If they are the Hunts ones they will all be leaky, some will be more critical than others. If the set is going to be used regularly replace them all. But I would start by changing the 0.03uf cap on the screen of V1, and dont forget the 0.001uf coupling the anode of V3 to the grid of V4 must be changed.

Mike

[Mike. Watterson]

4 is wrong. Where does it say that?
Actually since the Screen resistor and Anode resistor are HUGE values and a GOOD meter was 20K Ohm/V most service info doesn't state the voltages, or warns the 250V range or higher needs to be used.
Maybe you meant to type 40.
Anyway, the capacitor on g2 will be leaky. You can't measure the capacitor leakage with a DVM, you need a high voltage source limited by 2 M Ohms.

I've a couple of models very like this.

[crackle]

I admit that I have learnt a lot more about the heater circuit of these radios from this thread, and reading the "In your Worksop" article.
The circuit is cleverly designed and the valves are very delicate, I believe it has to rely on the components being on spec far more so than a conventional mains superhet.
I have a number of these Rhapsody radio models, but not one with this very early chassis.
In the past I have often found that one of these types of valve will not work in the set, so you replace it thinking it is duff, only to find it will work in another set with no problems. Looking back now it is probably associated with the varying conditions of components like the Hunts capacitors, and to some extent resistors, which I admit in my case if the radio has worked have not been replaced. (other than the Audio Coupling cap) But then I do not use the radios on a regular basis, they are displayed on a shelf around the upstairs landing.

Mike

[Mike. Watterson]

On the DF96 about 45V or more on g2 gives full gain. About 20V on g2 almost kills the gain, depending on Anode and g1 volts. The Hallicrafters S72 (using 1T4 = DF91) actually has manual RF gain knob which really varies Vg2 on the two 1T4 IF amps as well as the RF preamp. It has EIGHT valves (none are a rectifier) of the 50mA type. https://www.radiomuseum.org/r/hallicraft_s_72_s72.html
There is an export version with LW that's missing a SW band.
So while the cap between pre-amp Anode and output valve grid is the most "dangerous" as it can result in audio transformer failure (and wear out O/P valve and on a mains set damage mains transformer and rectifier) and "Tone Correction" cap on output anode is critical, the other leaky caps can stop a radio working or dramatically reduce gain. The one exception is the AGC cap which usually just results in maximum gain if leaky and only has a low voltage, typically 50nF or 100nF, though there can be two of them.

Paper dielectric caps even before WWII were known to have only about 10 years life. Very few (and not any of the rubber bung metal can "micromites") are hermetically sealed. The "oil" impregnation (waxy / Vaseline like except on very expensive types) gradually allows water in. They almost always test good on any DVM or capacitor meter, it's only with higher voltages the leakage is measurable. Even 1uA @ 40V is a disaster in high impedance valve circuits. It rarely matters on transistor sets as the HT is 5V to 20V and the impedances are 1/10th to 1/1000th. Leakage can be 10uA at 200 V.

[Chris55000]

Hi!

I'll just quickly mention in passing that because there were far fewer high–resistance testmeters in general use in the late 1940s to late 1950s when these valve battery/mains sets were produced, many makers often quoted "not measurable" or "2V" on Vg2 ang "6V" on Va for the diode/pentode AF valve, particularly the DAF91 50mA type!

Towards the end of production of these sets some makers did use a valve–voltmeter to take these readings, in which case Vg2 is typically 20–30V and Vs about 40–45V, but it's far less hasste to simply switch the set off, remove the diode–pentode and measure the anode (typically 1M) and screen–grid feed (typically 4M7) resistors between the valve pins and the +h.t. line using a dvm, where you can get at on the o/p transformer primary tags for the +h.t. point – either will do for the purposes of checking the feed resistors!

Obviously, I am in full agreement with what other Members say about the Hunts & other coupling/decoupling caps, turf the lot out unless you have a good–quality h.t. insulation tester!

I might add that uncalibrated/spares–only PAT testers often go very cheap on the various online sites, (I won a Seaward PAT1000X for £21 not long back!) etc., and these are a useful means of reforming caps/electrolytics if they have a test voltage selection facility down to 50 or 250V which some do, and one of these with it's built–in current limiting can also be used as the basis for a "semiconductor breakdown tester" (with a cheap 500V dvm module added across the test terminals) to test unmarked or unknown diodes and zeners & things!

There is a Mullard "Valves and their Circuits" publication (I thinks it's VTC23) that covers the theory of operation of directly–heated battery valves off the mains in series filament chains, and gives the formulae for calculation of the filament shunt resistors!

Chris Williams

[brightsparkey]

Thanks for all the advice. I'm lucky enough to have access to a well equipped lab, so measuring cap leakage is a matter of a psu and ammeter. I normally prefer to work out what's wrong and replace - the effort and impact on the set is already gone to, removing the offender. So I don't need to test usually, but have in the past.

Not every cap leakage current has a degrading effect, so I'd rather only change those that are necessary- partly out of laziness but also to minimise disturbance.

A pat tester is a bit brutish on 150V caps

Of course this set would fail pat test due to the 600v cap across the mains, which I will replace with an X type.

with my new understanding I can see that the v4 output tube is drawing probably ~9ma instead of a ~6ma, so caps are on order to replace the usual culprit. Because of the heater arrangement, the voltage at the pin wrt chassis is about right, but the cathode voltage has shifted - hence the incorrect initial diagnosis.

4v is on the Kolster manual's list of pin voltages. I'm using a meter with about the right impedance so the real voltage is probably more, but the tube has only 0.9V heater so it's probably running a bit low...

[brightsparkey]

Btw all electrolytic a were good - in fact the mains supply just worked including Siemens rectifier. The set has a nice multi tapped power dropper in vitreous enamel, measures spot on. Mains voltages all fine.

[Mike. Watterson]

About the only cap leakage on the non-electrolitics that doesn't degrade the radio badly are any decouplers on the filaments and the AGC filter. Everything else has a severe effect.

I use about 300V via 2 M Ohm in series with a neon with a 100nF across it. Simple and fast. Made from a single use film camera flash gun with the 300uF replaced by 1uF plastic.
Your average ceramic disk for transistor circuits pass on it, though I mostly use 150V met poly caps to replace battery radio paper caps. They pass, i.e. the neon might light once on larger values due to capacitor charging. Most paper caps have the neon on solidly, some flash, but even 1 flash per second is a fail.

[crackle]

Ah, the KB voltage chart figures were taken with a 1000 ohm/v meter.
With 1M on the anode and 3.3M on the screen that explains why they were given as so low for V3.
They will of course measure higher if using a more modern meter as you have found.

Mike

[Mike. Watterson]

25V is low for screen. Though some DMMs are 1 M Ohm and some are 10 M Ohm, so even with those you can't measure the screen voltage on a DAF9x valve. Even slight Cap leakage will hurt gain badly there and you'd need a specialist 100M input impedance to see the difference.

I'd only start measuring the valve pin voltages if it didn't work AFTER replacing all obviously paper dielectric caps.

[brightsparkey]

Well this set has certainly been hard going..

I spent a long time going around in circles, checking and replacing caps and resistors. Eventually I discovered that the resistors I had fitted to replace the heater ballast resistors, which came from my kit of E24s were all 1000x bigger in value than the rings on them say - 120ohms was in fact 120k - checking, the whole pack is mis-marked in that decade - absolutely infuriating !

In general there were many resistors high in value when measured - I check them out of circuit. Mohm resistors mostly 40-100% high on 20 or 10% tolerance. The previously mentioned ballast resistors, and one kohm type.

Capacitors have all been changed now, but going back to check them only 2 were actually leaky, the heater decoupler on V1 (drawing mAs and seriously messing up the heater circuit - probably the main culprit) and AGC capacitor which was preventing full gain as it leaked to 0V . 'That cap' was actually perfectly OK with only a couple of uA leak at 90V.

Receiving special mention:

• The rheostat was previously broken and the failed wire stuffed under the contact. I may repair the rheostat but for now it sports a tidy pot from farnell.
• There we 4 bad solder joints, all original - the grid 2 de-coupler from the mixer, a polystyrene cap, and most annoying of all the cap across the output transformer - this introduced a high order distortion above a certain volume level - thought it was broken, but turned out it was vibrating
• Some joker rewired the mains lead and reversed live and neutral...

Anyway - respectable sensitivity now restored on LW and MW - same performance give or take to my Roberts R66 which has the same line up. Just need to finish off the mains side and then back in its case - it'll need to cool off before I can look it in the eye again..

Set voltages now measured on my lab dmm with Gohms input impedance still show around 24V on the 1st audio pentode anode and looking at the tube data looks to be correct - I presume to get as much gain as possible - it can overdrive the output stage so it seems to be alright.

Kevin.