Weird fault on Akkord Pinguin K58 battery/mains portable.

[Tim]

Hi all.
I wonder if any of you could help with this.
The output is quieter than expected, slowly distorts and then disappears.
All valves OK, and all low value caps tubular ceramics which are usually very reliable. HT caps OKtoo. I haven't got any info for the set, but the distortion seems to be being generated in the DL96 stage. Waveform is fine up until the DL 96 control grid, but is 'orrible on the anode. Valve checked and Ok. All likely caps either polystyrene or ceramic so probably OK. Sound gets more distorted and disapears completely in a few seconds when DL96 grid volts measures -19volts. Connecting a scope to the grid results in a burst of working and a 200 k resistor grid to chassis increases the time to distortion to an hour or so Grid volts slowly becomes more -ve and HT drops. Rate of change about .1 volts every 5 seconds.Heaters are constant 1.3 volts.
Told you it was weird!
Any ideas 'cos I haven't!
Thanks
Tim

[AVO_VCM163]

Tim,
Sounds like possibly DK96. Does the fault persist with a replacement?

Odd that grid goes -ve. Could this be due to increasing hf (internal oscs) and rectification? 10nF grid to earth - any effect?

R

[dave_n_t]

Tim,

A couple of clarifications, plse, to aid 'distant diagnosis':

are you running from the (internal) mains supply; battery (i.e. replacement battery made from PP3s, etc.); or a (mains powered) battery eliminator? It might be good to eliminate these as possible causes.

You say that HT goes down - do you mean the power supply HT, or the voltage at the top end of the o/p transformer? What about the voltage across the output transformer (this is a reasonable measure of anode current in the DL96, since the winding resistance is likely to be fairly constant)?

And, where are you measuring the HT voltage (many of these sets have a 'self-bias' resistor in the HT -ve lead (around 470 ohms or so - if excessive current is being taken by the set, the voltage across this will rise.

My valve data says the grid voltage shd be about -5V; so with -19V the valve may well be cutting off....

dave

[Mark the Mender]

Now I had a fault very similar to this on a Vidor Lady Ann I did recently.
It would work for an hour or so then distort and fade, the more I tried the less time the thing worked.
In the end it was the main electrolytic smoothing can finally giving up the ghost.

its a start

cheers Mark

[Steve_P]

Reading this and the other post about these sets....

https://www.vintage-radio.net/forum/...ble+valve+sets

https://www.vintage-radio.net/forum/...ble+valve+sets

A look round on the net makes me think that your rectifier is past it's best. If it's only giving 50v on load that is very wrong. The sets I was able to find data on said 100v and 1.5v.

Change the diodes and, like Mark above me says, the smoothers. A photo of the diode and the way it is connected would help me work out how it's wired.

Cheers,

Steve P.

[Steve_P]

Just had the Vidor Lady Anne circuit up. Does the Mains Transformer secondary have a lead going to earth or not.

If it does, 2 diodes. If it does not, then 4 in a bridge format. I think.

Cheers,

Steve P.

[Tim]

Hi Steve.
Thanks for your responses guys.
I am using the mains option on the set. HT still low with DL96 removed and substitute smoothers(they tested fine anyway). HT remains for some time with the set switched off suggesting there is no short present.
With the valve in the anode voltage is about 5 volts lower than the screen grid.
I think the valve is being biased off too.
Is it possible this set uses one of the diodes in the block for -ve bias, and if my HT one has gone, this will stay at the same -ve voltage while my HT stays low.
I'll stick a 1N4004 across the diodes and see what happens.

Another strange thing is if I connect a 200 ohm resistor connected from the "bottom" of one of my 32uF caps to chassis restores HT to normal levels..
I think the mixer is fine as a good programme wave form is present at the DL96 grid, on all bands.

[Steve_P]

Sometimes you got the earths on the batteries separated by a resistor somewhere. Do the 'Bottom End' of the HT smoothers go directly to chassis earth?

Use an Ohmeter on a low range and check! Sometimes the earths are not 100% common.

Cheers,

Steve P.

[petervk2mlg]

D you have a schematic for this set?
Here is one in case not.
Peter

[Robert Darwent]

Hi Tim,

Further to the links provided by Steve pointing you to my valve portable set Success Stories, and in addition to what others have said I would suggest you also examine very closely the pins of all the valve sockets, especially that of the DL96.

It may not seem at all obvious but one or more broken pins making poor or intermittent contact may be the source of the 'weirdness' in your set rather than component failure. Especially as you say touching the DL96 grid pin produces a burst of operation.

The sockets used in these types of set are often prone to pin fatigue failure where one of the fork prongs breaks. They often look OK visually, but inserting a needle in each pin in turn will reveal which forks are broken and need replacing.

Its something you may not be too familiar with if your usual restorations are on mains sets rather than battery valve portables. I've found it can lead to some very strange problems!

Regards

[Tim]

The sockets are the paxolin type, but I'll have a look Robert. I think it's an effect though, as it does it at the other end of the wire linking the anode of the DAF96(via a capacitor) to the grid of the DL96.

Many thanks for the diagram Pete!!

[MichaelR]

Hi Tim,

I would check every component around the grid circuit of the DL96,including the 50Uf electrolytic in the powersupply. I think your instincts are right and that the grid bias to the valve is being upset. Also check the wiring in case somebody has been there before.

Any componenet that you suspect replace it with new

Mike

[kalee20]

It does rather sound to me as though the DL96 is being biased 'off', and that there's no DC return path for the grid. If the valveholder isn't making contact, there can still be enough capacitance to couple a signal to the floating grid, which in turn could slowly float to any DC level it wants (due to capturing the odd stray ion within the envelope).

Look at the waveform on the grid pin itself (not the valveholder contact) by removing the valves, winding a few turns of fine wire around the pin, and then plugging back in. Compare this with the waveform on the valveholder lug, looking at them both simultaneously with a dual-channel 'scope. Or failing that, just take the coupling capacitor and grid return resistor direct to the wire 'tail' just described!

[GeorgSc]

Hello Tim,
there is something rather special in this set, the
"Stabi St35".
This is a "Neumann Stabilisator", in fact it is a special
form of a Ni/Cd-Accumulator, with very low capacity, but
a high permanent loading current. They looked similar
to a electrolytic with axial leads.
(Neumann is the same as the "Microphone-Neumann",
they invented the sealed NiCd-cell, which was produced
under licence contract by DEAC in the 50ties, the "Stabilisator cells"
were marketed by Neumann)
So, in the Pinguin You could either use a primary cell or a DEAC NiCd as a
heater battery, or switch over to the "Stabi" when using
mains only.
Such "Stabis" are not available since decades, the Symbol (Diode)
in the schematic is not original, I suppose it reflects
one repair method, i. e. replacing it by a diode of some
1.4 Volt forward. (eg one selenium or two silicon diodes in series).
Another replacement is a A or AA NiCd-Cell, You have to
look for the permanent load current allowed when selecting one.
Are the selenium rectifiers working?
Very often they have a very high resistance. "The Tooth of Time" gnaws on them :=(
Good luck,
Georg

[Tim]

Thanks everyone.
Diagram supplied above seems to be right.
No progress today, set can only be made to work by connecting point shown in diagram below to chassis via 200 ohm resistor. It works, but I don't know why!
It does this with or without subsitute diode.

Seems to be stable,HT is correct and nothing is getting hot. Will investigate DL96 grid pin ASAP.
All electrolytics in PSU OK, including the 50uF.(checked by subsitution)
Anyone got any ideas, while I still have some hair!!

[Steve_P]

There's a 600 ohm resistor going between that point and deck. Well I think it's 600, my eyes aren't what they should be. Check it.

Cheers,

Steve P.

[Robert Darwent]

Originally Posted by Steve_P

There's a 600 ohm resistor going between that point and deck. Well I think it's 600, my eyes aren't what they should be. Check it.

I have a U58 set which is very similar to the K58 and I believe that that 600 ohm resistor is mounted on the thumbwheel pot tags mounted on the right hand side looking into the back of the set.

Your added 200 ohms resistor is certainly paralleled across it, so definitely worth checking as Steve suggests.

Regards

[Tim]

There's a 600 ohm resistor going between that point and deck. Well I think it's 600, my eyes aren't what they should be. Check it.

I have, and it's fine. I will try another though. Funnily enough the best value for the "bodge" resistor worked out at about 600-700 ohms............

[GeorgSc]

This 600 (500?) Ohms resistor maybe is
good when cold, but "opens" when hot,
that is my "theory".
What is in Your set, where the schematic shows that
"Stabi St35"? If it is a selenium rectifier, think about
replacing it. If this part fails, the heater voltage will
increase, maybe damaging some of the filaments.
Did You measure heater voltage already?
Regards
Georg

[Tim]

Georg.
Heater voltage is exactly 1.4 volts.