Grundig KS740 - no anode voltage

[Diniz Diniz]

Hello everyone!
I am repairing this set (grundig ks 740).
The set seems to be in a very good condition. Nonetheless, the sound is almost not audible, even with the ear next to the loudspeaker. It is really difficult to hear something.

When performing some measurements I realized that negative values of voltage where present in the anodes of ELL80.

For the ones who have access to the diagram, I already replaced the electrolytics C77, C78, C79, C65, C71. C73 and C74 also replaced. I checked several resistors (R77, R89, R88, R81, R82, R93, R87...).

I will show above some measurement and hope someone can give me a clue.

Measurements (with respect to the chassis):
Rectifier bridge DC = 308 Vdc
Positive terminal C79 = 308 Vdc
Positive terminal C78 = 184 Vdc
Positive terminal C77 = 183 Vdc

ELL80:
Pin 1: 184 Vdc
Pin 2: -1.4 Vdc
Pin 3: -4.9 Vdc
Pin 4: 0 Vac
Pin 5: 6.7 Vac
Pin 6: -2.4 Vdc
Pin 7: 4.24 Vdc
Pin 8: -2.9 Vdc
Pin 9: 185 Vdc

ECC83:
Pin 1: 89 Vdc
Pin 2: -0 Vdc
Pin 3: 1.02 Vdc
Pin 4: 6.7 Vac
Pin 5: 6.7 Vac
Pin 6: 93 Vdc
Pin 7: -0 Vdc
Pin 8: 1.01 Vdc
Pin 9: 0 Vdc

EAF801:
Pin 1: 65 Vdc
Pin 2: -0 Vdc
Pin 3: -0.9 Vdc
Pin 4: 6.7 Vac
Pin 5: 0 Vac
Pin 6: 158 Vdc
Pin 7: -0 Vdc
Pin 8: -0 Vdc
Pin 9: -0.85 Vdc

Any help will be much appreciated
Thanks in advance.

Diniz, from Portugal

[Chris55000]

Hi!

The Grundig KS740 is two single ended output valve stages for each of the stereo channels, altho' a quick glance at the diagram suggests they've drawn one channel of a push–pull amplifier, this is not the case!

I would be very surprised to find both output transformer primaries open circuit together, unless the set has been stored in damp or unheated conditions for many years!

Use your meter on "ohms" to check the wiring from the plus(+) tab on the contact–cooled bridge h.t. rectifier and pins 3 and 8 of the ELL80 double–pentode output valve you should get a reading of 400–500 ohms between each anode pin and the h.t. rectifier, and approximately twice that (800–1000 ohms) between pins 3 and 8 directly.

An infinite or very high reading between pins 3 and 8 indicates either or both of the o/p transformers (9048–078.01) is open circuit and needs replacement, whilst a reading across the two anode pins 3 & 8 but not to the h.t. rectifier simply means you've got a wire broken off somewhere!

If you are unlucky enough to find both the transformers defective, Ed Dinning in the UK can rewind these, or you can still get Multi–Ratio Valve Output transformers from RS!

These transformers are also available from eBay, etc., altho' you would have to amend the wiring as replacement transformers don't have the hum–cancellation extended primary on them!

Chris Williams

[Chris55000]

PS!

Running the set with no anode voltage on the anodes of the double–pentode output valve for long periods of time is very likely to permanently damage the valve as all the valve's cathode current has to flow out of the valve via the two screen grids, which would almost certainly be glowing bright red or orange under these conditions!

Once you have located and repaired your fault you should throw the ELL80 away and get a replacement, as running an output valve with no anode voltage risks overheating it's screen–grid to the point where the G2 wires can sag and cause an internal short to G3 & cathode or the control grid G1, causing further damage to the set!

I forgot to mention that if you need a replacement output transformer, this has to "match" the anode of the output valve to the loudspeaker, and for this purpose, the "Optimum Load" impedance, quoted in ohms, is always given by a reputable valve manufacturer for any output valve!

The turns ratio of the transformer is:–

✓(Anode load of output valve from datasheet/loudspeaker impedance)

For the ELL80, the load impedance is 10,000 ohms and the speaker 5 ohms, so the turns ratio is:–

✓2000 = 44.7 to 1,

. . .or 45:1 in round figures, this ratio suits the 6V6/6BW6 type valves, so a transformer sold for these valves or "45:1" will do – it's the most common ratio there was in the valve era!

Chris Williams

[Diniz Diniz]

Thank you very much Chris for your detailed and very helpfull answer. This is what keeps me doing this repairs, I am always learning

I measured the impedance between (+) rectifier bridge and pins 3 & 8 of output valve and I get infinite resistance. The same occurs between pin 3 & pin 8.
I performed some more measurements to verify previous results, and I invite you to see attached.

In my opinion the output transformers so not seem to be in "that" bad conditions.... and the set seems to have been stored appropriately.

Also I attached some photos of output transformers.

Thanks in advance

Diniz, From Portugal

[TrevorG3VLF]

The trnsformers are rusty and the soldered connections could be faulty.Try resoldering the connections and check that the transformer wire is not broken.
Use cored solder with proper flux, not plumbers acid flux.
If the wire has corroded through where old flux was, then you may be able to extend the dog end with care.

[Chris55000]

Hi!

The reason a special output transformer has been used with a tap on it's primary is to provide hum cancellation!

Because the output valve is fed directly from the non-smoothed H.T. supply, the 100 c/s ripple across the H.T. reservoir electrolytic will add a component of hum ih2 to the wanted signal anode current of the output valve - see attached diagram.

By feeding the H.T. supply into a tap on the output transformer primary and routing the remainder of the set's H.T. current via the small section 1-2 of the transformer primary at the top end, the receiver's hum current ih1 produces a hum-flux in the core which acts in opposition to that from the output valve, and therefore there is very little audible hum in the loudspeaker if the tap is correctly arranged.

However as these transformers aren't readily available unless rewound, it's necessary to connect the h.t. smoothing resistor directly to the top end of the output transformer primary together with the h.t. lead from the rectifier as shown in my second diagram, and in this case I recommend changing the smoothing resistor to 2k7 5W.

There is an advanced method of arranging hum-cancellation using a Wheatstone Bridge technique that doesn't need a special tapped transformer,
but this involves changing the O/P valve to operate from a separate G.B. resistor in the H.T.- lead, difficult to arrange in a printed-circuit built set!

Chris Williams

[Chris55000]

Hi!

Second diagram - replacement using standard O/P transformer.

Chris Williams

[Diniz Diniz]

Hi Chris,
Thank you for your detailed replacement suggestions.
Would this work for me ?
https://www.ebay.com/itm/2-Ausgangsu....c100009.m1982

Can you send me a link of one so called "standard OP transformer" ?

Thanks in advance,

Diniz from Portugal

[Diniz Diniz]

Also, I just remember that I have one telefunken bolero 2182 (stereo with two EL84) for spares. Are their OT's suitable for this set?

Thanks