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Old 20th May 2020, 9:08 pm   #1
Diniz Diniz
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Default Advice on output transformers

Hello everyone,

I apologize to come to this issue again.

I have this set, a Grundig KS740. We have already discussed about it here in this forum, and I found both output transformers damaged (primary in O.C.).

The transformers are two single ended (grundig ref. 9048-078.01) that work with an ELL80 (2xEL95 in the same envelope, I think).

A member of this forum provides me for free two OT that belonged to a Grundig (and came from Australia to Portugal), but it happens that they were push-pull type and work with a different output valve.

I recently acquired a chassis of a Grundig Mandello e-ST, hoping to find the transformers I need, but unfortunately they also are damaged
So, I plan to buy standard OT (with only 2 leads on the primary side) and follow Chris suggestion: 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, and change the smoothing resistor to 2k7 5W.

But I am not that familiar with transformers types, I was wondering if anyone can help me understand what is suitable and what is not.

I found this one (see link below), but the primary impedance is 5kOhm, and I believe a 10kOhm is needed for ELL80....
Also, I think the primary DC resistance must be lower than what I need...

https://www.aliexpress.com/item/4000...7e18466ccupONQ

Help me please, because now I have two sets waiting for suitable OT.

Thanks in advance,

Cheers from Portugal

Diniz
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Old 21st May 2020, 8:41 am   #2
cathoderay57
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Default Re: Advice on output transformers

Hello Diniz, RS Components provide a suitable transformer so long as you add the HT smoothing resistor. https://uk.rs-online.com/web/p/audio...rmers/2106475/ . The datasheet (copy attached) shows how to connect, depending on your speaker impedance. For the ELL80 the anode load is as you say 10k so for a 3 Ohm speaker connect speaker leads to A and C and HT and anode to 1 and 4. If it is 8 Ohm speaker the closest match you can configure is 9k which is probably good enough, using terminal posts D and B for speaker and 1 and 3 for anode/HT. Unfortunately the cost for 2 transformers is quite a lot. RS Components does international sales mail order and has an outlet in Spain but not Portugal. We are regular visitors to Portugal and one time we went to the Radio Museum at the site of your national broadcaster in Lisbon. I was lucky enough to be shown the reserve collection in the basement which was fantastic. The virtual on-line museum is also excellent. Cheers, Jerry
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Old 21st May 2020, 1:11 pm   #3
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Default Re: Advice on output transformers

For those who haven't seen it here is the link to the Lisbon RTP Radio & TV Museum https://museu.rtp.pt/
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Old 21st May 2020, 3:26 pm   #4
Diniz Diniz
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Default Re: Advice on output transformers

Thank you so much Jerry for you kindness and helpfull advice. I will order the components and let you know the final result. The speakers are 5ohm, so I think I can manage it.

And thanks for the advice on the museum, I have already heard about it but I didn't visited it yet. Now, I will definitely visit it, since I live close.

Once you come again to Portugal, let me know... I will be very pleased to meet you.

Regards

Diniz from Portugal
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Old 30th May 2020, 2:14 am   #5
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Default Re: Advice on output transformers

Hi everyone.
I recently get two OT from RS (the ones mentioned above).
I am facing a new problem...
I installed the first OT in the right channel. Terminal 4 of the OT to the anode (pin3), terminal one to the rectifier and to the screen through a 2.7kohm resistor, and terminal A and C (secondary) to the speaker (5ohm). With the balance to maximum right channel the sound is clear ... but when turning to left channel a very loud and intense whistle/squeak (I don’t know the exact term in English) starts. I installed the second OT (left channel) and immediately when I switch on the radio, the same whistle starts independently of the balance position (it no more possible to hear the stations) and I am forced to switch off the radio.
When I say very loud whistle, I mean it. I tested different ELL80 and the problem remains.

Any tips on what may be causing the problem?
Thanks in advance.
Regards.
Diniz from Portugal.
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Old 30th May 2020, 3:12 am   #6
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Default Re: Advice on output transformers

You have to reverse the primary OR the secondary leads.
You have made positive feedback, not negative feedback.
In fact you have made a quite powerful oscillator.

Best wishes
Joe
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Old 30th May 2020, 6:22 am   #7
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Default Re: Advice on output transformers

I do not know the circuit of that set, but if it has feedback from the loudspeaker output back into an earlier stage of the amplifier, it will be important to have the phase shift through the transformer correct.

If the transformer phase is the opposite of the original, instead of negative feedback, you will have positive feedback.

If this is the problem, it should be cured by interchanging which terminals the two anodes are connected to.

David
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Old 30th May 2020, 9:03 am   #8
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Default Re: Advice on output transformers

I think David is right, there is a 12k resistor attached to one wire of each OT secondary winding and the other end of that resistor is connected to the ECC83 cathode to provide negative feedback. Simply interchange the anode and rectifier wires on the output transformers should do the trick. Cheers, Jerry
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Old 30th May 2020, 12:16 pm   #9
Diniz Diniz
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Default Re: Advice on output transformers

Thank you so much. That’s it. The set is now playing in both channels.
But I am not still confident about one issue... the sound is nice at low volume, but it seems to be a bit distorted when I volume up. I don’t know if distorted is the proper term... the sound gets messy And with some back noise...
Would it be because the OTs are not yet soldered to the chassis ?? I just connected the wires.
The described problem is enhanced when using the P.U. Input.

Hope to hear your opinion. And once more thanks to all of you for the help and encouragement.
Diniz
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Old 30th May 2020, 12:55 pm   #10
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Default Re: Advice on output transformers

I don't know. Different people use different words to refer to effects on sounds. This is where a picture from an oscilloscope with a steady input signal would tell a lot.

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Old 30th May 2020, 2:46 pm   #11
Diniz Diniz
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Default Re: Advice on output transformers

Would you be open to teach me how to do that?

Thanks

(If a video of the problem can help, I can upload it on YouTube )
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Old 30th May 2020, 3:14 pm   #12
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Default Re: Advice on output transformers

Not sure if you have replaced any of the paper capacitors. I think the paper capacitors are marked on the circuit with 2 small dots to the left of, or above, the capacitor symbol (that is usually how Grundig marked them). Particularly it is worth replacing C68 and C69 (10nF) as well as C53 and C54 (3.3nF). Also small electrolytic cathode decoupling capacitors C71 (100uF) and C65 (50uF) can give trouble if leaky. Cheers, Jerry
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Old 30th May 2020, 4:18 pm   #13
Diniz Diniz
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Default Re: Advice on output transformers

I have already replaced some capacitors, some of them are c68, c69, c65, c71, but not c53 and c54. I plan to replace all paper capacitors and electrolitics. Hope that makes some improvement.

Another thing I have noticed is that the FM-stereo lamp don’t turn on when pushing the button. I tested the lamp and it is ok... This set is different from Grundig Mandello which have a grundig 6 decoder. Instead it have this apparatus on the socket (see attached image ).

Thanks so much.
Diniz from Portugal
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Old 30th May 2020, 7:27 pm   #14
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Default Re: Advice on output transformers

I think that is simply a 9-pin plug with a U-cable clamp and de-emphasis resistor/capacitor that plugs into the decoder socket when the decoder (presumably an option?) is not fitted. The resistor/capacitor serve to correct (de-emphasise) the rising response (pre-emphasis) used in FM transmission to achieve good signal-to-noise ratio. That would explain a lack of stereo light.
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Old 30th May 2020, 10:07 pm   #15
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Default Re: Advice on output transformers

I would try replacing c53 and c54 next, and see if the distortion problem is solved. Also check that the speakers sound OK at high volume on another receiver or amplifier. Using an oscilloscope would definitely give clues about the reason for the distortion but unless you already have an oscilloscope, it would probably be quite expensive and if you buy an old one you could have lots of problems trying to restore it before you can use it. Ideally you would also need a signal generator but you could use an audio tone generated from a computer app instead, and feed the signal in via the pick-up ( record player) sockets. It's also useful to measure the voltages on the ELL80 anodes, grid 2 and cathode and the ECC83 anodes and cathodes.
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Old 1st Jun 2020, 12:19 pm   #16
Diniz Diniz
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Default Re: Advice on output transformers

Hello to everyone.

I have already replaced paper capacitors (C68, C69, C73, C74, C63, C64, C56, C58, C54, C53, C51, C52, C28, C25, C23, C22, C21, C20, C17, C16, C15, C14, C31) and electrolytics (C65, C77, C78, C79, C29).

The sound seems much better now. Though in P.U. mode some "distrotion" is still noticed when volume is a bit high.

Also noticed that one of the OT gets a bit hot than the oder, i.e., after playing for 3 or 4 hours, one of them remains cold and the other is hot (but I can perfectly touch it without burning myself... I can keep the finger there witout problems). Is it normal?

As soon as possible I will mesure the anodes and cathodes voltages of ELL80 and ECC83.

Thanks again for all your effort and help.

Diniz
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Old 1st Jun 2020, 12:20 pm   #17
Diniz Diniz
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Default Re: Advice on output transformers

Quote:
Originally Posted by turretslug View Post
I think that is simply a 9-pin plug with a U-cable clamp and de-emphasis resistor/capacitor that plugs into the decoder socket when the decoder (presumably an option?) is not fitted. The resistor/capacitor serve to correct (de-emphasise) the rising response (pre-emphasis) used in FM transmission to achieve good signal-to-noise ratio. That would explain a lack of stereo light.
Thanks for the explanation, that may explain the absence of the fm-stereo light.
Quote:
Originally Posted by cathoderay57 View Post
I would try replacing c53 and c54 next, and see if the distortion problem is solved. Also check that the speakers sound OK at high volume on another receiver or amplifier. Using an oscilloscope would definitely give clues about the reason for the distortion but unless you already have an oscilloscope, it would probably be quite expensive and if you buy an old one you could have lots of problems trying to restore it before you can use it. Ideally you would also need a signal generator but you could use an audio tone generated from a computer app instead, and feed the signal in via the pick-up ( record player) sockets. It's also useful to measure the voltages on the ELL80 anodes, grid 2 and cathode and the ECC83 anodes and cathodes.
I have a oscilloscope and one day I would like to learn how to use it. However I do not have a signal generator

Thanks for all.
Diniz
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Old 1st Jun 2020, 3:58 pm   #18
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Default Re: Advice on output transformers

Hi Diniz. Concerning the OTs, if one is getting hotter than the other it indicates that one half of the ELL80 is taking more current than the other. This could be a clue to the distortion. Both pentodes inside the ELL80 valve use the same cathode resistor, R87 (=200 Ohms). That means the 2 pentodes inside need to be a matched pair to work properly. If one of the pentodes is drawing more current than the other, the biasing voltage produced by the current through R87 will be too low for one of the pentodes or too high for the other. That means that one of the pentodes is being biased such that it is working in a non-linear part of its characteristic. Is the distortion worse on one channel than the other? Because the 2 cathodes are connected together inside the valve (=pin 7) unfortunately you cannot get around this problem by replacing R87 with 2 separate cathode resistors! You could try another ELL80 and see if the distortion is any better. The anode voltages of the ELL80 might give us a clue if one of them is biased badly. There is quite a bit of info on the Web explaining how to use oscilloscopes. Here are a couple of links to get you started https://www.wikihow.com/Use-the-Oscilloscope and https://www.youtube.com/watch?v=niBG...Y&spfreload=10 . As I said for audio work you don't need a signal generator. Just connect up a jack plug to the phones output of a laptop or iPhone and use this app http://onlinetonegenerator.com/ Hope that helps, cheers Jerry P.S. Also worth checking the ELL80 grid 2 resistors R58 and R59 - should both be 2k

Last edited by cathoderay57; 1st Jun 2020 at 4:22 pm.
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Old 2nd Jun 2020, 12:16 am   #19
Diniz Diniz
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Default Re: Advice on output transformers

Hi. I just performed the following measurements (against the chassis) with FM selected.

ELL80:
Pin 1 (Grid 2') --> 247.7 Vdc
Pin 2 (Grid 1') --> 150 mVdc
Pin 3 (Anode') --> 286 Vdc
Pin 6 (Grid 1) --> 130 mVdc
Pin 7 (cathode) --> 8.35 Vdc
Pin 8 (Anode) --> 282.5 Vdc
Pin 9 (Grid 2) --> 248.0 Vdc

ECC83:
Pin 1 --> 130 Vdc
Pin 6 --> 130 Vdc
Pin 3 --> 1.8 Vdc
Pin 8 --> 1.8 Vdc

At the exit of the rectifier (B+ I believe is the term) the voltage was 301 Vdc

Unless I am mistaken, everything is within the specified values...

Hope to hear from you.

Thanks for the tips about oscilloscopes, I'll take a look.

Cheers, Diniz

(PS: I believe you meant R88 and R89 in your previous post).
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Old 2nd Jun 2020, 9:19 am   #20
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Default Re: Advice on output transformers

Hi Diniz, I tend to agree that the voltages don't appear to show any big problem. I was expecting the ELL80 anode voltages to be very different but that is not the case. For comparison (and for other members who do not have access to the Grundig service data) here are the quoted voltage figures for the ELL80 in brackets:
Pin 1 (Grid 2') --> 247.7 Vdc (MW 286v FM 248v PU 226v)
Pin 2 (Grid 1') --> 150 mVdc
Pin 3 (Anode') --> 286 Vdc (MW 276v FM 260v PU 264v)
Pin 6 (Grid 1) --> 130 mVdc
Pin 7 (cathode) --> 8.35 Vdc (MW 9.8v FM 6.5v PU 7.7v)
Pin 8 (Anode) --> 282.5 Vdc (MW 276v FM 260v PU 264v)
Pin 9 (Grid 2) --> 248.0 Vdc (MW 286v FM 248v PU 226v)
Yes, I got the resistor numbers wrong, apologies. The scope is probably the best next step unless anyone else has better ideas. Cheers, Jerry
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