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Old 10th Jun 2020, 8:56 pm   #41
Diniz Diniz
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Default Re: Advice on output transformers

Hello Jerry, thanks for your comments.

I followed your advice and I installed a 100k pot in the 6.3V heater supply (see Fig1 and Fig2). The 10k resistor was installed in the pot middle lead.

With the grundig level set to low-medium, I obtained the following signals: Fig3 is for the potentiometer set to value close to 100k; Fig4 is for the potentiometer set to a value close to minimum resistance. In both grid 1 and grid 1' the results were similar.

After that I set the pot to its minimum and have increased the grundig volume. I was not confident to set it to maximum because the sound was very loud and started to be distorted. I don't if I have reasons to, but I was also afraid of the voltage readings in grid 1 and grid 1'. The results are shown in Fig5.

I also tried the different OT connections configuration but the sound was worst, so I resoldered it back to the original.

I guess I should definitely try different speakers... I will try the originals when I have a chance.

Thanks so much for your kind help and your effort.

Kind regards,
Diniz
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Old 10th Jun 2020, 9:56 pm   #42
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Default Re: Advice on output transformers

Hi Diniz, thanks for the update. At least we have got a clean 50Hz supply going in, to test the response of the Grundig. Assuming that Figure 5 is the speaker output it seems to show that the output is saturating - indicated by the flattening of the upper waveform. I can't work out what the peak-to-peak voltage is showing as I'm not familiar with this scope. However, if the input signal is very high then it might be that the output valve is clipping i.e. there is insufficient voltage swing left on the anode to faithfully reproduce the upper sine shape. Alternatively the OT core might be saturating, although, with only 18mA anode current passing though the primary, that shouldn't be the case. Somebody more knowledgeable than me might be better able to comment on this. You've been very patient and no doubt spent a lot of time - hope it has been fun! I'll look forward to hear how you get on with different speakers. Kind regards, Jerry
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Old 13th Jun 2020, 10:33 am   #43
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Default Re: Advice on output transformers

Thanks Jerry for your comment and explanation. I’ve been learning a lot Just to clarify, the measurements in my previous comment were at grid 1.
Next week I will be able to test the original speakers, and I’ll let you know the performance of the sound.
And thanks again for all your patience and guidance. This has been a very fruitful project

Kind regards,
Diniz
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Old 13th Jun 2020, 11:40 am   #44
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Default Re: Advice on output transformers

Hi Diniz, OK, thanks for the update. Because your Figure 5 was measured at the grid of the ELL80, what I said was not correct. The distortion of the upper part of the sinewave in Fig 5 was happening at the anode of the ECC83 that is coupled to the ELL80 grid 1 via a capacitor. The distortion is probably because the test input signal was a little bit too high at the PU socket and therefore "overloading" the ECC83 pre-amplifier. If you get the opportunity to repeat the test, reduce the 50Hz input signal until you get a clean 50Hz waveform at the grid of the ELL80. Then transfer the scope leads to the speaker connections on the OT. That should then show you whether any distortion is happening in the ELL80 stage and whether the OT is saturating. Let's also hope some different speakers sound better! Cheers, Jerry
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Old 15th Jun 2020, 3:43 pm   #45
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Default Re: Advice on output transformers

Hi everyone!
Ok, I tested the original speakers and there is no improvement

I repeated the 50Hz signal test, and I monitored grid 1 of ELL80 in pin2 (Fig attached) and pin6 (Fig attached) until I get a clear signal. I moved the scope leads to the OT secondary and the result is in the Figure labeled as "OT secondary".

I would like your and other members opinion on what may be causing the distortion.

Thaks so much for your help.

Regards,
Diniz
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Old 15th Jun 2020, 4:22 pm   #46
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Default Re: Advice on output transformers

Hi Diniz, can you work out from the scope settings what the AC peak-to-peak voltage is at grid 1? I can see 50.0v indicated bottom left of the screen but not sure what that relates to. If the voltage is very high at the grid it is possibly higher than the ELL80 can handle. Alternatively, there is a problem in the output stage. Do both channels show the same nasty output waveform? In another thread I found a spec showing the frequency response of the OT http://oep.co.uk/page-content/datash...0issue%207.pdf Seems nothing unusual in the response at around 50Hz, and the anode current is well within spec. The only other thing I can suggest at this point is to try disconnecting the negative feedback resistor and take another look at the waveforms. It might give a clue. The curious thing is that your previous measurements at 400Hz and 1000Hz were OK so the problem only seems to happen at low frequency. Cheers, Jerry
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Old 15th Jun 2020, 6:26 pm   #47
Diniz Diniz
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Default Re: Advice on output transformers

Hi Jerry,

Which one is the negative feedback resistor? Do you mean R81 and R82?

Diniz
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Old 15th Jun 2020, 6:57 pm   #48
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Default Re: Advice on output transformers

Hi Diniz, yes, R81 and R82, that's correct. Before you do that, I had another thought. Can you check the connections to the grid 2 resistors R88 and R89. One end will be connected to grid 2 and grid 2' via the PCB tracks, but it is the other end I'm worried about. It is possible when you swapped the OT wires around to cure the previous howling feedback problem that R88 and/or R89 have been left connected to the wrong side of the OT primary winding. The end of the resistors must go to the OT connection to the rectifier (i.e. HT positive line). If, instead, one or either of those resistors is connected to the anode side of the OT primary then that could be causing distortion. Kind regards, Jerry
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Old 16th Jun 2020, 10:25 am   #49
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Default Re: Advice on output transformers

Hi Jerry,

Bad news!

I checked all connections of R88 and R89 and everything was ok. One lead goes to grid 2 (or grid 2') and the other goes to the OT and is connected together with HT supply from the rectifier.

After that, I followed your suggestion and disconnected R81 and R82 and take a look at the signal at secondary OT (fig 1 below). The signal showed no improvement and the peak-to-peak voltage was 6.40V.

So, I reconnected R81 and R82 willing to check the peak-to-peak voltage in "business as usual" case. Something very strange happened!

The only thing I heard is a kind of "spiky rain " in the loudspeaker. It was not continuous...it goes and comes... (kind of: tic..tic.....tic..tic.tic.....tic....tic.tic.....t ic..)
No sinewave was detectable in the scope. I also noticed that the Magic eye (that usually goes blue when starting the set) didn't seemed to be fed.

When I was trying to find out the problem, I think I measured 347V in the positive lead of the rectifier. I thought something was wrong with C79, so I replaced it with a 100uF/250V (the same ratings of the one that was installed there).
When I turn the set on, the positive lead of C79 starts to sparkle and burn...I was able to see the sparkles and I immediately turn the set off.

I can’t figure out what could have happened, since I ONLY have disconnected R81&R82, and reconnected them

Could a damaged rectifier do that to C79? Could a damaged rectifier increase its DC supply? Could it be causing the previous distortion we were trying to fix? Any thoughts?

Regards,
Diniz
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Old 16th Jun 2020, 2:14 pm   #50
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Default Re: Advice on output transformers

Hi Diniz, this Grundig is definitely not giving up without a struggle! The best thing to do is to disconnect the positive lead from the rectifier and measure the dc voltage between the rectifier output lead and chassis. Before applying power, with mains disconnected, you can also check it using a multimeter on Ohms range. From the AC input pins you should only get continuity in one direction to the positive lead and also to chassis. Cheers, Jerry
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Old 16th Jun 2020, 2:52 pm   #51
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Default Re: Advice on output transformers

I should add that if you try measuring the resistance as described in Post #53 with a digital multimeter the results will be meaningless. Don't bother doing that unless you have an analog meter. It sounds like the selenium rectifier marked B250 C100 https://www.radiomuseum.org/tubes/tube_b250c100~2.html has failed. They can sometimes fail as a random occurrence - probably not because of anything you have done. I'm not sure about the effect of a failing rectifier on distortion - yes, it does have impact but usually only when it is delivering lower than required voltage and current and the voltages you measured previously (when the radio was working) seemed OK. If the rectifier has failed you can easily get a suitable replacement semiconductor bridge rectifier and they are not expensive. If the rectifier measures short circuit (i.e. conducts in both directions) or if you are getting very low or no dc output then switch off immediately because it is possible to damage the mains transformer.

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Old 16th Jun 2020, 4:01 pm   #52
Diniz Diniz
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Default Re: Advice on output transformers

I followed yout advice in a hurry at lunch time.

I have disconnected the positive lead of the rectifier (the one that goes to C79) and measured 242Vdc against the chassis. The AC input was 258Vac.

With mains disconnected I checked for continuity between the positive lead and AC inputs (no continuity found). Also checked for continuity between negative lead (Chassis) and the AC pins (no continuity found). My multimeter is digital.

I resoldered the positive lead of the rectifier and the problem prevails --> sparkles on the positive lead of C79. (nonetheless, it keeps the value of 100uF).

Later today I will recheck the solderings of R89&R88 leads, R81&R82 leads. I should also recheck the measures of R93 and C73&C74 to see if they are still ok. I am guessing if something is not properly soldered...

You dont need to worry about what I am going to say because I am very carefull doing my repairs. I am only adding it because it may say something about the problem (the one that is not related to the fact that I am a newbie ).... Was it to supposed to get electrical shock touching the anode of ELL80 with the main disconnected (power supply off)? I believe it was C79 discharging ... am I right?

Hope to get the grundig back to life!

Cheers!
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Old 16th Jun 2020, 4:24 pm   #53
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Hi Diniz, please do be careful, the electrolytic capacitors often hold an electric charge even after the radio is switched off. Always test high voltage points with your multimeter before touching! Well, it sounds like the selenium rectifier might be OK. The lack of magic eye glow means that there is probably a short circuit on the HT line somewhere. With power off make a careful re-check of the work you did disconnecting and reconnecting R81 and R82 and make sure you have not accidentally dropped melted solder across any PCB tracks. If that all looks all OK, the most likely failed components are C79, C78 and C77. If you are using C79 rated at only 250v that is much too low. Before the valves warm up and start drawing current, the voltage on C79 is probably going to exceed 340v and it sounds like C79 is failing short circuit (producing the sparks and ticking noise in the speakers). You need to replace it with a capacitor rated at least 350v and ideally 400v. Use 350v capacitors for C78 and C77. C79 probably still tests showing a capacitance of 100uF because the capacitance test is done at very low voltage. C79 can still be failing at high voltage. Cheers, Jerry

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Old 18th Jun 2020, 1:53 am   #54
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Default Re: Advice on output transformers

Hi,
The Grundig is back to life! After resoldering C79 leads, I saw some sparkles in the ELL80 socket (with the tube removed). I thought that something was not ok, so I remove de socket and re-install it. It happened that probably when I soldered a wire to monitor pin 1 of the ELL80, the contact between the socket pin and the upper PCB track was lost. Then I install a wire between pin 1 and pin 9 to assure the contact... and the set came back to previous condition. I guess it was a random coincidence the fact that it happened immediately after disconnecting and reconnecting R81 and R82.
I must remind you that no improvement was observed when R81 and R82 were disconnected...However I realized that those resisters are rated 12kohm in the schematic but they actually measure 1.9kohm (both). They seem to be the original ones, since the set appear to be its original conditions when i start the restoration.

The following is intriguing me...the next pictures were taken with the "test potentiometer of 100kohm" set to minimum...the signal from the heater supply was fed to the PU socket (as previously done).
Fig1 shows the measurement at OT secondary (peak to peak 64.8V).
Fig2 shows the measurement at PU socket (i.e. after the 10kohm resistor installed in the middle lead of the 100kohm pot). Is it possible that the peak to peak voltage is 182V? Because the scope is saying so I thought that the maximum should be below 6.3v

Does anything of this help the diagnostic?

Now it is a question of pride...I would really like to know the cause of the distortion.

Thanks so much for your kind attention and for the knowledge you're sharing with me.

Regards
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Old 18th Jun 2020, 6:43 am   #55
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Default Re: Advice on output transformers

Hello
I don't think you are going to get far with using 50HZ as an input signal.
With small output transformers there is bound to be distortion at that frequency.
You need to inject a 1KHZ.
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Old 18th Jun 2020, 8:41 am   #56
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Hi Diniz, I am getting to the point where I am running short of ideas! All I can suggest at present are:
1. If you are still using a 100uF 250v cap for C79 you definitely need to replace it with one rated at 400v, for the reasons I explained in Post #56. I know you said that the one you found fitted was 250v, but that was maybe not the original capacitor. In the circuit diagram C79 is marked as 350v working voltage, 385v peak.
2. The 50Hz input scope waveform voltage is very puzzling. It cannot be 182v pk-pk so it must be something to do with the scope settings. Maybe it would help if you showed an image of the scope controls setting and identify the scope model.
3. Peter makes a fair point in Post #58 but we know from previous tests that the waveform seems undistorted at 1kHz and the pk-pk levels normal - and yet there is still audio distortion at medium to high volume settings.
4. It's tempting to think that the replacement OTs could be causing the problem - however, they are being used well within their specification, and I have used them in several radio restorations in the past with no distortion problems at all. They are small transformers but I suspect no smaller than the original ones?
5. The measured values of R81 and R82 are a concern. Can you show an image of them or describe the markings (e.g. coloured bands)
6. The most usual causes of distortion at high volume would be: a) the ELL80 grid coupling capacitors C68 and C69 but you said you had replaced them. What voltage rating were the replacements? b) the rectifier has aged and lost efficiency so not producing enough current when the volume is increased. If no other reason for the distortion is found, as a last resort you could try replacing the rectifier with a silicon bridge device such as a DF08M. These are cheap to buy (such as eBay). You will see they are rated at maximum repetitive peak reverse voltage = 800v and this might seem very high but is necessary because when C79 charges up to +300v while the rectifier ac input voltage falls to its negative peak of maybe -300v you can see the extreme peak voltage across the rectifier. If you use one of these then include a 100R 5 Watt resistor between the rectifier positive leg and C79.
Lots of questions but hope that helps! Cheers, Jerry
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Old 18th Jun 2020, 10:30 am   #57
Diniz Diniz
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Default Re: Advice on output transformers

Thanks for your detailed and careful explanation. I will follow your recommendations.
Would a KBPC610 bridge do? Its specs. say 700V RMS reserve voltage.
Thanks.
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Old 18th Jun 2020, 10:55 am   #58
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Default Re: Advice on output transformers

I also have a GBU6K (Maximum Repetitive Reverse Voltage 800V). I think it is suitable. Regards.
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Old 18th Jun 2020, 12:58 pm   #59
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Hi Diniz, either KBC610 or GBU6K will be OK. Don't forget the 100R series resistor because the silicon bridge will produce a higher HT voltage than the old selenium one and is able to deliver a much higher current. Unless you use the series resistor, it could cause more problems at switch-on! Kind regards, Jerry
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Old 3rd Jul 2020, 10:47 pm   #60
Diniz Diniz
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Default Re: Advice on output transformers

My apologies for the delayed response. I was confined 2 weeks (prophylactic) and unable to go out for the components.
But now everything is back to normal and I replaced the rectifier bridge and the electrolytics with proper voltage ratings. There is no improvement, the results are similar to the ones reported in post #55.

I think we've done all that is possible, which taking into account my expertise, was a lot.
The result is quite good, so... I'm moving for the next one

I would like to thank you for all the patience, for all the help and guidance.

Tomorrow I'll make a video to let you see the final result.

Kind regards,
Diniz, from Portugal
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