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Old 15th Apr 2017, 7:07 pm   #1
Diniz Diniz
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Default Grundig 3095 / 56

Hi everyone!

I am doing this new set (grundig 3095 / 56) which seems to be in bad condition.

I first realized that the rectifier bridge (reference B250 C100) was burned, or at least no DC voltage was measurable.

I replaced it (I dont know if properly) by a rectifier bridge BY179 which I believe withstands 250V and 1.5A, according to its datasheet. But I am not sure it is suitable for this set.

I list above the replacements:
- C80 (the 50uF electrol. capacitor)
- C76 (the UL84 cathod electrol. capacitor)
- C71 (I believe it is the "that" capacitor in this set)
- R49 (was missing ).

I would very much appreciate you advice on the following:
The AC voltage at AC terminals of the rectifier bridge is 268V~.
The DC voltage at the DC terminals of the rectifier bridge is 333V=.
According to the schematics it should be 280V=

If I disconnect the + (plus) terminal of the rectifier bridge the measument is 248V=. If I disconnect only the electr. capacitor C80, (keeping the connection to the output transformer) the measurement is 261V=.

(see attached part of the schematic)

Any comments, please?

Diniz
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Old 15th Apr 2017, 8:32 pm   #2
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Default Re: Grundig 3095 / 56

So far, all seems OK. Don't forget voltages +/- 20% of those measured may not indicate
a fault condition, and voltages will all be a bit higher if you are using a modern digital
multimeter.

Note to UK members; ensure you fit a small series wirewound resistor in the ac feed,
as some Grundig sets did not have a 240 volt setting.
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Old 15th Apr 2017, 9:16 pm   #3
Diniz Diniz
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Default Re: Grundig 3095 / 56

This set does not have 240V setting, indeed. The maximum it have is 220V.

The grid voltage in Portugal is 230V.

Do you think a dissipative resistor is needed to drop the AC feed? If so, where should it be placed (in what point of the diagram)?

Thanks in advance!!!

Diniz, PORTUGAL
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Old 15th Apr 2017, 10:47 pm   #4
'LIVEWIRE?'
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Default Re: Grundig 3095 / 56

The usual method, when replacing a selenium rectifier such as the B250C100 with s modern silicon one such as the BY179, is to include a suitable resistor in series withe DC side of the new rectifier -this can be either in the positive or negative line. Values of between 100 & 500 ohms may be suitable, the ides is to get the DC HT voltage as near as possible to that shown in the service manual. In this case that means dropping about 50v, and the resstance value needed can be calculated, using Ohm's Law by dividing 50 by the HT current. If this is (say) 50mA, then 50/0.05 = 1000 ohms. A 3 or 5 watt resistor would be needed, as the dissipated power would be 50x 0.05 = 2.5 watts
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Old 16th Apr 2017, 12:21 pm   #5
Leon Crampin
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Default Re: Grundig 3095 / 56

Better to put the resistor in one of the AC leads to the bridge. A suitable resistor can protect the transformer in the event of diode failure.

Leon.
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Old 16th Apr 2017, 2:12 pm   #6
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Default Re: Grundig 3095 / 56

If the resistor is in the DC out of the bridge it will need to be at least 3 times, preferably more, than the calculated wattage due to ripple current. Better in the mains side where the is none.
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Old 16th Apr 2017, 2:13 pm   #7
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Default Re: Grundig 3095 / 56

Surely the resistor will be dissipating the same amount of power, no matter where it is in a series circuit?
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Old 16th Apr 2017, 2:17 pm   #8
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Default Re: Grundig 3095 / 56

I was disabused of that fact several days ago, I will let someone else have a go, and they will!

In fact it is correct, the ripple current also makes measuring the DC voltage with a DVM difficult.
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Old 16th Apr 2017, 2:42 pm   #9
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Default Re: Grundig 3095 / 56

Well, most DVMs can't follow a waveform fast enough to get an accurate RMS reading (there are three kinds of A-D converter: accurate but slow, fast but rough and eye-wateringly expensive); so they have to work on the extremes, then multiply by a bodge factor to get what the RMS figure would be if the waveform were sinusoidal. And this bodge factor is wrong for most non-sinusoidal waveforms.

A resistor upstream of the rectifier will dissipate the same amount of power as the same value resistor downstream of the rectifier. It has to, per Kirchoff's Laws.
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Old 21st Apr 2017, 6:56 pm   #10
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Default Re: Grundig 3095 / 56

I would like thank all the comments above and apologize for having not let you know the progress of this restoration, but I have been away from home this week so there haven't been any progress.

Although, I'll tell you what I did last Sunday.
I've placed a resistor (390ohm 11W) in the AC side of the BY179 bridge, and the voltages are now pretty much close to the ones mentioned in the schematics, in the EL84, EABC80 and EF89 pins.

But regarding ECH81, no voltage was measurable in neither of its pins. So I realized that a resistor (I believe is R13 = 500ohm) was burned and out of range. (See immage attached which i found in the internet but is equal to what I found in my set).

After the replacement for a 500ohm (5W) resistor I got a strange behaviour: R49 and the resistor I introduced in the AC side of the rectifier ran very hot and smoke was noticeable!! So, i measured the AC current on the AC side of the BY179 and the result was 172mA!!! And the voltage after R49 (+ side of C81) was reduced to 50V compared to the initial 280V.

So I replaced the new resistor for the "burned" R13, and correct voltages came back, but still no voltage in ECH81 pins....

I dont know what to do regarding this R13 behaviour.... any tips for when I meet the radio again?

Thanks in advance. The help provided for this forum is being much appreciated!

Diniz
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Old 21st Apr 2017, 7:28 pm   #11
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Default Re: Grundig 3095 / 56

See attached R13 in schematics.

Thanks.
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Old 21st Apr 2017, 7:37 pm   #12
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Default Re: Grundig 3095 / 56

From what I can make out R13 feeds the mixer anode circuit of the ECH81 on AM and also the HT supply to the FM tuner on FM, do the resistors you referred to smoke when the radio is switched to AM or when switched to FM?

Lawrence.
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Old 21st Apr 2017, 9:20 pm   #13
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Default Re: Grundig 3095 / 56

I've had cases of almost shorted decoupling capacitors (C35)which leads to a high current consumption and burnt resistor(R13)
Yes,they were Grundigs.
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Old 26th Apr 2017, 3:45 pm   #14
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Default Re: Grundig 3095 / 56

I would like to apologize for the delay on the reply, but I am still away from home (unfortunately) and I've not been able to do any progress.

Lawrence, thanks for your question and interest. I am not 100% sure but I believe it happened when the set was switched to FM. Because I was hoping to get some signal on FM, since I realized that one "strange ferrit antena" was missing on the back of the set, and I was hoping that it would not affect the FM reception. (I dont know if that antena is also needed for the FM )

Roffe, I have replaced almost al the paper capacitors and I plan to replace all the "blue" capacitors as soon as I have the opportunity. Indeed, C35 has not been replaced yet. Thanks for your comment.

Best regards from Portugal

Diniz
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Old 26th Apr 2017, 7:08 pm   #15
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Default Re: Grundig 3095 / 56

Ferrite Rod Antennas are only used for AM (MW &LW) reception, so, if this is missing it will have no effect on FM (VHF), but you will not be able to receive any AM stations. The coils on the Ferrite rod form part of a tuned circuit. On the big Grundig Valve radio the rod is mounted on apanel which can be rotated for best reception. Going back to VHF(FM), there should be sockets on the rear of the radio to which a suitable aerial, usually a dipole, connect.
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Old 27th Apr 2017, 5:12 pm   #16
Diniz Diniz
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Default Re: Grundig 3095 / 56

Thanks Livewire for your explanation/clarification. Indeed, the set has a dipole aerial and respective sockets to connect. As soon as I have the opportunity I will continue the the restoration focusing on FM. I will let you know when it happen.

Thanks again!
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Old 15th May 2017, 6:42 pm   #17
Diniz Diniz
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Default Re: Grundig 3095 / 56

Hi all,
This weekend I was able to bring the set back to the bench. I've replaced C35 and the radio plays amazing (perfect tuning of several stations and loud enough sound).
I have also replaced C32, C33, C49, C66 and C65 in addition to the previous replacements mentioned in this thread.
After the replacement of the capacitors the radio plays nice, but for a while I was not able to reduce the volume to the minimum, i.e., the minimum was still very loud...
Now it came back to normal (almost no noticeable sound at minimum volume) but since I've done nothing regarding this ....do you have any idea to what should I look at, if the problem returns

I'd like to thank, once more, all of you that helped me in this project.

Cheers,

Diniz, PORTUGAL
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Old 15th May 2017, 11:21 pm   #18
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Default Re: Grundig 3095 / 56

It is probably a poor connection in the volume control (wiper). If the problem comes back you should spray some switch cleaner into the volume control.
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Old 16th May 2017, 5:52 pm   #19
Diniz Diniz
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Default Re: Grundig 3095 / 56

Thanks PJL, I will try some contact cleaner!
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