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Old 18th May 2019, 9:15 pm   #1
sexton_mallard
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Default Grundig 3028 3D

After feeling a bit jaded with slow progress on other projects, I finally gave in and dragged the big Grundig 3028 languishing as a ornament in a spare room out to the shed to be reactivated! This set was given to me about 12 years ago from a elderly neighbour. I always remember he served on HMS Suffolk - during the attack and sinking of the Bismarck, the memory of which will always be associated with this radio. Back on topic! This set was untested and after getting the back off I noticed the internal fuse was bridged with a bit of fuse wire and was pushed in the 100V clips! .

I changed this for a 2A slow blow clipped into the 220V clips and carried on basic checks such as main lead polarity check, plug fuse, continuity and so on.

I was expecting the worse to be honest, the mains bridge rectifier cremated and the main TX toasted but both looked OK and checked out OK on the DMM. With the chassis out, a external speaker was connected and power was applied via a 42w lamp limiter which glowed brightly for 30 seconds and then dimmed down. The mains electrolytics were warm but not excessively. A 150w bulb was swapped and the set was tried out on all wavebands for the first time. SW came in with good sensitivity, MW very good but LW was dead for now. A lot of noise and instability from scratchy switches and controls were expected. Switching to FM resulted in silence. A wiggle of the ECC85 brought Radio 2 and the Eurovision song contest blasting out. The sound quality is excellent and the tone control responds. Even the EM34 gives a bit of a green glow for good measure. This is a very promising radio and now to work through the circuit end to end weeding out dodgy blue and cardboard cylindrical caps. There is a bit of damage, some of the scale in the top corners appear to have clear tape which has pulled the black print off. There is also a rust stain in the middle of the superb grille cloth suggesting a period stored in damp conditions.
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Old 19th May 2019, 12:22 pm   #2
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Default Re: Grundig 3028 3D

That was a bit of luck with transformer! A friend of mine has one of these in fully restored condition and it sounds amazing.

Regards,
John Joe.
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Old 3rd Jun 2019, 10:13 pm   #3
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Default Re: Grundig 3028 3D

Thanks John Joe. The 100V marking was a misnomer. There is a wirewound resistor in series to drop the mains from 240v to 220v. It looks like a sticker that covered the embossed legend has long disappeared covering this up perhaps.

After reading other forum stories about the same radio I decided to err on the side of caution and restuff the selenium rectifier with 4 1N5399 diodes I had handy. The can was cut open to reveal the stack of plates and a slight but very distinctive smell which brought back memories. The strange smell reminded me of a childhood memory of my grandad recharging large boat batteries ready for the new boating season. The large battery charger used to occasionally emit a loud snap followed by this smell... the large finned metal rectifier in the charger must have been breaking down under protest, but the charger soldiered on for many more years. Back to the topic!

After testing my bridge rectifier was actually working from a 24v bench supply AC source, I replaced the can and placed a 4w 150R wirewound resistor in series. I have about 240-250V at the EL84 with a current draw of 74-78mA at the rectifier. The wirewound resistor gets very hot - almost too hot to touch. Being rated at 4w I would expect this to just get warm. I have it placed in series between the DC output of the rectifier and the mains electrolytics. Should I have it placed in series between the TX and the rectifier instead?
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Old 3rd Jun 2019, 10:53 pm   #4
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Default Re: Grundig 3028 3D

The location won't make any difference to dissipation. The calculation for power is complicated as the waveform is far from sinusoidal, you would need to measure the voltage drop with a true RMS meter, but wire wound resistors are ceramic for a reason and will get very hot at full dissipation.
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Old 4th Jun 2019, 3:48 pm   #5
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Default Re: Grundig 3028 3D

That poor old selenium rectifier, what had it ever done to you?

I've also read about the supposed demise of these rectifiers and I don't go along with it. I've got a 3028 that I've had for years and it's still got its original rectifier, and I wouldn't in my wildest dreams consider digging the insides of it out and replacing them with diodes, unless it had failed, of course. I've not personally known them fail with a bad smell, although there's always a first time for everything. Old TV triplers used to fail with a horrible smell, and I've had a few of them go that way. The flat rectangular and the square modular selenium types just seem to fade away without any drama and a lot of these are, dare I say such a thing, change on sight, particularly in the Bush SRP31 record player. I personally think with regards to the round selenium types failing in these Grundigs, that there's a bit more to it and it's more likely down to having been subjected to excessive HT current being drawn due to bad capacitors in the audio output stage. If your radio has been out of use for many years and you've sorted the leaking capacitors around the EL84, then I don't see any particular reason for the original rectifier to fail any time soon.

Regarding your particular radio, there are several capacitors around the EL84 and the EABC80 triode section that really need to be replaced in this set. I'm not 'big' on capacitor changing for no good reason, but in this particular set there are several, and they are of a type that tend to suddenly go virtually short circuit for no good reason. The obvious one that you've probably already done is the grid coupling capacitor "that cap" on the EL84. I haven't got a circuit in front of me, but there's several others that you should be able to identify by studying the circuit diagram. There's one in particular that's in series with a resistor to deck and as it starts to leak badly, it burns up the resistor. The naughty thing about this is that as the resistor burns, it doesn't go high in value, but drops in value and I can't quite remember without checking the circuit, but I seem to think that the capacitor comes from the anode of the EL84, so is likely to take out the primary of the output transformer, and of course your selenium rectifier if it was still there. Now that you've replaced it with silicon diodes, then the mains transformer would be next in line.

As regards to that 4 watt series resistor - throw the calculations out of the window for this one, it wants to be 7 to 10 watts and no less. Basically, think of a number and double it, or rather what I should say is calculate the number and double it! We know these resistors are given a design wattage rating, but look at the old TV dropper resistors that were given a design wattage rating but still failed all the time, also scorching everything around them including the cardboard back covers as in some radios. Something like around 90% of all TV call outs back in the day were to replace burnt out droppers that had been designed to a calculated wattage rating by the manufacturers - I rest my case.
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Old 4th Jun 2019, 8:00 pm   #6
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Default Re: Grundig 3028 3D

Thank you Techman and PJL. There does indeed seems to be some very unstable caps in this set. On switch on there is a disconcerting cracking/rustling from the speaker even before the the valves warm up. I have already swapped a few caps around the EL84 already. The 50mfd cathode electrolytic was actually getting hot the last time I had the set running when I was measuring HT! The mains electrolytic is developing a hot spot and has a stalagmite of dried electrolyte on the end where there is normally a plug or rivet in the top of the can. The wiring and valve sockets seem to be quite fragile.
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Old 4th Jun 2019, 8:33 pm   #7
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Default Re: Grundig 3028 3D

Hi Dom,

Since I posted earlier I've remembered that the capacitor that goes very leaky and takes out a resistor must connect directly or indirectly to the EL84 anode, so will be taking excess HT current through the output transformer primary. I was going to post again with the fault symptoms, but you've beaten me to it - yes, the immediately noticeable fault symptom is slight crackling and noise through the loudspeaker from first switch on and before any of the valves have warmed up. Don't run the set again until you've replaced both the capacitor and its associated resistor.

The main smoothing electrolytic may recover, but if it still seems to be getting warm then it may have to be replaced. I have to admit that the double smoothing electrolytic can in my set has the same dried electrolyte leakage evidence. Several years ago the reservoir section went open circuit and as a temporary fix I just connected a single electrolytic in place of the dud section and it's still working perfectly like this, possibly five years later. The warming of the can of your capacitor does indicate that it could be terminal, but keep an eye on the temperature and it may recover, however, the fact that it's blown its end seal slightly does indicate that it's not in a good state.
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Old 4th Jun 2019, 9:28 pm   #8
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Default Re: Grundig 3028 3D

C62 and R41 as of the circuits on pages 118 and 119 of Radio Servicing which is probably the circuit you're using.
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Old 4th Jun 2019, 10:18 pm   #9
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Default Re: Grundig 3028 3D

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Originally Posted by Techman View Post
C62 and R41 as of the circuits on pages 118 and 119 of Radio Servicing which is probably the circuit you're using.
I replaced C62 and R41 looks undamaged and measures bang on 10K, the rustling/crackling on switch on has gone. Phew and thanks. The old cap crumbled very easily.
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Old 18th Jul 2019, 10:42 pm   #10
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Default Re: Grundig 3028 3D

Is that an ERO branded capacitor? My current Telefunken project is full of the things, which appear to resemble the Philips bitumen ones with the addition of a paper sleeve...
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Old 29th Jul 2019, 9:54 pm   #11
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Default Re: Grundig 3028 3D

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Originally Posted by Nanozeugma View Post
Is that an ERO branded capacitor? My current Telefunken project is full of the things, which appear to resemble the Philips bitumen ones with the addition of a paper sleeve...
Yes it was indeed a ERO cap which actually seem to fall apart.

The radio is now back together after a re-cap of all paper blue and cylinder types. I used 2 4w resistors in parallel in the end with reasonable heat dissipation and the correct HT voltage. I soon discovered the hazard of a solid state rectifier and a total of 100mfd charged capacitance if the set is switched on then off before the valves have heated up to discharge the HT... . I shorted the EM34 socket accidentally and there was a loud crack! Fortunately I did not get in between the short.

I restrung the ferrite bar aerial 'roto-twerkulator', cleaned the dial backplate, valve sockets and the graphic equaliser thumbwheels and scale. The red rubber band had long crumbled away. Upon reconnecting the cabinet speakers for the first time (since I had the chassis connected on the external connector to a external speaker), the mid/bass driver was doing a good whoopee cushion impersonation. After removing, it appeared the dust cap had crumbled to dust and has filled the voicecoil magnetic gap. A bit of tapping and careful vaccum cleaning, the speaker was then working fine again.

One unfortunate cosmetic defect is the top corners of the dial printing has been pulled away by two strips of what looks to be a sellotape material which has crinkled up. I cut out two pieces of overhead transparency plastic and clamped the pieces of sellotape still with some scale print with a few tiny dabs of superglue (which dried transparent in a test beforehand - some cyanoacrylate glue can set with a whitish appearance. The repair is ok(ish) but the damage can still be seen as some of the printing has gone altogether. Leaving two patches of clear glass would have looked even worse.

The set is now back together and is on soak testing by entertaining me with a big smooth sound on FM in the kitchen. The MW and LW switches are a bit intermittent but I will keep working them with the hope any remaining tarnish on switch contacts gets polished off with use.
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Old 30th Jul 2019, 8:24 am   #12
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Default Re: Grundig 3028 3D

Dom

I too have the sellotape issue on one of my 3028's. It is a common problem.

Having spoken to my silk screen printer he suggests using glass paint as a possible solution. Yet to try, it is water based and although available in a wide range of colours, I hoping that the basic set can be mixed to suit.

My other set has not yet suffered and so I'm going to try and remove the tape using a liquide adhesive remover, not WD40.

Should any member have views on my suggestions or other solutions they would be very welcome.

Chris

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Old 30th Jul 2019, 8:58 am   #13
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Default Re: Grundig 3028 3D

I have three of these sets which all have broken or damaged glasses. My lifetimes work seems to be trying to recreate a dial glass for them, here is the result so far, it may be of some use to you although the bit that says roto twerkulator is missing as it is still a work in progress.
Hope this helps,
Don.
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Old 30th Jul 2019, 11:29 am   #14
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Default Re: Grundig 3028 3D

Don

Thank you so much for sharing your work to date.

Knowing how the original dial was laid down onto the clear glass would be my starting point. I do have some expertise in silk screen printing, as an end user, and this would require laying down each colour one at a time and knowing exactly the CMYK colour equivalent to the original.

Members experience in using a transfer seems variable because of light bleed through but given a reproduction from Radio Daze costs upwards of $139 it's worth trying.

My printer "guy" is away just now but next time I see him, I'll see what he can do. Maybe if there is sufficient demand and we can use the laser printer, some alternatives can be offered.

I would be happy to share this information as soon as it is known

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Old 31st Jul 2019, 9:23 pm   #15
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Default Re: Grundig 3028 3D

Quote:
Originally Posted by don200002 View Post
I have three of these sets which all have broken or damaged glasses. My lifetimes work seems to be trying to recreate a dial glass for them, here is the result so far, it may be of some use to you although the bit that says roto twerkulator is missing as it is still a work in progress.
Hope this helps,
Don.
Thanks Don.

Here are some more detailed pictures of the dial showing the damage, the captions over the knobs, and the main two types of capacitor that were replaced. The set is currently being 'debugged', the volume pot has worked loose in the chassis, the faring mid/bass driver problem has returned and a few other tidying up jobs still need to be done.
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Old 31st Jul 2019, 9:55 pm   #16
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Default Re: Grundig 3028 3D

Those capacitors are terrible and WILL all be leaky. Replace them ALL.
The red/brown resistors are usually OK.
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Old 31st Jul 2019, 10:05 pm   #17
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Default Re: Grundig 3028 3D

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Those capacitors are terrible and WILL all be leaky. Replace them ALL.
The red/brown resistors are usually OK.
Thanks Tim

All capacitors of the two types shewn were replaced. I just pictured one of each type for illustration. I also found all resistors were OK as well.
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Old 1st Nov 2020, 6:02 pm   #18
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Default Re: Grundig 3028 3D

Thread re-opened at OP's request
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Old 1st Nov 2020, 6:12 pm   #19
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Default Re: Grundig 3028 3D

The bass driver reverted to rubbing the voice coil and was causing dreadful distortion. After a few more attempts I gave up and started trialling with a replacement from a few spare drivers including 8 inch car audio coaxial units. For the craic I cut out a plywood adaptor baffle and bolted it in onto the original baffle using the existing speaker mounting studs. A water damaged B&W speaker yielded a 6 inch mid-bass driver. This was mounted and sounds good with the original tweeter and midrange speakers in the cabinet ends. The little EL84 copes well driving this lot with no changes to the crossover setup.
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Old 2nd Nov 2020, 8:59 am   #20
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Default Re: Grundig 3028 3D

Quote:
Originally Posted by linescan87 View Post
That was a bit of luck with transformer! A friend of mine has one of these in fully restored condition and it sounds amazing.

Regards,
John Joe.
I remember back in the 70's I took the one I had restored to work ,the radio/t/v workshop, after demonstrating the set to workmates and boasting how good the sound was ,one of the older engineers put a Bush acoustic table transistor radio AD161/2 's next to mine , I had to admit the bush sounded fantastic beat the socks of the Grundig , needless to say I was some what deflated , Hat's off the the designers of the Bush radio ,If you get chance to own one of these you wont be disappointed, well done with the restoration Dom ,Mick.
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