Grundig 4070U/St Stereo radio restoration

[don200002]

The tuning clutches are an easy fix if they are the same as earlier models, Simply cut a circle of rubber from an old cycle inner tube, cut a hole in the middle and a slot in one side so you can slide it over the shaft without any disassembly, then fix it in place with super glue.
Best of luck with the restoration,
Don.

[brightsparkey]

Yes I was lucky - the connector was a snug fit, and I did have to modify the paper board.

[brightsparkey]

Thanks for the hint Don. I shall try that and see how it goes. Looks like the AM side is nice and clean, so should work out, and I have some rubber sheet from my last radio repair.

Kevin.

[vidjoman]

Originally Posted by brightsparkey

The fuse holder is of the potentially lethal type. I put it on the Neutral side as the metal top is connected to the supply when the fuse is OK. Its inside the box when the cover is on, but not ideal. I may find a suitable modern holder to replace it just in case.

Not a good idea - If the fuse fails and the set is still connected to the mains then the set is still at mains potential to earth. In the event of failure you will have removed the neutral not the live.

As it's shielded when in use I can see no problem with leaving it as it was. You can always add some heatshrink sleeve if you are concerned about touching it when it's out of the box.

[brightsparkey]

Of course with its 2 pin original lead, it could have been either way around.. I think I shall source a modern insulated one and replace it before I'm done, just to be on the safe side (literally).

Kevin.

[brightsparkey]

Well I think I found out why that poor cap failed!

The previous replacement of the main rectifier caps made an error. The 3 caps in a common can have six terminals - three are -ve and 1 positive each. The caps were all replaced, but the 8uF and the 50uF were swapped, and worse, much worse, the three grounds were left disconnected from each-other.

The PCB relies on them being connected together, thus leaving the cathode resistor isolated from ground. Not nice for the cap as the cathodes can float around. Also the bias is poorly defined (see snip of schematic)

I'm going to fit some less huge new caps, so they will take a few days to arrive.. oh and connect the grounds together !

Had a go at the clutch - am now works OK after inserting a rubber sheet, but now the FM side is playing up sometimes - and the old material is still present in the middle, inside the spikes, preventing the same treatment. Back to head scratching on that..

The previous repairer saw fit to add wires to the springs to force the FM part harder together. Removing this makes it slip all the time, but doesn't make room for new material.. I really don't want to un-string it.. <sigh>

I can see that previous repairs made to this set were probably done in a rush to save cost, so I'll try and reserve judgment, but I'm amazed more damage wasn't done by the screwed-up earths.

Kevin.

[brightsparkey]

Attaching schematic..

The cap is grounded to a different land than the resistor so the cap was connected but the resistor o.c. due to the missing ground connection.

It looks like the cathode would have risen in voltage until the capacitor breaks down, and then a constant current would flow though it. Nasty for the cap, but oddly it probably worked for quite a while..

Kevin.

[brightsparkey]

Fitted the new capacitors - much tidier now - and with the correct wiring the audio is much cleaner and the hum has all gone. In fact the set seems to work well on both AM and FM now, with respectable sensitivitty. Not even worth bothering to align.

Going over the check-list:

(1) mains lead etc. - now safe with new connector and fuse holder replaced
(2) Smashed valves and incorrect replacements - replaced
(3) Clutch - both side now have new rubber and it works ok
(4) Boards are covered in grease - cleaned up OK - not perfect
(5) Check 'that cap' - only 2 wax papers in the set on the audio transformer replaced other caps all seem OK.
(6) Speaker wires are in poor shape - replaced
(7) ferrite rod antenna - re-affixed and glued with super-glue - seems to be OK but haven't tried with external AM antenna yet.
(8) Scratch on case top needs polishing out (minor) - to do
(9) Bruised rear corner - still left to do - acquiring thinners and finish..
(10) stereo decoder - one of the transformers was ejected from the board - replaced but not sure its correct - decoder seems deaf..
(11) The EAF 801 is probably gassy - it actual is OK - you ever can tell!
(12) the scale lamps - waiting for some to appear on ebay from german seller..
(13) the tone control lever - its actually balance so can live without but hoping for one to appear..
(14) the chance of the set being well aligned - not too bad in fact

So the list has dwindled to a few round tuits and I have a functioning radio. Happy result. I need to knuckle down to the casework next, but I need to get my workshop in order first...

Kevin.

PS a few pics to follow..

[paulsherwin]

It is rarely necessary to realign the IF stages unless somebody has been twiddling.

[brightsparkey]

Agreed - even with new valves, the tuning should be stable. In this case I was just concerned that it had been given a hard knock and that stuff would have been disturbed, but it seems to have fared quite well.

Just as well, as there are no instruction I've found for alignment. I'm in the process of documenting the stringing so that I have a record of it in case they snap. Will share once its done.

It seems there is a generic alignment process for these types of radio that flows along the lines of:
AM IF and detector
AM Oscillator tuning (usually to get the oscillator to match the dial scale by trading L and C tunings)
AM Pre-selectors
FM IF (and detector)
FM oscillator
FM preselector and neutralisation if any

During adjustment of the SABA, the IFs were spot on, but the oscillator was a little off
The FM oscillator was OK,but the FM neutralisation was well off. This was probably due to a change of front end tube (more sensitive at 100MHz).

What's nice on the SABA is the transformers have separate coupling adjustmnets. You can un-couple the two windings, and seperately tune each for resonance, and then recouple for critical coupling.

The Grundig just has two slugs for each transformer, one for each coil, so adjustment is more interactive and really needs a swept oscillator to check for the filter shaping, whereas the SABA needed nothing more than a voltmeter and a modulated signal source.

Kevin.

[brightsparkey]

I stumbled across some info that led me to discover that the Grundig 3070 and 4070 share the same chassis designs, and that the service manual is common. So I now have a source of service information found here:

https://www.vintageshifi.com/reperto.../Grundig-2.php

It shows schematics for both 3070 and 4070 radios and gives a stringing diagram and tuning instructions.

Kevin.

[brightsparkey]

So the saga of the scale lamps has become a nightmare! I have acquired a pair of Grundig lamp holders of the same type as seen in the video of this set linked to previously (although they are red not white..). They are plastic with a slot that fits over the lug on the scale backing plate positioning the lamp in the plane of the lug and above it. They fit just right and don't foul the tuning wheel or anything else, unlike any other sort.

So I was all set to put the chassis back in the case having fixed up the stereo decoder, but to my immense irritation the scale lamps protrude too far out of the front of the chassis and it wont fit back in - not by quite a bit.

There is a small relief in the board above the scale which is meant to accommodate the lamps so I guess it would be tight anyway.

The lamps I have are the normal type of MES scale lamp with quite a long glass part - is there some other type of lamp that was used in these sets? I looked on the usual auction site and could find no other 7V 0.3A lamps that would suit.

I am now foxed again on the scale illumination!

Kevin.

[brightsparkey]

some pictures to illustrate..

1st 2 pics show the chassis in correct position but with no lamps in.

Hard to see clearly even here, but with no lamp in, there is only a small gap between the lamp holder and the case.

Kevin.

[brightsparkey]

Originally Posted by brightsparkey

Attaching schematic..

The cap is grounded to a different land than the resistor so the cap was connected but the resistor o.c. due to the missing ground connection.

It looks like the cathode would have risen in voltage until the capacitor breaks down, and then a constant current would flow though it. Nasty for the cap, but oddly it probably worked for quite a while..

Kevin.

I was double checking this now that I have the service manual which shows the PCB layout. Even more curious! The track had been cut to isolate the two grounds. I have no idea why - it was clearly cut with a scalpel or some such.

So the cut in the track was responsible for isolating the resistor. I had established there was no connection, but the track cut was hidden under the wiring so I didn't notice before.

If only the set could tell you whats been done to it in the past !

Kevin.