Grundig 4070U/St Stereo radio restoration

[brightsparkey]

For my next project I have acquired an early 1960's Grundig Stereo radio complete with stereo decoder. It's a model made for US export, but found itself in the UK. It has FM 88-104MHz and a dual pentode output for either push pull of 2x single ended output using two transformers and a cunning phasing of the drives.

This may not be as simple as my previous restoration, largely because Hermes delivery saw fit to brutalise it in the post (not my choice of company!). Despite quite good packaging, it suffered a number of areas of damage, including smashing the output valve which is an annoyingly costly and unusual ell80.

Whilst I wait for the replacements, I started on the mechanical aspects. It has a clutch in the tuning mechanism that is missing some friction material so AM tuning doesn't work. Some improvisation required I think.

The first thing to fox me is the scale illumination lamps. They were both dangling free in the case. The holders have clips, but I can't work out how they fix to the scale pate. Does anyone have a similar set and can show me how they are meant to be fixed?

Photo's coming soon !

Many thanks,

Kevin.

[ukcol]

Radiomuseum have service info for this set if you haven't got it Kevin.

https://www.radiomuseum.org/r/grundig_4070ustereo.html

[paulsherwin]

You certainly can't be accused of taking the easy option! It was very unfortunate to lose the ELL80 as you say. It's possible to modify these sets to use a pair of separate pentodes if you google, but it involves a lot of bodging.

Early PCB sets like these are prone to a number of specific problems, most notably contamination and charring of the board and lifting tracks.

[brightsparkey]

Thanks for the link. I had seen this and the schematic is useful (albeit a little blurry). None of the photos show the scale lamp mountings.

I've attached some pics for illustration..

The scale has two lugs either side of the the tuning cap wheel that look like possibilities, but the clips on both lampholder dont dispose the lamps in such a way as to fit. The right side - lamp holder1 - point the lamp forward, but the it would foul on the case (too far forward). The left one - lampholder 2 - can either foul the tuning wheel or face upwards and foul the cord.

Both holder are different, and the currently free one has n provision for a second terminal. I'm foxed!

[brightsparkey]

Always one for a challenge

[paulsherwin]

The first holder looks like a replacement from a scrap set, suggesting that this radio has been worked on before, so watch out. The holders clip on a metal edge. You should be able to find some indication of their location - scratches etc.

[brightsparkey]

Thanks for the hint - yes the set has received attention in the past - there are some nasty blue 'splices' on the speaker wires and the main electrolytics have been replaced with what look like 80's or 90's vintage caps.

One of the tubes is incorrectly replaced too - a 12AT7 in place of an ecc83 which should be 12AX7. Probably worked OK as its the audio amplifier first stage.

Not much other evidence of tampering, but I haven't got too far yet.

Some more pics attached..

So far the issue list is:

(1) Frayed 2wire mains lead with Grundig 'interlock' plate needs making safe
(2) Smashed valves and incorrect replacements
(3) Clutch for AM/FM tuning needs attention in AM mode
(4) Boards are covered in grease - must have lived in a kitchen for some time
(5) Check 'that cap' or in fact several of them
(6) Speaker wires are in poor shape and someone has added blue automotive splices to them - need rewiring
(7) Hermes brutalised the ferrite rod antenna - I'll have fun working out which wire goes where and what the spacings should be
(8) Scratch on case top needs polishing out (minor)
(9) Hermes brutalised the rear corner - this will be interesting to fix, involving lifting and under-filling the veneer and a re-polish. Not looking forward to that!
(10) Hermes also brutalised the stereo decoder - one of the transformers was ejected from the board - another 6 wire puzzle!
(11) The EAF 801 is probably gassy (no getter metal on one side) so must be replaced.
(12) the lamps are free and I cant work out where they go
(13) the tone control lever that goes behind the main knob on the left of the three is missing. This may be irrecoverable and some form of fabrication may be required..
(14) the chance of the set being well aligned after being thrown around is low.

That should keep me busy

On the plus side:

most of the case is nice including the fabric
the PCBs look to be OK (one is slightly curled but not much)
The chassis and transformers look OK
The speaker look in good shape
Its complete apart from the tone control knob.

[AC/HL]

The first bulb-holder has one contact earthed, whereas the second is isolated. Are they wired in series?

Edit: No, they aren't: http://grundig.pytalhost.com/3070s/G...urhelfer-5.JPG

[brightsparkey]

Hi Bill,

yes they are meant to be in parallel, and there are twined wires, but no tag on the second holder.

The schematic is for a different model, but the lamps are the same. Mine has no LW band but has 2x SW as it was for the US. The MW is called 'BC' as well. The Ferrite antenna is different, by the look of it and the front end tuned circuits are switched differently. I'm slightly surprised that there is a 1k series resistor in the external antenna path, which can't be good for sensitivity.

The FM is 88-108MHz in fact, so can cover the full modern band. I may need to adjust the de-emphasis time constants too unless this has already been done - interestingly the decoder has a preset pot for this, but the internal mono filter is fixed.

Kevin.

[paulsherwin]

You should be able to simply glue the ferrite rod together with superglue, though as you say you will need to work out the connections. This is likely to be fairly simple as there will only be a single MW coil, plus maybe a winding for the external aerial connection.

A radio of this era may not use wax caps - ceramics and early plastic film types were taking over by then. If so then they may well be OK.

The difference in de-emphasis may be completely inaudible on a radio like this. The difference isn't obvious even with hifi gear, and many low end FM tuners use a single compromise de-emphasis setting for all markets. There's no reason not to correct it if it's easy to do of course

[ms660]

Bulbs & Holders...37 Secs in:

https://www.youtube.com/watch?v=VGbTroO1DiY

Lawrence.

[brightsparkey]

Lawrence,

nice find! That answers that one then! Neither holder are original..

Looks like the old plastic holders probably fell to bits. They look like they were indeed fixed onto the lugs on the back plate as I first guessed, in which case a more elegant retrofit is required.

Many thanks for digging that up. There are a few you-tubers with this set - some of them really should turn off their set and give it to someone who knows what they're doing before they wreck the transformers! But nice of them to show the insides..

Kevin.

[usradcoll1]

Originally Posted by paulsherwin

You certainly can't be accused of taking the easy option! It was very unfortunate to lose the ELL80 as you say. It's possible to modify these sets to use a pair of separate pentodes if you google, but it involves a lot of bodging.

Early PCB sets like these are prone to a number of specific problems, most notably contamination and charring of the board and lifting tracks.

Lucky thing it isn't a ECLL800 valve. Those things were nothing but trouble and are extremely rare and really expensive. Two penthodes and a triode stuffed in a 9 pin envelope. Those were so troublesome that the German manufacturers shipped an extra one with each set shipped.

[brightsparkey]

Not much difference - Watford valve have ECLL800 for £30. Best price I could find for the ELL80 was £25 - same price as I paid for the whole set more or less!

[MotorBikeLes]

The ELL80 is a wee bit prone to early failure as previously mentioned. One manufacturer (Korting I believe) supplied a "double egg cup" which plugged in place of the ELL80, and held two EL95s in the upper 7 pin holders.
Alternatively, the two "single ended" pentodes are normally preceded by a triode pair (ECC83 or ECC808), so a bit of rewiring will allow an ECL86 into each holder, giving slightly better reliability.
This is what I have done to my NordMende Parsifal.
Les.

[paulsherwin]

A bit tricky with a printed circuit set like this, though not impossible.

[brightsparkey]

I've ordered the replacement ELL80 to keep things simple.

I'm going to sort out the mains next. The grundig interlock seems to be on many models. Is there a recommended way of replacing it? I considered removing the plug and replacing it with a type 5/6 iec cloverleaf plug which has similar dimensions. That would permit an earthed chassis and 3 core mains lead.

I notice that the mains wiring runs the full length of the chassis as the tranny is the opposite end from the switch! The wires look too small to pass PAT, so I may rewire those too.

Kevin.

[paulsherwin]

You could simply wire a permanent 3 core connection.

[brightsparkey]

So I had a go at sorting out the mains wiring - wasn't too bad in the end.

I replaced the wire from the input mains plug to the switch with 2 core 3A oval flex. Very similar diameter to the original sleeving, so it fits nicely under all the clips and went in OK. A bit tricky at the switch end to navigate the tuning cord etc.

I then disconnected the connection to the mains voltage selector until the metal shell with the mains plug was free. I drilled out the heads of the rivets and removed the connector. The PCB behind the connector was going to be in the way so I cut out the area I needed to pass the IEC connector. Once this was done the connector was a neatly snug fit into the shell, and the fixing holes were nearly the same separation as the original. I popped out the two rivets and used some M2.5's to secure the new connector to the shell.

End result is pleasingly tidy, as if the connector was meant to fit. I also added some Y caps and took the earth to a tag to connect to the chassis. -Refitted and put it on the PAT tester - all passed OK.

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.

Once this was done I powered the set up gently on the variac. All seemed OK. Once up to volts the heater lit and noise from the speakers. Tuning FM resulted in audio! Good step forward. Lots of hum tho' and the electrolytic in the cathode of the output pentodes went pop after a minute or too.

Main supply caps look OK so, hopefully just that electrolytic that needs attention. Some excavation required as its under the main supply capacitors..

Onwards another small step..

Kevin.

[paulsherwin]

Nice neat job.