Started this week on the Graetz beast.

[boater sam]

The Graetz Melodia 4R/418 was taking up bench space so it just had to jump the queue and get fixed.

Its a bit of a monster, 3 speakers, long, medium, short waves and FM.  Ex80 valve line up with an EM80 eye and a selenium rectifier, uncommonly an AEG wire ended part in a tube.
Lots of odd things rarely seen in western sets, twin tuning mechanism from one knob by multiple cord and gear drives selected by pushbuttons, rotatable ferrite rod aerial, speech button, gram button, and a very odd mains switch linked to the off button.

First look was a heart stopper. 2 of the fragile wave wound coils under the coil pack had dropped many turns off into the space under the chassis.
I managed to get them looped back together despite the wire being very thin and secured them with a dab of glue.

The skeleton  bass control pot was seized solid. Had to remove it from its cord drive to the indicator, take it out of the chassis completely so that I could heat the boss and carefully work the spindle out. Polished the spindle and reassembled to restore operation. Fiddly but pleasing to get it done.


Run up on the limiter produced no bangs or smoke/flames and on full power it works well on all bands. Even FM, the ECC85 for once is a good one. In fact the valves all appear to be the original Siemens ones. Selenium rectifier seems so far to be working well.

The grid coupling cap to the EL84 was leaky of course, I changed this as a priority as +2.6 volts on the grid is a bad idea.
Caps are horrible black tar torpedoes with paper labels . Not possible to re-stuff these.

Smoothing can triple electrolytic was getting  warm so that will be re-stuffed. Its an odd can, Graetz made, wire ended, twist locked into 3 slots one of which is soldered to the chassis. Not simple to remove from a very crowded under chassis full of components and very thin connecting wiring. It contained 3 caps, two 50mf and one unconnected 4uf. Most strange as there is another 4uf cap close by in a separate can. I will re-stuff with a pair of 47uf 450v electrolytics.

The wiring is all pvc but very thin and solid cored so avoiding it breaking off valve tags etc. is a priority. All component leads are wound through tag holes and round and round before soldering. I will avoid unsoldering any of these as it is just asking for them to get broken off.

All in all a most interesting and challenging chassis to work on.
The cabinet is whole and relatively undamaged, some re-polishing will be required to sort out the sun damage to the top and one side.

[Mike Watterson]

Lots of odd things rarely seen in western sets, twin tuning mechanism from one knob by multiple cord and gear drives selected by pushbuttons, rotatable ferrite rod aerial, speech button, gram button, and a very odd mains switch linked to the off button.

All very common on many West German brands. I've had 3 different makes like that and French model with rotating wire drum aerial.
A few UK/Irish models also had the rotating ferrite, a pull cord switch linked to push buttons and gram,

Smoothing can triple electrolytic was getting warm so that will be re-stuffed. Its an odd can, Graetz made, wire ended, twist locked into 3 slots one of which is soldered to the chassis.

Try reforming at 30V DC, current limited and replace the selenium with 1N4007 + resistor as it's likely passing AC and will fail.

Grundig was fond of skinny solid wire with PVC.

[boater sam]

The can is already stripped out, I don't think it would have reformed as the top has a hole in it from new with a bit of clear tape over it which has vented electrolyte at some time.
Agreed rotating ferrites are fairly common on continental sets but the intricate mechanism of this one is a joy to behold.
Graetz were an East German company of course until quite late, I am assuming that this set was made in the later West German factory.

[boater sam]

A bit more progress today. made a cradle for the chassis to sit in upside down to avoid breaking the ferrite aerial off.

Electrolytic can rebuilt and refitted, the 4uf section was rewired to the EL84 screen grid as it was originally and the extra cap that had been fitted under the chassis  removed. This section of the original cap must have failed some time in the past, I am surprised that the whole thing was not changed at the time.

The tone correction capacitor on the EL84 anode has split, massively leaky so it got changed for a 1500v item, don't want to risk the output valve should a lower voltage one fail.

The agc line was not what it should be so the reservoir cap was changed. The eye now works fully but strangely there is a slight delay when tuning before the eye catches up. I have never noticed this before on an FM set.

Investigated the selenium rectifier a bit more closely. It does not get too warm after a couple of hours and the HT voltage is what it should be according to the circuit diagram.  I will leave it working as it is and see if it degrades. Seems a shame to replace it with silicon diodes but I can re-stuff the tube if the need arises.

Short wave is producing nothing other than mush but there is no proper aerial in the workshop, just 10 feet of wire in the ceiling so I am not expecting much.
The coil pack is very complicated and fragile so I think I will leave well alone as far as alignment but it would be nice to hear something on the short wave.

[Mike Watterson]

I only heard mush on SW today, but it was on a short vertical.
The 4uF (or similar) is + to chassis on the FM detector?* It does respond a bit slower than AM (typically 0.1uF and it is on the AGC line), though sounds unusually slow. The FM likely has no AGC.

I'd check the HT on scope, not just the DVM and maybe measure the AC current on Electrolytic. The Selenium rectifiers can pass AC without getting too hot.

Yes, I usually put 1KV or 2KV XY ceramics for "tone corrector" (really a snubber) as the voltage is AC out on top of HT and any noise spikes can be huge due to cut off of current causing big volts from inductance of transformer.

[* I've forgotten why it's that way, but obviously the magic eye needs 0V for no signal and increasingly negative for stronger signals. Also the FM Voltage can be -20V or more, so often a DC attenuator on FM vs AM mode to the Magic Eye]

[boater sam]

The 4uf is not the discriminator detector cap, its the screen grid decoupler of the output valve. Its in the can with the reservoir and smoothing caps.
The detector cap seems to be OK and yes I know it is pos to ground.

I don't know why there is a time delay in the eye responding when tuning. The eye does nothing on AM bands.

[Mike Watterson]

Sorry for being confusing. I wasn't meaning the screen decoupler. I was assuming the discriminator had an approximately 4uF and wondered had someone changed it.

Does it pickup on AM and what voltage is on the AGC line with no station and a strong station on AM? Maybe the switch is dirty as the AM and FM feed to magic eye is switched.

[boater sam]

On AM the agc ranges from 0.1v  to 2.7v depending on the signal.

Thinking about the selenium bridge rectifier passing AC onto the HT, I wired a single 1N4007 diode in the HT+ off the bridge as belt and braces.

Before the diode I have 267v DC but after the diode I have only 218v.  Measured with a digital meter.  Why?  
Surely the diode cannot drop 49v across a single junction without going slat?
The radio still works as before.

[Mike Watterson]

Look with a scope. The ripple will give strange results on some meters.

On AM the agc ranges from 0.1v to 2.7v depending on the signal.

That should vary the eye unless the FM DC attenuator is in circuit or eye triode grid not being switched to AM AGC line. I presume -2.7V on a signal and 0.1V is no signal.

Check anode resistor of eye, the anode and deflector share same pin on most eyes. Target is always a separate pin and not so critical of value.

[Radio Fixer]

Following along Sam: interesting as I have not worked on any of these later (to me) big German monsters.

Re the diode: before fitting HT was 267V but after was 218V. Only thing I can think of to explain it is the bridge rectifier was allowing a lot of AC thru and confusing the meter used to measure it. I have a true RMS one that can measure DC and AC separately but a scope would also shed light on it.

Best Gary