Grundig 80U

[Amie]

I recently started the repair of one of dad's "to do pile" radios. A Grundig 80u from approx. 1955.

It wasn't in an actual pile, it was living in a bubble wrap lined box and is in excellent exterior condition though not working. It was an online purchase and a note in the box states that it was sold not working, but complete and original. (That last description was not true). There was no plug and the note warned that it tripped the mains breaker when tried.

I was attracted by this set because it's small and has only three valves, giving the impression that it may be simple to repair. I've found some information online that describes its circuit operation. The set is a MW/UKW, (FM) superhet that uses the RF amplifier section of the first valve in a reflex circuit that also acts as first IF when in FM mode.

So not simple, no way!

Under advice, I connected the set up through an isolating transformer and series lamp, short version, the rectifier and 50uF reservoir were unoriginal and dead short, the rectifier was a silicon diode, BY 127

After replacing the dead cap and diode (diode with 4007) I tried the set, now the lamp limiter flashes bright then barely glows. On MW after warm up I had distorted sound for a moment that soon disappeared, nothing seems distressed, but the only way I can get sound is by switching off for a while and then on again. Injecting a signal into the pentode section control grid, gives a healthy buzz from the speaker, so I guess all's good from there on.

>>> HERE COME THE QUESTIONS<<<
The circuit seems odd to me as the triode and pentode sections have a common cathode and the bias for each section seems a little unusual, the neg supply has a parallel resistor cap combo that supplies the common of the rest of the set at a positive voltage to that of the output section grid, the only part of the set biased to the full negative supply. I don't have a spare ECL 113 to hand to swap, and I have no details for that valve, I do have some "new" ECL 80  types that I could use if they were electrically compatible, they are also common cathode, (would also need a base change), but is it more likely that there is another cause? Would it be unlikely that the triode section has failed in a common cathode valve with a good pentode section?

I have replaced the triode grid bias resistor 20M with two 10M in series as the old one was wayyy high, but with no improvement.

Thanks,
Amie.

[pwdrive]

Try and check the voltages when in the fault condition.

The grid bias for the triode section is by/due to grid current, the high value grid return resistor being the giveaway for that type of bias arrangement, the bias for the pentode section is provided by the current flowing through all the valves, that current has to flow through the 70 Ohm resistor (R16) The voltage developed across that resistor being the pentode's grid bias voltage, supplied via the 1 meg and 1k resistors, R15 & R14)

https://nvhrbiblio.nl/schema/Grundig_80U.pdf

Lawrence.

[ppppenguin]

Amie, I presume you're now back from the USA. Hope you had a great time.

Data on the ECL113 can be found here: http://www.mif.pg.gda.pl/homepages/frank...ECL113.pdf and here: http://www.mif.pg.gda.pl/homepages/frank...ECL113.gif Not a valve I've encountered before.

Initially I'm more concerned that the set was tripping an RCD. An isolating transformer will certainly stop that happening but doesn't solve the problem. A bench mains supply really needs its own RCD so when it trips you don't annoy anyone else. Not always possible but something to bear in mind. The power supply arrangements in that set are interesting. A transformer to supply 6.3V heaters plus halfwave rectified HT straight from the mains. The sort of set that's live chassis but might fool people because it's got a mains transformer.

[Amie]

Amie, I presume you're now back from the USA. Hope you had a great time.

Data on the ECL113 can be found here: http://www.mif.pg.gda.pl/homepages/frank...ECL113.pdf and here: http://www.mif.pg.gda.pl/homepages/frank...ECL113.gif  Not a valve I've encountered before.

Initially I'm more concerned that the set was tripping an RCD. An isolating transformer will certainly stop that happening but doesn't solve the problem.  A bench mains supply really needs its own RCD so when it trips you don't annoy anyone else. Not always possible but something to bear in mind. The power supply arrangements in that set are interesting.  A transformer to supply 6.3V heaters plus halfwave rectified HT straight from the mains. The sort of set that's live chassis but might fool people because it's got a mains transformer.

Hello Jeffrey.
Yes, just home. Thanks had a fantastic time, it was great to see my friend again, I was sad to leave

I had started on this before I left on holiday, but am now interested in getting on and fixing it if I can.

The setup I use for powering is an isolating transformer with one side of its output earthed separately from the mains earth and an rcbo on its output, one of dad's bits of kit, I don't think the radio tripped the rcd, just the mcb.

I've read that you can't just replace the original selenium rectifier with a modern diode in a mains derived supply and that it should have a series resistor, this set had already had a silicon diode put in with no resistor and had proven that point. What value would be good?

The grid to cathode voltage (triode section) is zero, but there's no short. It should have a small negative voltage, I would have thought? Its cathode is common with the pentode, and that seems to work fine.

Thanks for the info and PDFs

Amie

[Mike Watterson]

Typically about 150 Ohms, maybe 5W, I forget.
If it's a German market Grundig and not for UK and doesn't have a 240V setting, then you need a resistor on the input too. Check valve heater voltage.

Any paper caps on grids are usually bad.

[Mike Watterson]

The grid to cathode voltage (triode section) is zero, but there's no short. It should have a small negative voltage, I would have thought? Its cathode is common with the pentode, and that seems to work fine.

Thanks for the info and PDFs

Amie

Unless you have a 100M Ohm VVM, you won't see a voltage as the DVMs are mostly 1M (cheap ones) or 10M Ohm. Clue is the 20 M Ohm grid leak bias.

[Amie]

The grid to cathode voltage (triode section) is zero, but there's no short. It should have a small negative voltage, I would have thought? Its cathode is common with the pentode, and that seems to work fine.

Thanks for the info and PDFs

Amie

Unless you have a 100M Ohm VVM, you won't see a voltage as the DVMs are mostly 1M (cheap ones) or 10M Ohm. Clue is the 20 M Ohm grid leak bias.

Thanks.

German version, I think, made for 220v supplies? Valve heaters run at 6.55 on home juice. I've got it on the isolators -20 tap at the moment, heaters at 6.26

Measured grid voltage with "valve" volt meter.

Will check capacitors, I think they're resin cased thingy's, (leaky or open, you think?)

Amie.

[reset]

Amie
We can thank lawrence for the datasheet it will make the testing easier.
The teacher says get the DC bias correct first and then look at the AC. Consider the amplifier as a voltage divider with the load dropping voltage at a current controlled by the bias. Measuring the plate voltage will tell us quite a bit about the health of the circuit.

   

Measure the plate voltages and record them. We can look at them and see how healthy your circuits are.
1. If they are normal DC bias is well
2. No plate voltage indicates an open load OR screen supply.
   Missing screen voltage will put the valve in cut-off.
3. Equal DC supply indicates no drop across the load.
4. Abnormally low plate indicates high current which could be caused by a leaky coupling cap placing some plate voltage from the previous stage on the grid or a load increasing in value.
ETC.

Greg

[Amie]

Try and check the voltages when in the fault condition.

The grid bias for the triode section is by/due to grid current, the high value grid return resistor being the giveaway for that type of bias arrangement, the bias for the pentode section is provided by the current flowing through all the valves, that current has to flow through the 70 Ohm resistor (R16) The voltage developed across that resistor being the pentode's grid bias voltage, supplied via the 1 meg and 1k resistors, R15 & R14)

https://nvhrbiblio.nl/schema/Grundig_80U.pdf

Lawrence.

Thanks for the link Lawrence, I've tried to download it, but is still "queued"  I will look at what Greg attached and go from there, the capacitor at the top of the volume control, plus the one to the grid seem ok, but I've temporarily replaced them anyway and there's no difference.

Amie

[Amie]

Amie
We can thank lawrence for the datasheet it will make the testing easier.
The teacher says get the DC bias correct first and then look at the AC. Consider the amplifier as a voltage divider with the load dropping voltage at a current controlled by the bias. Measuring the plate voltage will tell us quite a bit about the health of the circuit.



Measure the plate voltages and record them. We can look at them and see how healthy your circuits are.
1. If they are normal DC bias is well
2. No plate voltage indicates an open load OR screen supply.
   Missing screen voltage will put the valve in cut-off.
3. Equal DC supply indicates no drop across the load.
4. Abnormally low plate indicates high current which could be caused by a leaky coupling cap placing some plate voltage from the previous stage on the grid or a load increasing in value.
ETC.

Greg

Ok. So there was no volts on the triode anode, the 200K was replaced as it was OC. At switch on the pentode anode volts crashed and the resistor I'd just replaced fried 

I suppose from your fault finding guide points I should now replace the 2n2 capacitor. I'll do that tomorrow and report, I've cases to unpack now

Thanks all,
 Amie