Hi guys,
I have this beautiful Telefunken Concerto 8.
It still works but has a nasty 100Hz hum.
Bag of Doritos for size (sorry.)
Now, I checked the obvious things. These are my initial observations:
1) without the EL84, the hum is gone.
2) with the EL84 but without the last preamp tube, hum is still there.
So, the problem must be in the EL84 stage, right?
Here are the voltages I measure:
Apart from everything being a tad low (note the voltage switcher is set to 240V, while we have 230V here), everything looks good.
B+ is supposed to be a 270V rail with a 50uF cap. I'm measuring 240V with 3.7V Vrms ripple.
B2+ is supposed to be a 230V rail with a 50uF cap for the preamp tubes, I'm measuring 209V with 3.1V ripple. I measured the supply caps, they are spot on 50uF. Didn't measure ESR.
I checked the resistors around the EL84, they are spot on after 60 years!
Current through the EL84 is about 40mA, that's good (checked the cathode resistor, it is 120 ohm).
Current throught the g2 resistor is around 30mA, that's correct.
So it looks like the EL84 stage works fine.
I then thought that the ripple was coming in through g1.
So i shorted the point between W26 and W27 to ground (that's why I wrote 0V).
I was expecting the hum to go away, but it actually gets a bit louder! Now I'm confused...
So, voltages around the EL84 stage are good, currents are good, ripple is reasonable. But the hum's there, even if I short the input to ground.
What am I missing here?
I have this beautiful Telefunken Concerto 8.
It still works but has a nasty 100Hz hum.
An externally hosted image should be here but it was not working when we last tested it.
Bag of Doritos for size (sorry.)
Now, I checked the obvious things. These are my initial observations:
1) without the EL84, the hum is gone.
2) with the EL84 but without the last preamp tube, hum is still there.
So, the problem must be in the EL84 stage, right?
Here are the voltages I measure:
Apart from everything being a tad low (note the voltage switcher is set to 240V, while we have 230V here), everything looks good.
B+ is supposed to be a 270V rail with a 50uF cap. I'm measuring 240V with 3.7V Vrms ripple.
B2+ is supposed to be a 230V rail with a 50uF cap for the preamp tubes, I'm measuring 209V with 3.1V ripple. I measured the supply caps, they are spot on 50uF. Didn't measure ESR.
I checked the resistors around the EL84, they are spot on after 60 years!
Current through the EL84 is about 40mA, that's good (checked the cathode resistor, it is 120 ohm).
Current throught the g2 resistor is around 30mA, that's correct.
So it looks like the EL84 stage works fine.
I then thought that the ripple was coming in through g1.
So i shorted the point between W26 and W27 to ground (that's why I wrote 0V).
I was expecting the hum to go away, but it actually gets a bit louder! Now I'm confused...
So, voltages around the EL84 stage are good, currents are good, ripple is reasonable. But the hum's there, even if I short the input to ground.
What am I missing here?
Sorry, I'd like to help you, but as the pic doesn't load, I can't.
Note that there's always more or less hum to be expected from SE designs that are fed off the 1st filter capacitor.
Best regards!
Note that there's always more or less hum to be expected from SE designs that are fed off the 1st filter capacitor.
Best regards!
Hi, thanks!
Does a direct link work?
https://i.ibb.co/NWsww1N/concerto8.png
Anyway, I agree some hum is to be expected, but this is definitely too much.
Does a direct link work?
https://i.ibb.co/NWsww1N/concerto8.png
Anyway, I agree some hum is to be expected, but this is definitely too much.
Ok, you said the filter capacitors are ok. Is the hum still present when you connect the EL84's grid (where you've written 0V) to ground? Do the resistors W36 and W40 have their nominal values? Did you replace the cathode capacitor C61?
Another reason might be a weak EL84 which would disturb the hum compensation arrangement.
Best regards!
Another reason might be a weak EL84 which would disturb the hum compensation arrangement.
Best regards!
How do you know the filter caps are okay?
They may look fine, but the low voltage and high ripple says otherwise.
They may look fine, but the low voltage and high ripple says otherwise.
The hum doesn't disappear when I short the grid to ground.
W36 measures 120.1, W40 measures 985 Ohm. That's well within spec.
C61 was NOT replaced, but I don't immediately see how this would cause this.
Both B+ and B2+ caps measure 50uF. Leakage is not too high (>1MOhm).
Ripple looks like this on both B+ (240V) and B2+ (210V).
An externally hosted image should be here but it was not working when we last tested it.
It's measured wrong on the scope, but ptp ripple is ~10V. That's about 5% of the supply. Is that too high?
Don't bother measuring it, just replace it. Selenium rectifiers are dangerous .
Just note your B+ will be higher.
As for the caps, they measure okay with no voltage present, once voltage is applied they will measure badly you would need a proper ESR tester to be sure. Replace them.
Just note your B+ will be higher.
As for the caps, they measure okay with no voltage present, once voltage is applied they will measure badly you would need a proper ESR tester to be sure. Replace them.
If I remember correctly, Telefunken radios of that period have the AEG cylindrical selenium rectifier. It does increase internal resistence over time, overheat and eventually fails, taking the power transformer wit him. Later models have the more reliable rectifier with square shape. Replace the bridge rectifier with a silicon rectifier and a resistor that must be tuned to get the voltage on the schematic. 220 ohm could be a starting point. Add furter filter capacitor at the positive terminal of the bridge rectifier (33uF could be a good value). Add a 100uF filter capacitor on pin 9 of the EL84.
Well spotted on the selenium rectifier! After decades, the question isn't if it will fail, but when does it fail instead.
Rectifier translates to »Gleichrichter« in German, and there's a pun on this: Gleich-riecht-er, soon it will stink, as failed Se rectifiers produce that certain smell similar to rotted eggs. I'd replace any Se rectifier in any gear that I intend to operate in the future.
Best regards!
Rectifier translates to »Gleichrichter« in German, and there's a pun on this: Gleich-riecht-er, soon it will stink, as failed Se rectifiers produce that certain smell similar to rotted eggs. I'd replace any Se rectifier in any gear that I intend to operate in the future.
Best regards!
Yes, good point, this is the cilindrical one.
I checked, unloaded B+ (before the tubes warm up) is about 300 VDC. It drops to 240VDC loaded.
As we're at around 75mA consumption, that would mean 800 Ohm series resistance of the rectifier. I know the transformer is also responsible for a part of that, but still it seems too high.
I don't like to replace components unless I know they really are bad, because it's not that easy in these ptp chassis. But this one seems something plausible and not too difficult to try out. I'll change it tomorrow by some beefy diodes and a series resistor. Let's see if it helps. Thanks!
4 tiny UF4007 diodes or a small rectifier bridge are enough. It should be possible to leave the original rectifier in place (disconnected) to keep the original look of the chassis. A fuse to protect the high voltage transformer winding and a check for coupling and filter capacitor leakage is good thing to do on this radio because it does have quality speakers and output transformer, so it is likely to be turned on often if the power supply and audio amplifier section are restored back to factory specification. The EL84 and EABC80 tubes may be exchanged with known good ones, but if they still are the original Telefunken tubes, they may still be good today (check the cathode current).
The EL84 cathode current appears quite a bit low at about 39 mA, but it depends on the plate load (OT's primary impedance) if it fits or not. The EL84 datasheets also show some 9 watts operating points at 250 V/36 mA/7 kohms.
Best regards!
Best regards!
Yes, you're right and I'll have to wipe my eyes. So we've got a weak EL84, as yet assumed, plus a questionable rectifier there.
Best regards!
Best regards!
OK..... I've restored TONS of these german/european radios for past customers over the decades.
Simple answer......
IT NEEDS A COMPLETE RESTORATION.
From the power supply through to the coupling caps.
Do NOT attempt to re-align any tuning stages - it is a very involved job.
All tubes must be tested and replaced if weak.
A mechanical overhaul/lubing of the tuning mechanism is also required. - including the rotating AM antenna (if equipped).
The pushbutton system also needs to have corrosion/tarnishing cleaned out.
The fuseholder is also probably tarnished and needs attention.
There is no other option, nor any "shortcuts" available.
Simple answer......
IT NEEDS A COMPLETE RESTORATION.
From the power supply through to the coupling caps.
Do NOT attempt to re-align any tuning stages - it is a very involved job.
All tubes must be tested and replaced if weak.
A mechanical overhaul/lubing of the tuning mechanism is also required. - including the rotating AM antenna (if equipped).
The pushbutton system also needs to have corrosion/tarnishing cleaned out.
The fuseholder is also probably tarnished and needs attention.
There is no other option, nor any "shortcuts" available.
If the radio does have collector or emotional value, wiseoldtech just wrote the right thing to do. It is a lot of work (if done properly) but the radio will work fine for the next decade and more. The RF section is the most difficult part, but I guess that a up to spec reception is more a requirement in USA than in Europe. Here the AM radio is basically dead, and FM has been extended to the current 88-108MHz after the tube radio era; classic radios cannot get a full FM reception anyway. Most people use the restored radio as powered speaker trough the phono inputs, so having a working RF section is not always a requirement. Before committing to this, I believe that a quick minimal repair such as the one PsychoM is attempting can be useful - if done properly with limited disturbance to the original circuit. It will give an idea about the potential of the radio, because not every old device is worth the time and effort needed to restore it. The fuseholder mention is a important one. The fuse on the primary side of the power transformer must be replaced with a new one of the proper value listed on the schematic, or added if missing. The conductors on the primary side of the power transformer (including the power cord) need to be inspected troughly and distanced, if possible, from the secundary side. Do not connect anything to the line or phono input of the radio or touch the metal chassis before assessing the insulation: the chassis could be live at mains voltage due to design choice, faulty components or old improper repairs.
Thanks for all the inputs, guys!
First of all, this is not my radio. It belongs to an old lady and has some emotional value, but it recently started to hum. I volunteered to take a look at it.
I agree full restoration would be the way to go if it was my radio, i've done it before on some old tube amps as well. But here, it does not need a full restoration, for the following reasons:
- she barely uses it.
- she doesn't care for the highest quality sound. The hum that is there now, is really annoying though, and it must mean something is way out of spec.
- Analog radio is almost dead in Belgium. AM has been dead for years. FM is planned to disappear in 2022 or 2023 (though it might get postponed again...). This would make the radio worthless to operate (for her at least).
Also, I'm doubting she would want to spend the money for an entire restoration. Without doubt it will extend the lifetime of the radio by another half century, but that won't be necessary, unfortunately.
So the first thing I want to do, is to get rid of the hum, to see if I can fix the actual problem. I'll try the rectifier first. Afterwards, I'll try another EL84 (i don't have one laying around right now). By the way, I miscalculated the cathode current, I forgot to add the g2 current to the anode current. I thought 4.7V on the cathode was OK, but indeed it is a bit on the low side. Not catastrophical, I would say.
Afterwards, I wanted to see if there are some other latent failures. The fuses are a very good point, also the wiring insulation. But from an initial optical inspection, the radio looks in very, very good shape. Actually, I've never seen one this clean before that hasn't been restored.
First of all, this is not my radio. It belongs to an old lady and has some emotional value, but it recently started to hum. I volunteered to take a look at it.
I agree full restoration would be the way to go if it was my radio, i've done it before on some old tube amps as well. But here, it does not need a full restoration, for the following reasons:
- she barely uses it.
- she doesn't care for the highest quality sound. The hum that is there now, is really annoying though, and it must mean something is way out of spec.
- Analog radio is almost dead in Belgium. AM has been dead for years. FM is planned to disappear in 2022 or 2023 (though it might get postponed again...). This would make the radio worthless to operate (for her at least).
Also, I'm doubting she would want to spend the money for an entire restoration. Without doubt it will extend the lifetime of the radio by another half century, but that won't be necessary, unfortunately.
So the first thing I want to do, is to get rid of the hum, to see if I can fix the actual problem. I'll try the rectifier first. Afterwards, I'll try another EL84 (i don't have one laying around right now). By the way, I miscalculated the cathode current, I forgot to add the g2 current to the anode current. I thought 4.7V on the cathode was OK, but indeed it is a bit on the low side. Not catastrophical, I would say.
Afterwards, I wanted to see if there are some other latent failures. The fuses are a very good point, also the wiring insulation. But from an initial optical inspection, the radio looks in very, very good shape. Actually, I've never seen one this clean before that hasn't been restored.