I have a Tecnico radio and recap it.

Also changed a few resistors.

The set is working now.

However, I notice that the B+ creep up after the set is switch on.

B+ initially is 360v then down to 205v. I understand this is because the valves need time to warm up.

What I don't understand is why the voltage creeps up again slowly?

The rectifier is 5Y3GT and power amp is 6V6GT.

Is this normal and why?

Thanks

Kevin

It depends how much it's creeping. A few volts doesn't matter but if it's quite a lot it could be a resistor going out of spec as it heats up, or a valve drawing less current as it gets hotter. You need to check all the voltages when it's cold and then compare them to the same measurements when it's hot to narrow down the area.

I've had sets where the B+ has changed by 5 volts and the bias voltage by 0.5 volts as it warms up for no obvious reason. All voltages are still in spec with no effect on performance. Tired valves maybe but all else being well I don't bother chasing it.

Did you replace the electro's during your recap and check the transformer for leakage?

Cheers,

Warren

Need to know what model to (hopefully) get the voltage B+ should be. What normally happens is that with a B+ of 250V (some Technico's not that high) it will surge to close to 500V then settle.

I have seen some dodgy 450V caps and with a 250V rail I will not use less than 500V caps on a 5Y3 / 80. As for Scraps.... Need to know the extent of replacements.

Need more info.

Marc

Hi Warren,

The voltage change is in 5 to 10 volts range. I replaced all the caps except mica and adjustable ones.

Hi Marc,

It is a model 1050. Unfortunately, I have no voltage info, only the circuit.

What confusing to me is when the voltage goes up, also the current goes up too.

The 6V6GT cathode self bias goes up and so does the voltage across the output transformer.

The power meter tells the same story. The set's current draw goes from 198ma to 205ma.

In fact, that's how I noticed this symptom.

I will look it up: Current draw goes up through a resistance if the voltage applied increases. The transformer should be on the 240 -260 tap if it has one? Will look.

198mA sounds like a leaking B+ cap but that is not entirely consistent with the volts.

Marc

200ma is about 48 watts.

The valves are 6BE6 6BA6 6AV6 6V6GT and 5Y3GT.

This power consumption is normal?

In the circuit there is two tap, 210 and 240v

The filament voltage looks ok so it should be the correct tap.

Ok the parts list is on page 313 and it appears to be consistent with the last Technico I had dealings with.It is a typically slack Australian circuit, when compared with a European one, as they tended to list the voltage & current on just about everything. We get......

The following may not be pretty: But you need to check the wiring first. Including the tap on the transformer.

What I believe I am seeing if it is not a transformer (doubt) (The Technico mentioned cooked its) is the result of a monumental PSU wiring stuffup or a short.

If you look at the circuit you will find that the centre tap (most negative point) does not go directly to ground (chassis). It does so via R9. That means that the first cap c26, was, if chassis mounted, sitting on a bakelite plate and insulated from the chassis. It must be insulated from the chassis as it's negative goes to the "floating" centre tap, not the chassis.

This system is "back bias". If back bias is correct CT to K of the 6V6 should be right.

Further the C26 positive should be on the filament of the 5Y3. connected to that same pin should be a 60mA choke. So this is perhaps missing,shorting out.or something is pulling too much load (C26?) Choke should be around the size of the speaker transformer. Often it is replaced by resistors when it cooks.

If it is missing the set will get too many volts & draw too much current and you will fry the transformer.

The previous Tecnico ran 190V B+. Looking at the 6V6 cathode resistor R19 and R9 that is consistent with the 6V6 running at round 180 - 190V with a bias of -8.5V. So I would expect about 8V across R19. 6V6 current (the biggest draw) would be in the order of 31mA.

If there is a heater cathode short in a valve that tends to fry R9. If a heater is intermittent in shorting that may cause the valve to loose conduction. Do not run 5Y3 sideways.

You can only buy 450V caps commonly. There are other suppliers. Modern caps do not normally carry a surge rating, so at 360V surge I would not go below 450V for C36 & C31. All Electrolytics & paper caps should have been replaced by this point?

Just because it is a new cap (& not wired backwards...we hope) does not mean it can't fail. I have seen several of those El Cheepo 450V one's fail quickly. Wiring them backwards stuffs them. I have had these get to a certain voltage & then draw current at a hyperbolic rate. Such fails are noted also on the American Forum.

It might pay to check, albeit that it could be visual from the lights, the mains & see if it is fluctuating? Do use the right meter & care for that exercise. Meal times in a City can be interesting. I have not seen a plug in voltage meter, like those little plugs you get to test if the socket (ext cable) is wired right, or faulty.

Weekend project: Check all of this and get back to us.

Marc

Checked the radio during the weekend and made no progress.

The input capacitor is under chassis so it is ok.

The 198ma is the mains current draw for the whole radio.

Tried another 6V6 makes no difference.

Don't have another 5Y3 go try, however, it may cause the issue.

Check the taps and it is correct as it is the higher dc resistance one.

Don't know what to do next.

May have to live with it.

No voltages on circuit sucks.

The 6v6 plate draws about 34ma by measure the dc resistance of the primary of the OT and voltage drop. The voltage is from 8 to 9.4v. It changes when the mains voltage changes.

The mains voltage changed from 228 to 244 v during the day.

The power consumption of the radio changed from 42w to 51 w.

The current from 181ma to 205 ma.

34mA is in the ball park for a 6V6. That's a big fluctuation in the mains voltage but I don't really see a problem with your measurements. If the voltage increases there will be a corresponding increase in current and your measurements back this up. Marc gave your a lot of good tips in post #8, how did you go with all those checks?

Cheers,

Warren

And pray tell how you deduced that the input cap by being under the chassis in the pan, was ok? I have seen several "in the pan that have exploded" and new underrated fail & crap quality ones die inside two years.

Shotgunning & wild assumptions do not fix radio's. Methodology fixes radio's and one starts at the power supply first.

Monitoring the mains current, is not a reliable way of determining where a fault is. With a 16V change in voltage it might be worth an email to Energy safe to see what the acceptable variation is?

As said I would expect a sag at meal time, but expect higher volts later, but it looks like the system may have a fault, or is stressed under load.

If you get the valve data sheets for the tubes (eg Franks electron tube pages) while many only list the common 250V rail many are more detailed. B+ after the choke (screen (pin4) of 6V6) is the B+ rail.
Add the plate & screen (Pentodes) currents of all valves & that should be the current.

The catch 22 here is that you need the current at a stable 240V. 5Y3 normally dies slowly and reduces voltage steadily. The only other common internal cause is thermal. eg overheating resistor, faulty heater, faulty filter cap. I do have a list of common causes somewhere, put out by a Californian radio club.

The normal behaviour of a set with a filament rectifier on start up with a B+ of 250V (after the choke), is to surge to a voltage close to 500V (which is why I use 500V caps and the oldies 525 surge volts) and then stabilise on 250V after the valves have come into conduction & stabilised. 20% variance, either way, is considered normal. However, low voltage indicates overload and high voltage is indicative of a valve/s losing conduction, sometimes that is a heater, more often a component.

Most Pentagrids other than 6SA7 are voltage sensitive and will wander if the voltage is not stable. Their general behaviour is also to drift high until they stabilise (Radiotron Manual). It will be almost impossible to make a proper assessment of this set with an unstable mains, so you need to make your measurements at a point where it is at its most stable, or find another source.

Marc

Thanks Marc, will try to do something when I can find a stable power source.

Kevin

I would start with Energy Safe as fluctuating voltage on mains can cause problems They may be able to get a monitor onto the line (if the poles & wires mob have not got an auto one on it?). Motors in general are more likely to be damaged by very low volts than voltages a bit over.

There are a few things that you can look at even it is unstable. As said the cathode to chassis volts on the OP tube is a good guide to to what is going on current wise, within the valves of this set.

One of the comments in a tester manual, I have here, is that the maximum draw of a HV Electrolytic is 10mA. Another says C= 0.1CV; V being voltage applied; C in mfd & current in uA.

Albeit the valve tester has a tester for B+ & Cathode caps. I built a reformer into my bench PSU when I built it, and use that for finding elusive ferals that will get to a certain voltage & than start drawing current at a hyperbolical rate, even before the cap gets to rated value. New old stock electrolytics should be reformed before use (old dry ones often won't, so we do not bother... toss). Loss of form (polarity) causes many Electrolytics to present as a short.
That is one reason why you do not power sets that have been sitting, unused, for ages.

Marc