Diora Pioneer U3 dropping mains voltage help please

[lloydwells]

I have one of these not actually unattractive polish radios this one's dated 1953 the set uses a UY1N rectifier two UCH21 and a UBL21 output valve the set it designed to run on mains up to 220v and the rectifier and output valve are getting exceptionally hot on our mains I could therefore do with dropping the voltage a little suggestions please attached is the relevant piece of the schematic.

[PJL]

If you don't want to mess around with the set then a Bucking transformer is the way to go.

[lloydwells]

It's pretty much in bits so I really don't mind adding something in

[TrevorG3VLF]

Valves get hot. Measure voltages to get a better idea of correct working conditions. Add a resistor if necessary.

Check the voltage at the grid and cathode of the output valve and the voltage across the output transformer. A leaky grid drive capacitor could generate these symptoms.

[PJL]

You need to use a true RMS meter to get the additional dropper right. How you wire this will depend on how much of the current 1xx, 2xx settings you want to retain.

[lloydwells]

All the caps in it are new all the resistors are in spec there's expected hot and theres £### me that's red hot hot which these are .

[Herald1360]

Just try a 120R 10W resistor in series between the switch and internal resistors as shown. It should be near enough and will reduce the set's dissipation (though it will just move the excess to itself, so the total heat in the box won't drop much!).

[Herald1360]

Originally Posted by lloydwells

All the caps in it are new all the resistors are in spec there's expected hot and theres £### me that's red hot hot which these are .

Measure the total HT current. If it's reasonable compared with the typical total operating currents for the valves in the set (about 90mA) then that's just how it is. Check the cathode current for the UBL21 too- it's just possible that the valve or its holder may have internal grid leakage problems if the grid is not getting any coupling capacitor leakage trouble.


You could cool things down a bit by increasing the cathode R for the UBL21 if you like.

Edit: Oops, no cathode R for UBL21, set uses a negative supply dropper between HT- and chassis for bias supplies. Do check the value of this and the state of its decoupling capacitor.

[lloydwells]

The decoupling cap is new the resistor is nigh on 1 meg I've inserted the aforementioned 10w 120r in the supply now the rectifier is tolerably hot the UBL21 is still red hot

[TrevorG3VLF]

So, measure the voltage across the output transformer to determine the UBL21 anode current
Herald1360 says the bias is supplied by passing the set current through a resistor. I do not have this part of the circuit. It is not on the disc which I have. This resistor will be bypassed with an electrolytic capacitor. Is this connected the right way round with the POSITIVE to chassis?
Measure the UBL21 grid/cathode voltage with a high resistance meter. It should be near 0V.

[ColinB]

Does the new additional resistor take you to around 220V across the mains input to the set? If not, a larger resistor is needed. Obviously, switch everything on before measuring

[Herald1360]

Originally Posted by TrevorG3VLF

.....

Measure the UBL21 grid/cathode voltage with a high resistance meter. It should be near 0V.

NO!

Since the bias is applied to the grid and the cathode is taken direct to 0V, the voltage on the UBL21 grid should be negative wrt its cathode and in the region of-10 to -13V. Even a 10meg input DMM will read this about 10% low.

The bias resistor should be around 120R, the grid leak is the 1M or thereabouts resistor that the bias will be applied through. It's the tapped resistor bypassed by the electrolytic (positive to chassis) just above the rectifier in the circuit snippet in post #1.

[TrevorG3VLF]

Originally Posted by Herald1360

NO!

Since the bias is applied to the grid and the cathode is taken direct to 0V, the voltage on the UBL21 grid should be negative wrt its cathode and in the region of-10 to -13V. Even a 10meg input DMM will read this about 10% low.

The bias resistor should be around 120R, the grid leak is the 1M or thereabouts resistor that the bias will be applied through. It's the tapped resistor bypassed by the electrolytic (positive to chassis) just above the rectifier in the circuit snippet in post #1.

I realized my error when going to sleep last night.
I cannot find my valve data books to see what typical voltages are. I have been tidied up.

[TrevorG3VLF]

The data for the UBL21 suggests 200V anode and screen with 55mA anode and 9.5mA screen. The grid should be at -13V relative to the cathode.
Measuring the resistance of the output transformer and the voltage across it will give the anode current.

Getting the bias right will be a better solution than dropping the HT voltage.

[Silicon]

Is the device marked 'Limiter 100mA' a thermistor? They often run very hot in normal operation.

[PJL]

The heater voltages add up to 145V + dial lamp.
The two resistors in the heater chain add up 450ohms giving a 45V drop at 0.1A.
The 100mA limiter must be a thermistor and must have been designed to drop 220-145-dial lamp-45.

What meter are you using to measure AC voltages? As the HT current is comparable with the heater current, if they share the same droppers the waveform will be distorted. A true RMS meter is the only way you can check the heater chain voltage.

[Herald1360]

The heaters appear to be on a separate chain from the HT rectifier but with no clear indication of the voltage selector switch function it's hard to be sure. They may be separate on 120V and commoned on 220V if the HT isn't simply left to vary with the supply voltage. There is a connection from rectifier anode back to the selector switch which suggests the dual approach.

[PJL]

The heater chain must be split for 120V but it is highly likely the HT is run through the droppers when set to 220V. I agree that the 120 ohms should roughly drop the 20V although 150 ohms might be safer. An averaging AC voltage measurement is not going to give meaningful results.

It is often desirable to reduce the anode dissipation as set designers always pushed the limits but any change to the HT current is going to impact the heater voltage.

[ms660]

So far as I can make out.....

220V setting:

HT...Switched Live>125 Ohm>anode of rectifier.

Heaters...Switched Live>400 Ohm>Rect.>Mixer/Osc.>250 Ohm (hot thermistor)>AF output>IF/1st AF>pilot lamps>Switched Neutral.

120V setting:

HT...Switched live>anode of rectifier.

Heaters...Switched Live>50 Ohm>then split into two parallel circuits:

1)...>250 Ohm (hot thermistor)>AF output>IF/1st AF>pilot lamps>Switched Neutral.

2)…>400 Ohm>Rect.>Mixer/Osc>Switched Neutral.

Lawrence.

[hannahs radios]

Unless this set was was genuinely meant for export to the US the 120 volt tapping might actually be for 127 volts this was a voltage used in Eastern Europe worth mentioning even though I don't know this set.