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Old 6th Oct 2019, 12:44 pm   #81
ms660
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Default Re: Philips BX281U20 valve radio

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Originally Posted by cdm1christopher View Post
Lawrence do you mean R36 as this is connected to C75 and C109 on output valve UBL21?
No, insofar as R36 does not develop the bias voltage, R75 does due to the current flowing through it.

No current flows through R36, its job is to connect the bias voltage that's developed across R75 to the control grid of the sound output valve in order to bias the grid -ve with respect to the cathode, that resistor is a relatively high value in order to "isolate" the signal on the control grid from the bias source, that's because the grid bias source is bypassed by C75 which would otherwise present a very low impedance to ground for the signal if the grid was connected directly to the bias source.

Remember that normally the control grid of a valve that's suitably biased will not pass current unless the peak value of the signal voltage during its +ve half cycle on the grid exceeds the grid's bias voltage.

Lawrence.
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Old 6th Oct 2019, 1:11 pm   #82
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Default Re: Philips BX281U20 valve radio

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Originally Posted by cdm1christopher View Post
Again completely confused here which is the right way round on mains plug after re checking again.
First...Unplug the fused mains plug from the domestic mains socket, next...to avoid any confusion remove one of the valves and turn the receivers on/off switch to on, then measure the resistance between the Neutral pin of the fused mains plug and the chassis of the receiver, if the Neutral is connected correctly the resistance reading should be approx. 490 ohms.

Lawrence.
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Old 6th Oct 2019, 1:31 pm   #83
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Default Re: Philips BX281U20 valve radio

Well initially it went over 400r then settled at 137r. So I do have it the right way round? As no reading on +ve side. So job done Lawrence you need to write a book.
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Old 6th Oct 2019, 1:51 pm   #84
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Default Re: Philips BX281U20 valve radio

Yep fine 460r now removed ubl21 valve shot up .
Thanks again Lawrence.
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Old 11th Oct 2019, 12:26 pm   #85
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Default Re: Philips BX281U20 valve radio

Taken some voltage readings.
Bit difficult as pins not easily readable.
Rectifier
Anode 217v
Heaters
Pin 1 170vac
Pin 8 115vac
Pin 1 -24vdc
Pin 8 -36vdc.

UBL21
Heaters bit high bit low
Pin 1 89v
Pin 8 24v

Trouble finding heaters on uch21.
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Old 11th Oct 2019, 1:14 pm   #86
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Default Re: Philips BX281U20 valve radio

Em interesting since just taking readings keep losing rf signal completely then bit fiddling on knobs pulls rf back in!
I think I might need to clean up the radio wave selector wipers see if that's the problem as I'm getting lot of static switching back and forth!

If not solved I might change the resistors as I've had it running for 3 maybe 4 days can't remember without problems so now everything has had time to heat up and run as it should maybe it's causing weak spots in circuit to break down starting to cause problems just a thought!

Capacitors are fine so either resistors, wipers or valves. Got NOS rectifier so could be UBL21 don't think it's UCH21 as there not really stressed as such like the UBL21 or Uy1n.

Any thoughts?

Cheers Chris
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Old 11th Oct 2019, 1:48 pm   #87
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Default Re: Philips BX281U20 valve radio

The heater voltages require to be measured as the voltage drop across the heaters; ie the voltage between pins 1 and 8.

The heaters are run from an AC series circuit so should be measured on an AC range

I assume the readings you have posted are between heater and chassis. In this case they indicate a voltage of 65V across the heater of the output valve. From the datasheet, this shows the heater is being overrun by almost 20pc as it should be 55V but it would be worth measuring across the pins to be sure.
.

It is likely that this is because the set is designed for 220V operation. It is also likely that the other voltage points on the DC circuit measured wrt chassis will be high.
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Old 11th Oct 2019, 1:57 pm   #88
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Default Re: Philips BX281U20 valve radio

PS An additional dropping resistor will be required to bring heater voltages into tollerance as discussed further up the thread.
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Old 11th Oct 2019, 3:21 pm   #89
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Default Re: Philips BX281U20 valve radio

Yes the measurments are in AC between heaters and chasis so 20v higher than should be. Not sure where to put an additional dropper as I cannot find the discussion on it in this thread. Or do you mean trying to find a new dropper to go in place of the original but increasing the ohms ratings? If that the case I would have no idea how to calculate the right values as they are 75 150 and 190 respectively. Plus I would assume the dropper is around the 25watt mark? Or could I put a shunt resistor in along the heater chain? Or would I need a diode as well?
Or just change out the old resistors to see if that brings the voltage down. Just plucking things out of the air.
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Old 11th Oct 2019, 3:40 pm   #90
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Default Re: Philips BX281U20 valve radio

If you want to drop some volts use Ohms law to calculate, eg: for 20 Volt drop @ 0.1 Amps = 20/0.1 = 200 Ohms......Power dissipated (Watts) = Isquared*R = 0.01*200 = 2 Watts dissipated.

Lawrence.
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Old 11th Oct 2019, 4:31 pm   #91
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Default Re: Philips BX281U20 valve radio

You need to measure the voltage between pins 1 and 8 on each valve

Uy1n -50v
Uch21 -20V
Ubl2- 55V


By doing a quick sum and working out the differences between pins 1 and 8 from chassis as i am assuming that these are the readings you posted, i believe you have (170-115) = 55V on the UY1N and (89-24)= 65v across the UBL21 heaters

But you should measure directly across those valves (pin 1 and 8) to be sure - and take the same measurements for the uch21s

I believe that if you do this, and add all of the overvoltages together you will arive at a figure close to the difference between 220V and your own mains supply .

This figure is the one which will require to be dropped -and will likely be 20-25V -just as in Lawrence's post.

To have the set running as designed you should also consider the HT circuit,which will also be over voltage, and consider the HT (DC)current in your calculation- in reality something ballpark 200-250 ohms would be expected, but it is worth doing the measurements and the sums.

Please dont go changing resistors for the sake of it as this is unlikely to yield improvement. The overvoltage measurements show the set is performing as expected in this respect because you are using a 220V set on 245V mains. Likewise there are many other possible causes of the intermittency you describe than a resistor. Proper diagnosis is required.

The additional dropper would go in after the mains switch.
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Old 11th Oct 2019, 6:21 pm   #92
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Default Re: Philips BX281U20 valve radio

Thanks guys more work ha ha ha.
Yeah your right it has 245v going through it and as your saying it was designed for 220v so it's going to run a bit hot although anode pin 3 was 217v.
I'll leave resistors alone have a few measurements tomorrow and I'll mark off the pin locations underneath beforehand as near on impossible to read them as well as access as it's very cramped.
Em good news so in essence just adding an additional resistor with right wattage to bring voltage down.

All the intermittent is I'm using my make do outside aerial so going to pick up all sorts.
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Old 12th Oct 2019, 1:36 pm   #93
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Default Re: Philips BX281U20 valve radio

Okay measurements pins 1&8
Rectifier pin 3 bang on 220vac

UBL 21
Chassis readings pin 1 90vac pin 8 24vac. Should be 55v.
Measurment across both pins 65vac so should be 110v? is that correct?

UCH21
Chasis readings pin 1 116vac pin 2 90vac. Should be 20v seems well out of spec.
Measurement across both pins 92vac so should be 40v? Is that correct? So again if so well out of spec.

No readings on pins 1&8 on 2nd UCH21 is that correct?

Interesting when touching pin 8 which goes to voltage selector unit goes lovely and quiet with just a slight hum?

So these readings seem a bit off or not?

I've put in a NOS UCH21 in today.

Cheers Chris
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Old 12th Oct 2019, 2:13 pm   #94
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Default Re: Philips BX281U20 valve radio

Check your heater voltage measurement between pins 1&8 with the heater voltage given in the valve data, that should tell you if all's well or not.

Lawrence.
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Old 12th Oct 2019, 3:53 pm   #95
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Default Re: Philips BX281U20 valve radio

It is difficult to make sense of some of the voltages in your last post.

For this exercise we are interested in the voltage between the two heater pins (1 & 8)rather than their relationship with the chassis. My previous calculations working this out using voltage taken wrt chassis appear to have confused matters,so for the moment we want to focus on what voltage exists between the start and end of each heater.

To measure this directly one probe of the meter goes to
pin 8 and the other to pin 1 on each valve

From the datasheet the following results should be expected:

Uy1n -50v
Uch21 -20V
Ubl2- 55V

But from the measurements you have already taken wrt chassis and the sums i have done, i would expect the results to be significantly too high (eg the UBL 21 65V instead of 55V)

For each of the four valves measure,on AC volts range, voltage between pin 1 and 8 and let us know the results.

Remember also that all of the pins are at a mains voltage at some point above earth, so touching some of them with your fingers will be a shock hazard depending on where they are in the heater chain. Do not touch the chassis when taking the readings as,again this puts you at risk of shock.
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Old 12th Oct 2019, 4:03 pm   #96
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Default Re: Philips BX281U20 valve radio

I think found culprit one of the uch21 valves on prodding stroke tapping is causing static/distortion loud through speaker so could be something loose but think it's prob on way out! Or it might be worth changing the small cap and resistor first. Not sure it's that though as I've already given them a thorough prod with no problems. Plus these are all the original valves apart from 2 already changed.

All others fine.
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Old 12th Oct 2019, 4:19 pm   #97
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Default Re: Philips BX281U20 valve radio

I'll take readings again although it was set on ac as opposed to dc. Measurements taken both off the chasis and across the pins.

As you can see bit high but could not get a reading off the 2nd UCH21!

Ah did not take reading for pins 1&8 on rectifier although on touching pin 8 set goes quiet.

As said above something wrong with 1st UCH21. I'm assuming these are linked to the RF signal which would explain poor reception at times and no reading at the time off the 2nd UCH21! Just a thought.

Cheers Chris
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Old 12th Oct 2019, 8:59 pm   #98
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Default Re: Philips BX281U20 valve radio

The most likely explanation is poor connections on the valve pins. Remember the heaters are in series, so the current has to flow through all of them; and if there is a break anywhere in the chain, the current will be cut off and the valves will go cold.

Both the UCH21s are involved in the radio side of things. One changes the frequency from the tuned frequency to the IF (Intermediate Frequency). From this point on, everything is working on a fixed frequency. The other uses the "H" bit as an IF amplifier and the "C" bit as an audio preamplifier. The UBL21 contains the detector diode and the audio power amplifier, and the UY21 is the power supply rectifier.

A poor connection almost anywhere will create an audible noise; a brief disconnection of the heater chain will not give the valves time to cool down, but the change in heater current may induce noise elsewhere in the circuit. If one particular valve is especially crackly, it may need its pins cleaning. Get some proper contact cleaner, and use it sparingly on each of the pins. The socket contacts are spring-loaded with a sharp edge that should scrape away any dirt and oxide as the valve is inserted and removed -- be careful doing this, as there is the equivalent of about 50kg. of air pressing on the glass on top of any forces you may exert.
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Old 12th Oct 2019, 10:34 pm   #99
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Default Re: Philips BX281U20 valve radio

All valves and sockets where thoroughly cleaned on first obtaining the radio. All wiring has been checked and renewed where needed. All cold solder joints re done. All resistors checked as well as small capacitors all in spec. Wiring, valves, resistors, checked re checked. All wiring connections tested for continuity no breaks. RF checked, output transformer checked, tuning capacitor removed cleaned re wired re checked before re install, speaker checked for output all new main capacitors re placed, new tunning dial cord, new y2 caps installed on aerial connection, chasis thoroughly cleaned especially areas of rust. UCH21 removed installed in opposite socket same problem on tapping no difference made when tapping other valve installed on the other socket. So must be a short in the valve somewhere. These valves although look clean now where actual rust buckets until I cleaned them and don't forget their all original so firing them up and running for around 4 days caused problems and maybe that valve now being run in has started to fail. Just my assumption.

Cheers Chris
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Old 13th Oct 2019, 12:59 pm   #100
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Default Re: Philips BX281U20 valve radio

Well another check again ew.
Pic of the 6.8meg resistors. Completely broken down. Think shows when you have these old girls running for a while how things start to break down although visually and measuring wise seem perfectly fine. R36 gone as well triple it's rated resistance. UCH21 on end is not linked to the heater supply chain just for reference.
Thermistors are fine although they looked over heated.
Pic of back panel art work not great but looks tidyer.

Cheers Chris

Ordered new UCH21 valves got pair for 14 quid including postage so think I'll wait for them then re take readings. The faulty UCH21 I did notice yesterday started hot plating like 500w light bulb.
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