Bush PB53 Output stage problems

[Radio_Dave]

I seem to have a resistance of 66K between the H.T. and the chassis. I can't see anything on the circuit diagram to justify this Should it be there?

David

[paulsherwin]

Originally Posted by Radio_Dave

I seem to have a resistance of 66K between the H.T. and the chassis. I can't see anything on the circuit diagram to justify this Should it be there?

I don't imagine so. That wouldn't explain all the extra current though, unless I've forgotten Ohm's Law

Best regards, Paul

[Radio_Dave]

Hi,

I seem to be getting somewhere today . There was a tiny piece of tinned wire on the back of the tag board which was connecting the TDD4's anode load resistor to the chassis, with this removed I have got rid of that 66K between the HT and chassis and more importantly it is picking up stations

The voltage on the reservoir cap has now fallen to 251V and the smoothing cap has rissen slightly to 223V. I am trying to measure the HT current but I'm not sure how to do it . I disconnected the smoothing cap from the HT and connected my DMM between them, set it to the 20mA range but when I turned the radio on all I end up with is zero How am I supposed to check it properly?

David

[Ed_Dinning]

Hi Dave, the easiest way, without disconnection is to measure the voltage dropped across the fiels coil resistance/resistor , then use ohms law
V/R to work out the current.
Ed

[Radio_Dave]

The resistance is 600ohms and the voltage drop is 28V so I work the current out to be 0.046 amps.

Is this HT current consumption ok or have I got more problems to look for?

Many Thanks

David

[Ed_Dinning]

Hi Dave, without a service sheet it is not possible to be accurate, but it would seem about right for a standard superhet. You may neet to check the DDT anode load as it may ahve suffered from the higher current that was being drawn due to the fault and gone high in value.

Ed

[Radio_Dave]

Hi Ed,

I'm not sure what I need to find on the service sheet. The only thing I can find, which might be of relevance is the Mains Consumption and that is 80 Watts

HTH

David

[Sam]

You can (I think ) work out the HT current from the sum of the anode and screen currents. These are in the valve analysis table. Im the case of this Bush, it is 90.3mA. However, that is with the proper valve line-up. This may well be different with the alterations you say this set has had!

Sam

[mjizycky]

46mA with the valve line up given is probably about right - pity the set has been "got at" so! One thought to consider, once the other problems are resolved, is that you have rather less magnetising current for the loudspeaker than desirable as it's now devoid of its "8 watt output" , as Bush called it!

This is why there's a resistor straight across the HT in an Ekco AD65...

Just a thought - try looking for a scrap Bush SW45. That used the same output stage so would yield a suitable transformer and other parts. The LS has the same field resistance in that.

[Radio_Dave]

I have actually been offered a PenB4, matching output tranny and speaker from a forum member and I am negotiating a deal as we speak

But as for now I am going to continue with what I have got. This is because, as Paul Sherwin correctly guessed, the replacement mains transformer doesn't seem to have a high enough output . The original had 660VAC across the windings the one I have only has 525VAC.

I need to get around another 25V on the HT to use the AC2/PenDD and another 50V if I want to use a PenB4

Paul Sherwin suggested using a couple of diodes in place of the rectifier, would using a bridge rectifier give me more or less volts to play with?

My mains supply is 240V and I am using the 250V tapping on the transformer, if I used the 230V or 200V tapping (still using the rectifier valve) would this increase the HT?

As soon as I get the HT voltage right I'll be able to check the current drawn by the output valve

Many Thanks

David

[mjizycky]

I have actually been offered a PenB4, matching output tranny and speaker from a forum member and I am negotiating a deal as we speak

Excellent stuff. You won't be disappointed!

Paul Sherwin suggested using a couple of diodes in place of the rectifier, would using a bridge rectifier give me more or less volts to play with?

You'd get far too many with a bridge (in theory, 525 x 2[sup]0.5[/sup] - about 730V) and would have to " lose " some somewhere; usually via a resistor. However I have to confess that I'm not very keen on semiconductor diodes in radios at the best of times, primarily because the HT is applied to the main electrolytics instantly and so they are subjected to a much higher voltage than normal until the valves heat up and start drawing current. For what it's worth, I don't much like direct heat valve rectifiers either!

My mains supply is 240V and I am using the 250V tapping on the transformer, if I used the 230V or 200V tapping ( still using the rectifier valve ) would this increase the HT?

Yes-and every other voltage too, including the heater volts. And over-running valve heaters is definitely not good for them!

Somehow you need to get hold of another scrap chassis for a PB53. Might I suggest subscribing to Chas Miller's auction catalogues? Often Chas has scrap chassis featuring in his auctions; I once had a complete rustbucket chassis for a SUG52 (the console version of the PB53 - another potential source of parts as these sets are rather unloved by collectors) but before you get too excited, it had already been robbed of the useful parts

Keep persevering with the PB53 - this is a very much underrated radio because it isn't loved by the fashionistas. Your time and effort will be rewarded, trust me. When you've got it sorted, drop me a PM with your e-mail address and I'll e-mail you a scan of the instruction book

[jim_beacon]

However I have to confess that I'm not very keen on semiconductor diodes in radios at the best of times, primarily because the HT is applied to the main electrolytics instantly and so they are subjected to a much higher voltage than normal until the valves heat up and start drawing current.

I have to agree - this is valuable advice that I would wholheartedly agree with, I've been trying to get this message across for ages (a voice in the wilderness ) You end up applying the peak voltage (less the rectifier drop) to the electrolytics - there will be no volt drop from a resistor in series with the rectifier, as, neglecting leakage, no current is being drawn. Compare this with a valve rectifier, where the rectifier has a drop of several tens of volts, and the rectifier is normally the slowest valve to warm up, ensuring that the set will draw current as soon as it is available.

If you compare sets with valve and semiconductor rectifiers, you will find the electrolytics generaly have a higher voltage rating in sets with semiconductor rectifiers.

Jim

[Radio_Dave]

Ok I hear what you say I there another way of raising the voltage using the transformer and rectifier valve?

As I mentioned earlier my mains voltage is exactly 239V which puts it between the 250 and 230V tappings. Is it worth trying the 230V tapping if I watch the heater voltage?

What about if I reduce the resistance of the speaker field some more?

If I could find another 25V on the HT I think I'll be ok

Thanks

David

[quantum]

I wouldn't recommend using the 230v tapping - overdriving mains trannies is bad practice and depending on its power handling capacity and the HT demand of the set (as it is not the original tranny) you could cause it to overheat and possibly burn out a winding.

Even if your HT is currently 25v down, are you getting nothing from the set with whichever output stage you are using? The audio stages should work on this slightly lower voltage and unless your oscillator stage is a bit flaky the Frequency Changer should work too - many mains sets I've worked with could drop the HT by up to a 100v and still the AF stages would be OK, and even the FC stage would often work on the MW/LW bands at least...

[Radio_Dave]

Originally Posted by quantum

Even if your HT is currently 25v down, are you getting nothing from the set with whichever output stage you are using?

Yes the radio is now working (well sort of) When I rebuilt the tag board I thaught I removed everything but unfortunately I left behind a tiny wire on the back which later connected the TDD4's anode load resistor to the chassis . Once I had removed this things were alot better.

Anyhow can anybody suggest what I do next? I seem to be making more problems than fixing them

Many Thanks

David

[Radio_Dave]

Hi

I just want to post a quick update. The radio is now working on all freqencies and all the push buttons are working .

I can't do any more with the output stage and the low power supply it is therefore very quiet But now for the good news! A very kind forum member (hotbulb) is sorting out a PenB4, output tranny, speaker and mains transformer all from a Murphy A50 When the parts arrive I'm sure I'll be able to get this radio working as it should

Many Thanks

David

[mjizycky]

A very kind forum member is sorting out a PenB4, output tranny, speaker and mains transformer all from a Murphy A50

MURPHY?! he said, in best Dennis-Hopper-in-Blue-Velvet mode, commenting upon Heineken> Yeuch!!!! You can't go round fitting sub-standard parts to a Bush! Actually, I can't think of a better donor set other than another PB53 or maybe an SW43 (for the mains TX at least) or SW45.

Greetings, Before you fit the parts from the A50, check the feedback circuit (including chokes) used by Murphy - the one on this set would give Philips a good run for their money. It did work well though!

Regards, Leon.

[Radio_Dave]

Hi,

I've fitted the mains transformer, output transformer and the PenB4 from the Murphy A50 (Thanks hotbulb). The HT is now a very healthy 265V , The speaker field is dropping 54V so I've worked the HT current to be 90mA which again is good . However volume from the radio is not great. I suspect it is either the alignment or the speaker.

It seems to be a waste of time checking the alignment, as yet, because I have just noticed that instead of a TH4A I have a TH4B I will have to get the correct valve first

My second suspect is the speaker. Unfortunately the speaker from the Murphy was no good so I am using the (incorrect) speaker which came with the radio. I've had to put a 900ohm resistor across the speaker field to bring the resistance down to 600ohms and 2.7ohms across the speech coil to try and match the output tranny I think I am right in beliving that this is hardly an ideal solution so I want to try Quantums suggestion of using a PE speaker. Can I use a resistor to replace the speaker field or does it have to be a coil?

Many Thanks

David

[mjizycky]

A resistor will be fine if you're using a PM speaker - it's just that the hum level will be slightly higher. You'll need a 10W wirewound for that job. (54^2 / 600 ~=5W, so make it 10W to allow for the initial charging surge at switch-on)

I wonder what that odd energised speaker I've got in the shed is...?

Mike.