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Old 4th Mar 2020, 11:32 am   #1
Kentode
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Default PYE R33 Over Voltage.

This one is puzzling!

It's been partially restored and the metal rectifier replaced with an unmarked diode ( no dropper resistor) and works but with loud hum so I replaced the triple can capacitor, C25-C27-C28.

That's cleared the hum, but the voltage at the cathode of the replacement diode was reading 280vdc using both DMM and analogue meter.

I disconnected one side of the diode and tacked in a 1N4007, but I still need to drop 55 volts!

Using the ERT 1431 Service Sheet I've checked for short circuit from HT to chassis and there are no resistors or capacitors that are wildly out of spec, no heater/cathode shorts.

I've included a few pictures of the board wiring and my sketch.

I'd much prefer to cure the problem rather than insert a big wirewound resistor.

Any ideas?

PS Apologies for the poor picture of the sketch.
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Old 4th Mar 2020, 11:55 am   #2
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Default Re: PYE R33 Over Voltage.

I suspect the cure will be that WW resistor. The forward drop of any Si diode will be less than that of a metal rectifier.

Of course, there is the question of "does it matter anyway?" Provided no caps are being overvalued or resistors overheated it's unlikely to cause any problems, even less if the use is limited compared with the often 16/7 running that would once have been its lot.
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Old 4th Mar 2020, 12:33 pm   #3
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Default Re: PYE R33 Over Voltage.

There are three causes of excessive HT voltage:

1) excessive mains voltage, these sets were designed for AC mains 195-250V so your mains maybe at the top of the input voltage range, have you selected the 221-250V dropper tap?

2) Too little voltage drop along the path from mains, in this case the dropper, a 22ohm resistor, and the loss in the old rectifier.

3) The set is taking too little current.

Check the dropper tap is correct.

The current can be checked by measuring the cathode voltage of the output pentode across R14. It is also worth checking R14 is still 330 ohms. If the voltage is low then you need a replacement UCL83.

Because this is a Printed Circuit Board radio, I would not want the UCL83 to overheat and damage the board so I would add a wirewound dropper to the HT in series with the 22 ohm to get the HT voltage down to the service sheet value.
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Old 5th Mar 2020, 3:32 pm   #4
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Default Re: PYE R33 Over Voltage.

Thanks for the replies, I'll be in the workshop this weekend and Will let you know the results.
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Old 5th Mar 2020, 4:08 pm   #5
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Default Re: PYE R33 Over Voltage.

When you replace an old metal/selenium rectifier with a modern diode, you must always fit an appropriate value and wattage resistor in series with it - there's absolutely NO exception to this rule!
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Old 5th Mar 2020, 4:34 pm   #6
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Default Re: PYE R33 Over Voltage.

I repaired one of these about 5-6 years ago, silicon diode and WW resistor, forgotten the value but has Techman states it is something that is required.
I also had the UCL83 drawing grid current and that required replacement, also a few capacitors.
Nice little radio when it was sorted out.
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Old 6th Mar 2020, 11:55 pm   #7
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Default Re: PYE R33 Over Voltage.

According to the ERT service sheet, the voltage at the cathode of the pentode section of V3 should be 10.4 volts. It's probably worth unsoldering the anode lead from the output transformer and inserting a milliammeter - the anode current should be in the region of 26mA. if it's a lot lower than this either R14 has gone high or, more probably, the valve itself has lost emission. Either way, the result would be that the HT voltage was too high.
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Old 7th Mar 2020, 1:12 am   #8
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Default Re: PYE R33 Over Voltage.

Is it not possible to use the output transformer as a load resistance to get the anode current without disconnecting anything?
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Old 7th Mar 2020, 3:48 am   #9
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Default Re: PYE R33 Over Voltage.

When I did the same thing I increased the value of R16. I can't recall what to but do remember that it wasn't a massively high-powered resistor and it fitted on the PCB where the original had been.
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Old 7th Mar 2020, 10:07 am   #10
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Default Re: PYE R33 Over Voltage.

The R33 with the UCL83 is very sensitive to high HT. The valve tends to run into grid current at the slightest excuse.
As mentioned by Techman, you must wire a suitable surge limiter resistor before the silicon diode. You may need to go as high as 75ohm to get the HT down to the stated level. Trial and error. Do not exceed the value in the service manual. If anything aim for around 10% less. John.
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Old 7th Mar 2020, 10:16 am   #11
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Default Re: PYE R33 Over Voltage.

Hello there, when I repaired my R33 I found that I needed to dramatically increase the value of R16 from 22 ohms to 270 ohms. This is a massive increase I know, but this was the only way I could get the HT anywhere near normal. I too used a 1N4007 to replace the faulty metal rectifier. It is important to have the HT normal so the (good) UCL83 is not overloaded. For R16, I used a Welwyn wirewound 3 watt device. It runs warm, but you can still touch it. Below you can see it mounted in the position of R16, just below the speaker magnet in the third picture. I also incresased the UCL83 cathode resistor (second pic) R14 to 470 ohm from 330 to reduce the dissipation in this already hot little valve. The HT and cathode voltage (about 9.9v) of the UCL83 is very slightly below the published data. The set is used daily for several hours and has been perfectly fine for a couple of years now.
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Old 12th Mar 2020, 1:42 pm   #12
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Default Re: PYE R33 Over Voltage.

I've fitted the 1n4007 diode and a 150 ohm, 10 watt wirewound resistor.

Most of the voltages are slightly lower than required apart from V1 UCH 81 pin 8 which is now 271vdc when it should be 84vdc!

C7 showed 112vdc on the chassis side so I replaced it with no improvement.

This is my second failure this year and I'm not sure if I can carry on. The components surrounding V1 seem to measure okay and I'm struggling...

Anybody want to buy a complete workshop and several dozen radio's?
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Old 12th Mar 2020, 2:20 pm   #13
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Default Re: PYE R33 Over Voltage.

Don't worry:
That's the oscillator anode with lots of RF on it and a DMM is likely to be upset trying to read it. (I killed one meter doing that.)
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Old 12th Mar 2020, 3:23 pm   #14
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Default Re: PYE R33 Over Voltage.

DMM's are of little help when repairing old equipment. They can give odd readings due to their very high impedance. You can't beat a good old AVO 8 used in conjunction with a DMM. They make a good couple.
Most manufacturers readings, listed in period service manuals stated that the voltage readings were taken with a 20.000 ohms per volt AVO 8. Similar confusion arose when the AVO 8 was introduced around 1953. The old model 7, the staple of most radio service dept's is only 1000 ohms per volt and drops quite a voltage when applied to a high impedance source.
Does the R33 actually operate OK? If so, what is the problem now you have reduced the HT to a more manageable figure. Hope you get it sorted. I have a couple of these and they perform very well once you have strapped down the UCL83! John,
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Old 12th Mar 2020, 4:41 pm   #15
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Default Re: PYE R33 Over Voltage.

I'm now getting a reading of 75vdc using an analogue meter on pin 8, thanks for that!

Unfortunately when I turned up the volume all I'm getting is hum with the signal (Five Live) sounding "pumped" in for a second then disappearing, repeatedly.

The radio was working before I replaced C7. I must have cooked it as it's now 800 pf. I tacked it onto the underside of the PCB, could this be the problem? Also I'm not sure if it's a 63 volt capacitor.

I had a large turkey sandwich with next year's cranberry sauce that tasted great so I'm not totally useless!
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Old 14th Mar 2020, 1:45 pm   #16
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Default Re: PYE R33 Over Voltage.

Are there any tests l can make with the valves removed? Can I power up without the valves in?

I've got a Hameg 203 oscilloscope, which is earthed, can I disconnect the earth and use it to look at the signal? I'd need some help as I've never used it for live testing.

I understand that the Pye's chassis is connected to neutral as would be the chassis of the Hameg, would that be ok? I've just checked and there is no path from the earth clip of the probe to the neutral of the 'scope.

I'm not in the habit of leaning on the radiator while licking the chassis!
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Old 14th Mar 2020, 2:08 pm   #17
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Default Re: PYE R33 Over Voltage.

Quote:
can I disconnect the earth and use it to look at the signal
I wouldn't. The scope case will likely float up to half or full mains via its filter circuitry. Shock hazard. When you connect the scope ground to the radio chassis the whole thing will be at neutral. Safer but the correct advice is to treat a mains neutral as though it were live. One bad connection and the entire rig could rise to mains voltage.

Quote:
I've just checked and there is no path from the earth clip of the probe to the neutral of the 'scope
Good, there shouldn't be. The earth clip of the probe is connected to the earth pin on the scope 13A plug. The live and neutral will go to the power supply in the scope. Don't confuse neutral and earth. The terms are not interchangeable! Scopes don't usually have live chassis construction...

I'm an experienced engineer (day job) and I never lift earths on things unless I'm well away from them and have no intention of touching anything. Good workshop practice. For live chassis scope work, I have an isolating transformer which goes on the item under test.
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Old 14th Mar 2020, 2:17 pm   #18
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Default Re: PYE R33 Over Voltage.

Quote:
Originally Posted by Kentode View Post
Are there any tests l can make with the valves removed? Can I power up without the valves in?

Only continuity checks component value measurements and those only if any parallel connected components are known and can be allowed for.

I've got a Hameg 203 oscilloscope, which is earthed, can I disconnect the earth and use it to look at the signal? I'd need some help as I've never used it for live testing.

Short answer is NO it's not safe!

A longer answer is that you could power the set from an ungrounded isolation transformer and then use the normal grounded 'scope it to look at the set.

If the 'scope has the capability to display "A-B" and the A and B trace inputs have adequate individual voltage to earth ratings you could use that mode whereby the earth clips are isolated (taped up or removed) and the A probe connects to the signal you wish to see with the B probe connected to chassis (0V).

I understand that the Pye's chassis is connected to neutral as would be the chassis of the Hameg, would that be ok? I've just checked and there is no path from the earth clip of the probe to the neutral of the 'scope.

There will be no such connection in the 'scope since it would be dangerous to say the least!

I'm not in the habit of leaning on the radiator while licking the chassis!

A good thing too! I nearly electrocuted myself once operating a live chassis radio with no knobs whilst I leant on a bare brick wall with my other hand. I knew the chassis was connected to neutral but unfortunately the cowboy who had wired the fusebox in my workshop had crossed over the line and neutral feeds !
Take care!

Edit: Crossed with Kevin- don't think we've said anything contradictory!
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Old 14th Mar 2020, 3:11 pm   #19
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Default Re: PYE R33 Over Voltage.

Okay, so I'm a few years from using the oscilloscope in anger which takes me back to post#15.

How can I determine the current fault?
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Old 14th Mar 2020, 3:51 pm   #20
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Default Re: PYE R33 Over Voltage.

Also, while searching for more information about the subject of oscilloscope use l found a 2008 post on this forum by Neil Purling " Using oscilloscope on live chassis radio".

Apologies for not hyperlinking, that's something else l need to learn how to do using a mobile phone!
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