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Old 15th Oct 2020, 2:40 pm   #61
Kentode
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Default Re: PYE R33 Over Voltage.

Just checked R1,3,4 and 5.

R5 1M actual 1M08 - that's ok.
R4 1M actual 1M14 - that's ok.
R3 47K actual 75K - I'll replace unless told not to.
R1 1M actual, well l pushed the probes up against the legs on the top of the PCB and noticed a slight movement inward - see picture below.

To be replaced.

Also I cleaned the the valve pins and seats yesterday.
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Old 15th Oct 2020, 2:50 pm   #62
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Default Re: PYE R33 Over Voltage.

Also check C26. Sometimes I have seen caps in this position fail in some fault conditions (when EHT develops across the transformer in the output stage)
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Old 15th Oct 2020, 3:15 pm   #63
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Default Re: PYE R33 Over Voltage.

Point-to-point wired sets had an excellent solid 'groundplane' available everywhere in the form of the chassis.

PCB sets invariably used single-sided boards with only one layer for tracks. track crossovers were inconvenient and cost money. Grounding was done via whatever area was left over and stitched together with jumpers or long spindly tracks.

Consequently there were issues with ground interactions and stability was often marginal. Manufacturers mucked around with layouts a bit trying to dodge the issues they ran into, but they were learning to use a new technology. It was a step forwards in cost saving and convenient manufacture, it was a step backwards in stability and RF isolation.

With time and the inevitable change in component characteristics, discovering that some designs may have had less margin than you'd hoped isn't surprising. There may be a component gone sour, but things are less tolerant than they were.

Even double-sided PCBs with a solid grounplane on one side often get that plane badly slotted to get tracks crossing. This is where multilayer PCBs with pretty solid groundplanes come into play. It all costs money and a lot of stuff is forced ono single-sided boards, and SRBP at that, so we get water absorption. The fix is either fibreglass if the budget will stand it, or simply hoping thet water absorption trouble is well out of warranty.

Laying out a single sided board with a good chance of success is still a bit of a dark art.

Aligning a superhet isn't a dark art at all. A reasonable signal generator and a methodical approach will get you there. The key is in knowing the method. It isn't difficult, but it can all come crashing down around your ears if you try short cuts.

In general, the mica capacitors in the RF sections of radios are reliable and rarely need changing, but a few do fail and once you've been through everything else you have to check them. You should be able to get modern replacements close enough to get signs of lif from the set, but fine adjustment of alignment is likely needed to get full performance. Of all the RF capacitors in a radio, the ones in the oscillator and the ones you least want to change. It's not just a matter of getting the tuning scale to read correctly, they usually are chosen to give low drift over temperature and that's a whole new can of worms.

If you can't get mica, then NP0 of C0G grade ceramics are the best if you can get the voltage rating.

On the resistor front, those PCB mounting resistors are not renowned for stability, or longevity, or low noise, and some of them in your photos do look rather crabby.

Edit.... I'm a slow typist, posts crossed.... that resistor is beyond crabby!

David
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Old 15th Oct 2020, 7:14 pm   #64
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Default Re: PYE R33 Over Voltage.

I changed the resistor R1 and kinda hoped it would be the cure - it wasn't!

Tomorrow I'll change the other, but l don't think that's going to do it either.

Al, I've already changed C26, along with the electrolytics.

Will a mica cap be the cure? I'm running out of ideas.
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Old 16th Oct 2020, 8:50 am   #65
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Default Re: PYE R33 Over Voltage.

A large positive reading like you are getting could well be what Paul commented on. Electrical leakage on the PCB could be the answer. Remove the valves and then measure the voltage when you switch on. A reading obviates any of the valves and really condemns the PCB to the bin!
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Old 16th Oct 2020, 10:47 am   #66
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Default Re: PYE R33 Over Voltage.

I'm not sure Trevor which large positive reading you mean? I'll remove the valves and take a voltage reading but is there a best place or places to do this?

I'll order some mica caps to bring the chassis back to original and have some left over for anyone else who makes the same mistake as me.
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Old 16th Oct 2020, 11:21 am   #67
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Default Re: PYE R33 Over Voltage.

Hi.
The 75v you have on the grid at the opposite end of C7 or have I missed something?
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Old 16th Oct 2020, 11:43 am   #68
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Default Re: PYE R33 Over Voltage.

Hi.
Looking at the circuit for the first time C7 is the feedback cap for the local oscillator. It connects between the Anode and the tuned circuits that then are returned to the grid via C11 on pin 9. When the oscillator runs you will read a DC voltage on the grid its quite normal, in fact tuning the radio should make that voltage vary, in some instances putting the meter on pin 9 will result in a zero voltage as the meter damps the oscillator but not always. The fault sounds like instabity.
I'd replace the following one at a time to ascertain what cap has made a change if any.
C5 and C6. I'd still remove V1 and check the voltage on pin 9 the oscillator grid and in this instance it should be Zero, if not the PCB or the valve base is electrically leaky but very much doubt that it will be. 70 odd volts on the grid will be fine as like I say is normal when the oscillator runs, it's a mean level average of the AC voltage on the grid, a result of oscillation.
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Old 16th Oct 2020, 2:06 pm   #69
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Default Re: PYE R33 Over Voltage.

What I would say is the DC voltage on the grid with respect to ground/chassis should be negative a positive voltage is highly unlikely and if so this means you have a fault. I've done a YouTube clip and will upload and post the link shortly.
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Old 16th Oct 2020, 2:29 pm   #70
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Default Re: PYE R33 Over Voltage.

Here we go.
https://youtu.be/bwQSPch-yLY
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Old 16th Oct 2020, 4:02 pm   #71
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Default Re: PYE R33 Over Voltage.

A big thank you for that Trevor, I'll watch it a few times and get back to this on Monday.

Somewhere in the back of my mind l read about distortion on top of a signal only, but a brief search has not yet found it.
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Old 16th Oct 2020, 6:52 pm   #72
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Default Re: PYE R33 Over Voltage.

Quote:
Originally Posted by murphyv310 View Post
Great stuff!

Are you Scottish by any chance

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Old 16th Oct 2020, 10:31 pm   #73
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Default Re: PYE R33 Over Voltage.

Ocht aye. Born in Walton-on-Thames but been in Bonnie Scotland since 1957 ye Ken.
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Old 19th Oct 2020, 2:37 pm   #74
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Default Re: PYE R33 Over Voltage.

Progress of sorts. There is no volts on V1 pin 9 so the PCB isn't conductive.

Using Trevor's video as a guide l tested and saw a negative 10 volts using my Pro'sKit MT-2017 set to +/- 25 VDC.

Here's the thing l can't explain. I tuned the radio to a station and it burbled away, l applied the probe and got the -10V but the signal went , as suggested . So far so good.

Then I tuned away from the station, with the probes still on, and the station came back stronger about 3/4 of a turn further on with a hetrodyne beat that l could almost, but not quite, tune out.

Lifting the probe made the station disappear. Does this give anyone a clue?

I'm waiting for the mica capacitors for C7 and I'm going to use polypropylene caps for C5 and C6.

If feels like I'm very close to a solution...
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Old 19th Oct 2020, 8:29 pm   #75
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Default Re: PYE R33 Over Voltage.

Hi.

That's correct as the capacitance of the probe etc is tuning the oscillator, you would have had to tune it closer to 200 meters to receive the station you were receiving beforehand.
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Old 20th Oct 2020, 9:21 am   #76
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Default Re: PYE R33 Over Voltage.

That makes sense Trevor. If only I could buy some sense on eBay!
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Old 20th Oct 2020, 11:43 am   #77
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Default Re: PYE R33 Over Voltage.

I've replaced C5 10nf, the original measured 8.4nf on my LCR meter, and added 1nf in parallel with the 1N4007 diode. The problem remains.

Replacements for C6 and C7 will be here Thursday afternoon along with new tips for my Weller iron so I'll be reading up on NFB until then.
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Old 20th Oct 2020, 4:58 pm   #78
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Default Re: PYE R33 Over Voltage.

One other thing to try is to get a screening can around the UCH81 and ground it to the metalwork of the set. If that removes thebl problem it's instability. Check all earth's, print for cracks and earth links from the metalwork to 0v, ground points on the PCB.
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Old 22nd Oct 2020, 9:46 am   #79
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Default Re: PYE R33 Over Voltage.

I tried a valve cover over V1, earthed to the chassis, no improvement. The afternoon post should bring spare parts and I'll take a good look at earths and the PCB.
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Old 22nd Oct 2020, 12:58 pm   #80
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Default Re: PYE R33 Over Voltage.

Right, the parts have arrived. The picture below shows C30/C14. Can anyone with a PYE R33 confirm where the tag highlighted goes on the PCB?

Please ignore the second picture.
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