Notifications
Clear all

GEC BT1252 restoration

Page 1 / 3
 
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137

Hello all,

Picked up this little GEC the other day from Marc, who was kindly storing it for me.

Looks like this will be a bit more of a challenge than the last 2 repairs!

The cabinet doesn't look too bad, with only the feet missing. There's now felt pads underneath to stop scratches.

PSX 20180825 003030
PSX 20180825 003600
PSX 20180825 003518
PSX 20180825 003320
PSX 20180825 003137
PSX 20180825 003100

The real fun starts inside. Here's some pictures.

PSX 20180825 004440
PSX 20180825 004405
PSX 20180825 004332
PSX 20180825 004220
PSX 20180825 004136
PSX 20180825 003933
PSX 20180825 003800
PSX 20180825 003710

The line o/p valve (N339) is never working again, so if any members can help point me in the right direction in regard to replacement, I'd be obliged. My tube booster worked its magic last week, so the tube base is temporarily removed, hence the cut wires. The poor old height pot has separated, too. There's something awry with the voltage selector/dropper section.

I think a run around with a vacuum cleaner and paint brush will be in order, before I delve deeper into things. The newly built Reformer of Boing! will get to work on the smoothing block over the bank holiday weekend methinks!

Looks like I'll have weeks of fun with little telly 🙂

If anyone's got any tips or observations,  please don't hesitate to share  ? 

All the best,

Tony

ReplyQuote
Topic starter Posted : 25/08/2018 1:38 am
Topic Tags
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4980

Hi Tony, looks like quite a challenge and I'm sure you will enjoy restoring this set. It may be worth testing the LOPT before replacing any components. 

I'm tempted to say that the N339 could be replaced by a PL81....it looks like a PL81 is actually fitted in your pic.

However, if you look on ebay, there is a selection of N339 types available.

A nice little set, and well worth the work IMHO.   ? 

 

 

ReplyQuote
Posted : 25/08/2018 8:51 am
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4980

Oh I nearly forgot...check the barretter (the lamp type device in the black can at the top) it is rather a fragile but useful item used to drop the heater volts. It will run rather hot in use, but the filament should barely glow.

Hope I'm not teaching my grandmother to suck eggs lol  ? 

ReplyQuote
Posted : 25/08/2018 10:57 am
Nuvistor
(@nuvistor)
Famed V-Ratter Registered
Posts: 4158

If one is available a PL81 is worth trying, not an equivalent but very similar. 

 

ReplyQuote
Posted : 25/08/2018 12:56 pm
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137
Posted by: PYE625

Hi Tony, looks like quite a challenge and I'm sure you will enjoy restoring this set. It may be worth testing the LOPT before replacing any components. 

Hi Andrew,

Yes, it looks like there will be fun to be had!

I was going to get my new function generator to ring test the LOPT,  but the silly thing died before I had a chance to test the LOPT  ? 

Ah well, I'll just HAVE to make up a LOPT ringer now..... Honestly, I'm building more stuff than fixing! 

I'm tempted to say that the N339 could be replaced by a PL81....it looks like a PL81 is actually fitted in your pic.

However, if you look on ebay, there is a selection of N339 types available.

A nice little set, and well worth the work IMHO.   ? 

You were correct about the valve being a PL81. I've already bought a N339 from a seller close to me. The set certainly looks nice..... More on my adventures with it a bit later.

All the best,

Tony

ReplyQuote
Topic starter Posted : 28/08/2018 10:50 pm
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137
Posted by: Nuvistor

If one is available a PL81 is worth trying, not an equivalent but very similar. 

 

Hi Frank,

Thanks for advice, but seeing as I could get the original type....

Cheers anyway  ?  ? 

ReplyQuote
Topic starter Posted : 28/08/2018 10:53 pm
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137

Hello all,

I used the bank holiday to attack the little GEC.

After a run around with the vacuum cleaner, it was time to remove the lid and delve in.

1st stop, The EHT section...

PSX 20180827 134408
PSX 20180827 133424

 

Oh dear!

Anyone know where an EHT cap like this can be had?

The U43 has had it as well...

PSX 20180827 134319

So, I've ordered both a U43 and N339.

Next, the smoothing block and rectifier. Here's the smoothing block in situ. All puffed up!

PSX 20180827 132424

After hoiking out the block and rectifier, we had this...

PSX 20180827 132238
PSX 20180827 132305

February 1956 was not a good vintage for caps, as you'll see. I started off slowly cranking the voltage on the Reformer Of Boing, whilst trying to keep to at, or below 1mA of leakage. Here we are, trotting on for 50 whole volts.

PSX 20180827 132154

After about 4 hours, I got the outer section up to 200v, for a leakage of 700uA, so went up to 250v..... That's when the luck ran out! The leakage shot up past 2mA and stayed there. Even back down at 200v, the leakage was now above 1mA. Onwards!

The inner section was even worse. It took over 4 hours to get up to 50v, for 1mA. Obviously, the block is caput. I tried going up to 150v, just for curiosity, and got 4mA.

So, I'm in the market for a smoothing block as well!

It wasn't all doom and gloom though. Whilst I was waiting for the smoothing block not to reform, I cleaned up all of the pins and bases for all of the valves.

I also gave the CRT another bop on the Booster Of Boing. The emission improved by about 5%, so I reunited the CRT with the rest of the telly.

PSX 20180828 234042

Note the changed 22k resistor. The original was over 33k!

Remember that disintegrated and burnt height pot? Well, after scraping away the charred insulator and rebuilding, we had something like this.

PSX 20180828 234112
PSX 20180828 234139

I did a 500v insulation test between the metal body and pins. The insulation resistance was over 20 meg, so the original pot is back in situ now  ? 

PSX 20180828 234222

The last thing I done was to try and test the selenium rectifier, MR2. Now I don't have a variac, but I do have a variable DC supply, in the shape of the Reformer Of Boing. So what I did was to take a note of applied volts and what the resultant output volts were from MR2. I did this for both polarities and a range of voltages, as below.

PSX 20180828 234346

In this case, MR2 was reversed biased with 63v, which yielded an output of 42v. A drop of 21v.

The data I collected, was collated into a hastily written table, as below...

PSX 20180829 003905

From this data, I'm happy to try bigger loads, say a 15w bulb to start, up to about 60w, which is about the 250mA draw in the circuit diagram that I have.

I can make up a sort of variac by using a potential divider network using different wattages of lamps.

So there we are... A few steps forward, a few back and some more test kit to build... Oh what joy!

All the best,

Tony

 

ReplyQuote
Topic starter Posted : 29/08/2018 12:53 am
Nuvistor
(@nuvistor)
Famed V-Ratter Registered
Posts: 4158

The EHT connector looks standard, perhaps one from a faulty tripler.

 

ReplyQuote
Posted : 29/08/2018 8:51 am
Marc
 Marc
(@marc)
Famed V-Ratter Registered
Posts: 2761
Posted by: Nuvistor

The EHT connector looks standard.

 

Hi Frank, 

No it's not a standard EHT cap. If I remember correctly the EHT cap on Tony's set is female and takes a very thin, almost like a screw thread, spike on the CRT. I think the whole cap assembly is only about an inch in diameter too, odd little thing. 

But with my luck I've probably got it wrong, no doubt Tony will be along soon.  ? 

ReplyQuote
Posted : 29/08/2018 12:42 pm
Nuvistor
(@nuvistor)
Famed V-Ratter Registered
Posts: 4158

You are correct, I enlarged the photo of the crt and I thought it looked standard but it isn’t, still you may be able to reuse the clip portion of the connector and fit it into a new or reclaimed shroud.

ReplyQuote
Posted : 29/08/2018 12:52 pm
Marc
 Marc
(@marc)
Famed V-Ratter Registered
Posts: 2761
Posted by: Nuvistor

you may be able to reuse the clip portion of the connector and fit it into a new or reclaimed shroud.

Absolutely  ? 

ReplyQuote
Posted : 29/08/2018 1:40 pm
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4980

Well Tony !  You might have to try your hand at re-stuffing that smoothing block....plenty of fun there !

But first, it might pay to temporarily connect suitable modern capacitors in circuit to begin with for testing the set. The selenium rectifier will not withstand an excessive current draw in the set, so go careful with this. A pop followed by a nasty pong will tell you.  ? 

A 1N4007 and a 25 ohm (or so) dropper resistor in series might prove worth the while, at least until you are happy the rest of the set is operational and normal. 

ReplyQuote
Posted : 29/08/2018 10:31 pm
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137
Posted by: PYE625

Well Tony !  You might have to try your hand at re-stuffing that smoothing block....plenty of fun there !

I did try restuffing many moons ago. It worked, but looked awful. I think I'm better at stuffing my face 😉
In the end I bought a similar sized block. We'll see what happens.

But first, it might pay to temporarily connect suitable modern capacitors in circuit to begin with for testing the set. The selenium rectifier will not withstand an excessive current draw in the set, so go careful with this. A pop followed by a nasty pong will tell you.  ? 

I'm well with you on that one... The smell of selenium never leaves you!

A 1N4007 and a 25 ohm (or so) dropper resistor in series might prove worth the while, at least until you are happy the rest of the set is operational and normal. 

Could do. I was wondering about the higher than normal HT as the set warms up, i.e. when there's little load. I was thinking of putting in a NTC in series with the dropper to avoid inrush. Does that sound like a good idea?
Fingers crossed that the selenium rectifier is OK!
All the best,
Tony

BTW, I picked up a mint N339 from a chap in Bexley called Bob. He's got a garage absolutely full of NOS valves that he's trying to offload, if anyone's looking.

ReplyQuote
Topic starter Posted : 29/08/2018 11:58 pm
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4980
 
Posted by: Boingy
Could do. I was wondering about the higher than normal HT as the set warms up, i.e. when there's little load. I was thinking of putting in a NTC in series with the dropper to avoid inrush. Does that sound like a good idea?

In theory it is a good idea, but probably not necessary. The 25 ohm resistor will limit surges in any case, plus any other series resistors in the dropper section will help too. With a thermistor, you might have to experiment with different values to arrive at a suitable hot resistance to get the correct HT level. Also, the HT will dip as the valves warm up and then waiting for the thermistor to heat will delay operation further. Not only that, HT level might vary with ambient temperature causing undesirable effects. In a heater circuit, slight variations don't really matter, but the HT lines are more critical.

Initial power up via a lamp limiter will show any nasties.

ReplyQuote
Posted : 30/08/2018 5:21 pm
Nuvistor
(@nuvistor)
Famed V-Ratter Registered
Posts: 4158

A resistor and silicon diode was a standard replacement for selenium rectifiers, didn't cause any problems, though perhaps a capacitor across the diode would have been a good idea to stop spikes damaging the silicon diode.

 

ReplyQuote
Posted : 30/08/2018 5:46 pm
PYE625
(@pye625)
Famed V-Ratter Registered
Posts: 4980

That's good practise Frank, a small cap (470pf-1000pf) across the diode.

ReplyQuote
Posted : 30/08/2018 7:03 pm
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137

Thanks chaps for the replies. Most appreciated.

I'll probably have a go at ring testing the LOPT over the weekend, once the 555 ICs arrive. After that, I'll cold check the line o/p stage to see what happened.

Thanks again 🙂

ReplyQuote
Topic starter Posted : 30/08/2018 9:59 pm
PYE625 liked
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137

Hello all,
So yesterday I got to grips a little more with the TV. This time around I concentrated on the LOPT, putting it on my freshly made LOPT ringer.

Once the LOPT was isolated, I popped on the ringer and 'scope to see what waveform I'd get.
Here it is 🙂
(50us and 200mV per division)

BT1252 LOPT2

Not bad, considering the age of the TV (1956).
I'm reasonably happy that the LOPT is OK, when compared with my T-9 portable...

T9 LOPT

(50us and 200mV per division)

Other bits and pieces have sorted themselves out now as well. The EY51/U43 valve arrived on Saturday and I picked up the NOS smoothing block, which is reforming as I write. So far, one of the 200uF sections is down to 640uA at the stated working voltage of 300v.

I checked the barreter and it seems OK. The proof of the pudding will be when I eventually expose the TV to full mains potential. Talking of which, the left voltage selector above the scanning coils had been set to 200v. This left selector switches the droppers that feed the selenium rectifier and smoothing block. Oh joy!
Anyway, I'll be starting the process of swapping out capacitors (especially the waxies and the ubiquitous 'orrible Hunts) this week. So far the scrap pile consists of one resistor, 2 valves and the smoothing block. This will change soon 😉

All the best,
Tony

ReplyQuote
Topic starter Posted : 04/09/2018 12:12 am
Boingy
(@boingy)
Trusted V-Ratter Registered
Posts: 137

Hello all,

So I got to spend more time on the little GEC on Sunday. Some more steps forward, with new valve and smoothing block fitted...

PSX 20180910 234924
PSX 20180910 235052
PSX 20180910 235149

The LOPT was refitted after a successful ring test...

PSX 20180910 235311

There was also another setback. The CZ1A Brimistor has lead a hard life, the upshot of which is that it reads over 23K at room temperature. Way too high!

PSX 20180910 234840

There's a seller on eBay, but it's in Turkey, with a lead time anything up to November!

If anyone has got a CZ1, or CZ1A going, I can find it a good home, for a suitable recompense 😉

There were also a couple if little mysteries, one solved and one not.

Solved mystery first.

I pulled out the 16u+32u smoothing block to see about reforming it. The poor thing looked very sorry for itself...

PSX 20180910 235350

So got to work, cleaning the outside and noticed that it was covered in wax. "Funny", I thought. This cap couldn't have leaked wax!

PSX 20180910 235433

Searching around revealed the culprit...

PSX 20180910 235617

Yet another waxie to change!

The 2nd mystery involves an extra valve on the main deck...

20180911 000332

It's not in any of the manuals in the library, so far as I can see. Does anyone know what it does?

Lastly a quick look at the rear of the chassis, with new caps fitted.

PSX 20180910 235230

More news when it arrives. If anyone can help with the CZ1A, I'd be obliged. 

All the best,

Tony

ReplyQuote
Topic starter Posted : 11/09/2018 12:44 am
freya
(@freya)
Noble V-Ratter Registered
Posts: 1283

Your mystery valve is V6 double diode D77

V6A is vision interference limiter, V6B is to eliminate line sync pulses 

ReplyQuote
Posted : 11/09/2018 8:07 am
Page 1 / 3