New find Thorn BRC 1580
I've just picked up a 1580 chassis with a Ferguson badge. These TV's are very similar in appearance to the much more common 1590 chassis. The external clues are no DC input or headphone socket and a vertical speaker grille, however internally a completely different design, the 1590 is all transistor but the 1580 is hybrid using a PL81 line output valve. It's not in great condition but I'm sure it'll clean up. The CRT doesn't look good on the B&K but I'm hoping it'll wake up, I've not applied power yet as I'm looking for a lamp for my limiter, cold measurements show the filament chain is intact and mains fuse intact and HT rectifier OK.
They really squeezed the electronics into that cabinet.
Nice find, I’ve been on the lookout for one for a long time. A mix of 1500 Signals and 980 timebases.
Replacing (not changing, it had just fallen out) the PCL805 restored a raster on this set. The video coupling capacitor was open and the video driver transistor collector resistor had gone high, both replaced but still no snow. I've bypassed the tuner and injected IF direct, still nothing, some incorrect DC voltages around the IF amp and ACG stages will need further investigation, but looking good so far.
Anyone who wishes to offer help or just follow what's going on but does not have access to the data, here's a snippet of the relevant part of the circuit under investigation.
That's a super little rarity you have there. Plenty of 1590s around but almost no 1580s. I wonder exactly when these date from as I think the 1590 was in brochures by 1973. The 1580 can only have had a fairly short production run- maybe 1971/2? It would be interesting to have examples of both sets side by side to see how they compare performance-wise.
Best of luck with the repair. Should be interesting!
The strange DC voltages were due to four faulty lockfit transistors in the IF stage, VT1,3,4 and 5 all open base-emitter junction. I replaced VT1,3,4 with BF159s which I had to hand, I had to use a BC212 for VT5 as its the only PNP silicon I had in stock. This gave a weak picture with poor sync still bypassing the tuner. I'll investigate a better replacement for VT5 and I'm now suspicious of the rest of the lockfits and will check them all.
Oddly enough, I don't think I have ever come across a faulty lockfit to date.
Obviously, other's clearly have though. ?
Seems odd they all are popped....I wonder if there was something that could have caused it?
It wouldn’t surprise me if failures in Lockfits was mainly down to the batch they were from. Some seem to work well after years others more susceptible. Storage is perhaps another common factor.
. Storage is perhaps another common factor.
Couldn't agree more. I had to replace all of them in a BRC1500 that had been stored for years in a basement or cellar. I think they can now be generally be regarded almost as suspect as elderly caps.
I think they can now be generally be regarded almost as suspect as elderly caps.
I guess I'm a misfit then! ?
I accept locfits in some circumstances have been found by some folk to be troublesome. Where I disagree is that they are always suspect.
I can only speak as I find and for me the proof is in the pudding. I have 50+ year old 1st gen colour, hybrid sets happily running with a full compliment of locfits. This, despite being told numerous times, I must replace them all.
I have thousands of transistors an ICs that I bought from a no longer trading TV repair shop, I bought their entire stock. It contains countless locfits and I have had no hesitation in using them and will continue to do so with confidence. I've used them on many repairs, all of which are still running perfectly fine.
I think there are two factors, perhaps three. As you rightly point out storage is likely the key. Another is they are not a good to remove, test and then put back due to their locking nature. Having said that I have done exactly that, it just takes great care and patience. Finally as Frank mentioned quality control on batches, might have a bearing.
As I said at the start, I'm not denying there is some evidence of problems but I do think locfits get an unfair reputation. I've discussed this before with local member "The Teleman" and he feels the same. So with Andrew, The Teleman and myself, there are at least three of us who have differing experiences.
So I would say be guarded but don't condemn.
With so many gone in the IF stage of Marks TV, I'd be more inclined to think storage or surge/flash-over.
I think I had to change all of the BF194's in my GEC 2040 ... all the other types were fine though... A good handful of lockfits had failed in my N1500 too...
... I had to use a BC212 for VT5 as its the only PNP silicon I had in stock.
I had almost forgotten about the 1580 chassis in spite of the fact two customers had them. Both were reliable apart from the odd valve replacement.
Looking at the diagram provided by Crusty, is VT5 NPN (TVT5 or BF197)? BC212 would be fine for VT8 (TVT13) or have I misunderstood?
LockFit transistors used to have the odd failure, even when new, but these are now 50 years old and good old entropy catches up.
To quote Captain Mainwaring "I was wondering who'd be the first to spot that !"
Yes, I did in fact replace VT5 with a BC212, I had been looking at VT8 ! Surprisingly the picture on the set in my photo was with the wrong transistor in ! I've since fitted a BF196 in the VT5 position with disappointing results, I've noticed its collector voltage is very high along with HT5 which feeds the IF section and is about 30v read with a fluke. I'm wondering if this caused the demise of the lockfits ? I will check the voltage with an AVO and investigate why it is high.
It is possible BF194 and BF195 could have been damaged by HT5 being high as the transistor data indicates 30V max.
HT5 is derived from the field output valve cathode V3(b) and to a certain extent this voltage depends on current loading of I.F stage and associated circuitry. Estimate about 52mA with everything working properly. It will be fairly easy to do a quick check using a bench power supply to power and monitor HT5 current with the set unplugged.
Re VT5.... we have all done this at some point, I know i have!
Update: Yesterday I lost the picture and noticed just the line output and boost valves lit C74 filament chain decoupler had gone short, luckily the heater overrun does not seem to have damaged them as a replacement cap restored a picture of sorts. The video output decoupling capacitor has also been added to the growing pile of dead components, further updates to follow.
Just last Thursday, I retrieved my 1580 off the shelf where it lives nestled between a 1590 and a couple of 980s. Some time ago, the sound stopped working on it and a few weeks later I powered it up and there was a mad signals overloading fault with wild sync issues and streaking and a shading from left to right. Tackle this first I thought. It looked like a kind of AGC lockout to me and sure enough, the 4uF cap (C54) that acts as smoothing in the AGC system was open circuit. With stability restored, a nice picture was once again displayed.
By some happy coincidence, the lack of sound was caused by an identical electrolytic, this time C67 in the FM ratio detector section. These were both branded Callins. No "Lockfit" issues yet! There are more "Callins" branded electrolytics inside so no doubt the cover will be off it again!
Those Callins capacitors gave us work in the 70’s when they were quite new and hadn’t done many hours work.
I often wonder if they were made to a set price and that made them poor or if Callins just couldn’t make good capacitors. Somehow I can’t believe they didn’t have the know how.
Anyone know if they made military grade items that were decent?
I cleared the no sound fault on my set, it was VT9 BE open like the rest, that caused HT5 to increase to about 30V, a replacement restored it to about 22V. However now VT2 has joined its compatriots and gone BE open and I've run out of BF196's so I'll need to order more I'll do the electrolytics too. The contrast pot is open as well so this little TV is starting to look like a rebuild..