Here’s a nice dinky little B&O 3300 which was kindly donated by Chris to make room for other acquisitions. It’s a 20AX set but one of the very few to use the 20” version of the CRT. Taking off the back and looking inside shows a very compact chassis and would appear to be a nightmare to work on but B&O have done an excellent job, it’s been very cleverly put together. The only difficult bits to get to are the customer controls and loudspeaker. The wires of the harness are incredibly thin as in my ITT FT110 but not one has parted company with it’s plugs while working on it.
Powering on produced a pretty lacklustre green picture, the red beam switch was turned off and there was no drive to the blue cathode of the CRT. Looking for the output transistors led me to a small PCB with three TO-92 transistors, no normal heatsinks as you would expect as with any other video output transistor running in class A. Turning over the board revealed the lack of blue due to dry joints but what surprised me the most was the rather large collector lands doubling up as the heatsink.
Resoldering brought up a colour picture lacking the correct amount of green and with some E-W pincushion distortion. Now this set was I believe one of the earliest to use auto greyscale and the RGB stages are rather complex. Before getting too involved in the RGB stages the old trick of swapping over the drives of the cathodes proved that the fault did not lie in the circuitry but was a low emission green gun in the CRT. Setting up as per the instructions and adjusting the gain controls to suite made a tremendous difference and replacing a 470uF plastic ROE capacitor C21 restoring the correct picture shape. All in all an excellent little set.
One fly in the ointment after a few hours running the EHT cap broke down and began arcing. It’s one of those translucent types which unlike the flat grey ones seem to degrade with heat.
John.
Hi John,
Ah, this is good, reports from the Chez 'Jayceebee' bench, bet you're chuffed to bits now having the workshop fully up and running, very well deserved in my opinion. Bet you can't wait for the winter evenings, a toasty workshop and a tasty project like your A823.
Well, this B&O turned out to be a fairly simple fix! Must admit, all I did when I got the TV was power it up to see the condition, then put it on the "to do" pile, then never got around to it. As you point out, what a novel way of heat sinking. A lovely little set and if your tight on space, won't take up too much room in the collection. I believe I have a NOS LOPT for one of these in stock, so if you have trouble just ask, not sure if they were ever troublesome though.
Look forward to further tales from your bench 👍
p.s.
Just noticed, I thought you were a Metcal man, what gives with the Weller? Not that Weller's are bad, i've many myself but since it was you who moved me on to Metcal, I was surprised to see it.
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Hi Chris and thanks for gifting me this lovely little set and how I wish I’d built my workshop years ago. I am a Metcal man but due to the cost of tips it only comes out on special occasions, I’m down to one brand new of my favourite tip type.
The faulty EHT lead is a screened affair and is extremely long, it snakes back and forth inside the set and if damaged should be replaced as a complete item. I thought about reducing the length when I fit a new cap/connected but I wonder if the screening plays a tuning role? There are also two 22K resistors in the cap which will need to be retained. More later.
John.
Re the resistors, the cap and the screening -
Is there any sign of heat around the tube in which the resistors live? If so, the tripler will fail soon.
The screening is there to allow the resistors to live in the cap, else the lead would radiate. Most replacement triplers have the resistors (which aren't critical) in the tripler itself.
I'd suggest a replacement tripler. I know a source if you don't.
Also, have a look at the tuning cap from the collector of the line output to ground. If it's one of those pink mixed dielectric types it is likely to fail soon. The green Philips ones are OK.
Note you cannot use just any cap with the necessary voltage rating in this position. But you probably know that.
This set uses a DST instead of a tripler and there is no sign of damage to the resistors themselves. I’m pretty sure the problem is just heat/age related to the plastics used.
John.
These EHT leads are very similar to those from the Philips G11. The one I have in front of me measures approximately 85cm long and with a resistance of 30kΩ.
Its a great looking set John
Best regards
Mike
I seem to remember grafting a G11 one into one of these sets with no issues back in the 90's...
I seem to remember using just ordinary EHT leads culled from duff Lopts with the softer rubber type caps with no problems, even missing the in built resistors out, never got any bouncers as far as I recall
Hi Baz, I thought about using the EHT cap off a brand new TX10 focus unit I still have as they are of extremely high quality. Seems a shame to destroy but even if I did add a TX10 to my collection I seriously doubt it would still have the original type used in very early production. Never ever had to replace the later type.
John.
How about this....
http://markhindes.easywebstore.co.uk/EHT66_A21HV9.aspx
Might save spoiling the TX10 unit.... You just never know when you might need it. 😉
To understand the black art of electronics is to understand witchcraft. Andrew.
Thanks Andrew, that may just fit the bill. The annoying thing is I'm fairly sure I disposed of the failed tripler from my FT110, the cap from that was in decent condition. I'll dig out the set a bit later and have a look, it may be still inside.
John.
Well some not good news I'm afraid, thankfully I hadn't thrown away the old FT110 tripler so swapped the EHT cap and even though I say it myself it was a pretty neat job. Around the final anode and it's insulation sheet was given a good clean and everything put back the way it was.
On switching on I fully expected the sound of EHT coming up but no, just a brief hum of auto-degauss. I was sure I'd got the power off before any serious occurred damage previously but no. BU208 line output transistor s/c, two 1k resistors on the PSU toast, but worst of all an earth strip of print also on the PSU had been vapourised. Things like the latter worry me as the fault currents can cause catastrophic damage elsewhere. Will need to go through the PSU with a fine toothcomb, It's complex but not OTT.
John.
If the August 1985 Eugene Trundle Television article is not sufficient, and you need the service data scanned, I have the B&O 3300 circuits.
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