Amplifier transistor substitution.

The key bit about that part is high voltage with low cob, and relatively high ft. Its cob is around 2pf

The BF472 has a voltage rating of 300V. It has a companion part BF470 with a voltage rating of 250V, its just possible that the design don't need 300V. You would have to check.

So, look for a PNP 300V, 2 Watt, cob < 4pf ft > 50MHz, I > 50ma

-- Kevin Aylward

- SuperSpice

** A nice old Hameg CRO, worth keeping going in anyone's book. You likely did not check the WES catalogue.

It lists the BF472 for $0.86 + gst.

FYI:

I'm not your pal, but nor am I a psycho.

.... Phil

Whatever sub you choose, it would probably be a good idea to replace the other 472 with the same type as well. A matched pair would be nice.

Thanks for that.

Sylvia.

Another possibility is do a parametric search on the 2SC types.

Archives of CRT VGA monitor schematics could provide some useful leads. Video output transistors commonly go up to 600MHz - the faster ones are less likely to have the voltage rating you need.

A lot of monitors ran the horizontal driver from a 250V HT rail - plenty of driver transistors with enough voltage rating, but many not fast enough.

2SA1381?

Yes, that does seem a possibility.

I've simulated part of the circuit from T705 onwards, using 2N3904 and

2N3906 (ignoring the voltage limitations), and with capacitors between the collector and base. Even with 2pF (as with the BF472) the sweep is not very linear at the highest rate (80ns), and worse with the 3.1pf (as with the 2SA1381) but that's right at the limits of the scope's abilities anyway, with the Y amplifier bandwidth not really being up to that. At lower sweep rates, the capacitance has little effect.

Sylvia.

Sylvia, as mentioned, those are low-capacitance high-voltage video transistors. All the rage when CRTs were in heavy use, but dropped like a rock thereafter. Most were in TO-126 packages. You should replace both transistors, and maybe the associated bypass cap, in case it was the problem. I've a good collection of TO-126 video. There's still a modest supply at various places. But if you don't easily obtain replacements, let me know and I'll mail you a few.

--
 Thanks, 
    - Win

Yes, at the time there were high voltage transistors available that could be used for other interesting purposes.

Thanks for the offer.

Sylvia.

Hmm....

They may have supplied a circuit diagram and board layouts in the user manual, but this thing is clearly not designed to be repaired. There are multiple wires soldered directly to the boards, including some of those from the power transformer, and to the side of the board that's not readily accessible until after they're been desoldered. How did they assemble this thing? With some kind of right-angled soldering iron?

Sylvia.

Win, could you possibly post a few part numbers for these devices here, please?

I'm x-posting this to a more appropriate forum, Sylvia, the guys there will be better able to assist you with this...

This is not proving as straight forward as I'd hoped. I've removed T711, and now R739, which is the counterpart of the failed R740, at the top right, is starting to emit smoke. The voltage across R732 (attached to the base of T708) is about 30 volts, which explains the pain experienced by R739, but is not otherwise easy to explain.

I saw a similar voltage across R737 (bottom right) until I removed T711, but then the voltage returned to normal.

I find it hard to believe that two transistors would fail randomly so close in time, and some kind of common mode failure seems more likely, but I'm stumped if I can see what it might be.

Any thoughts?

Sylvia.

Short in deflection plate or its wiring?

--

-TV

What, you want to know what's in my secret stash?!! 200 to 300V video transistors are pretty amazing. They made broadband high-voltage amplifiers, taking advantage of the low-capacitance deflection-plate load, only a few pF. Most are gone now, along with the CRTs they powered. But a few are still here. To find them I type TO-126 into my 'Everything' search engine, maybe adding a voltage like 300V, and check their availability on Octopart.

Whoa, I see we can still buy Toshiba's 2SC5460, 800V, 2.2pF. Quest has inventory, $0.39, qty 50.

KSA1381 pnp 300V 3.1pF 7W Fairchild $0.25 Digi-Key KSC3503 npn 300V 2.6pF 7W Fairchild $0.26 Digi-Key KSA1142 pnp 180V 4.5pF 8W Fairchild $0.25 Digi-Key KSC2682 npn 180V 3.2pF 8W Fairchild $0.11 Rochester KSC5026 npn 800V 35pF 20W Fairchild $0.45 Digi-Key MJE350 pnp 300V 6.4pF(meas) 21W, multiple sources MJE340 npn 300V 4.1pF(meas) 21W, multiple sources

The MJE340 and MJE350 datasheets don't mention capacitance, so I had ignored those for years. But eventually I measured two ON Semi parts, Cob at 100V, and was pleasantly surprised. There's also the MJD340 and MJD350 in DPak.

--
 Thanks, 
    - Win

Thanks, Win. I'm going to stock up while there's still some left out there. I may never need to use any, but I find a comprehensive and copious stash of discretes helps compensate for my unfortunate lack of spirituality. :-)

That's worth checking. Thanks.

Sylvia.

You may be able to find your BF472s sourced from India:

I did a search and no BF numbers showed up, however they did manufacture the device recently according to the referenced PDF. You may have luck tracing the part through CDLI's distribution network. It never hurts to ask!

I would also check T710 (BF458) and T707 (BF199) as well though before ordering anything. A simple gain test should suffice. Try to find a reliable source for these transistors, there are a lot of Chinese (etc.) fakes out there!

Have you upgraded the electrolytic capacitors in your scope? They are most likely well past their Best-Before date!

John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup) 
John's  Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games) 
                      www.flippers.com 
        "Old pinballers never die, they just flip out."

Is Indian stuff kosher? Like others here I've had big problems with fake semis from the far east; voltage regs in particular.

Oh yes, see above!

They were *all* totally fine! I mean I'm sure they're not as they were when new, but none of us are. ;-) The fault with that scope (if it's the one I'm thinking of) turned out to be a diode in the SMPS section that someone had subbed with an inferior part. It needed a fast recovery diode and someone had stuck in the first spare diode the could find I'd imagine. It would have worked fine at 50Hz but this was 20kHz. One of the guys who posts here spotted it from a photo I posted!