Me thinks it's time I gave some input to assist in solving this tale of woe and desperation!
the first thing is, don't worry about the 158 VAC from your mains transformer h.t. secondary, I'll come to that later!
Repeated failure of R427 means the excess current path can only be via Q404/Q407/Q408 on the Y+ top side of the circuit or Q409/Q410 or Q412 on the Y– lower half of the circuit, or C415 is shorting when the h.t. is applied each time the oscilloscope is switched on!
Begin by removing Q411 and Q412 and switch on again – if R427 still burns up your fault can only be C415 or a short in the PCB itself!
The two PNP transistors Q407 and Q410 form a constant–current load for the driver stages Q408 and Q410.
The quickest and best way to fault–find this type of directly coupled circuit is to invest in one of those little colour screen Chinese TC1 Component Testers, one of these will run through all the transistors in half–an hour or so!
How are you for desoldering equipment? – I presume you have access to something as you've mentioned replacing numerous resistors, etc.
If you find no fault with the transistors on a low–voltage tester, you can either connect the collector/emitter of each transistor across the scope's +140V h.t. supply via a 100K 2W resistor to see if any are breaking down at high voltage – connect.the collector to h.t. via a 100K 2W and short b/e together and connect to 0V for all the "2SC" NPN devices, and short b/e together, connect to +140V with collector connected to 0V via 100K 2W for the "2SA" or "2SB" PNP types, or alternatively, order new replacements for them of you prefer not to use the oscilloscope's h.t. supply to test them – it's easy to set up a 100K 2W resistor with your DVM across it and two leads plus a jumper for b/e (outer two legs on most Japanese transistors) on a small plug–in type breadboard, into which you can insert the transistors!
Either way, the voltage across the 100K resistor should be negiglible if the transistor is a good one – any transistor that shorts out partially or completely will show up as a high voltage across the test resistor, and should be binned!
The same check can also be carried out for all the small non–polarised ceramic and mylar film capacitors , again trying them across the +140V supply via your 100K 2W resistor!
This is a completely symmetrical circuit from both the Y+ and the Y– sections, so resistance checks between each point and the +140V and earth (0V) in turn should help you locate obvious discrepancies between each half!
All the small 1N4148 diodes and zeners, etc., are best replaced in one go rather than trying to check them all one by one in this sort of complex d.c. coupled amplifier arrangment – they're cheap enough!
Finally, do make sure you've not got a short to earth on either of the CRT Y–plate connections!
Regarding the high voltage from the 145 V h.t. secondary of your mains transformer, it will be wound with very thin wire and the percentage voltage regulation will be fairly poor, so with your vertical circuit faulty, it will rise quite a bit on no load compared with the maker's quoted nominal figure!
For a 145 V r.m.s. (nominal) secondary, you can expect
(1.4 × 145) = 204 Volts across C901, so if you can get a 250 V rated electrolytic of the same diameter as the 200 V component I would obtain and fit one!
Pintek's drawings aren't the last word in clarity, but I cannot fault their helpfulness in providing the information in the first place, and my thanks to them and youself for attaching them so we can help with your queries!
Chris Williams
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