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hi all i have a sick tek 2215 scope,the issue is the trace is too low on the screen( both channels affected ),i can only position the trace about halfway up the screen using the position controlls,i have checked the r378 r379 r389 and r 388,they are as follows 378=314 ohms 379= 309 ohms 389= 321 ohms and 388= 316 ohms,the two resistors on the deflection plate wires are 57 ohms not 51 as per manual,any ideas of whats wrong,im sort of stuck,also q387 and q386 test ok.,thanks in advance .

Replace the four resistors you asked about with . Mount these resistors off the boards a few mm to allow air flow. The 51 ohm types can be . Ordering 100 of the Xicon will give you enough stock for a while.

Replace the RIFAs off the mains input! These will be a clear amber colour. These are an X2 type. Get the ones with the highest voltage you can, 300VAC or higher. These RIFAs WILL fail. You will know it when they do!

Mark

well q387 and q386 test good,the resistors seem ok to me bar the ones measuring 57 ohms,would being 6 ohms high cause this?

On Thu, Oct 21, 2021 at 08:24 PM, <nanovnauser@...> wrote:

well q387 and q386 test good,the resistors seem ok to me bar the ones
measuring 57 ohms,would being 6 ohms high cause this?

Resistors are not that far off to cause the issue you are seeing.

First check if the vertical deflection plate is connected to the output of the vertical amp. Sounds unlikely but there were cases of wire coming off one of the plates giving exact symptoms you have.

If this doesn't solve the problem set both channels to GND, set Y position knob to mid setting. Compare the following voltages with what you see in the schematic (and report back if you need some ideas):
Voltage at bases of Q350, Q360, Q376, Q386
Voltage at collectors of Q377, Q387
Voltage at the bases of Q370, Q380
Voltage at the collectors of Q370, Q380


Ozan

I currently have the crt removed,is it ok to power the cope on with the crt removed,or do i need to connect the anode eht to prevent arcing?

Well i hooked up the eht to the crt and moved the whole lot to one side,heres a few readings i took,on q387 base i get 8.8v,it should be 2.6!,r290 wiper measures as follows,fully cw=3.44,fully ccw 3.4v,r190 is fully cw3.4,fully ccw 3.4v,dont know if this helps?

On Mon, Oct 25, 2021 at 12:02 AM, <nanovnauser@...> wrote:

Well i hooked up the eht to the crt and moved the whole lot to one side,heres
a few readings i took,on q387 base i get 8.8v,it should be 2.6!,r290 wiper
measures as follows,fully cw=3.44,fully ccw 3.4v,r190 is fully cw3.4,fully ccw
3.4v,dont know if this helps?

---
Base voltage at Q387 doesn't point to a failure by itself. We need voltages at other nodes to tell which component failed. This was the list in the earlier message:

Voltage at bases of Q350, Q360, Q376, Q386
Voltage at collectors of Q377, Q387
Voltage at the bases of Q370, Q380
Voltage at the collectors of Q370, Q380

R290 and R190 voltages look fine if you are measuring voltage at the wiper (center) terminal.

Ozan


Ozan

the voltages on the vert position pot wipers is as follows fully ccw=3.6v,halfway point =1.2v,fully cw =3.6v.

heres some more measurments,at the junction of r354 and 364,i get 8.4v (should be 8.6v)
q360 emitter reads 1.129v,q350 emitter reads 0.574v both should read 0.8v !

q360 collector reads ccw -4.761 cw = -1.45
q360 emmitter= cw .47v ccw 1.125v
r350=cw=.001 ccw=3.989
r360ccw=0.173 cw= 4.353
both measure 221 ohms.

I know you said earlier that Q386 and Q387 tested OK but the measurements you have made around Q360 do point to one or other of Q386/Q387 being the culprit. It is not easy to make definitive measurements in circuit with all the relatively low value resistors around Q386/Q387.

If you centre the vertical position pot you should get the bases of Q350 and Q360 both close to 0V, this doesn't need to be precise. Then measure the following voltages wrt ground:

Q377 collector Q387 collector
Q377 emitter Q387 emitter
Q376 collector Q386 collector (these should be the same as the line above but if you can make a second measurement it confirms no faults on the PCB)
Q376 emitter Q386 emitter (these should be the same but check as above)

If it is not obvious which transistor has failed open circuit then additionally measure the voltages on the bases of Q386 and Q387 (and Q376 and Q377 if you want to be thorough).

This doesn't distinguish between Q386 having an open collector-base junction and Q387 having an open emitter-base junction, at this point you probably need to remove at least one device.

Apologies to Ozan for repeating a large fraction of what he said earlier!

Regards,

Roger

Heres what i measure in respect of ground
q377 C=1.8v E=-2.1v B= -1.5v
q387 C= 21v E= 9.4v B= 9.9v
q376 C= -2.3v E= -5.6v B= -4.97v
q386 C=9.5v E= -5.7v B= -3.1v

just tested q 376 out of circuit it says npn but only a beta of 13.

Q386 has an open BE junction if there's no typo in that line. 2.6V VBE = dead.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

q386 tests as a back to back diode on my tester,does that sound right?,also can you recomend a sub to replace it with ?,tia m3vuv.

What junction voltage does it report in that diode test, and at what current? A not uncommon failure mode is extremely high base resistance (on the way to becoming infinity, perhaps), so some multimeters will tell you that you have a junction, but you need to pay attention to the numbers. Many diode test functions will actually tell you the junction voltage when it's testing a diode. If yours is like that, you should be able to see what I'm talking about.? In any case, your earlier voltage measurements tell us that the transistor is dead. I haven't followed your thread, but your voltage measurements say that your trace should be wedged over to either to the top or bottom.

As to a replacement, the BFR96 seems to be available from auction sites and such. But for a quick verification of your diagnoses, you can throw in a 2N3904 and see if that restores functionality. If not, you have other faults to chase down.

-- Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

the tester is a atmega bases one from china,was going to attatch a pic but cant see how on here.

Wouldn’t tell me much anyway! Need to know how it tests.

Just try a 2n3904 or pn2222. Faster than trying to understand your tester!

Tom

Sent from my iThing, so please forgive brevity and typos

Generally, the algorithm is to run each pin of the device through two resistors, one about 680 ohms, one about 470K.? Each of the ends of those resistors goes to a separate I/O pin.? Each device pin also goes directly to the A/D converter.

Depending on which pin is connected, (with an active pullup, or not, or with an active pulldown or not (IIRC),? you can measure the forward voltage drop of a diode through either about 8 ma or about 10 ua.

Once you identify the base by diode tests, you likely pick the collector (10 ua into the base, 680 ohms collector, likely ground the emitter), as the lead that gives the highest beta.

Clever.? The schematics are on the web, but the algorithms are not.

Not super accurate, but useful...

Harvey

Tom, Harvey,

This problem has threads running in parallel on eeVBlog and the UK Vintage Wireless Forum. On eeVBlog the generic component tester shows two back to back diodes, one with Vf ~ 700mV and the other with Vf ~ 1.2V, actually quite an informative result if you know what you are about.

Roger