7603 working .... then blank


Dave Hills
 

U973E should be turned OFF, its base is 4.3V negative with respect to its emitter. It has a 180k collector load resistor so with a high impedance voltmeter its collector voltage should read something similar to the +50V supply ie +0.7V. In fact its collector voltage is more negative than its emitter which is distinctly suspicious.

Probably due to forward current through CR981 and the collector of U973B.


Roger Evans
 

Dave,

Thanks for that clarification,

Roger


tinkera123
 

Hi,

U973D ..... should read ON ..... my typo

U973B is ON .... (see Dave's comment above).

I have probed all voltages around the +50V supply this afternoon and would like to have a good think about it before commenting too quickly. But in brief ..... all transistors appear ON ..... at least to some degree ..... b to e >+0.5v and c voltages all more positive than b However, CR875 does show a 'short' with Diode Tester ..... yet the V at pin P900-6 (Base of Q876A) is +0.7V .... not sure how this is possible and will re-check in morning.

Cheers,
Ian


Roger Evans
 

Ian,

To check CR875 you will need to remove P900 in order to isolate faults elsewhere between +50V and ground.

The most common failures are electrolytic capacitors, particularly some tantalum types, and the less headroom in the rated voltage the more likely are they to fail. I have only spotted two electrolytics that are across the +50V supply, C9 on the main board which is rated at 125V and C492 on the vertical amplifier board which is rated at only 50V in my version of the manual. C492 has a 2.7R series resistor which is broadly in line with drawing 200mA at +0,7V. Removing P450 on the vertical amplifier board will give you a check on C492.

Roger


tinkera123
 

Hi,

+50v PSU

Firstly, a correction to what I wrote yesterday ……. not all transistors are ON (you probably knew that already ….. 😊)
Q863 is ON (e -12.5, b -11.9, c -0.6)
Q860 is OFF (e -12.5, b -12.5, c 2.6)

Dave ….. how did you calculate Ve in your Post on Feb 23??? Estimated Ie???? Datasheet spec???

Roger … in your Post on Feb 19, you stated that Q860 and Q863 have very small Vce. Can you please explain that further??? Are you referring to the voltage available between e and c of approx. 55 volts in total??? Or a Datasheet spec????

CR875

Re-tested CR875 and then removed the various Plugs and re-tested.
The ‘short’ is ‘downstream’ of P971. After removing this Plug, CR875 measured normally as a Good diode.

Not sure if this is good news or not …. as I am unfamiliar with what is ‘downstream’ …. but I guess I am about to find out ….. 😊

Cheers,
Ian


Roger Evans
 

Ian,

What I meant in the Feb19th post when I wrote it was that the voltage between collector and emitter would be too low for the transistors to conduct the required curent. This was a mistake in my logic since in this circuit when the collector voltage (nominally +5V) drops the emitter voltage also drops until the transistor can conduct the current. Frankly a lot of my attempts to understand the interplay of the power supplies were wrong and are probably best ignored. At least I have learned a little even if I have confused things for you. Apologies.

Tracking down the 'short' circuit on +50V should not be too difficult, the first step is to isolate the various boards by unplugging the power in and power out ribbon cables, note the orientation but they should be marked with an arrow on the connector and on the PCB. In most cases the short circuit can be measured with the power off just using a multimeter to measure resistance between +50V and ground on each board. On the circuit schematics you are then looking for components connected between +50V and ground either directly or with very small value series resistors. Electrolytic capacitors are prime suspects, but you might find mechanical damage or a foreign object. If you have a suspect component then unsolder one lead and test the component again. If it is difficult to unsolder one end then you can at your discretion snip one lead, test, and remake the cut lead with some solder.

Regards,

Roger


Dave Hills
 

On Sun, Feb 25, 2018 at 10:54 pm, tinkera123 wrote:


Dave ….. how did you calculate Ve in your Post on Feb 23??? Estimated Ie????
Datasheet spec???
OK. A lot easier to do than explain, but I'll give it a go. No data sheets, just calculations
based on some assumptions derived from experience.

First make some assumptions based on observed conditions:
Collector current very small, based on large emitter and collector resistances.
Base current negligible for purpose of estimating operating points.
Vf CR861 ~ 0.6v, assuming a small signal diode.

Step 1) Simplify the circuit by setting the +50 v sense feedback voltage (+0.7v), to zero and replacing
the divide by two network (R858 & R859), with a 20K resistor and a -25v source in
place of the -50v sense.

Step 2) Calculate the loop current through R864, CR861, and my 20K equivalent resistor.

Step 3) Calculate the base voltage for Q860, Vb = ILoop* R864 = -12.4v.

Step 4) Calculate the emitter voltage for Q860 & Q863, Ve = (-12.4v - 0.6v) = -13.0v.


Dave


tinkera123
 

Hi,

Thank you for that Dave ... key word "experience" .... well, I am gathering it slowly ..... :) I haven't totally grasped the above yet ... but I will.

Some good news ... my 'beast' is now running ... traced the 'short', or more correctly, the very low R across the +50v supply to Ground, all the way to the Vertical Amp board .... and then to .... yes, you more "experienced" TekScope people know the answer already ... C492 .... tantalum(?) 1uF 50 v cap across a +50v Supply .... mmmmm .... in my el cheapo scope maybe ... but in my Tek scope ....????

Replaced it with a 1uF 100v electrolytic ... and she has been running for an hour or so already. Perfect traces?? .... no .... but all functionality seems to be there. I will let it run for awhile to flush out any more issues ....:)

I haven't yet had the time to search this SIte, but I'm sure that it contains some suggestions for replacement caps for these tantalums .... there are 4 on this board. I will be away for a few days, so I will tackle the next phase of getting this Scope running properly then.

A huge THANKYOU to all those who responded, and lurked, for all your assistance. I like to do things like this slowly and learn heaps along the way.

Roger, either you are a 'night owl' or you aren't located in USA .... ???

Cheers,
Ian


Roger Evans
 

Ian,

Delighted you have the scope fixed despite my best efforts to cause confusion. No, I am not a night owl, I live in the UK.

If you browse around here and elsewhere you will find the general recommendation when replacing tantalums with aluminium (UK spelling!) electrolytics to approximately double the capacitance to ensure low ESR so 2.2uF at 100V as and when you can get hold of one.

Enjoy your now working scope,

Roger


 

On any machine, if there are tantalums to be replaced, I always use film capacitors of the same value.
My preferred choice is Wima MKS2, which are similar in size and profile to an electrolytic, so there is generally never any issue of "Will it fit?".
Easily available from Mouser or Digikey.

Menahem Yachad
CondorAudio
Israel


tinkera123
 

Thankyou Roger and Menahem for your recommendations ... my Scope has run trouble free for quite few hours now ... and I am discovering its functionality is more complex than my el cheapo scope .... :)

Cheers,
Ian


Jordan
 

I realize that this thread is a few years old, but I wanted to say that my 7633 scope has a number of caps which are electrolytics when the same caps in my 7603 are tantalums. For example, there are 4 x 1uF tantalums on the readout board of the 7603 whereas the same caps on the 7633 readout boards are 4.7uF electrolytics. IIRC, these boards have the same Tek part number...