HP745A switch circuit - frustrating square wave generator problem

[tonycox01]

Hi,

I'm trying to fix up an HP745A AC calibrator. The main square wave generator is an astable multivibrator that feeds a 1kHz square wave in to a bistable multivibrator to produce  30v p-p 500Hz square wave. See first pic. The astable section produces a nice 1Khz 29v p-p  square wave but the bistable section generates nothing at all.

It is a frustrating problem. I replaced the three transistors (Q3-5) as they were only 2N3904's and also the diodes. Initially I found that the -25V rail was not working properly but that is fixed now and checked as working. The +/-15v rail, the -25v and +22v rails all seem to be working fine - although the +22v is a little high at nearly +26v.

If I put a scope both sides of CR2 I see the incoming +/- pulses from C4 on one side and only negative rectified pulses on the other side (see 2nd pic) - which is what Id expect. But if I do the same to CR1 I see the incoming +/- pulses on one side but nothing at all on the other side (see 3rd pic). Both diodes are new so I know they are good.

I'm struggling to understand how these rectified pulses disappear for CR1.

Any advice/help would be very welcome!

many thanks

Tony

[duak]

I think there won't be any pulses on the anode of CR1 if Q4 is turned on.  What are the voltages on the collectors and bases of Q3 & Q4?

I see from your schematic that CR1 & CR2 might be  1N645.  I don't know if this is correct and wonder why the designer chose these parts as they appear to be rectifiers rather than signal diodes.  If so, there may be more to this circuit than meets the eye.  ie., the resistor and capacitor values might be fairly critical.

The circuit is not symmetrical as there is a buffer that brings back some current (through R16).  Have you checked Q5 and the parts around it?

[tonycox01]

Thanks Duak.

If Q4 was turned on it would have to be all the time to see no pulses and in which case I should see some output which I do not. I'll check the base voltages however to be sure.

CR1/2 are rectifiers according to the parts list and I replaced them with similar spec 1N4002's. I don't think these are the problem as the originals checked OK too. I agree that this circuit seems to be doing more than is obvious. I thought about disconnecting the buffer stage to see if the core multivibrator circuit itself still functions.

I really am running out of ideas - and parts to replace....

thanks

Tony