HP 1740A Oscilloscope Delayed Sweep Fault

[jpierre]

Hello,

This is to tell you how perseverance pay, this is the little story of the repair of my HP1740A, he failed after I putted it back on for a radio repair that was waiting for long (I retired recently).
I started the repair with the vertical amplifier as the trace was out of screen, replaced A5A1 (purchased on eBay but have to confirm it was faulty) A5Q1 and A5Q3 (1853-0354 as BC557), followed by horizontal main sweep that was disrupted on output (A11Q2 and A11Q5 1853-0354 replaced by 2N2905). Then the next default was the one of this thread, as soon the delayed time base was putted on, the magnified trace was not limited in time and delayed mode was not starting (spot locked on left, out of screen), DELAY was working.
I made an intensive use of the HP Operating&Service Manual and this is the only scope I own, so the static measurements was the only way.
On schematic 10, I followed the service manual to set the scope, but DLYD SWP START input was not correct, I noticed I had to play with MAIN TRIGGER LEVEL to have the correct value to block A9Q3, then the voltage on A9Q4 emitter was negative and was blocking it, it should had been conducting. I removed Q3 to have it clear, no change, so I suspected A9Q1 as it should be providing the right status on Q4 (I had it removed as some others before for a static check) and I checked the resistors on his collector. This was the moment I checked R5 as it is near R3 and big surprise, R5 was cut completely, nice aspect but no resistance. Hopefully I could find one in my scrapped components and this solved the problem.
I hope this will help you, I4CY and someone else.
Even if the delayed function is not often used, I wanted it running as I had used it in the past when searching for bits in PCM frames.

I faced a very strange situation in the repair, as I had the A9 board removed, I was checking some values and I removed the U1 from his mounting (metal can package), when I putted it back, I noticed one pin was not more than 3 mm long, guess what, the rest of it was in the mounting, of course it was not an unused pin.

regards to all
Jean-Pierre.

[jpierre]

Hello i4cy,
Looking at your third picture with collected measures I suqggest at least Q4 is dead as it should be passing current for Q5, input on Q3 should be near 0v but this what i found also before moving the MAIN TRIGGER LEVEL pot, this putted the right level. Dont forget to follow HP instructions for static measurement.

Regards,
Jean-Pierre

[niknak1]

Hello I am new to the forum and would like to first of all thank jpierre for his post.
Last week I purchased a used HP1740A scope. Despite it's age it is a really nice Scope.
On going through the operation manual I discovered my scope too had the dreaded "Delayed Sweep Fault" as disussed in this forum. Symptoms:-> No Highlighted points on the sweep. No Sweep with DLY'D key depressed, No Sweep with MIXED key pressed.
Well I was also really dissapointed while although the scope is fully useable without this feature, It is a feature that I also wanted to have.
After reading jpierre's post, I opened up the scope and removed the A9 DLYD SWEEP ASSY PCboard. I too measured R5 and found it was open circuit! It should be a 3.32K Ohm resistor 1% tollerance and .5Watt Power Rating.
Not having the resistor available in my collection I joined a few of my available 1W resistors in series to achieve the exact 3.32K ohm +/- 1%. I removed the faulty R5 from the A9 DLYD SWEEP ASSY PCBoard and replaced with my cobbled together resistor.......and to my absolute joy and delight the problem was solved.
Now the delayed sweep is working in all opperation modes as well as the highlighted position marker beam is visible on the sweep signal.

I hope this will help others as it has helped me.

Regards,

Nick

[__i4cy__]

My apologies for not earlier replying to "jpierre" in the original thread https://www.vintage-radio.net/forum/...d.php?t=123718, which has now been closed, hence the update here. His suggestion to check/change R5 and endorsed by "niknak1's" confirmation that changing this resistor also worked for him, inspired me to have another go at fixing my scope.

After removing the A9 board again, I did indeed measure R5 to be around 6kohms, not o/c but certainly faulty. I also found R3 much higher in value than expected. These were both replaced with 2W 5% metal film resistors with the closest E024 value. As already mentioned the originals were specific 1% values in the E48 range, but I cannot see why HP decided on these specific values other than getting the biasing totally spot on.

While I had the board out on the bench, I checked all the transistors and diodes making sure that each was tested properly by effectively removing each from the board. All checked out ok except Q7 which had gone to silicon heaven. "Radio Wrangler" in the original thread suggested trying 2N2369A, but in the end I went for a 2N2222A instead which is supposed to be another equivalent of the 1854-0019 HP part number. For good measure I also went over all the A9 board solder joints just in case there was a dry joint lurking, and cleaned interconnect contacts with switch cleaner.

After carefully putting it all back together I am pleased to say the delayed sweep finally seems to be working now. I get the bright band on the trace, with MIXED and DLY'D working as expected. A big thanks to everyone who helping me get this one sorted.