On 12/14/2018 5:29 PM, Yves Tardif wrote:

> Hi,

> I bought a 3577A several years ago and I had a few times the UNLOCK

> message that appeared on the screen with a BEEP sound and lasted less than a second.

> I thought then that it was the instability of the PLL 10MHz when

> powering on, and the problem disappeared after a few minutes.

> Over time, the problem was more common and seemed independent of the

> temperature or operating time of the device. But that did not prevent

> me from using it.

> Recently, the problem became more serious, and the UNLOCK message

> appeared with the indication of a possible failure on the A5, A6 or A7 PCBs.

> So, I decided to look for the problem so I checked the A5, A6 and A7 PCBs.

> The power supplies were stable and rated, the problem was that the

> phase detector was no longer receiving feedback from the VCO.

> The circuit is a little complex, but I realized that everything was

> pointing to the synthesizer, a custom HP circuit.

> I noticed when scanning the VCO spectrum that sometimes it stops for a

> fraction of a second or more without UNLOCK and then suddenly continues scanning.

> On the screen the sweep continued, or sometimes stopped like the VCO.

> Several other more rare symptoms were added like, the frozen screen

> and a PRESET solved the problem, unwanted characters on the screen,

> message ERROR SYTEM # 24 ...

> I thought then maybe the processor card was the cause of the problem.

> I had already checked the test points at the main power supply and

> everything was ok. So, I put an oscilloscope in ROLL mode and monitor

> the different voltages for a period of time where there was the

> problem ... everything was stable.

> So, I decided to monitor the voltage all over the motherboard, and

> finally, I discovered that during the intermittent problem, + 5V

> suddenly decreased during the period, as low as + 3.9V.

> The + 5V of the motherboard comes from a pair of AWG12 or AWG14

> soldered wire that goes to the main power supply where it connects via

> a connector (it passes 10A by this link).

> The connector was not damaged, no sign of overheating.

> After several unsuccessful attempts to repair this connector, I

> decided to directly solder the pair of wires from the motherboard

> directly to the power supply.

> Since that time, all the intermittent problems that they have disappeared.

> My 3577A has never worked so well.

> For those who have a 3577A and have this kind of intermittent

> behavior, the J8 connector may be the cause.

>?

> Yves

>?

?