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8566B SA all PLLs unlocked

 

Hello,

I turned on my 8655 yesterday and found out that I don¡¯t see the staircase noise floor. PL1,PL2 M/N, YTO, REF all shown as unlocked. Instrument responds to first key press, and then display freeze. It does always respond to the green instrument preset key thought. No good, last time I used it is 3 month ago and it was working.

First suspected the CPU and front panel boards but nothing seemed wrong there. After some troubleshooting, the culprit was found to be the negative regulator where -40 is dead. I pulled a neg regulator A18 from a spare unit and swapped, now it¡¯s great again ¡ª whew.

I¡¯m trying to figure out why the -40 is dead as the A18 is quite expensive on ebay. I don¡¯t see anything obvious wrong. The fuse is ok suggesting the -40 crowbar is probably ok. Did diode test on Q5, Q6 and the diodes around the -40 regulator circuit, seems ok to me. Have anyone seen this?

Thanks,
Calvin
 

Hello Calvin,
did you check the 6 power resistors? It did happen to me that a broken one on A1A6 once killed an opamp in the IF/display unit.
best regards,
Peter
 

I too have seen a failed power resistor. ?Other failed item maybe be a leaky diode that then shuts down the power supply. ?If the -40v supply is other than 0, -40v suspect a leaky diode, or shorted or leaky cap on the affected supply. ?
You will also get a similar symptom if you have an ext 10 MHz ref and it drops out, or you lose the internal 10MHz, or the 100MHz loop is down.

Don Bitters
 

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Fixed one last week ¡­ all loops unlocked ¡­ bad pass element in the +10 VDC supply.

td

?
 

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Finally got a chance to work on it and found the culprit. It¡¯s the SCR for the -40V crowbar.

The SCR (CR25) conducts, or leaks while Vgc is zero. This is a false over-voltage condition tripped the over-current protection circuits(Q6) therefore shuts down the -40V supply but current leakage on the SCR is not strong enough to blow up the fuse for the -40V. Removing the SCR then the -40V is well and alive but of course I lose the over-voltage protection which is a no-no as two of the consumers of the -40V is the YTO, YTX main coil drivers, expensive ones.

Now that CR25, 2N4186 is a MOTOROLA silicon control rectifier which is long obsolete, what would be a good substitute?

Thanks,
Calvin


On Jul 20, 2018, at 10:16 PM, Calvin Guan <guancalvin@...> wrote:

Hello,

I turned on my 8655 yesterday and found out that I don¡¯t see the staircase noise floor. PL1,PL2 M/N, YTO, REF all shown as unlocked. Instrument responds to first key press, and then display freeze. It does always respond to the green instrument preset key thought. No good, last time I used it is 3 month ago and it was working.

First suspected the CPU and front panel boards but nothing seemed wrong there. After some troubleshooting, the culprit was found to be the negative regulator where -40 is dead. I pulled a neg regulator A18 from a spare unit and swapped, now it¡¯s great again ¡ª whew.

I¡¯m trying to figure out why the -40 is dead as the A18 is quite expensive on ebay. I don¡¯t see anything obvious wrong. The fuse is ok suggesting the -40 crowbar is probably ok. Did diode test on Q5, Q6 and the diodes around the -40 regulator circuit, seems ok to me. Have anyone seen this?

Thanks,
Calvin

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