Finally getting somewhere with the HP 5248M, bit tired tonight but will explain more tomorrow. ![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif.pagespeed.ce._JElyJQqdB.gif)
Well I went through testing all the transistors in the 5MHz to 10MHz multiplier stage (A26) but couldn't find anything that was causing the low output voltage, I came to the conclusion that the crystal oven might be the cause.
To verify the board was OK before troubleshooting the oven, I disconnected the DIV output from the oven & used a Philips RF source in it's place (which was set it up using the oscilloscope for the correct rms level), well that made a big difference as I was now getting the correct output from the multiplier.
![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif.pagespeed.ce._JElyJQqdB.gif)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1119310)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1119314)
Before & after at A26(6),
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1119318)
Oh dear not another faulty HP crystal oven I thought, the only accessible adjustments are the fine/course for the frequency & a filter for the main 5MHz output (BNC).
The manual only tells you to adjust the 5MHz filter if the output (BNC) on the back of the counter is below 2.5Vpp, which I had checked & it was well above. I had also checked the input to the A26 board was around 70mV or so I thought, turns out I should have checked the rms level not peak to peak
![Banging Head |O](https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif.pagespeed.ce.8iLbFYV4Bc.gif)
and this was a bit low.
After looking at the circuit I couldn't work out why the DIV output was low if the main output was fine. The 5MHz filter adjustment is in this part of circuit so I made a note of the original position before adjusting it, to my surprise the A26 board starting working as it was supposed to and I had well over the required 5Vpp at A26(6)
![Phew :phew:](https://www.eevblog.com/forum/Smileys/default/phew.gif.pagespeed.ce.n2uLOVFYMP.gif)
, I guess the original owner must have twiddled with it instead of fixing the separate oven power supply.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1118916)
The self check was finally useable, note the two LSD are still faulty (probably the last parts in the repair queue), still not getting anything through from either input, which I expected as the function control board still needs working on.
David