Hello I am repairing the HP spectrum analyzer type 141T and the unit HP8555A.
There were many defects in it but these have all been resolved. One problem remains and that is about 12 dB too low a reproduction.
After several measurements, I think this error is in the 50 MHz AMPL ASSY A2A5 (HP 08555-60060).
The input of this unit is a JFET (Q1) and 4 transistors (q2-q5) type 2N7179.
In the service documentation (SERVICE SHEET 5) a number of voltages are specified with a tolerance of 0.1 V. The measured voltages at the JFET on the Source is 2V (must be 2.8V) and on the Drain 13.00V (must be 10.06V .
Also all other voltages stated on the transistors Q2-Q5 are about 2 Volt higher on the Emtter and Base. The voltage on the Collector is good (19.9V).
What is striking is that the JFET is very warm > 60 degrees.
Unfortunately no AC gain value has been specified. I measure a gain on the first unit Q1-Q2 of 30 dB. The +15 dB and +5 dB circuit work well.
Suspicious is of course the JFET but it is difficult to buy and very expensive. In order to have a better look at this 50 MHz Amply A2A5 and possibly replace parts, you should remove the PCB.
The service documentation SERVICE SHEET 18 describes the REMOVEL AND REPLACEMENT PROCEDURE of the THIRD CONVERTER A2 . In order to remove the entire module housing and board A2A5, the wires at the bottom of the housing must be soldered loose.
What is not clear to me is whether when removing the housing of A2A5 the connections A2C8, A2C10, A2C11, A2C12, A2C8 and A2C13 on the board are solder points or whether these are pin connectors.
When I have removed the screws from the A2 board and removed the connection 50MHz input and the soldering to J4 and J2 I can not get the PCB out of the housing. The PCB remains attached to connections A2C8 to A2C13.
So my questions are:
1st Does anyone know if the JFET q1 gets that hot
2nd Are the connections A2C8 to A2C13 solder joints or connectors.
Thank you in advance for your response, best regards,
L.R.
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