[time-nuts] Re: HP 5065A repair

[Matt Huszagh]

Well I think I was wrong about A3 being an issue. I compared this unit
to a working unit and they behave pretty similarly. The A1 synthesizer
5.135 MHz output is a little lower on the (thus far) non-functioning
unit, but it's not a huge difference and the RF excitation signal into
the RVFR looks pretty similar. A3TP2 is low on both units (93 mV on the
working one). I expect the working unit probably just needs an
alignment.

Anyway, I'm back to de-flooding.

Matt

Matt Huszagh <huszaghmatt at gmail.com> writes:

> I'm back to troubleshooting the A3 multiplier assembly because if the
> 60 + 5.315... MHz signal to the SRD isn't correct, I wouldn't expect to
> know if and when de-flooding succeeds.
>
> The 5.315... MHz component is a bit weak - in the neighborhood of 5mVrms
> instead of the expected 20mVrms. Additionally, the AGC DC voltage (TP2)
> is 70 mV, but should be around 4.5 V.
>
> I've got A3 hooked up in isolation on my bench with 5MHz at 1Vrms to J2
> and 600mVrms to J5 at 5.315MHz. Any suggestions for how to troubleshoot
> this? I measured Q4 and C12 to ensure they're not shorted - they're
> fine. I also checked different points in the circuit and compared them
> with the voltage annotations in the schematic. Unfortunately, I'm having
> some trouble understanding this circuit, and some of the annotations
> confuse me.
>
> According to the annotations, the base of Q4 (the AGC amplifier), is at
> +0.7Vdc for no AGC and +6Vdc for full AGC. But there's no emitter
> resistor so how does this get to +6Vdc? Moreover, I don't see a DC path
> to supply current to raise the DC voltage. In case it's helpful, Q4 is
> apparently a 2N2484.
>
> Another example is Q9 base is labelled with 2.4V. I think that's dc
> since I see Vrms in a number of other places. But I don't see how that
> could be true. This is fed via a +20V supply, and the base is biased by
> a resistive dividier with 18k and 2k, so it should be at most +2V, but
> then there's the parallel path of the BE junction and emitter resistor,
> so it would be even lower than this. So, where are they getting 2.4V?
>
> I checked a number of points in the main signal path and some looked
> more or less in line with amplitude annotations, but others didn't. I'm
> not totally sure how precise these annotations are supposed to be,
> though.
>
> Finally, I wanted to clarify how the RVFR SRD bias works. The only DC
> path to the bias test point is through J4 (which provides the
> 60 + 5.315... MHz signal to the SRD). But, the RVFR schematic does not
> show any DC bias applied to the SRD. I presume that's there but just not
> shown. Is that right? That would make sense, since the pot there (R40)
> would divide down the bias.
>
> Any suggestions would be immensely appreciated.
>
> Thank you!
> Matt
>
> Matt Huszagh <huszaghmatt at gmail.com> writes:
>
>> I've decide to start by addressing the cell flooding. Despite the
>> incorrect AGC DC voltage, the 60 MHz signal looks roughly ok and from a
>> quick look I don't think I can really perform the loop alignment without
>> some sort of RVFR signal.
>>
>> I've attached a current-limited supply at 5V through a 5ohm power
>> resistor to the TED wires and have the unit powered with the meter set
>> to 2nd harmonic. The meter indication almost immediately started
>> deflecting off 0. It's been just a few minutes and the readings are
>> getting almost to 10 at max deflection, though deflecting back and
>> forth. I'm hopeful this is a good sign. I'll update as this unit has
>> more time to de-flood.
>>
>> Please feel free to make suggestions if I should be following a
>> different approach at the moment.
>>
>> Thanks!
>> Matt
>>
>> Matt Huszagh <huszaghmatt at gmail.com> writes:
>>
>>> There are a few other things I wanted to mention.
>>>
>>> I checked the DC voltages around the cell and lamp heaters. J15P3 (cell
>>> heater) measured 15.4V and J15P5 measured 14.4V. P4 measured 28.6V. The
>>> specified nominal voltages for P3 and P5 are 18V, so these are a bit
>>> off, but since the heater currents look ok as measured by the front
>>> panel meter, I got the sense that this is probably ok. Thoughts?
>>>
>>> Also, as part of the pre-emptive maintenance I replaced the lamp
>>> assembly film resistor and, in the process, disassembled part of A12
>>> including the lamp assembly. Everything looked fine to my eye and I have
>>> pictures I can share if desired. In particular, the lamp assembly was
>>> not charred.
>>>
>>> Matt
>>>
>>> Matt Huszagh <huszaghmatt at gmail.com> writes:
>>>
>>>> Hi,
>>>>
>>>> I have an HP 5065A received in unknown condition. After performing some
>>>> pre-emptive maintenance (basically Corby's recommendations here:
>>>> https://www.eevblog.com/forum/metrology/inspecting-aligning-and-testing-an-hp-5065a-rubidium/
>>>> and the first aid advice from Luciano here:
>>>> http://www.timeok.it/hp5065a-corner-3/) I turned the unit on. +/- 20V
>>>> supplies look good and cell and lamp ovens drop down from pegged in
>>>> under 45 min to about 32 each.
>>>>
>>>> However, the 2nd harmonic reading is 0 and of course the unit does not
>>>> lock.
>>>>
>>>> This unit was left unpowered for some time (I got the sense it was at
>>>> least several years) before I received it, and therefore cell flooding
>>>> is likely. However, I have noticed a few other discrepencies.
>>>>
>>>> The first is that the A3 TP2 AGC DC voltage measures 71 mV, instead of
>>>> the expected >4.5 V. I also checked the 60 MHz signal into 50 ohms and
>>>> see about 30 mW. One place in the manual says this should be >1V (which
>>>> it is), but another place says 300 mW. Is loading this with 50 ohms
>>>> correct? If so, maybe 300 mW is a typo and it should read 30 mW? In any
>>>> event the AGC DC voltage is wrong and I suppose needs to be addressed.
>>>>
>>>> The second issue, and probably a more serious one, is that the PHOTO I
>>>> meter reading is 20 (less than the specified 25) and the actual current
>>>> measured out of A12 is -10 uA (should be 50 uA or greater). There are
>>>> two issues here: (1) the actual current disagrees with the meter
>>>> indication and (2) the current is low. Is the low current possibly
>>>> explained by cell flooding and or incorrect/insufficient 60 MHz RF
>>>> signal application?
>>>>
>>>> Thoughts on next steps? I believe I need to address both issues
>>>> above. Addressing cell flooding is straightforward, but will take some
>>>> time. The troubleshooting section for A3 says to perform the loop
>>>> adjustment procedure if the AGC DC voltage is wrong. Which should I do
>>>> first? Or, should I start with something else?
>>>>
>>>> Thanks!
>>>> Matt