HP 8901B Modulation Analyzer - Error 06 repair

[Tony_G]

HI All,

I asked this question on the HP User Group but I thought I'd ask here as well.

Recently bought a non-working 8901B because it shares parts with my 8902A (I needed to fix the A17 L8 inductor, some mechanicals from when I dropped it off my work bench, etc) - I finished the process of transferring all the parts and I was just going to throw it back up on eBay but I thought, "Why not try and repair it first".

The unit is consistently displaying "Error 06" which is an over power error even with nothing connected to the unit.

I tacked on some wires to read signal levels from U3A - Pins 1, 3, 4 & 10. The 5V rail (pin 10) was good and what I saw was that I was no longer getting Error 06 - The unit didn't function but I wasn't getting the error. I did notice that on pin 4 it was 5V with the occasional pulse to 0V - So I dis connected my wires and then put the unit back together and I started getting Error 06 again - This time intermittently.

So I open the unit back up and tack a single wire this time to each of pins 1, 3, & 4 of U3A - Basically tack wire, turn unit on, watch scope, turn unit off then move to next pin.

Now I'm seeing Pin 3 at +2.6V, which is as it should be. Pin 1 is at 5V and pin 4 is at 0V. So according to the service manual the overpower detector is not detecting an over power situation - The service sheet shows OVERPOWER (L) and the textual description talks about the output going low in an over power situation (pg 8F-18, Service Sheet 4).

So it seems to me that the over power detector isn't the problem and I'm left looking for a different culprit. Pin 1 of U3A goes out to  pin 21 on the motherboard and to LO Control Assembly.

I haven't checked that assembly yet but looking at the schematic shows it connecting to U15 pin 1 and through C23 to ground (U15 is an SN74LS138N which is still available from Digikey and my local NTE distributor as a NTE74LS138). The other area it could be is in Q16 & CR10 which propagate the error status to A13.

I won't get around to check A20 till this evening but I thought I might just post this here incase someone has dealt with this error condition before and has any tips, thoughts, guidance, etc.

TonyG

[Tony_G]

Just took the board out for a quick check and I guess the replaceable parts list is wrong - It's actually an SN74LS175N (NTE74LS175 both parts available still) which makes more sense when you look at the circuit schematic (doh!)

[Tony_G]

Had a bit of time to continue to poke around - Here is what I found and posted to the HP user group:

----
This morning I discovered that I clear failed repair 101 - I didn't notice the jumper missing from the A28 RF Motherboard to the A25 Audio Motherboard:



So I whipped off the covers of my working 8902A and used the jumper from it - Turned on the 8901B and, damn, Error 06 was still displays.

While I had the working 8902A open I thought I'd swap the A20 LO Control Assembly and the A13 Controller Assembly (apart from A15 RF Assembly these seem to be the only place the OVERPOWER signals get acted upon).

Still no dice, Error 06 on the display.

I needed the 8902A for something else so I swapped back this bits, reassembled my 8902A and ordered a small jumper cable to replace the missing one (surprisingly cheap on eBay given that I didn't have either the ribbon cable or the connectors and would have had to order those to make the cable myself anyway).

While I had the 8901B on it's back to remove the jumper cable I finally realized that I could measure the pins directly (doh!) so I set about to measure the RF & LO assemblies.

What I saw was the following:

P/0 A28 RF Motherboard Assembly

A15 RF Input Assembly
Pin 21 (I)   OVERPOWER (L)   3.857V
Pin 17 (E)   OVERPOWER (H)   0.300V

A20 LO Control
Pin 29 (I)   OVERPOWER (L)   3.856V
Pin 27 (E)   OVERPOWER (H)   0.300V
pin 4 (Q)    OVERPOWER PROTECT STATUS   4.58V (but jumping around)

According to Service Sheet 21 in normal operation Q16 should just be a solid high unless an overpower condition is detected. What I see on pin 4 (connected directly to the collector of Q16) is in fact a series of pulses - But they aren't a consistent train:




Probing the other pins shows a solid voltage at the measured value.

So I checked Q16 and Q28 to see if they were good and according to my el cheapo transistor checker they're both fine.

I'm at a bit of a loss to work out what the flaw is here and where to look next? If Q16 was turning on and off then I should see that reflected in OVERPOWER (H) as it drives the base of Q16 but I don't.

I do see a crappy random waveform on AMP STATUS (L) - Pin 35:



Any suggestions on where i should look next?

Thanks,

TonyG

[Tony_G]

And voodoo like the unit just starts working...



While I was trying to work out the cause I was flipping the unit back and forwards to get to the top and bottom. I guess this caused whatever the issue was to go away.

It also caused a small noise and when I eventually investigated, I discovered the missing jumper cable. Reinstalling that has made the unit work.

I'm not happy about that because I don't have a good root cause for it and it could just manifest again - I'll need to point that out in the eBay add when I go to sell this unit (it's basically a less functional version of my working 8902A Measuring Receiver).

TonyG

[BFabio]

Hello, I have an HP8901A that works for me up to 650 Mhz, well but from that frequency up it does not recognize any frequency anymore, someone could guide me to repair that fault. Thank you.

[Tony_G]

Sounds like there is an issue with either the VCO or the LO divider. I'd grab the service manuals from Keysight and start from there. Check out BD1.

TonyG