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HP 8673D synthesized generator - need help identifying the two PC boards attached to A1A11 power amplifier

 

I'm on (the other) HP 8673D repair project now. This one never worked in the time I have owned it, and I'd been storing it as a parts mule, but decided (now that I have some experience with disassembly and repair) that I'd like see if I can bring it back to life.

Primary symptom: Very low output (rarely approaching 0 dBm, at some frequency ranges much less), unleveled indicator (as the available RF would never reach the setpoint of the ALC circuit), yet never an "unlocked" indication.

Fault isolation so far: The service manual vol 2 08673-90022 (in section BD5) walks through several tests; it turns out RF level (so-called "baseband") 2.0-6.6 GHz out of YTO (then through a bias tee, modulator for pulse modulation, and a high pass filter) measures with adequate RF on all YTO frequencies (spec is >= 10 dBm; this is satisfied with about +18 dBm at bottom end of range and staying above +10 dB to the end of the range.) This signal is sent to the YTM assembly; output from YTM is very low and fluctuates widely across frequencies. The service manual details RF level tests at the high pass filter output (what goes to the power amplifier which is the start of the YTM system) and RF level tests at output of the YTM (this output is very low and uneven across frequencies, resembling output of the 8673D overall, so fault is in this area.

Further attempt at fault isolation: The YTM assembly includes a power amplifier (A1A11, part number in my (preliminary) manual is 08673-67004. The popwer amplifier in this 8673D part number is 08673-67011. The manual provides zero detail (that I can find) detailing expected output of the power amplifier that feeds the YTM itself. As that connection was SMA, I connected A1A11 power amplifier output (via attenuation) to the power meter and found ~0 dBm output (varying widely across frequencies), and after a few minutes in this configuration, A1A11 amplifier output went to zero and never returned.

Unsuccessful search for details in service manual: Attached are photos of the A1A11 amplifier and two associated PC boards, part numbers 08673-60140 (provides DC to the A1A11 power amplifier via pins E1 through E12) and 08673-60139. The DC test points on 08673-60140 (upper/larger board in photo) expect +5, -3.5, -6.5 and -9 VDC; I measured these (in system while running) at +5, -6.5, -6.5 and -9 (+/- some mV), so the -3.5V test point's voltage was incorrect.

I'm not able find schematics of either board in the (large/heavy) service manual, hence my posting here - does this ring a bell with anyone that may have encountered these modules in another context, has worked on an 8673D in this area, or has seen these mentioned in the service manual volume 2 (document number 08673-90022)?

In modern times, getting more gain and power at 2.0-6.6 GHz would not be at all difficult, however, the YTM is sensitive to the level of RF input, and it appears visually that the circuitry around the A1A11 amplifier is at least in part allowing the CPU to control output from A1A11 to keep drive within some (undocumented) acceptable level window.

Thanks for any clues!

Dave

8673d-a1a11-top.jpg
8673d-a1a11-bottom.jpg

 

What a beast. I’ll have to find one as a project.

Peter

On Mar 18, 2025, at 9:35?PM, David Feldman via groups.io <wb0gaz@...> wrote:

?I'm on (the other) HP 8673D repair project now. This one never worked in the time I have owned it, and I'd been storing it as a parts mule, but decided (now that I have some experience with disassembly and repair) that I'd like see if I can bring it back to life.

Primary symptom: Very low output (rarely approaching 0 dBm, at some frequency ranges much less), unleveled indicator (as the available RF would never reach the setpoint of the ALC circuit), yet never an "unlocked" indication.

Fault isolation so far: The service manual vol 2 08673-90022 (in section BD5) walks through several tests; it turns out RF level (so-called "baseband") 2.0-6.6 GHz out of YTO (then through a bias tee, modulator for pulse modulation, and a high pass filter) measures with adequate RF on all YTO frequencies (spec is >= 10 dBm; this is satisfied with about +18 dBm at bottom end of range and staying above +10 dB to the end of the range.) This signal is sent to the YTM assembly; output from YTM is very low and fluctuates widely across frequencies. The service manual details RF level tests at the high pass filter output (what goes to the power amplifier which is the start of the YTM system) and RF level tests at output of the YTM (this output is very low and uneven across frequencies, resembling output of the 8673D overall, so fault is in this area.

Further attempt at fault isolation: The YTM assembly includes a power amplifier (A1A11, part number in my (preliminary) manual is 08673-67004. The popwer amplifier in this 8673D part number is 08673-67011. The manual provides zero detail (that I can find) detailing expected output of the power amplifier that feeds the YTM itself. As that connection was SMA, I connected A1A11 power amplifier output (via attenuation) to the power meter and found ~0 dBm output (varying widely across frequencies), and after a few minutes in this configuration, A1A11 amplifier output went to zero and never returned.

Unsuccessful search for details in service manual: Attached are photos of the A1A11 amplifier and two associated PC boards, part numbers 08673-60140 (provides DC to the A1A11 power amplifier via pins E1 through E12) and 08673-60139. The DC test points on 08673-60140 (upper/larger board in photo) expect +5, -3.5, -6.5 and -9 VDC; I measured these (in system while running) at +5, -6.5, -6.5 and -9 (+/- some mV), so the -3.5V test point's voltage was incorrect.

I'm not able find schematics of either board in the (large/heavy) service manual, hence my posting here - does this ring a bell with anyone that may have encountered these modules in another context, has worked on an 8673D in this area, or has seen these mentioned in the service manual volume 2 (document number 08673-90022)?

In modern times, getting more gain and power at 2.0-6.6 GHz would not be at all difficult, however, the YTM is sensitive to the level of RF input, and it appears visually that the circuitry around the A1A11 amplifier is at least in part allowing the CPU to control output from A1A11 to keep drive within some (undocumented) acceptable level window.

Thanks for any clues!

Dave







<8673d-a1a11-top.jpg>
<8673d-a1a11-bottom.jpg>
 

Peter - yes - beast (trivia - back in about 1981 I was one of hundreds of volunteer extras (person-sized bird characters) for a movie being filmed north of Los Angeles called "the beastmaster" - never saw the movie!.)

A recently purchased (earlier this week) VEVOR TFD15 hydraulic lift table has made 1000% improvement in handling these heavy/old machines (8660, 8350, 8673) which use linear/transformer power supplies and are made of 100% pure cast iron straight from the mine. Highly recommend such a thing for those of us "of a certain age" that still wrangle electronics by the metric ton.

Determined course of action at this point --- going to build extender mechanism to allow the pictured module to operate outside the synthesizer. 6x2 3.94mm card edge connector, already have PCBs with 3.94mm pitch traces for making these sorts of things, and 14-pin IDC DIP male plugs to make the 2-port ribbon cable that appears to go off in the direction of the YTO, are both on order, couple weeks til delivery, so machine is back to it's "hangar queen" location until I am ready for testing.

Barring any other adds to this discussion, I'll surface again with the topic once I am able operate the module(s) outside the synthesizer chassis.
 

The lift table sounds like a useful boat anchor manipulation device.? I am of "that age" but am still very good at lifting (everyone is good at something in the gym).? However, sidelined a couple of months due to having to go to the maintenance facility for a shoulder repair for a different injury and the doc telling me no heavy lifting for a bit.? So for now, circuit development and debug of tiny stuff like 0402 SMD parts.

I have to get into my 8660D which I still like for lower frequency RF work as other more modern units I have (modern is relative) don't go below 300 kHz.? The 8660D mainframe is alright but could use a tuneup but the low frequency plugin has no output.? I will probably need to make an extender.? I do have parts and harnesses from a scrapped 8660 mainframe I grabbed that someone had left as trash at the end of a hamfest so maybe can make an extension cable with those Winshester-looking connectors.

Most of the alignments and adjustments in the 8660 can be done without extenders but some steps can't be done without.

Due to how busy I am I think this will need to wait a few months but it will happen!

Peter

On 3/19/2025 4:49 PM, David Feldman via groups.io wrote:
Peter - yes - beast (trivia - back in about 1981 I was one of hundreds of volunteer extras (person-sized bird characters) for a movie being filmed north of Los Angeles called "the beastmaster" - never saw the movie!.)

A recently purchased (earlier this week) VEVOR TFD15 hydraulic lift table has made 1000% improvement in handling these heavy/old machines (8660, 8350, 8673) which use linear/transformer power supplies and are made of 100% pure cast iron straight from the mine. Highly recommend such a thing for those of us "of a certain age" that still wrangle electronics by the metric ton.

Determined course of action at this point --- going to build extender mechanism to allow the pictured module to operate outside the synthesizer. 6x2 3.94mm card edge connector, already have PCBs with 3.94mm pitch traces for making these sorts of things, and 14-pin IDC DIP male plugs to make the 2-port ribbon cable that appears to go off in the direction of the YTO, are both on order, couple weeks til delivery, so machine is back to it's "hangar queen" location until I am ready for testing.

Barring any other adds to this discussion, I'll surface again with the topic once I am able operate the module(s) outside the synthesizer chassis.

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