Dear all
I have seen posts in the past about this issue
My friend has acquired a unit with the output stage defunct.
Any ideas about what the problem is likely to be and the solution?

Thanks
Richard

Richard Hosking wrote:

Dear all
I have seen posts in the past about this issue
My friend has acquired a unit with the output stage defunct.
Any ideas about what the problem is likely to be and the solution?

Thanks
Richard


Try the test-equipment reflector at qth.net

Bob, W6NBI

Richard Hosking wrote:

Dear all
I have seen posts in the past about this issue
My friend has acquired a unit with the output stage defunct.
Any ideas about what the problem is likely to be and the solution?

Thanks
Richard


Try the test-equipment reflector at qth.net

Bob, W6NBI

In article ,
says...
Dear all
I have seen posts in the past about this issue
My friend has acquired a unit with the output stage defunct.
Any ideas about what the problem is likely to be and the solution?

Thanks
Richard

They tend to get fried by reverse power application. Best solution
would be to replace it with a part from a donor unit. Next bet thing
would probably be a 50-ohm MMIC like a GALI-5 that's capable of similar
gain and output power. I don't remember if the 8640B's output-power
sampler is built into its output stage, though; if it is, then some
careful hacking might be necessary.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

In article ,
says...
Dear all
I have seen posts in the past about this issue
My friend has acquired a unit with the output stage defunct.
Any ideas about what the problem is likely to be and the solution?

Thanks
Richard

They tend to get fried by reverse power application. Best solution
would be to replace it with a part from a donor unit. Next bet thing
would probably be a 50-ohm MMIC like a GALI-5 that's capable of similar
gain and output power. I don't remember if the 8640B's output-power
sampler is built into its output stage, though; if it is, then some
careful hacking might be necessary.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

I'm pretty sure it's just an amplifier. I took a very quick look at
the schematic this morning for my HP8640M, which should be the same.
I believe it's 18dB gain. As you say, it must be capable of the
output power. GALI-51 would be a bit closer match for the gain, but
not quite enough output power?

Cheers,
Tom

Parallel a couple.

W4ZCB

I'm pretty sure it's just an amplifier. I took a very quick look at
the schematic this morning for my HP8640M, which should be the same.
I believe it's 18dB gain. As you say, it must be capable of the
output power. GALI-51 would be a bit closer match for the gain, but
not quite enough output power?

Cheers,
Tom

Parallel a couple.

W4ZCB

Tom Bruhns wrote:
"Ian White, G3SEK" wrote in message
...

Is there also an issue about harmonic content? ISTR there is no
significant harmonic filtering after that amp, so any replacement needs
to be operating well below its quoted maximum power output (which will
be at 1dB compression, where significant harmonic output is guaranteed).

Depending on the detailed schematic, might the input and output match
also be important? Are any filters or attenuators relying on this amp
for their wideband 50R source load or source? If there is no fixed
attenuator pad at the output, then the variable attenuator is relying on
the accuracy of the amp's output impedance (at least at high output
levels, ie low attenuation).

This might be a case for using push-pull, to reduce the even harmonics,
if you can find suitable wideband transformers. Four GALIs of some kind
in push-pull parallel might do the job nicely... but if you want to get
it as right as HP did in the original design, it ain't trivial.

I shouldn't have said that - I'm now too scared to switch on my 8640A!

All good points, Ian. Indeed there's no filtering after the
amplifier, so if you want to keep the harmonic content low, you will
want a decent amplifier there. Also, the output power should be at
least +23dBm if you want to have the same performance as the original.
However, I believe the output is leveled (or at least monitored), so
the output impedance is irrelevant, except perhaps on the highest
range where the source impedance may depend on the amplifier output
impedance.

Good point.

I could have a closer look at the schematic (and in the HP
Journal article about the 8640B) if anyone has a serious need to know
some more details.


Are those old HPJs on the web anywhere?

It may have been in this group a while ago that mention was made of
the way the 8640 generates its output bands: an oscillator from about
250 to 500MHz, with digital divide-by-2 stages followed by LC filters
to attenuate the harmonics. Someone said, I believe, that there is
only one filter per octave. That is not the case with my 8640M, which
on the higher bands has two filters per octave, one for the low half
and one for the high half of the band, which are somehow automatically
switched. I didn't look into just how the switch point was
determined.

There's a mechanical switch on the slide-rule tuning mechanism.

But the true glory of the 8640 series is the way it keeps the FM
deviation constant when more dividers are switched in. Who else but HP
would have used a differential gearbox?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

Tom Bruhns wrote:
"Ian White, G3SEK" wrote in message
...

Is there also an issue about harmonic content? ISTR there is no
significant harmonic filtering after that amp, so any replacement needs
to be operating well below its quoted maximum power output (which will
be at 1dB compression, where significant harmonic output is guaranteed).

Depending on the detailed schematic, might the input and output match
also be important? Are any filters or attenuators relying on this amp
for their wideband 50R source load or source? If there is no fixed
attenuator pad at the output, then the variable attenuator is relying on
the accuracy of the amp's output impedance (at least at high output
levels, ie low attenuation).

This might be a case for using push-pull, to reduce the even harmonics,
if you can find suitable wideband transformers. Four GALIs of some kind
in push-pull parallel might do the job nicely... but if you want to get
it as right as HP did in the original design, it ain't trivial.

I shouldn't have said that - I'm now too scared to switch on my 8640A!

All good points, Ian. Indeed there's no filtering after the
amplifier, so if you want to keep the harmonic content low, you will
want a decent amplifier there. Also, the output power should be at
least +23dBm if you want to have the same performance as the original.
However, I believe the output is leveled (or at least monitored), so
the output impedance is irrelevant, except perhaps on the highest
range where the source impedance may depend on the amplifier output
impedance.

Good point.

I could have a closer look at the schematic (and in the HP
Journal article about the 8640B) if anyone has a serious need to know
some more details.


Are those old HPJs on the web anywhere?

It may have been in this group a while ago that mention was made of
the way the 8640 generates its output bands: an oscillator from about
250 to 500MHz, with digital divide-by-2 stages followed by LC filters
to attenuate the harmonics. Someone said, I believe, that there is
only one filter per octave. That is not the case with my 8640M, which
on the higher bands has two filters per octave, one for the low half
and one for the high half of the band, which are somehow automatically
switched. I didn't look into just how the switch point was
determined.

There's a mechanical switch on the slide-rule tuning mechanism.

But the true glory of the 8640 series is the way it keeps the FM
deviation constant when more dividers are switched in. Who else but HP
would have used a differential gearbox?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

Looking at a cicuit, this issues seem to be
The attenuator presumably relies on the output impedance being 50 ohms
The hybrid has a gain of 16dB with an output of about +23dBm. Monitoring and
AGC is separate from the hybrid, so all it has to do is provide the gain and
output!
Supply voltage is 44V
There is no post amplifier filtering, so harmonics will depend on the amp.
None of the conventional MMICs seem to fit directly - it might be necessary
to make up something from discrete components

Richard

Tom Bruhns wrote in message
m...
"Ian White, G3SEK" wrote in message
...

Is there also an issue about harmonic content? ISTR there is no
significant harmonic filtering after that amp, so any replacement needs
to be operating well below its quoted maximum power output (which will
be at 1dB compression, where significant harmonic output is guaranteed).

Depending on the detailed schematic, might the input and output match
also be important? Are any filters or attenuators relying on this amp
for their wideband 50R source load or source? If there is no fixed
attenuator pad at the output, then the variable attenuator is relying on
the accuracy of the amp's output impedance (at least at high output
levels, ie low attenuation).

This might be a case for using push-pull, to reduce the even harmonics,
if you can find suitable wideband transformers. Four GALIs of some kind
in push-pull parallel might do the job nicely... but if you want to get
it as right as HP did in the original design, it ain't trivial.

I shouldn't have said that - I'm now too scared to switch on my 8640A!

All good points, Ian. Indeed there's no filtering after the
amplifier, so if you want to keep the harmonic content low, you will
want a decent amplifier there. Also, the output power should be at
least +23dBm if you want to have the same performance as the original.
However, I believe the output is leveled (or at least monitored), so
the output impedance is irrelevant, except perhaps on the highest
range where the source impedance may depend on the amplifier output
impedance. I could have a closer look at the schematic (and in the HP
Journal article about the 8640B) if anyone has a serious need to know
some more details.

It may have been in this group a while ago that mention was made of
the way the 8640 generates its output bands: an oscillator from about
250 to 500MHz, with digital divide-by-2 stages followed by LC filters
to attenuate the harmonics. Someone said, I believe, that there is
only one filter per octave. That is not the case with my 8640M, which
on the higher bands has two filters per octave, one for the low half
and one for the high half of the band, which are somehow automatically
switched. I didn't look into just how the switch point was
determined.

Cheers,
Tom