Hewlett Packard HP461A Wide Band Amplifier

[Peter_O]

Not shure if this one should be here or in 'Beginners'.
It's grown up lab equipment, but so far my knowledge might be kind of beginners level. 

2023 this little guy needed to be rescued for 30 Euro.



The 461 provides a gain of 20 dB or 40 dB with a frequency response of +/- 1 dB from 1 kHz to 150 MHz and a max output of 0.5 V RMS into 50 Ohm.
https://www.keysight.com/us/en/assets/9018-02904/user-manuals/9018-02904.pdf

Obviously there have been some repairs in this one already.

 



What bothers me are these points:

1)
The PSU voltage is in spec, but the ripple is 8 mV RMS instead of < 1 mV spec.
Would you change the caps?

2)
The bode plot shows quite OK behavior below 10 MHz but goes wild above that.

 

Following the service manual I tried to adjust the inductors, but there was no change in the frequency response at all, so there might be broken coils.

3)
The transformer is running quite hot with 60 °C as well as some resistors. Is this normal for this kind of device or do I need to search for a fault?

 

Has anybody of you been an one of these points before?
Hints etc. appreciated!

[Stray Electron]

  Nice save!  What's the date code on that unit?

   Those small silver capacitors shown in picture #4 (and some in Picture #2) are wet slug tantalum caps and are VERY expensive and very high quality and rarely fail. I would not touch them unless you can prove that they're bad.  But I would check those large silver electrolytic caps.

  I've never worked on one of those but IMO the transformer should not be getting hot. 

  Looking at the second thermal image it looks like that capacitor that's partially hiding under the black heat sink is getting very hot. I sort of doubt that it's bad but I think something is overloading it.  (Possibly that amplifier is oscillating and is dumping high frequency into the capacitor).

[Peter_O]

  Nice save!  What's the date code on that unit?

Thx for your ideas! 
It's a 0946... .

   Those small silver capacitors shown in picture #4 (and some in Picture #2) are wet slug tantalum caps and are VERY expensive and very high quality and rarely fail. I would not touch them unless you can prove that they're bad.  But I would check those large silver electrolytic caps.

Will check the PSU caps.

  I've never worked on one of those but IMO the transformer should not be getting hot. 

  Looking at the second thermal image it looks like that capacitor that's partially hiding under the black heat sink is getting very hot. I sort of doubt that it's bad but I think something is overloading it.  (Possibly that amplifier is oscillating and is dumping high frequency into the capacitor).

Will check the temperature of the cap and try to do some more measurements.

[coppercone2]

mine had a cracked attenuator resistor that was driving me nuts. Its slightly out of spec now by 1dB but I can live with that.

[Stray Electron]

  Nice save!  What's the date code on that unit?

Thx for your ideas! 
It's a 0946... .

  Wahoo! Built in 1969.  Nominally the "46" week of the year but HP actually marked a date about 8 weeks (IIRC) AFTER the actual production date to allow time for shipping, etc.  So your's was probably being built at about the same time that Apollo 11 was landing on the moon!

[factory]

  Nice save!  What's the date code on that unit?

Thx for your ideas! 
It's a 0946... .

  Wahoo! Built in 1969.  Nominally the "46" week of the year but HP actually marked a date about 8 weeks (IIRC) AFTER the actual production date to allow time for shipping, etc.  So your's was probably being built at about the same time that Apollo 11 was landing on the moon!

It wasn't built in 1969, the serial prefix code is the date code of the last major design revision.

Why else would it have four components showing 1978 date codes and another showing 1977? There are probably many more not visible from the pictures.

There are other clues such as the printed HP logo, post 1970 changes, IEC socket & white plastic power indicator light and later cover colors.

David

[Swainster]

I've got one of these, sort of - sadly a previous owner had plundered the output transistor and attenuator for parts. I've got it partly working by trial and error substituting various RF transistors for the missing HP unobtanium part. Other than the missing bits, the main issue was that a lot of the carbon composite resistors had drifted out of spec. As this operates up to the VHF band, I wasn't convinced that metal film resistors would be suitable replacement for carbon composite so I put SMD resistors on some sliced up copper clad as the closest substitute that I could come up with from my 'stash'. Lastly, I can say that if the RF tuning slugs are stuck, don't force them as they turn back to dust as the slightest provocation. Still a (stalled) work in progress, as the flatness is out of spec (crumbled slugs not helping here) and the attenuator is still missing some mechanical bits.

BTW, member Tony_G did a youtube video on his example.

Regarding your bode plot results, could the issue be a dodgy BNC connector?

[Peter_O]

Let me add a clear schematics pdf.

I've checked two of  the bigger PSU caps C1, ... C5:

They should be 65 µF and are
82 µF / 0,5 Ohm and
79 µF / 0,6 Ohm
at 100 Hz.

So I think, all five should be fine, or do you think the ESR is to high?

C7, one of the smaller PSU ones:

It should be 20 µF and is
22 µF / 0,11 Ohm.

-----

And I took another look at the hot 220 Oh resistors R52 and R53 near the output emitter follower Q9. They are still hot at 80°C.

snip: Looking at the second thermal image it looks like that capacitor that's partially hiding under the black heat sink is getting very hot. I sort of doubt that it's bad but I think something is overloading it.  (Possibly that amplifier is oscillating and is dumping high frequency into the capacitor).

This is C30, which is the final output coupling capacitor. Having a closer look, it definitely stays cold.

[MathWizard]

So if this was built with discrete 1970's part's, besides the expensive caps someone mentioned, how good of a version can be built with today's jellybean parts? What about for say $50, case/hardware not included ?

I want to try it with my cheap parts, I haven't looked at the schematic yet, maybe it's some JFET buffer input like the JFET buffer probe I made. My cheap JFET's might be ok for 50-100MHz

[factory]

   Those small silver capacitors shown in picture #4 (and some in Picture #2) are wet slug tantalum caps and are VERY expensive and very high quality and rarely fail. I would not touch them unless you can prove that they're bad.

The Kemet T110 are hermetically sealed solid tants, can't tell what the others are from the picture. Have you checked for any that might be shorted? it does happen, just not as often as the infamous bead type.

Does the transformer still overheat if the secondary windings are disconnected and did you measure power consumption? Also check the fuse is the correct value for the voltage setting.

David

[David Hess]

So if this was built with discrete 1970's part's, besides the expensive caps someone mentioned, how good of a version can be built with today's jellybean parts? What about for say $50, case/hardware not included ?

A modern inexpensive implementation would use several operational amplifiers in series and have a frequency range from DC to several MHz, similar to the example below.

[Someone]

So if this was built with discrete 1970's part's, besides the expensive caps someone mentioned, how good of a version can be built with today's jellybean parts? What about for say $50, case/hardware not included ?

I want to try it with my cheap parts, I haven't looked at the schematic yet, maybe it's some JFET buffer input like the JFET buffer probe I made. My cheap JFET's might be ok for 50-100MHz

Its 50 ohm input, so just find a MMIC amplifier with suitable flatness down to DC. $1-2 ?

[Peter_O]

   Those small silver capacitors shown in picture #4 (and some in Picture #2) are wet slug tantalum caps and are VERY expensive and very high quality and rarely fail. I would not touch them unless you can prove that they're bad.

The Kemet T110 are hermetically sealed solid tants, can't tell what the others are from the picture. Have you checked for any that might be shorted? it does happen, just not as often as the infamous bead type.

Does the transformer still overheat if the secondary windings are disconnected and did you measure power consumption? Also check the fuse is the correct value for the voltage setting.

David

Yes. I woll disconnect Amp from PSU and check separately in the next step.
Thx for the hint to check the cap(s) for shorts, David!
Will do and come back here.

[coppercone2]

did you do a continuity check on the coils?

if so, desolder them and LCR test? its a bit of a wild goose chase. i bet you can replace the inductors.

[Peter_O]

The main fuse is in spec: 250mA.

Disconnected secondary side of transformer.
Resistance is
primary:
1-2 240 Ohm
2-3 0 Ohm
3-4 270 Ohm
secondary:
5-6 31 Ohm
6-7 34 Ohm

Voltage selector is OK:
1-4 is 230V
Secondary Voltage is
5-6: 44 V RMS
6-7: 44 V RMS

Disconnected Trafo stays cold.
Scope with differential probe shows same voltages ...

 

As R52 and R53 were getting hot, I followed David's (factory) advice and checked C29 (orange).
It's 2.2µF bang on and not shorted.



With connected transformer and seperated +15V and -15V lines between PSU and amp,
I put a 2k resistor as a load in between.
Output is +14.6 and -16.2V.
Trafo stays cold.
But R2 and R3 (yellow) are getting hot quickly (100°C).

Next step is to better understand this PSU. 

And to put an lab PSU at the amp and see, what happens.

[coppercone2]

i mean the tuner coils

[Peter_O]

i mean the tuner coils

Sure.
Tuning the amp will be the next step.
After having solved the problem of running 'too hot'.

[David Hess]

Another thing to think about when looking at this design is that transistors were the most expensive part, so all effort was made to minimize how many were used.  A modern discrete implementation could use cascode stages, doubling the number of transistors, but producing a flat response without the coil peaking.

[coppercone2]

i mean the tuner coils

Sure.
Tuning the amp will be the next step.
After having solved the problem of running 'too hot'.

I just meant to check if there is broken wire on the coils maybe it snapped. see if there is zero ohm continuity across it

[coppercone2]

Another thing to think about when looking at this design is that transistors were the most expensive part, so all effort was made to minimize how many were used.  A modern discrete implementation could use cascode stages, doubling the number of transistors, but producing a flat response without the coil peaking.

i noticed smart people are united against coils lol. like seriously, they wanna get rid of coils. the silicon design guys get angry at inductors

i wonder if they will have advanced meta material or refined structure inductors one day that was used by anti transistor zealots in RF designs. or some kind of 'super inductor'

but this amp weirded me out. I used it with an antenna on a tecsun radio. it seemed to help the signal quality (but this was one with a damaged attenuator). I heard some guy from the west coast talking on HAM and it felt very eerie, like he was present at my house. it made me wonder about psionics. He also happened to be talking about the nature of perception lol. Then I took it apart to fix it and recently finished fixing it. I am just waiting to find a transcript some where that the conversation that I heard took place in 1965 and that the man has been dead for 50 years. Or that he was aware his transmission was being used a test signal thousands of miles away by a certain  and that my bank account details go. is this box amplifying more then photons?? turns out someone lifted the schematic from a classified CIA fun file titled 'stargate'. I never had the feeling someone might guess my bank account details because of an amplifier but I swear I turned it off because it felt like some how the dude was in my house lol

[David Hess]

I noticed smart people are united against coils lol. like seriously, they wanna get rid of coils. the silicon design guys get angry at inductors

i wonder if they will have advanced meta material or refined structure inductors one day that was used by anti transistor zealots in RF designs. or some kind of 'super inductor'

Until we get superconductors, inductors just do not perform well compared to capacitors.

More advanced designs of that era used inductors, but not for extending the bandwidth of each stage in such a simple way.  Tektronix had the secret of bridged t-coils for many years.

[coppercone2]

I think there might still be something with like powder technologies related to inductor improvements. There is a SURPRISING amount of science behind powders that most EE are not aware of, save for maybe super capacitor / battery process people, an a surprising amount of liberty taken in device production that ignores MANY details/possible measurements of even unprocessed feed stock used to make some kind of structure, let alone the assembly process. Not sure what effect any of it would have but it would be expensive and time consuming to determine / control some known factors. and I bet there is even more behind magnetic properties of powders.

[factory]

This probably won't help, but here are some pictures of my 462A, the circuit diagram in a previous post shows the differences, it's a bit older, there are some Ge transistors in the earlier ones, the coils are well sealed to prevent adjustment.

The manual only mentions Q9 having a heatsink fitted, mine is exactly as described, the 461A seems to have a load of extra heatsinks for unknown reasons.

I don't like the use of a 250mA fuse for both 115V & 230V operation, especially with that tiny transformer.

David

[MathWizard]

Well I'm going to make something like this, I'm just trying to get a model working to spec's in LTSpice with my transistors. I'm trying to find a higher quality image of the BJT list. I can tweak the biasing a bit if needed. Or try to, I've never tried to calculate anything for such a baising, I wonder what those DC and AC eqn's will be.

I'll power it from a bench PSU. Or maybe try making a low noise, low power split rail PSU just for little test devices/probes.

I ordered some 0805 LCR SMD sample-books, with lots of small nH inductors and C0G/NP0 caps. I made a JFET buffer probe that works ok. Now I need to miniaturize it and fit it in some metal tube and get some coax.

[MathWizard]

Can anyone make out the BJT's in the part list ? Q4 is 2N3478, I could try S9018 for most of these. But for the PNP, I don't think I have anything fast like a 9018.

I would like to try adding JFET's to the input, like a buffer probe, but for the class AB amp, I still need faster PNP's. I just made a big Digikey order, but I forget fast PNP's

[Peter_O]

This probably won't help, but here are some pictures of my 462A, the circuit diagram in a previous post shows the differences, it's a bit older, there are some Ge transistors in the earlier ones, the coils are well sealed to prevent adjustment.
The manual only mentions Q9 having a heatsink fitted, mine is exactly as described, the 461A seems to have a load of extra heatsinks for unknown reasons.

It's always nice to have pictures for comparison. 
My 461A has been tinkered with, no question. Soldering, additional heatsinks, the makeshift one at least, scratched pin labeling at the power transistor, removed seals at the inductors, ...

I don't like the use of a 250mA fuse for both 115V & 230V operation, especially with that tiny transformer.

David

Yeah. For the moment I have the IR camera at hand when playing.
I'm not sorted in these big US style fuses but in 5x20mm only. 

[Peter_O]

snip
Can anyone make out the BJT's in the part list ?

Here's better scan of the service manual.

[factory]

My 462A seems to have been made just after 1970 (SN 947A-01167), some parts dated 6952 and 0-13 (1970 week 13), it disobeys the manual changes (947-01160 & below) as it still has a red plastic neon indicator & original cover colors, guess they used up what was in stock first and/or had shortages of the later parts. But has a factory fitted IEC, with a big crack across it, from whatever impact cracked the side frame.

Some of the smaller carbon resistors have been disturbed by a previous owner.

Transistor in italics don't quite match anything in the manual, or change sheet, for mine at least, some resistors & capacitors values are different for the 462A, which is optimized for pulse use.

Q3 is 1850-0106 (Ge)
Q4 is 4-073 (aka 1854-0073, 2N3478)
Q5 to Q7 are SS98 (aka 1854-0305)
Q8 is 2N3663 (aka 1854-0219)
Q9 is 54-19 (aka 1854-0019)

As usual some unknown spec. selected parts with custom numbering for HP, can't rely on the NSN lists for the Ge one, as it mentions several Si parts on the same page.

Earlier ones used 1854-0031 (aka 2N2218A) for Q4 to Q8, they state to use the newer part list it any fail & replace as a set of 5.

The broken tuning slugs are a problem, they are usually made from slightly different materials depending on the the frequency.

Fuses are quite well stocked here, some modern ones have nuisance blow proprieties that the older ones didn't (in a 711A), I bought some older Littelfuse/Bussmann ones from a seller in Greece a few years ago, old enough to be packed in metal tins (Qty. 5), price in quantity was better than RS at the time.

David

[MathWizard]

Ok I missed your replies, thanks. Yeah I wondered if Q3 was some old Germanium thing with a low VBE.

[factory]

The custom parts had their required spec detailed in HP lists, one or more vendors would provide devices that met the specs, employees had access to these lists on Microfiche, which seem to have mostly been lost to time, but someone sent CuriousMarc some, none of these are available to view/download anywhere that I know of.
Transistor 1854-0019 is shown at 9.33 in the video, it doesn't have a Jedec part number, but does have some specs that should help find an regular part.

https://youtu.be/MshAFrOgW1A?feature=shared&t=573

David

[factory]

Note: the various circuit diagrams cover two different versions, the older one covers the unit with 3 Ge transistors (Q3 & both in the PSU), the later ones are all Si (there are two of these with differing voltage readings for Q9).

Seems the OPs newer unit doesn't follow any of the available manuals for the PSU section, it has two differing values for R5/R6 in the PSU, but the manuals show them both as the same value, which may or may not be wrong.  As they changed from Ge to Si transistors, Zener is slightly different voltage too.

David

[Tony_G]

I don't think I ever took a photo of my 461A but I grabbed this from a video I did on the repair of it:



Assuming that it wasn't touched then the original only had a single heatsink as well.

The manual only shows 1 heatsink as well:



I never did get around to fixing the missing slug in the first inductor from memory...

TonyG

[factory]

Yours seems a little newer than mine, but both have Ge transistors for the PSU, interesting that they fitted one capacitor instead of the five that most have.

The custom parts had their required spec detailed in HP lists, one or more vendors would provide devices that met the specs, employees had access to these lists on Microfiche, which seem to have mostly been lost to time, but someone sent CuriousMarc some, none of these are available to view/download anywhere that I know of.
Transistor 1854-0019 is shown at 9.33 in the video, it doesn't have a Jedec part number, but does have some specs that should help find an regular part.

https://youtu.be/MshAFrOgW1A?feature=shared&t=573

David

It seems some of the HP component lists on Fiche, from the 1980s are now available to download, thanks to CuriousMarc for adding them here; https://archive.org/details/@curiousmarc

David

[Tony_G]

I can't remember if I ever saw a schematic for the new power supply but I did end up getting the unit just in cal at +/- 0.5 dB for 20 & 40.

Look forward to hearing that you have yours working well too.

TonyG

[Peter_O]

I will continue with this project. 

At the moment time is short. My mum is relocating from hospital to a nursing home, and there is a lot to deal with. 

[Peter_O]

I just returned to the bench.

From a lab PSU the amp draws 100mA per rail: +15V and - 15V.

The ripple of the orginal PSU is 8mV instead of <1mV acc. to spec.

Now I'm not sure:
a) is the amp drawing to much current and cause the psu ripple out of spec?
b) are the filter caps out of spec?



b: I checked all of the filter caps C1-C5:

	spec µF	real µF	ESR Ohm
C1-C5	65	81	0.8
		78	0.8
		70	1.0
		79	0.7
		80	1.1

and also C6 and C7:

	spec µF	real µF	ESR Ohm
C6	20	24	3.2 (problem?)
C7	20	22	1.9

ALL resistors are OK.

With the output disconnected, the PSU delivers 30V OK, the tranformer stays cold, but R2 and R3 get very hot at about 110°C in about a minute.

Voltage over R2 and R3 together are 60V (!) which would be 0.8W per resistor.

Unfortunately I shorted the -15V rail to ground with the DVM probe while measuring the voltage over R2. 
So for the moment the damage has gotten worse and do I qualify as a rooky obviously (see first post).

That fault aside, the central question reg. the psu seems to be: Is 2x44V RMS at the transformer outputs OK?
I had checked the 110/230 switching and that was OK, delivering 230V to the outer contacts of the primary side.

Maybe someone of you owning a 461 or 462 can check the transformer output voltages?

If the 2x44V of my one are wrong, I would give futher tries to get the puppy running a miss for the moment. Otherwise I would need to fix the damage of the short first. 

[coppercone2]

I am angry at the thought of opening that thing up again after having to remake the springy thing for the attenuator, but I think you should hold off working on it until you buy some multimeter probe sleeves or insulated multimeter probe tip attachment. I destroyed (and repaired) a 30A 30V psu that way. Fluke has some stuff but you can probobly do better. Before you make the mistake again and destroy the transformer or something like that.

I keep a miniature plastic organizer box of all the DMM slip on tips and stuff on my bench because slipping that little plastic on the probe (conveniently) can easily save you 30 hours work or a throw away.... also recommend a pressurized can of spray alcohol and cotton wipes in the DMM probe attachment kit (cut into little squares) so you keep the probes clean because alot of the time the problem that causes slipping is excess pressure to compensate for dirty probes. Fiber scratch pen and q-tips can be helpful to for cleaning probe points on the old meter before you work so you can just glance it and get a good measurement. and I have been meaning to make some partially cut probe sleeves so I can expose one side of the probe if I need to do the measurement along the shaft

there is gold probes too but that is expensive i cant suggest buying that for most people lol but the probe tip attachments and sleeves are a requirement for safe work. But you need to set it up so its conveniant and you chose the optimal probe configuration for every measurement you do without it being a hassle

gooseneck lamp or head lamp too, way more useful then blowing budget on lab lights


I always end up breaking stuff if I do electronics while there is something stressful going on, and I also think there is something infuriating about the design of that amp that effects my judgement lol

Maybe I will open it later but hopefully someone else that has less history with the unit can do it.

[Peter_O]

Yes. 
The problem is standing in front of the drawer with probes and accessories like DMM probe sleves.    

Maybe it's nesserary to get a kick in the ass from time to time.

First I will check the transformer again.

[David Hess]

I keep a miniature plastic organizer box of all the DMM slip on tips and stuff on my bench because slipping that little plastic on the probe (conveniently) can easily save you 30 hours work or a throw away.... also recommend a pressurized can of spray alcohol and cotton wipes in the DMM probe attachment kit (cut into little squares) so you keep the probes clean because alot of the time the problem that causes slipping is excess pressure to compensate for dirty probes. Fiber scratch pen and q-tips can be helpful to for cleaning probe points on the old meter before you work so you can just glance it and get a good measurement. and I have been meaning to make some partially cut probe sleeves so I can expose one side of the probe if I need to do the measurement along the shaft

I have started keeping my multimeters and other portable gear in hard cases, where I can also keep the probes and accessories, as shown below.  For my oscilloscopes some of my oscilloscope carts have drawers, but for other accessories, I have settled on divided plastic boxes.

there is gold probes too but that is expensive i cant suggest buying that for most people lol but the probe tip attachments and sleeves are a requirement for safe work. But you need to set it up so its conveniant and you chose the optimal probe configuration for every measurement you do without it being a hassle

I am very happy with the test lead kit I bought for my old Beckman Circuitmate multimeter from Probemaster for about $50 total.  They were able to supply it with non-safety right angle plugs to fit my multimeter.

https://probemaster.com/8000-series-kits/

[Peter_O]

Yup. I have drawers of test leads etc. The trick is to use them.   

Back to the topic:

I fixed the PSU so far.
The transformer is still alive, showing the same resistances as before. 
In the end only the zehner diode whas shot. 
With one 0.5W chinesium one from my stock the PSU is running again; but the zehner is getting hot very quickly because it's only 0.5W instead of 1W.

Next step is to purchase proper zehners: Acc. to lists it's a HP 1902-0202 or ZS11213-191; 15V, 1W.

On the way I found a fault that came with the device:
Output-PIN 6 and 7 (one secondary coil) of the transformer had been connected the wrong way.
That's interesting because the wire connection at the transformator looked original 70's style and the wires looked original. Is it possible, that it had been wired wrongly at the factory?

Here the faulty wiring:



Anyhow. Now it matches basic logic, the schematics and the manual too, which states "red" at PIN 6.

When the proper zehner is in, we'll see, if the transformer stays cold.

[coppercone2]

picture 2 almost looks identical to my probe boxes lol

you are missing BNC to screw terminal for $1.27. so it fits a twisted pair more perfectly then a binding post


actually never mind, its not similar, because I made a solid oak box with cut outs for most of the coaxial connectors. Banana stuff is in a box like that still. I poured silicone rubber compound (2 part) into drill holes to make safe platforms to hold connectors. I thought they are too valuable to stay in the plastic box. I think I need to make a third box now  . But I keep the unterminated connectors in a box for cable making, not adaptors

[factory]

Yup. I have drawers of test leads etc. The trick is to use them.   

Back to the topic:

I fixed the PSU so far.
The transformer is still alive, showing the same resistances as before. 
In the end only the zehner diode whas shot. 
With one 0.5W chinesium one from my stock the PSU is running again; but the zehner is getting hot very quickly because it's only 0.5W instead of 1W.

Next step is to purchase proper zehners: Acc. to lists it's a HP 1902-0202 or ZS11213-191; 15V, 1W.

On the way I found a fault that came with the device:
Output-PIN 6 and 7 (one secondary coil) of the transformer had been connected the wrong way.
That's interesting because the wire connection at the transformator looked original 70's style and the wires looked original. Is it possible, that it had been wired wrongly at the factory?

Here the faulty wiring:



Anyhow. Now it matches basic logic, the schematics and the manual too, which states "red" at PIN 6.

When the proper zehner is in, we'll see, if the transformer stays cold.

I've still got mine here (462A) if you need any readings, it seems obvious now but must have been the last thing you suspected, mine has the centre tap going to the red wire and the two outer ones (the same colors) going to the rectifiers.

David

[Peter_O]

Thx for providing the pic, David!

Interestingly your's are different pcbs:
The PSU board showing the vintage HP-logo and different connections,  and the amp board with rounded tracks.
The wire colors are the same.


The zehner diode is announce to arrive today.

[factory]

The PSU should be the same design, the older one in my 462A is the "OR" or original version, the newer board in your 461A is revision A. Interesting the older PSU seems to have all the traces on the back, the newer one a mix with them on both sides.

The amp boards have different part numbers, one being 00461-66502 (Rev C) & the other 00462-66502 (Rev A) for obvious reasons. The later boards all got redesigned at some point, to use straight traces & sharp corners, most products from the 1960s that were made for many years, had these changes.

There are more the pictures of my 462A attached to reply #22, if you want to see the differences.

David