HP Barney Oliver Amplifier - Output Transistor Selection - Page 1
Forum section: electronics / projects

author: HotGlass
author_rank: Contributor
timestamp: November 02, 2021, 12:59:36 am
content: Hello all,First post here, but have been watching the channel for ages. Mostly I work with tube stuff, either repairing, restoring, or designing and building amplifiers. I'm not a complete idiot with solid state amplifiers, but they're certainly not my high point. When I used to repair amps professionally, I always found the solid state amps took too much time and frustration to be profitable as a repairman.Anyway, I've purchased and am in the process of restoring an HP Barney Oliver amplifier. For those not aware, Hewlett Packard's head of the Research Division in 1972 was Barney Oliver, and he designed a very high quality amplifier which was made in one or two runs, with a total of about 500 units produced. It's a gorgeous piece of equipment. (PICS at bottom)So I've mostly repaired it at this point. A complete replacement of all electrolytics except the two large main cans (which test fine but will probably get replaced in the end) was required. There were at least 4 leaky caps and several with climbing ESR. The result was a blown 100R carbon comp resistor and Q1 were toasted. That's all fixed now and the voltage rails look healthy again.This brings me to the subject of this post - the output transistors. This is where my lack of familiarity with this sort of transistor is not helping things. If only these were pentodes... Alas... So the amp is a class B amp with tons of feedback, and the output section uses complimentary pairs of BJT transistors - MJ802 and MJ4052. Two of the original transistors were already replaced when I got the amp, and two had the original 1972 date codes on them. In the process of troubleshooting, I decided to pull these transistors out to both save them from a malfunctioning driver section as well as to test them out of circuit. The hFE of these transistors (at least as measured by my little cheapo DROK tester) showed that 4052 were wildly out of spec. Datasheet says a hFE of 25-100 and these tested 186 and 290! The 802 meanwhile tested within spec (but not terribly close to each other).So, I decided to order 3 new pairs of 802/4052 from NTE direct and they arrived Saturday. Much to my disappointment, the situation almost seems the same, but even more strange. NTE sent me three NTE branded MJ802 that all tested within spec for hFE at 72,72,50. Meanwhile the MJ4052 tested out at 67,131,168. Two of these showing out of spec ( hFE > 100 ). And the 4052 were branded MOSPEC not NTE. Also, the pins on the MOSPEC transistors were over diameter and tinned with really gloopy solder. They looked like they could damage the transistor socket in the amp.Can anyone else help me make sense of what to do here? I realize that these aren't like tube circuits in the way you need to match output devices, especially when like 100db of feedback is used to deal with things like crossover distortion. Am I making something out of nothing? This is a very rare amp and a piece of history. I have the budget and the patience to restore it properly, but I don't want to get hung up on something I don't fully grasp. My impression was that older BJTs like this were often with pretty all-over-the-place beta, even within the same batch. But I'm surprised that a modern batch of 4052 from MOSPEC would be so random and that NTE would send me those along with a perfectly middle of the road set of 802.1 - Should I consider a transistor that has a higher hFE than the datasheet to be defective or unacceptable? Is my DROK tester even valid for this?2 - Am I being unreasonable in expecting NTE not to send me two different brands of transistor for complimentary pair use?3 - Is there a replacement part number that I might have better luck with or would be a 100% compatible upgrade? I see NTE also makes the 180/181, at a higher price despite identical specs...Any other sage advice for this unusual restoration?Thanks much!And now as promised, the eye candy...

author: bob91343
author_rank: Super Contributor
timestamp: November 02, 2021, 04:28:20 am
content: First, are you measuring current gain at the current on the spec sheet? Current gain is a strong function of collector current.As for complementary pairs, I doubt that the brand makes a whole lot of difference. In those days they didn't have the great selection we now have, and they took what was available. With that in mind, you are probably safe in making substitutions. You could even use MOSFETs if you bias them properly.My many years of repairing amplifiers tells me that you don't have to be too fussy. I almost always used the same part number for replacements, but when not available (and with customer permission) I would select transistors with proper voltage and current and power ratings, as well as gain and frequency. The bias network always had to be addressed.

author: HotGlass
author_rank: Contributor
timestamp: November 02, 2021, 09:25:56 pm
content: Thanks for the reply. To your first question, no I am using the hFE values from a transistor tester, and one of the questions I posed was if that was a valid way to test or not. Can I gather from your reply that it is not?

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 02, 2021, 09:39:45 pm
content: Those basic little Atmega based component testers punch far above their weight and are a super handy thing to have, but you must keep in mind that they will evaluate everything as a low voltage small signal part since that's all they're capable of.I'm just a hobbyist screwing around, but as I understand it is unwise to design a transistor circuit that's sensitive to beta to begin with (since it varies wildly even within a given gain group), higher hfe won't be a problem because the circuit will be designed to have a gain factor well below theminimumhfe of the specified transistor.Definitely wick off any goopy tinning before inserting them into a socket.

author: edavid
author_rank: Super Contributor
timestamp: November 02, 2021, 09:48:35 pm
content: Quote from: HotGlass on November 02, 2021, 12:59:36 amYes! You should put the original transistors back in, since they are apparently fine.QuoteNo and no.QuoteYes - NTE does not make any transistors, so you never know what brand you are really getting. If that is not OK, you should buy OEM parts. MJ802 and MJ4502 are still in production, so there is no reason to buy NTE.

author: HotGlass
author_rank: Contributor
timestamp: November 02, 2021, 10:20:47 pm
content: Yeah I had a feeling that my tester might not be so well up to the task of measuring devices like this. I could always make a rig with a power supply and a current bridge to figure it out. But thanks for the reminder about the circuit's overall gain, that's a point I knew at one point but had forgotten. I really don't work on solid-state stuff that often, and when I do it's always a struggle to remember stuff I learned in high school almost 30 years ago.If mistmatched betas are not such a big deal, I might just go back to the transistors that were in there when I got the amp rather than mix two different brands of new transistor. I think I need to breadboard the output stage and mess around with it a bit until I understand its operation a little better.

author: HotGlass
author_rank: Contributor
timestamp: November 02, 2021, 10:24:48 pm
content: Thank you for the concise answers. In tube world, a mismatch like this would really be a problem, but I had a feeling I might be making too big a deal of it. I will give the amp a try with the old transistors tonight and see how it goes. If not, I'll order some new ones from somebody other than NTE. I realize that there's no real reason to dislike different manufacturers, but I do and my OCD will not let it stand. Not for an amp like this anyway, made by the gods at HP.

author: bob91343
author_rank: Super Contributor
timestamp: November 03, 2021, 12:20:09 am
content: It's very simple to cobble up a tester for these transistors. One power supply and a milliammeter and a few resistors.I know of a circuit where replacement transistors must be low beta or oscillation threatens. This could also be true of your amplifier; I don't know. Gain is not a golden gift that must be maximized. So, if you are serious about replacing those, choose some that come as close as possible to the original specifications.Early units used alloy transistors; later ones went to epitaxial, and so on. Each type has its own peculiarities and when you substitute, you need to be aware of the pitfalls.As mentioned above, if they are good, don't replace them.

author: CaptDon
author_rank: Super Contributor
timestamp: November 03, 2021, 01:09:33 pm
content: If that was submitted to me for design review I would have insisted the volume control be moved away from the power supply section and placed over on the left side. As designed it seems more like a Barney Miller amplifier. What kind of output power does that thing do, or is it only a line out device? It looks like at least a 250VA capable transformer and huge energy storage filter caps. If it has heavy feedback for flat response it probably sucks at transient response? You would probably love the Krohn-Hite amplifier in my lab. 100wrms output D.C. to 1mhz. Yes, with D.C. response and the offset control you can use it as a power supply!!

author: edavid
author_rank: Super Contributor
timestamp: November 03, 2021, 03:02:20 pm
content: Quote from: CaptDon on November 03, 2021, 01:09:33 pm50WPC into 8 ohms. You can be sure it meets its spec.QuoteThat doesn't make sense to me. It was carefully bandlimited, so it should have excellent time domain response.

author: CaptDon
author_rank: Super Contributor
timestamp: November 03, 2021, 03:53:31 pm
content: I guess it depends on the overall phase delay of the amplifier in-to-out and how the feedback is derived. In some of our ultrasonic domain amplifiers (up to around 250khz) I used a bit of resistance in the emitter connection and sometimes shunted the resistor with 10pf to 47pf to keep sharp edges sharp but not enough to overshoot. You do suffer some gain loss with the emitter resistor but it sort of provides negative feedback compensation on a stage by stage basis without the phase delay associated with the 'reach-around' output to input feedback. Sometimes I also had to resort to Class-A push/pull low impedance output stages which waste power but make the amplifier flat and stable across decades of output impedance mis-matches. You do own a little known piece of audiophile gear which someone would pay dearly for even if they never turned it on!! It seems most solid state amplifiers make the music sound 'sterile' or better yet 'exactly how it sounded when we recorded and produced it'. I do still prefer the broader sound of a class AB push/pull tube amp with its added even harmonic distortion since it adds musically pleasing overtones to everything being reproduced. I had a tube type amp switchable to the monitor speakers just to compare A-B what the product sounded like with solid state amplification vs. tube and to a point the tube amp driving the monitors always got that "Wow, does that sound sweet" response from musicians

author: HotGlass
author_rank: Contributor
timestamp: November 03, 2021, 09:22:45 pm
content: Progress report -Per advice from folks here, I re-installed the old transistors last night. After some light testing, all seems well. The transistors clearly work. How well they work, I don't know yet. I haven't gotten set up to take meaningful measurements just yet... But I will be checking freq. response and THD, etc. soon enough. Sound-wise, it seems OK but I haven't hooked it up to high quality speakers yet.During my testing an interesting problem showed up. 120Hz hum kept intermittently creeping in and then going away. After scratching my head for a bit, I started prodding around the power supply board near the main filter caps, and was able to make the hum come and go just by poking at the board. Clearly a bad connection... So I recalled something I saw before I screwed the board down after recapping it, and forgot about. But now it seems to be an issue. There appears to be some metallurgical problem between the gold-plated PCB and the studs mounting it to the chassis. They seem to use the studs as a ground point. Interestingly, I do not see the same problem at the interface between the power caps and the board (aluminum to gold).So this means I need to come up with a way to replace these failed ground connections, hopefully in a way that doesn't alter the appearance too much. I'd wager that this problem is present on every single one of these amplifiers. My instinct is to get a length of wire and use it to ground the PCB to the chassis using a soldered connection. If anyone with metallurgical experience has any ideas on how to repair the old mechanical ground connections, I'm all ears. I don't think tinning it is going to help much...Thanks again for all the great advice!

author: HotGlass
author_rank: Contributor
timestamp: November 03, 2021, 09:51:28 pm
content: Sorry for double reply, still getting the hang of this board. Here's a better pic of the whole underside of the power supply PCB. I've recapped this board completely, and had to replace Q1 (+50V rail). The metallurgical issue appears to happen at all of the interfaces between the standoffs and the gold plated PCB traces. Of the 4 mounting points, all of which show this corrosion or activity, only two are used as grounds, the center top and center bottom in the image. The ones at the corners do not connect to anything. The more I look at this issue, the more I suspect that it probably plagues this amplifier design in general and is in need of a solution that can be shared with the other 500 or so owners of these. Honestly, I don't really see any way around adding a ground wire.

author: CaptDon
author_rank: Super Contributor
timestamp: November 04, 2021, 12:13:19 am
content: I had so many of those Sprague small electrolytics die in Broadcast Electronics and International Tapetronics 'triple stacks'. Have you replaced them? They are an accident waiting to happen. Mine were something like 10mfd at 25v and they went way down to about .1mfd. We finally just shotgunned all of those because they were dying like flies.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 04, 2021, 12:24:58 am
content: Interesting dissimilar metal corrosion, never occurred to me that was even possible with gold, perhaps patch it up with toothed washers?

author: Benta
author_rank: Super Contributor
timestamp: November 04, 2021, 12:26:08 am
content: Some comments:The Al standoffs are not a great choice as you've sen yourself. My suggestion would be brass, which is pretty neutral towards other metals.Your focus on transistor hFEand hfeis misplaced. But I understand where you're coming from.Earlier in my life I also worked on tube amps, and the gain of each tube was pretty precisely specified and open-loop circuits were normal.With solid-state amps, theonlyparameter is, that there is enough gain, feedback will take care of the rest.

author: Benta
author_rank: Super Contributor
timestamp: November 04, 2021, 12:28:23 am
content: Quote from: BrokenYugo on November 04, 2021, 12:24:58 amI don't think gold plays a role here, that's just plating. But Al-Cu can create some interesting problems.

author: HotGlass
author_rank: Contributor
timestamp: November 04, 2021, 12:50:35 am
content: Quote from: CaptDon on November 04, 2021, 12:13:19 amThose Sprague caps are all new. Other than the two large cans, I replaced all of the electrolytics with original Sprague 30D series caps. 4 were quite leaky when tested at full voltage, one was practically a dead short and was ostensibly the reason Q1 gave up the ghost along with R27 on the R channel driver board.Quote from: BrokenYugo on November 04, 2021, 12:24:58 amPossible, but I think it just postpones the problem to another day. They are already secured with steel screws with toothed washers from above, the holes are through-plated and gold plated on both sides. The corrosion doesn't seem to be happening on the top of the board with the screws, but it also doesn't seem to be making a good electrical connection via that washer and the steel screw. EDIT - It occurs to me that the corrosion may very well have severed continuity between the trace and the top side of that via where the screw fastens.Quote from: Benta on November 04, 2021, 12:26:08 amBrass is an interesting thought, but unfortunately the original standoffs are press fit and swaged into the chassis. I'd have to drill out the originals, and then I'd be left with holes too large for traditional brass standoffs. The holes are already about the diameter of the standoff itself. And thanks for the comments regarding beta. While I've repaired a lot of SS gear in years past, I've never really done any design work. I'm too obsessed with tube circuits so I devote all my time to that. Maybe one day I'll post some pics of my 28 tube monster amp. Why 28 tubes? Because it uses 9 rectifiers, of course! I mean, it only dissipates 700W at idle... I have some extra parts left over from this amp repair, so maybe I'll do a mockup of this power amp section and play around with it and try to understand its operation a little better. Barney Oliver wrote papers about the thing, but boy the math sure is dense in places.Thanks for all the comments and suggestions, folks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 04, 2021, 02:27:32 am
content: Tin plated or aluminum spacers are probably not a good idea in contact with gold. They diffuse together, forming brittle intermetallic compounds (in the case of aluminum, "purple gold" corrosion). Aluminum probably less so, on account of its surface oxide that acts as a barrier for the most part, but all it takes is a little wiping motion to push that aside and get some metal-on-metal contact.Compatible metals like steel (maybe not galvanized though?), stainless or brass are probably a good idea. Just scrape off whatever crud is present, patch it up as best you can if it's peeling or whatever, and bolt back in place.If you're really desperate, gold can be dissolved off the surface by soaking in solder; this is standard NASA procedure for plated components, for example (to avoid tin-gold intermetallics in solder joints). I doubt this is necessary, just that it's a possible option.Quote from: HotGlass on November 04, 2021, 12:50:35 amEnough height for washers in there?Tim

author: HotGlass
author_rank: Contributor
timestamp: November 04, 2021, 04:09:46 am
content: Hello again,So, I've spent some time analyzing the ground paths in this power supply and the chassis overall, and my conclusion is that I need to modify rather than repair this grounding scheme. Here's why:First, some observations..It seems that a star to chassis grounding scheme was attempted here. There are multiple places where grounds terminate to the chassis at the card sockets, and lots of signal grounds terminate there as well. In the power supply board, they attempted to follow this scheme and the main +/- 40V supply has a ground path via the standoff to the chassis. This standoff turns out to also be loose to the chassis, no doubt exacerbating the problem. But in reality, this isn't grounding to the chassis, the reverse is true. The center tap of the power transformer does not connect to the chassis, it connects directly to the PCB and is connected with a via to the ground trace bridging the main power supply caps on the left. So this means that in reality, the lowest impedance part of the grounding scheme is actually right here on the PCB where the center tap is connected and NOT the chassis.This brings me to my second observation; the second part of the power supply which contains all the transistors and is responsible for the 10v and 20v rails is solely grounded via the other standoff. This means that the ground path for this side of the circuit is asinine - current must leave the board via the standoff, travel via the chassis to the other standoff on the other side of the board, then re-enter the board to find its way to the center tap of the transformer. Unless I am misunderstanding something more clever than I can conceive of, this is a design flaw. No doubt HP knew what they were doing, but they're not immune to production design issues, and something with only 500 units made and was sold only to employees and with no warranty, well... You need some iteration to deal with things like this...So at the end of the day, I think fixing the mechanical grounding is a fool's errand. My time would be better spent making a permanent wired connection starring the chassis and the other half of the PCB back to the center tap. And to my tube amp world eyes, it really does seem strange that the center tap is tied to the power supply PCB and have to go through a rather tortuous path to get to the chassis. I never thought I'd be able to improve on this amp with my skill level, but I bet I can knock the hum floor waaaay down with a simple modification.Opinions?

author: HotGlass
author_rank: Contributor
timestamp: November 04, 2021, 07:54:18 pm
content: Hello again all.First off an apology for not posting this thread in the repair forum. Noob mistake.Anyway, last night I re-did the grounding on the amp. I pulled the center tap of the transformer off the PCB and soldered it along with two other wires into an eyelet. That eyelet is bolted to the chassis via one of the transformer bolts. One wire from the eyelet goes to the PCB where the center tap was previously (and therefore also to the main caps) and the second wire goes to the side of the board that grounds the 10 and 20v rails. I used mil-spec PTFE coated wire with silver plated copper conductors, so the connection will be very low resistance - certainly better than aluminum standoffs. All of these modifications are hidden under the power supply PCB so it's visually original.Testing seems good. I still hear a small amount of hum and some diode buzz in the speakers, but I have to get very close to them to hear it. Just using my ears, I'd say it's 3-6db lower than before (when the ground was actually working and not failing) and at least 12-18db lower than when the ground was failing. Poking at the board no longer changes anything. I'm not sure what's 'normal' for this amp in terms of hum, and I still haven't replaced the main filter caps even though they test fine. I suspect that perhaps I could get the hum a little lower with some new ones, and if I put modern diodes in there, I could probably eliminate a lot of the switching noise. Honestly, I'm likely spoiled by how dead-quiet a properly working tube amp is.

author: bob91343
author_rank: Super Contributor
timestamp: November 04, 2021, 11:54:33 pm
content: Since there still is hum, where does it originate? If you short the input, for instance, does it change?

author: HotGlass
author_rank: Contributor
timestamp: November 05, 2021, 12:19:37 am
content: Quote from: bob91343 on November 04, 2021, 11:54:33 pmI think I've got the hum licked now. The original grounding was really dependent on those standoffs as it turns out. After my grounding modification, hum came down, but not enough. After some poking around on the power supply PCB, I noticed that despite my previous comment to the contrary, the mechanical ground at the standoff where the filter caps meet the chassis was still an issue. This was really surprising to me, and suggests that perhaps the through-hole via that would otherwise ground this part of the circuit via the thick blue wire I added is not as conductive as I'd hope.With my FFT running on the speaker output, parallel with the speaker, here's the noise floor with the amp turned OFF.With my grounding modification and the screws on the standoff somewhat loose, this is the noise floor. 60Hz harmonics all the way up...When I tighten down the screws, things improve but not enough:So finally I decided to make another ground modification. After experimenting with a jumper wire, I decided to solder an eyelet directly to the PCB where the screw would normally ground that section via the standoff. I put another eyelet on the other end and bolted it down to the chassis at the power transformer along with the three wires I did in my first modification. Here's the mod:And here's the result:This is a HUGE improvement. The trashy diode noise is now completely inaudible even with my ear against the tweeter, and the hum is barely detectable with my ear against the woofer. Is there room for improvement? Perhaps... But at the moment I find this to be quite satisfactory, particularly because the HF trash seems to have vanished.WTH Am I doing wrong with my in-line attachments? Aaargh! Ok, I truly cannot figure out how to make that work after many edits... Anyway, long story short, adding another ground wire on top of the two I added gave me a 10db decrease in all of the 60hz harmonics pretty much across the board.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2021, 12:59:57 am
content: Inline attachments are disabled, because...reasons; use  tags to embed the link instead.Tim

author: HotGlass
author_rank: Contributor
timestamp: November 05, 2021, 07:02:17 am
content: Quote from: T3sl4co1l on November 05, 2021, 12:59:57 amThanks Tim, I'll give it a go in this post...So the plot thickens. Of course I couldn't leave well enough alone. I still think there is room for improvement on the hum floor, probably mostly because Barney Oliver was every bit as OCD about things like this as I am, but he was far more skilled. So ultimately, I am going to take it on faith that he would not have stopped here. And I think I have good cause to suspect that the hum can be even lower, because I think I've found another fault. It's late and I haven't tried to wrap my head around what it all means, but it seems that a 60Hz component is making its way into the +50V rail that is simply not present int the -50V rail.Here's the -50V rail. Note that the 120Hz component dominates, followed by even order harmonics of it. 60Hz is present, but is close to 30db below the 120Hz component. Yellow trace is the AC coupled signal from the rail, with the FFT in the background. Cursor is centered at 120Hz.Now here's the +50V rail. This rail had Q1 replaced because it was blown. An overcurrent happened to this rail, which I *thought* I had repaired the aftermath of. Perhaps not... Note that the 60Hz component is now greater than the 120Hz component, and if I'm interpreting this correctly, the 3rd order harmonics are now way more present as well.It's tool late in the evening for me to take a stab at solving this issue, but I suspect perhaps I've missed a shorted diode. Since the 50V rails are just 10V rails floated on top of the 40V rails, it's possible for weird things to happen and result in that bizarre waveform. I guess I will have to lift up the power supply PCB yet again... Anyway, my working theory at the moment is that this additional 60Hz component and its associated harmonics being present on the +50 and not the -50 rail is flummoxing the common mode rejection or cancelation or whatever you want to call it, that this balanced design relies on to be quiet.

author: HotGlass
author_rank: Contributor
timestamp: November 05, 2021, 11:44:46 pm
content: So I got around to working on the amp again today, and sure enough, bad diode. This was a strange way for a diode to fail. It tested fine on the circuit board with a Keithley 2015 meter in diode test mode; .6v drop in one direction, dead open in the other. It wasn't until I removed it from the board that I discovered a crack in it. Testing it out of circuit showed that it either worked or didn't, depending on the pressure applied to the lead. So ostensibly the crack in the diode was causing or allowing the lead to pull off the diode junction. I replaced the two rectifier diodes in the +50V rail and the two in the -50V rail with some fresh 1N4002, and now the ripple looks identical (albeit inverted) in both supplies. It became obvious this morning when I looked at those waveforms with fresh eyes and noticed that basically I was looking at half-wave rectification on one side, plus some weirdness because the supply is floated off the 40V rail. Once I saw that the period had doubled, the answer was obvious. The diode just wouldn't test bad on the board, but it HAD to be...This didn't seem to make any difference with the amount of audible hum, which as I said before is pretty satisfactory at the moment, I need to be within 6" or so of the speaker to hear it. However, playing around with paralleling *the existing grounds I already added* is showing yet further reduction in noise floor, which is puzzling as hell. It seems this part of the power supply wants as low a resistance to ground as it can get. I may poke at that further or just leave well enough alone.So that brings the total failed parts list on this amp to 5 capacitors, one diode, one resistor, one transistor. The failed caps likely caused all the rest of the failures.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2021, 02:12:31 am
content: Quote from: HotGlass on November 02, 2021, 12:59:36 amThe MJ802 and MJ4052 were the standard complimentary pair for servo and audio amplifiers for a long time in higher power applications than the 2N3055 would support. I see them in a lot of old designs and have a parts drawer full of pulls. Although last time I checked they were still available new for a reasonable price, a good more modern replacement is the MJ15003/MJ15004 which is only 20 amps instead of 30 amps, but 250 watts instead of 200 watts with a higher secondary breakdown rating.

author: edavid
author_rank: Super Contributor
timestamp: November 06, 2021, 03:32:17 pm
content: Just so no one is confused, the correct PNP part number is MJ4502.

author: AndrewBCN
author_rank: Frequent Contributor
timestamp: November 06, 2021, 04:37:31 pm
content: Just wanted to say thanks for the "eye candy". It is as over-engineered as one could expect from Hewlett Packard's head of Research.

author: coppercone2
author_rank: Super Contributor
timestamp: November 06, 2021, 04:46:05 pm
content: Well, when they do it that way, it works. If its done like a transformer-on-pcb cheaper HP style, you can get solder joints cracking near the heavy transformer. Its just not stiff enough. Thats a lovely problem too, trouble shoot a circuit near a giant transformer thats soldered directly to the PCB, complete with soldered bolts.Its not overengineered, it has not been cannibalized.You can say someone bothered to think about mechanical design and long term durability.. also guess which one is gonna fix easier with crappy tools, the better one, where they don't assemble-in-place a complex system ridden with heavy solder joints, easy to burn insulation, etc. When you say thats over engineered its like 'boss, I can be cheap!!!'BTW in test labs now, there are hold ups in inquiring certain test gear. Long delays. Guess what happens when production equipment breaks now? (i.e. cheap relay inside very expensive shit). One of those HP+++ boxes going haywire is like a department crisis. When the stakes are high enough that they would think nothing of sending thousands of dollars for a overnight shipped replacement while its being repaired.. you can develop a business contingency plan based on risk assessment (how much spare you have), the quality is supposed to make that risk go down.