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Old 26th May 2023, 11:25 am   #1
knobtwiddler
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Default Hitachi Amp Fusible Resistors (and general servicing)

Hi,

I have a Hitachi HMA-7500 that hasn't supposedly been turned on in many years. The PCBs are caked in thick dust, and my plan is to remove them from the chassis and fully clean before doing anything. I will also replace the smaller electrolytics, not because I am a dogmatic re-capper, but because I have a Hakko FR300 and the spare caps, so they can be changed in seconds (you can change small cap in under a minute on a single-layer PCB with the Hakko). I plan to sacrifice a goat to the capacitor gods in the hope that the sizeable filter caps are good... (will also reform them on a variac).

You can see the schematic here: http://bilder.hifi-forum.de/max/5886...ked_983566.png

The observant amongst you will note that the amp is based around the Hitachi Lateral MOSFET apps note - which can be seen here: http://jm.plantefeve.pagesperso-oran...k_condense.pdf

The actual circuit shown above was used by Maplin for their kit, which was highly successful and I have several of them that I built. Having the luxury of test loads and an analyser, I recently discovered that with plenty of bias (too embarassed to say how much...), one of my Maplins can drive 10W into 5R with the 20-20K THD barely exceeding -80dBV. When Baxandall claimed in the late '70s that decent SS amps had gotten to the point where you couldn't discern them between one another, he was right!

I have read many reports claiming that the 7500's fusible Rs need to be changed, and can suffer fatigue over time. Some techs seem to regard them like tants, replacing them on sight. As the schem shows, there are 3 per channel in the 7500 - around the current mirror / VAS.

I wonder why Hitachi fitted fusible Rs? They must have had faith in the O/P protection, as were one to blow it would create an almighty offset... And if the O/P protection is that snappy, why have fusible Rs in the 1st place?

Numerous OEMs in the 80s used the Hitachi topology, barely modifying it. I am thinking of H+H (the V800 - some of which some seem to omit the Zobel!!) and C-Audio (I have one of these monsters on the to-do list - I'd better fix it while I can still lift it...). There must be tens of others who utilised the design I am sure. It's appeared in numerous cookbooks. But only Hitachi used fusible Rs... What did they know that others didn't? Would you be tempted just to change them for MFs?

TIA!
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Old 26th May 2023, 3:58 pm   #2
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Be careful I have just done my HMA7500 a load of fusible resistors just failed The amp is loosely based on the Maplin circuit but the driver stage voltages are obtained in an odd way +v from one tx and neg from the other TX still a great amp but you will need to replace all the fusible resistors
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Old 26th May 2023, 4:34 pm   #3
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Thanks, Trev.

I believe the separate linear-regulated VAS PSU / unregulated OPS PSU (seen on other Hitachi amps) are intended to keep the quiescent current in the current mirror stable. In theory, if you run the whole caboodle from one rail, if the OPS are stressed by a loud transient, rail voltage sag can occur, thus changing bias in the VAS. There is a mod that I've fitted to my Maplin amps that puts a CCS in place of the R that feeds the collector of one side of the CM, which is designed to attain the same benefit. Entry no. 9917 on my list of things to do is to put this theory to the test, by loading an amp with / without the CCS and seeing how THD is affected. I have a feeling that, were the rails to be so loaded that voltage sag affects VAS bias, then your SQ is likely to be in trouble anyway!

Hitachi also repeated the separate rails for VAS and OPS on their Japan-only HMA-9500.

As per my 1st post, would it really matter if I just changed the fusible Rs for normal Rs? I am thinking that their purpose is to protect the valuable OPS' gates from gross over-voltage. I think Hitachi's designers knew what they were doing. I don't suppose you have a part no. for the Fusible Rs that you used? RS only have 18 of things in their entire catalogue!
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Old 26th May 2023, 6:50 pm   #4
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

I did not use RS but went with UK suppliers on e bay the resistors supplied all measured well and the amp was repaired without any problems
I tend to agree re separate power supplies for the vas and outputs etc
Any way good luck
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Old 27th May 2023, 1:12 am   #5
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Thanks, Trev.

I think I might be a bad boy and leave the fusible Rs out. There are tens of amps with a virtually identical topology, bar a few value changes and a tweak here and there. None / very few of them have used fusible Rs in the same spots from what I can remember.
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Old 27th May 2023, 9:24 am   #6
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

I tend to agree with you about 40 odd years ago I met with an early Sony amp worked on it for hours in the end the fault was faulty fusible resistors that's all just the resistors
Seems to be a fault waiting to happen
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Old 27th May 2023, 12:12 pm   #7
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Hi,

I have changed fusible resistors in Hitachi HA-250 and HA-5300 amplifiers. I posted the HA-250 work here:

https://www.vintage-radio.net/forum/...d.php?t=185766

I was tempted to replace with standard resistors but I found fusibles of the right size and fitted them. The fusibles probably saved my transistors as I made a beginner mistake setting the bias. Why Hitachi felt it necessary to fit these in so many models when other manufacturers didn't is beyond me but they must be responsible for large numbers of failures of their amps as they age. The amps now work perfectly so the work was worthwhile but I would have no hesitation in fitting standard resistors next time.

Here is what I fitted:

47 ohm (4 required) - RS part number 267-1610 (TE Connectivity FRN25J47R) = £0.86 + VAT for a pack of 10.

10 ohm (6 required) - Ebay seller Petelox. (TE Connectivity FRN25J10R) £5 ish for 10 including shipping.

120 ohm (4 required) - Cricklewood Electronics part number G120R - unknown manufacturer. £0.60 each. £6.30 total including shipping. Expensive but I couldn't find them anywhere else.

The prices given are from 2021.

Cheers.
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Old 31st May 2023, 10:51 am   #8
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Thank you, Mel and Trev - much appreciated.

Logically, considering that the Hitachi topology was used in so many amps of the 80s, with no others I've seen using fusible Rs in the VAS, what exactly did Hitachi know that the other OEMs didn't? The other OEMs would be smaller, independent types - whereas Hitachi was one of the world's largest electronic corporations.

I intend to power up the VAS and OPS using bench PSUs. Once I see that both channels are symmetrical in terms of current draw and stable I'll put rail fuses back in.

I am a little loathe to put unnecessary fuses anywhere at present, resistive or not, as I am grappling with the aftershocks of a client based thousands of miles away who received what I believe was a defective fuse. The unit came all the way back to my workshop and the fuse only blew when we connected one of our dummy loads. I suspect that the fuse was 'partially' blown when with the client, causing V to drop, which would've explained his problem. I might start a separate thread on this as I don't recall seeing it before...
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Old 10th Jun 2023, 9:22 pm   #9
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Update:

Fusible resistors? Fusible little Every single one was OC. Whether in the CM or in the linear stage for the VAS, they were all kaput.

I haven't had a chance to trim the HMA-7500 (at this point it is 'working'), but it seems to be ok and I've attached a plot showing it driving 7V into 4R7 (approx 10W). One side has more THD+N than other, which I am thinking is likely due to the EI transformer layout. I need to see how it's affected by loading to determine if the massive filter caps are passing ripple (I changed all the other caps as it took 5 mins compared to the fusible Rs )

Methinks its performance is bang-on for a 1978 design, and likely to rival just about anything in its day.

EDIT - There is a hidden fusible R under the PCB. It's bodged in there P-P. Be warned. Don't lose any hour with an eye glass looking for the little ****** like I did.
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Old 10th Jun 2023, 9:34 pm   #10
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Sorry i did not mention the hidden resistor I think it was a design mistake But so pleased you have done your amp A true design classic
Commercial hi-fi has gone down bank since this was produced
#All the best Trev
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Old 10th Jun 2023, 11:25 pm   #11
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Looks like I spoke too soon. I noticed one of the Zobels smoking at the workshop. I removed the header connector to the OPS and cleaned the pins, making it work fine. I now have it home and- aside from a dreadful hum (ground loop I think), it has started to smoke again. Am wondering if my speaker cable, which is too long and curled up, has precipitated it? Am back to using previous amp, which has a noisy toroidal. Am worried 7500 will work fine at the lab but not at home... aargh!!!!
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Old 11th Jun 2023, 11:30 am   #12
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Regardless of whether the speaker cable is over-long and curled up, no amp should become unstable when faced with such conditions (am now thinking that the hot Zobel on my bench was the same symptom, and cleaning the pins for the OPS didn't achieve a lot - it just happened to stop oscillating when I switched it back on, as is the case with instability, i.e. stable one day, unstable the next). The OPS are socketed, which makes me a little nervous if one considers that Lateral MOSFETs need little excuse to hoot.

Have attached the 7500 service manual. As it shows, it has two Zobels: 1 before and 1 after O/P inductor. Input conditioning is pretty basic, but the RC filter ought to be doing its job. I wonder if the various caps in the I/P filter and Zobel ought to be tested? There is a cap across the 27K FB resistor. I've seen ceramic caps fail with age. I ought to check all of the ceramics that pertain to stability. As it's a single-layer PCB, it will be quicker just to use a vacuum desolder machine and shotgun replace them (assuming they aren't in fiddly places, as with some of the blinking fusible Rs...).
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Old 11th Jun 2023, 11:56 am   #13
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Please let me know what you find It may have been the caps that have taken out the fusible resistors
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Old 12th Jun 2023, 11:57 am   #14
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

The twin Zobel arrangement is uncommon. Does it make you think that Hitachi knew this design could have a propensity to hoot? The design is almost identical to the Maplin DIY amp from the 80s, of which I've built many.

Were the ceramic in the FB loop at fault, I would have imagined it to have developed resistance, which would affect gain... Gain between LR is within 0.1dB, so not bad for 1978.

It's a relatively easy amp to work on, and I have a Hakko vacuum machine, so I can swap a cap in under a minute. The ceramics are normal values that I have on shelf, so although it seems moronic to simply shotgun the ceramics, in this case it can be done in a matter of minutes, so I'm tempted to do so. But I can't help feel that the coiled up speaker cable precipitated the problem, which makes me worry as to whether the design is that stable...

If it's flat to 20K, then that's better than my ears. I might look at a larger value of FB cap. In 1978, the big amp OEMs were in fierce competition and marketing flat BW to high ranges was probably en vogue. A little curtailing of BW shouldn't be a bad thing.
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Old 13th Jun 2023, 12:24 am   #15
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Update:

It looks like I have found the culprit. When fed with a sine to both I/Ps, everything looked fine. The problem came when I muted the left channel, feeding just the right. There was a nasty looking oscillation @ 200KHz (see scope shot).

I fed in a square @ 31.25KHz to both channels (as high as my best square generator goes). Looks pretty tidy. Methinks this is a stable amp.

I then put blanking plugs into the RCAs... This showed that the left channel had spikey anomalies in the FFT that weren't present on the right, which looked nice and neat, as a good channel with shorted I/Ps should do. This made me wonder if the shell of the left RCA were at fault... Lo and behold, it was floating. Despite not hooting when I/Ps were open or shorted, when one side was powered but not other, there was an almighty 200K hoot. Hitachi used the most ridiculous RCA sockets I've ever seen, where the outer shell was connected to inner solder pin via crimping... Years of wiggling RCAs had dislodged the crimp... It was designed to fail. I had to fashion a sheet of GRP with holes and mount new RCAs. I can now feed a loud sine to either side independently, with no hooting. Noise profile looks same for both channels.

The Hitachi does have a somewhat odd grounding scheme. It has 4 secondaries in total from 2 transformers, with each VAS powered from its own linear PSU, and either OPS from an unregulated arrangement. To prevent ground loops as a class 1 device, the chassis is earthed, but the signal ground floats. The L/R RCA shell signal 0V connections meet at the I/P. It's a fully 'dual mono' amp, but signal 0V meets at RCA I/P.
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Old 13th Jun 2023, 12:27 am   #16
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Square @ 31.25K (approx 10W - driving 4R7 dummy)
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Old 14th Jun 2023, 12:42 am   #17
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Further update:

i) Meter lamps. Don't talk to me about them. They are possibly the trickiest thing in the whole amp to change. I had to desolder a meter (always perilous, as you worry that the leads will detach inside if you keep heat on too long...) and open it on the bench to change bulb. I could have done it without desoldering, but it would have been fraught with other dangers, as the looming snake to the PCB is short and you would have to prise meter open whilst not damaging the thin snake wires (the meter PCB also houses ancillary circuits, not just metering, so it is very snakey). I didn't fancy having a meter movement exposed and flapping around outside the front of amp. You have to remove the front bezel to get to meters and it's a pig of a job. If I had more time I'd fit an LED replacement.

ii) An even bigger pig than the meter bulbs is adjusting bias. You have to desolder inaccesible links that are hidden under / between other components and fixtures, tethering your DMM via test points that are also difficult to access. I do not know what Hitachi were thinking...

I noticed that the right channel met factory THD spec into dummy, whereas left distorted far earlier. I hooked both channels to dummy and simply trimmed left to mirror right, leaving right well alone. The result gives 0.0005% THD @ 1KHz / 7V into 4R7 (~10W). Note how the harmonics all mirror each other nicely in attached screen-grab from testing. I kept an eye on overall current draw for amp while I did this, making sure that the left didn't increase overall draw at the mains once trimmed.

Peter Baxandall claimed that he felt that the 'better' SS amps had become indistinguishable from one another by the late '70s. I think the Hitachi here might fit the 'Baxandall criteria'.

edit - apologies for awful photo quality. I made a video, but realised I couldn't post it here without external linking, so took screen grab to convey gist.
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Old 14th Jun 2023, 5:29 am   #18
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Otto Zobel was one of the gods of filter design and synthesised a number of sections to realise pole-zero combinations. He was strongly involved in image-parameter techniques, and as these are one of the ways into designing diplexers, the diplexer used to create a high frequency disconnection between an audio amplifier and its speaker cables/speaker they seem to have acquired his name.

Diplexer filters are interesting and useful beasties. Think of a filter in general. Shove signals of a range of frequencies at it and not all frequencies come out the far end. Putting trust in the speed of light, the signals came out of your generator before they hit the filter, so there was energy on the move. If it didn't come out the far end, where did it go?

If your filter was made out of theoretically perfect components, inductors and capacitors are lossless, so the energy can't have been dissipated in them, so there is only one place for the stopband energy to go, and that is straight back out the input, as a reflection.

This can upset things, with devices like the output stage being sandwiched between two mirrors if the driving circuits are reflective to signals coming out the gate of the power FETs. Those FETs have forwards gain but may not have enough loss in the reverse direction to make the to-and-fro bounce an overall loss. Emitter followers and source followers are right devils for spurious oscillation without any visible loop around them. Some UHF oscillators in sig gens etc are unstable emitter follower style. So if you've got an emitter follower sandwiched between filter-y things then go looking for the stopper resistors to keep it sane.

So a diplexer filter pair can be used to take wanted band energy and feed it to your load, but the added part of it can take the stopband energy and dump it into a load resistor turning it into heat, never to be seen again. You have the laws of thermodynamics on your side this time!

This filter can be simplified embedding the load resistor in the filter. It may take a couple of resistors rather than a single one, but it saves on Ls and Cs which are far more expensive.

A smoking 'Zobel' network is a serious indicator of something oscillating.

The added R/C series pair shunting the output ahead of the expected full 'Zobel' looks like someone at Hitachi has had trouble in this area back in the early days of this amplifier.

I notice that the top power transistor has a base-stopper resistor, but the bottom one hasn't. Hmm-Hmm.

If you want to look at a more elaborate Lateral MOSFET amp, take a look at David Hafler's DH200 circuit. No fusible resistors, sensibly stable, higher voltage rails for the driver stages.

Unlike bipolars, MOSFET followers require several volts of Vgs to rev them up to their current capabilities. If you use the same supply rail for drivers and outputs, you lose several volts of expensive heavy current supply voltage. This offends feelings about efficiency and you don't get the full power capability of the output devices you paid good money for.

It looks like Hitachi's fusible resistors are causing more amplifier failures than whatever they were put in there to protect against. As said, no-one else seems to need them, so they might have been a flash in someone's pan and a bit of a home goal when seen in the long term.

Hafler uses paralleled Mosfets to shift more current than the base Hitachi applications note design, and this further complicates instability modes, but he uses gate resistors in all output FETs. He doesn't need that added shunt pair ahead of the usual 'Zobel'

My 'big beastie' that's sat in the lounge for 40-odd years is very similar to Hafler's take on the output stage. It runs a +/- 55v heavy current supply and has a pair of floating supplies making +15v above the +55, and -15v below the -55. Being floating, these supplies track variations on the 55v rails and any hum on them. Current in the VAS and its load resistor were calculated to limit the max Vgs to reach the output devices, and avoid the possibility of killing them.

Hafler used different value gate resistors for the N-channel and P-channel MOSFETs to accommodate their different Cgs values.

The big beastie can deliver +/- 15A to the speaker and +/-55v, but not quite both at once. It's an excessive design, but not quite that excessive.

Choosing not to have any form of 'Zobel' in the Naim amps was a bit of a prat-fall and the number of combusted amplifiers which had met boutique speaker cables is evidence. It's almost as if someone didn't know what they were there for. Coming at audio amplifier design from the direction of RF design brings with it a number of anti-hooting precautions and the understanding that they are precautions best taken as standard practice.

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Old 14th Jun 2023, 11:04 am   #19
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Thank you, David! An RF background helps your AF from turning RF -) Love your take on the 'black hole' of that which has been filtered.

I have to say that I am embarrassed not to have spied the lack of Gate R on the P-Channel device. It’s highly curious. Even the suggestions in Hitachi’s paper implement it!! http://jm.plantefeve.pagesperso-oran...k_condense.pdf

The good old Maplin version (I made several and still use them) has them, as did Hafler. I ought to do a trace of my 7500 to see if this is a schematic typo. They could have added them dead-bug-style, under PCB.

I wonder what mechanism made it hoot when the 0V RCA shield was disconnected on one channel and I alternated L/R I/P signals? The 31KHz square in previous post looks clean, devoid of any overshoot. I’ve seen far more ragged squares on items that are regarded as stable.

I can’t get my head around why they’d go to all the bother of dual-mono layout, but then common 0V at the I/P… (It has 8 secondaries in total, from 2 TFs – a typo in my earlier post). I think that a little more inspection of the PCB will reveal the reason for this. They used a 2-core cable with single screen for the RCAs... 0V is common at RCAs as well as when it hits VAS PCB. If you have floating PSUs for each channel and a speaker on the end, why not keep things fully independent and minimise potential for ground loops? The only reason I can see for this is the headphone O/P, where 0V for each side has to meet... But that could be switched.... Did they think someone might listen to speakers and HP at the same time?

Incidentally, the designer Bob Cordell has a new take on the Hafler. I plan to put a couple of them into a separate Hitachi amp chassis that I have: http://www.cordellaudio.com/poweramp..._Amplifier.pdf

From what I can make out, the Cordell implementation ought to have the best performance of any Lat-FET amp. I am looking forward to testing it.
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Old 14th Jun 2023, 12:05 pm   #20
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Default Re: Hitachi Amp Fusible Resistors (and general servicing)

Those funny earthing schemes like single-point are OK for avoiding hum problems, which is their reason for existing, but they can go seriously astray at higher frequencies and can contribute to instabilities.

You need a planned earthing ploy, but it has to segue into something more like groundplane or careful separation at higher frequencies. Once you go up in freq, you can never perfectly connect two things, or perfectly isolate them. The tracks and wires to that single-point turn into mismatched transmission lines and the wretched things couple as well.

That's an interesting development of the DH200. Cascoding the differential input stage is good, but he fails to do the same to the VAS. I went a bit further and had differential, cascoded VASs in the positive side and the negative side rather than current mirrors.

Cascoding reduces phase shift, extends frequency response and reduces the Early effect. It also takes the high voltage off of the transistors Vce and you can use separate high gain and high voltage parts. I was also trying to use as many transistors as possible (subject to them doing something useful) as a joke with a friend.

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