Troubleshoot And Repair This ADCOM GFA-555se Audio Amplifier

[@EriksElectronicsWorkbench]

I am forever grateful that Erik took on this repair of my 555SE. It has been back online for 5 months now and has run flawlessly. The failing device on the preamp board started out with just a random crackling noise once in awhile, hard to diagnose. Eventually it generated noise blasts that damaged my speakers. I believe that this failure to a great extent was driven by inadequate cooling of devices on the preamp board. Adcom provided no heat sinks, or enhanced air flow, to manage these temperatures. I have resolved that problem and the fix is detailed in this link. adcom555cooling.tiiny.site/ This fix should add years of life to your 555 series amp. -Ed B.

[@christopherspiro9857]

Thanks for that info. I had adcom around 12 years ago the preamp and 60002 but I unplugged them after power down and blow something fix it once but still one channel was not working. I travelled for 5 years and when I came back no one could help ( so I put them on the curb. I was thinking of buying gfa555se your model for a good deal (new) do you think I should? They offer just a two year warranty. Do you believe it was heating or design flaw of the amp? How good does it sound ? Thanks!

[@EriksElectronicsWorkbench]

@@christopherspiro9857 The 555SE is a very well built and powerful amp and is based on the previous two 555 versions that all sound amazing and have great specs. The amps do not have protection circuits and some transistors do run quite warm. I would not say it is a design flaw but perhaps a minor weak point. I own the 555 Mk II (first video I made is a repair on it) and it has performed perfect ever since the repair and sounds fantastic. -Erik

[@christopherspiro9857]

@@EriksElectronicsWorkbench Thank you for replying

[@edwardbuker1403]

@@christopherspiro9857 Chris, I think the 555SE is a great sounding amplifier and I would not hesitate to buy one again. The SE version has many upgrades, all new components (caps, resisters, circuit boards, and semiconductors) that have had a lot of improvements, due to learning over the last 30 years. I think the SE is the best choice of all the 555 amps, given the age of the earlier versions. The original Nelson Pass circuit design has stood the test of time, producing one of the best sounding amplifiers to ever be brought to the marketplace. It is superbly quiet, powerful, stable, while being able to handle low impedance speaker loads. I think my amp failure was due to a manufacturing defect in a semiconductor that was accelerated by heating. As long as you add the inexpensive fan mod I describe, you should be fine. I would also consider the change that Erik describes, that eliminates output transistor oscillation that takes place during the start up cycle. Hopefully Adcom will get wind of that error and make that change in the production of the 555SE. My only reservation with Adcom is how good their customer service is. As an aside, I also own a Parasound A23 Halo Amp and in A/B listening tests I think the 555SE sounds a bit better to me, the A23 seems to be "flatter", not as dynamic, the 555SE just continues to pleases me every time I listen to it. Hope this helps....Later Ed

[@iami9307]

Absolutely fascinating on the way you found the noisy transistor. Never have seen a technique like that used before

[@mohimadvani6425]

I have never seen such a thorough and informative video before. excellent job 👏👍❤😊

[@EriksElectronicsWorkbench]

Thank you for watching and the nice compliment.

[@tedrobinson3802]

I added a soft start module with fault protection to my 555 during the rebuild. A nice addition.

[@davidnguyen5028]

I would like to install a soft start module for the GFA5800, where can I purchase one?

[@tedrobinson3802]

@davidnguyen5028 it's been a few years but I believe I got a board /kit from ebay

[@JeffWeissman-d3x]

Erick the video was excellent, and informative. I haven't worked on bench repairs since 1979, and this reminded me of things I forgot. I liked your tip using the probe to find noisy parts. Keep up the great videos.

[@EriksElectronicsWorkbench]

Thanks for the compliment! There will be more videos coming :)

[@mechanoid5739]

I had that exact same problem with a noisy transistor in the pre-amp circuit of an amp I fixed years ago. When you said there was popping a hissing noises reported I thought, " I bet it's a noisy transistor in the pre-amp output!" However, I did see a repair on KZbin where a similar noise was caused by a resistor. I never realised that that could happen with a passive component!

[@EriksElectronicsWorkbench]

Yes resistors can become noisy and I've had it happen to mica capacitors as well.

[@GordonPyzik]

An Adcom needs a repair. Really. This is such a rare case.

[@alanmeyne7571]

You are a thorough professional!

[@EriksElectronicsWorkbench]

Thank you!!

[@Wil_Bloodworth]

So much excellent information in this video. Thank you Erik!

[@EriksElectronicsWorkbench]

Thanks for watching and for the nice compliment!

[@MrAKISPAPAS]

Unbilivable skills and patience - Bravo

[@EriksElectronicsWorkbench]

thank you!

[@jeremiahchamberlin4499]

Awesome video. I’m glad I hung in there till the end.

[@EriksElectronicsWorkbench]

Thank you! The video is a bit lengthy lol, lots to cover, glad you stuck around until the end.

[@HipocratesAG]

Brilliant presentation, thanks!

[@2tallB]

Thanks for that systematic walk-through and the explanations. Lots of tips here 👍🏼

[@EriksElectronicsWorkbench]

Glad you found the video helpful, thanks for watching!

[@2tallB]

That tone tracing probe was interesting. ( a fluke?). How well does that work for components other than transistors? And would a thermal camera also work for that, or is that used for a different type of fault? Thanks again!!

[@EriksElectronicsWorkbench]

@@2tallB Yes the tone tracer is by Fluke. It will pick up any noisy component, resistor, capacitor, etc. When you get near the defective part the static or crackle type noise increases a lot. A thermal camera would not pick up a noisy part, it would only show if the part was overheating. Of course it's possible to be noisy and overheating too. Here the transistor was not overheating.

[@2tallB]

@@EriksElectronicsWorkbench interesting, thanks for the info!

[@RGD-Audio-Repairs]

Came across you're channel tonight.. and honestly, Fantastic videos! I love how you take the time to explain things, and how the amplifier operates, How it works, What could potentially be at fault, How to troubleshoot etc... Excellent.... Subbed

[@EriksElectronicsWorkbench]

Thank you for the nice compliments! Glad you discovered the channel and decided to subscribe.

[@b.powell3480]

Great video!, transistor cascading noise !!

[@xyloeye]

Really great video. Thanks, Erik.

[@EriksElectronicsWorkbench]

Happy to know you enjoyed the video!

[@riccitone]

Great video, learned a lot here! Wanted to ask if you've seen this or might have knowledge of whether this has become a common problem with the newer version amps (555se) or if this is pretty well an isolated case? Was admiring and looking into these and then found this video. Thanks!

[@EriksElectronicsWorkbench]

The amp's circuit design is based very closely to the 555MkII design and I have not heard of the issues this amp had being a problem in other SE or MKII models. There are some transistors that run quite hot on the main circuit board (this model and previous model) and there is speculation that the heat can lead to problems. I was surprised the SE didn't have heatsinks on those transistors. The noisy transistor is probably an isolated failure. The instability (oscillations) are an easy fix with 4 components if an amp is found to have that problem.

[@markconway2677]

Erik, Really enjoyed this video. I learned a lot by watching. You give excellent thorough explanations. I'm starting repairing audio gear and wondered if you had advice on what equipment to acquire in terms of a good scope, multimeter, power supply, variac, etc. would be. Analog or digital scope? Workbench layout specs, tips. In fact, if you publish an ebook on the subject, I'm definitely a buyer. Thanks a lot.

[@EriksElectronicsWorkbench]

Thanks for watching and the nice compliment. Glad you found the video helpful. Some basics for your workbench I'd recommend: good quality DMM (I use Fluke) and consider an analog VOM (mine is Simpson 270), soldering and desoldering tools (Hakko brand is my go to), dual output DC power supply 0-30 volts at a few amps (I use a Keithley model as my main DC supply) , function or signal generator (BK Precision), dual channel digital scope (I use Tektronix, Fluke, HP/Agilent brands plus a Tektronix analog scope but I'd recommend to start with just a digital scope), variac and isolation transformer rated at least 1000 VA, and if you work on vintage vacuum tube gear build a dim bulb setup for current limiting. Peak brand capacitor tester, it's the small purple colored tester I use in my videos. Make a simple switched AC outlet setup on the bench for easy and quick power control. Several 8 ohm resistors at a few hundred watts to load test amplifiers. Parallel them for 4 ohm testing. If your main focus is audio gear the scope and signal generator don't need to have exotic frequency ranges but invest in quality test gear that you can consistently depend on. Plenty of bins or drawers to keep parts and tools organized.

[@markwroblewski6500]

Excellent troubleshooting and repair skills.How do you match a complimentary pair of NPN and PNP power output transistors?Thanks in advance.

[@EriksElectronicsWorkbench]

You can build a test circuit or use a transistor tester to measure the hFE or Beta. The hFE of a transistor, often called the DC current gain or beta (β) of the transistor, is a measure of how much the transistor amplifies the current from the base to the collector for a bipolar junction transistor (BJT). The hFE is given by the ratio of the collector current (Ic) to the base current (Ib).

[@Dutchamp]

Just found your channel, looks interesting. You did test the transistors right, but isn't true that it is tricky. Transistors measuring passive can be good. But under load they still can be bad. It is an indication and little more difficult to measure under load. Still a nice video nice work

[@EriksElectronicsWorkbench]

True of any component that the real test is how it performs in circuit. The end result is the amplifier works properly so all of the transistors are therefor functioning as they should. Hope you enjoy the content on this channel! Thanks for watching.

[@benjisan2656]

Hi Erick, could you share which manufacturer of output transistors you sourced for your repair?

[@EriksElectronicsWorkbench]

They are made by ON Semiconductor. The original transistors were made by Motorola and they sold their semiconductor division to ON Semiconductor a few years ago.

[@Foxtrot1967]

Greetings Erik! What a great video, thank you. Who'd of know you can diagnose certain issues with a toner. Learn something new each day. I have a Adcom GFA 5500 that has been great for years but slowly over time it has lost its bass output. Everything else seems fine with the amp, just poor bass. Is this something that can easily fixed or should I buy another amp?

[@EriksElectronicsWorkbench]

Thanks for watching, glad you enjoyed the video. The loss of bass implies the amps frequency response is poor on the low end. Are you sure the problem is the amp and not a pre amp or prior component? A coupling capacitor would be my guess but unusual if both channels have the same issue. A poor cable connection (high resistance) can also give the impression of weak bass.

[@Foxtrot1967]

@@EriksElectronicsWorkbench Hello Erik. I've tried switching pre amps and the RCA cables are of good quality. The DC offset and Bias are perfect. So frustrating because I know the amp has the potential to pushtons of clean powerful bass to my Klipsch RF 7's and La Scala's

[@EriksElectronicsWorkbench]

@@Foxtrot1967 have you bench tested the amp to see what its actual frequency response performance is? It should be nearly flat from 20Hz-20KHz. The actual output power at various frequencies should remain constant for a set input level. This would prove if the amp performs correctly or not. The issue could be in the speakers or room acoustics. If the amp does not have a flat response I'd suspect a capacitor or possibly a power supply problem but careful testing would be needed otherwise it's just guessing at the issue.

[@Foxtrot1967]

Hi Erik. It's almost impossible to find a local shop that knows how to properly repair vintage amps. Can you recommend one in Northern CA? If not, would you be willing to take on the task? I sure would appreciate it.

[@EriksElectronicsWorkbench]

@@Foxtrot1967 I don't have a shop to recommend, however I did reply to your email.

[@glenngutheil9950]

Good work

[@EriksElectronicsWorkbench]

thank you!

[@cristiansavin4062]

Hi Erik. Just found your channel, looks amazing interesting! As a new GFA-555se owner this video here is totally important for me. The internet is full of schematics for the GFA-555MkII, but I can't find any schematics for the 555se. Do you have any link that you can share?! Thanks in advance!

[@EriksElectronicsWorkbench]

Hi, glad you find my channel interesting! The 555se is nearly identical to the 555MKII except for the output relays and some other minor changes. I show the relay control schematic in the video. I do not know of a link for the schematics but I can email you what I have if you send an email to the address listed on my youtube page.

[@krisyoung447]

Great Video! My friend gave me a GFA-7000 that is showing a thermal protection LED on the center channel. I ran it for a couple hours and it played fine. Wonder if it is just a bad thermistor or something? I just ordered the schematics but this is low on the to do list. What was the model of that Fluke snooping speaker thing? I have a Hartke bass amp that I would like to try that on.

[@EriksElectronicsWorkbench]

The noise probe is a Fluke Networks Pro3000 Probe. On your GFA-7000 if it is not actually getting hot then yes the sensor might have a problem or a problem in the protection circuit. Usually a thermal fault shuts down the amp on that affected channel, so if it keeps working (and is not actually hot) seems like the control circuit might be the problem area.

[@thephonoguy]

What source would you recommend for the output/driver transistors and the insulators. I rebuilt a 555II 20+ years ago but don't remember where I got the components. Thanks for your help!

[@EriksElectronicsWorkbench]

The original Toshiba and Motorola (now ON Semi) devices have become hard to find. I was lucky to find new old stock in factory packaging for this repair, some sourced from eBay but be very careful about fake components on eBay. I talk about fakes in the video. NTE makes replacement devices that would probably work fine but check the tech specs. Sources like Mouser Electronics and DigiKey have mounting hardware.

[@JamesWilliams-ps6xz]

Have you had a chance to analyze the Klipsh Forte IV speaker? Or any other from the heritage line?

[@EriksElectronicsWorkbench]

Unfortunately I don't have experience with that model line of speakers.

[@sthengr]

Thanks!

[@JeffWeissman-d3x]

Erik, Just curious if you have worked on the Anthem MCA-225, or Rotel RB-1582MKll amps? If so which do you feel is built for music not home theater?

[@EriksElectronicsWorkbench]

I do not have experience working on either of these models. The specs are very close on both. They even weigh within a pound of each other. Both would be very capable amps for stereo music. Both are two channel (not multichannel for home theater) although the Anthem design allows for other multichannel designs because it is modular internally. I own a different model 2-channel Rotel amp and have always been happy with it's performance. Anthem looks to be high quality, made in Canada. The Anthem model can drive 2 ohm loads, Rotel specs to 4 ohms. Anthem has just slightly better SNR and a touch lower THD.

[@JamesBarone-o2r]

Is the GTP 400 a good pre amp. For a GFA 545II. WHAT WOULD YOU SUGGEST OR IS THE PRE AMP. GOOD ENOUGH FOR THAT AMP. ALSO IS THAT AMP WORTH KEEPING . THX.

[@EriksElectronicsWorkbench]

The GTP-400 has good specifications and would work well with a GFA-545II. Info for the preamp is here www.hifiengine.com/manual_library/adcom/gtp-400.shtml Yes the GFA-545II is a great amplifier. I own one and have been very happy with it's performance.

[@davidnguyen5028]

Hi Erik, I just subscribed. I learned a lot from your video. My Adcom GFA 5800 makes a loud popping noise at turn on through my speakers, what could be the symptoms? I have replaced the four large filter capacitors but the symptom remains. Thanks in advance.

[@EriksElectronicsWorkbench]

Hi, glad you found the video helpful and thanks for subscribing. The GFA-5800 does not have a speaker protection relay so turn on (and power off) noises can occur but should not be serious. If the popping is very loud it might be a failing component but tracking it down would require testing. There are many components that could create such a noise. If the popping sound is repeating (not just a single pop) it is almost certainly a failing component. Also remove the input cables to eliminate the noise coming from an external source. There is a NTC thermistor on the transformer primary to prevent turn on current surge and perhaps it has failed. Erik

[@f430ferrari5]

I have a GFA 5500 and I also got the Adcom ACE 515 power conditioner. It has a turn on and turn off delay. Helps with any turn on or turn off thump. I’ve had this combo plus Adcom pre-amp and 5 disc changer for over 25 years. Everything works great still.

[@hazevapeco8141]

I've got an original GFA-555, just finally had the chance to hook it up - and even though both speakers played, there was no lower frequencies! Switched to the L and R of my GFA-6000 and everything played perfectly. Any ideas what could be wrong with the 555?

[@EriksElectronicsWorkbench]

It's unusual for both channels to have this same fault because the amp is essentially two separate mono sections. I would have suspected the preamp. It's possible the input stage of each channel was damaged and you are getting essentially a weak audio signal which would sound like it is lacking in bass. The only way to find the problem would be to connect an audio signal generator to the amp and start probing through the amp with an oscilloscope to see where the signal loss is occurring.

[@hazevapeco8141]

Sounds like a great idea. Thanks for replying, and the advice! Also realized when I got home that it's actually a GFA-5500, not a 555 like I thought. 🤭@@EriksElectronicsWorkbench

[@MrAKISPAPAS]

Why do you use different type of transistors ? one side is BM2221 and the other side is BM2230

[@EriksElectronicsWorkbench]

The numbers you reference are not the part number but actually are lot numbers or possibly date codes. The part number is printed above the numbers you mentioned. But the output transistor sets are in fact different part numbers because one set is NPN type and the other set is PNP. This is the polarity of the transistor structure. These transistors are designed to work together and are known as complimentary pairs. The amplifier has both positive and negative power supply rails and that dictates using both NPN and PNP transistors. The reason for the two power supply rail polarities is so the audio signal can swing above and below ground reference and is inherent in most power amplifier designs.

[@unknownerror542]

Anyone else notice what the probe picked up @1:26:54? Barely faintly, sounds like a CW morse code conversation.

[@pareshpremjee8214]

Why dont you use the Fluke meters Diode checker mode to test the transistor? It so much easier then measuring resistance....

[@EriksElectronicsWorkbench]

At the start of the video I was looking for the failed component that was blowing the fuse (a total short circuit) and the ohm meter is ok for finding that. Then I found the transistor had completely shorted so there was no junction to forward bias in order to get a reading. If you are checking a good transistor the diode function is needed to forward bias the junction as the ohm meter would not provide a reading because the test voltage is too low. I do use the diode mode later in the video.

[@poormanselectronicsbench2021]

There are times, where, a small / high resistance leak in a transistor will not be found by using a Fluke DMM in diode check mode. That is where, a old school VOM like a Triplett 630, that uses 30 Volts in its higher range ohm resistance circuit will help pick those faults out.

[@deezsquuezeorg]

Can you repair a 5503 ? In Iowa.. thanks

[@EriksElectronicsWorkbench]

Hi, sorry not taking on additional outside projects at this time.

[@JamesBarone-o2r]

Where are u located

[@stevenburdick424]

I have the 5500. What is newer?

[@EriksElectronicsWorkbench]

The GFA-5500 was released around early 2000's. It's the MOSFET version of the 555. This amp is newer than that, being only several years old.

[@stevenburdick424]

@@EriksElectronicsWorkbench Thanks

[@v12alpine]

Looks like that output relay does not offer protection for DC, it's just a turnon/turnoff thump eliminator? I would be very wary of hooking this amp up to an expensive speaker.

[@EriksElectronicsWorkbench]

Correct, the relay is only for turn on/off thumps. The "no DC offset protection" is an ongoing debate on these 555 amps. They have never included that protection. The original and MkII versions had no relays at all.

[@akispapasavvas9307]

How hard is it to install DC offset protection? Does it degrade the sound? Is it really that dangerous?

[@vegaslimoguy2376]

@@akispapasavvas9307 i own about every model Adcom has ever made, never ever had a problem.

[@davesubers3415]

What’s a repair like this cost?

[@EriksElectronicsWorkbench]

the new parts cost $132

[@akispapasavvas9307]

And the labor, for fine tuning and testing?

[@akispapasavvas9307]

I have two units and am hesitating to use since I hear so many scary stories for unriliable quality control from Adcom. I wish I could send them to you for some modifications and functionality test adjustmens, but it would be a huge transportation cost. Do you believe that ADCOM will try and do these mods after the failures they had regarding the hot transistor temperatures and the current draw when switching on the unit?

[@EriksElectronicsWorkbench]

@@akispapasavvas9307 If the amp is out of warranty I doubt Adcom would offer any service for it. And under warranty they won't modify it, just service per the original design. If you want to upgrade some of the design take a look at the products offered here.. hoppesbrain.com/shop/

[@casio912]

Terrible device?

[@christopherspiro9857]

Bad design? They only offer a 2 year warranty.

[@christopherspiro9857]

Bad design? They only offer a 2 year warranty.

[@vegaslimoguy2376]

Bad design??? This is the Nelson Pass design that Adcom has used for the last 40 years plus! Educate yourself

[@David-hy5iq]

@@vegaslimoguy2376 Have you even watched the video? The amplifier oscillates like crazy on startup due to bad design choices. Nelson Pass designed the original 555. He has not been consulted for any changes made in the 555II- or 555se-versions.