So, my Adcom 555 journey started in 2015. I picked up an early rackmount version of a GFA-555 for 300.00. I had never seen one like this one, the speaker connections on the back are not the the red/black variety, but a reddish brown for both the positive and negative.
Upon receiving it, I took it to Jim Williams (Pro audio upgrades), who did his thing on the driver board, and the main PSU caps tested as good.
I hadn't used it much, but late last year, I started to use it more with some JBL 6332's. One day one side went out. I opened it up and I saw there were 4 fuses, which I carefully ordered the same type and replaced them. All was good until one day, one side would make a pop sound and then volume on that side would get really low, and then come back intermittently.
I found a local guy that took a look around, checked some values on some caps, and said he repaired "30 things" and tested another 250 things. He charged me 150.00.
I got the amp home and it sounded great! i was playing it at relatively high volume and then, POP again, signal gets low. Out of anger, and smacked the front of the amp around the power switch, and loud static, and it came back to life. Something is loose.
I have built some studio style mic preamps and a compressor back in the day, so I am not an expert, but can get around a bit. I called the guy back to see what his input was on this symptom, and I wasn't that keen on taking it back to him, and he did lift a trace while he was working and I now have an extra wire on the bottom of the board....He advised me to get a piece of wood and poke different parts of the unit to try and isolate where the loose connection is. I used a 6 inch long wooden q-tip and started poking around while audio was passing through. When i hit one of the fuse holders on one of the output boards, POP! I found the issue!
I disconnected the amp and re-flowed the solder on all four fuse holders, as well as rough them up a bit for solid connection. I also re-flowed some joints on the outputs. Felling like I really accomplished something, I decided to check the bias before I closed it up. As I was too impatient to wait for clip leads to arrive, I tried it with regular leads and while on one of the emitter resistors, it slipped and sparked, shorting to i believe either the orange or white wire pad right near the 2nd emitter resisitor. After this and before I turned it off, the driver board started to smoke just a little bit, so I immediately shut it off.
I did call another local guy, but his wait time was 6 weeks. After looking at may ADCOM troubleshooting/check out vids on you tube, and reading every thread here on Adcom 555, I have decided to tackle it myself!
When powered on the left distortion LED on the front panel stays on. I powered it down and used the multimeter to check for resistance on the output board ( not sure if it's the pos or neg side, but its the one with the green transistor at the top. There is no resistance on the Toshiba TO3's so I know that at least one of them is bad. I plan on taking this board out and removing all of the TO3's to test. One of the things bugging me on this is that the board does not have the silk screening, and the layout is close, but doesn't match the one that I have downloaded, and I have looked high and low for this board layout and cannot locate. The only one I couldn't get to was hifiengine, as they are closed for new registrations.
I would also like to address if possible the resistor that the guy moved to the bottom of the board and the wire trace.
Side note, while I was beating myself out for shorting the unit, i started to look around for another 555 locally, and found one 3 exits from me for only 280.00. I went to see and hear it, it his guy was the original owner! Unit is super clean, and is also an early version but with the red black horizontal terminals on the back. I opened this one when i got home, and it is pure untouched! so at least I have a back up if something goes south, but my hopes are to get the non functioning unit back on it's feet, and then may replace the electrolytics on the driver board, the one safety transistor that is a known possible failure point, and the filter caps on the bottom as Jim Williams did to my 1st unit.
This is a great community and I'm kind of glad I found it, as I'm starting to get the electronic bug again! Thanks to all in advance!
Upon receiving it, I took it to Jim Williams (Pro audio upgrades), who did his thing on the driver board, and the main PSU caps tested as good.
I hadn't used it much, but late last year, I started to use it more with some JBL 6332's. One day one side went out. I opened it up and I saw there were 4 fuses, which I carefully ordered the same type and replaced them. All was good until one day, one side would make a pop sound and then volume on that side would get really low, and then come back intermittently.
I found a local guy that took a look around, checked some values on some caps, and said he repaired "30 things" and tested another 250 things. He charged me 150.00.
I got the amp home and it sounded great! i was playing it at relatively high volume and then, POP again, signal gets low. Out of anger, and smacked the front of the amp around the power switch, and loud static, and it came back to life. Something is loose.
I have built some studio style mic preamps and a compressor back in the day, so I am not an expert, but can get around a bit. I called the guy back to see what his input was on this symptom, and I wasn't that keen on taking it back to him, and he did lift a trace while he was working and I now have an extra wire on the bottom of the board....He advised me to get a piece of wood and poke different parts of the unit to try and isolate where the loose connection is. I used a 6 inch long wooden q-tip and started poking around while audio was passing through. When i hit one of the fuse holders on one of the output boards, POP! I found the issue!
I disconnected the amp and re-flowed the solder on all four fuse holders, as well as rough them up a bit for solid connection. I also re-flowed some joints on the outputs. Felling like I really accomplished something, I decided to check the bias before I closed it up. As I was too impatient to wait for clip leads to arrive, I tried it with regular leads and while on one of the emitter resistors, it slipped and sparked, shorting to i believe either the orange or white wire pad right near the 2nd emitter resisitor. After this and before I turned it off, the driver board started to smoke just a little bit, so I immediately shut it off.
I did call another local guy, but his wait time was 6 weeks. After looking at may ADCOM troubleshooting/check out vids on you tube, and reading every thread here on Adcom 555, I have decided to tackle it myself!
When powered on the left distortion LED on the front panel stays on. I powered it down and used the multimeter to check for resistance on the output board ( not sure if it's the pos or neg side, but its the one with the green transistor at the top. There is no resistance on the Toshiba TO3's so I know that at least one of them is bad. I plan on taking this board out and removing all of the TO3's to test. One of the things bugging me on this is that the board does not have the silk screening, and the layout is close, but doesn't match the one that I have downloaded, and I have looked high and low for this board layout and cannot locate. The only one I couldn't get to was hifiengine, as they are closed for new registrations.
I would also like to address if possible the resistor that the guy moved to the bottom of the board and the wire trace.
Side note, while I was beating myself out for shorting the unit, i started to look around for another 555 locally, and found one 3 exits from me for only 280.00. I went to see and hear it, it his guy was the original owner! Unit is super clean, and is also an early version but with the red black horizontal terminals on the back. I opened this one when i got home, and it is pure untouched! so at least I have a back up if something goes south, but my hopes are to get the non functioning unit back on it's feet, and then may replace the electrolytics on the driver board, the one safety transistor that is a known possible failure point, and the filter caps on the bottom as Jim Williams did to my 1st unit.
This is a great community and I'm kind of glad I found it, as I'm starting to get the electronic bug again! Thanks to all in advance!
I have a login for hifiengine so grabbed the 2 service manuals they have there for the original 555 (not II).
Transistor testers test at <2 volts. So does DVM diode scale. Output transistors and drivers fail at 10 to 85 volts. Failing a transistor tester condemns a used transistor. That does not prove it good. I find an Iceo test at 12 v has some validity on used transistors. 12 series 47 k resistor series current scale of DVM series transistor C to E. Base is left open. I<10 uA, okay. I= 12v/47k=255 uA, the transistor is garbage.
Usually instead of taking transistors or diodes out, techs insert a dim bulb load series the amp AC input do limit damage from shorts. 60 w tungsten bulb series the AC input. Mine is in a grounded steel box with a circuit breaker, so if the wire pops off the edison socket it blows the breaker instead of setting a fire or burning my hand. The lamp should light up briefly the go dim, if the amp is okay. Steady bright lamp, the amp has a short.
Then the transistors & diodes can be tested with DVM. Vbe is +-0.6 v except relay and lamp drivers. Vce>1v except relay and lamp drivers.
Never test voltage with 2 hands on 2 probes. >25 v across your heart can stop it. Use an alligator clip lead to attach the black DVM probe to the analog ground, then take voltages with one hand and write them down. Voltage across a component is voltage 1 - voltage 2.
Never wear metal like a ring on hands, wrists or neck. 1 v at high current through a ring can burn your flesh to charcoal.
Wear safety glasses. Parts can explode, and solder can splash in the eye. Especially desoldering.
As you found out, a naked meter probe is dangerous to the product. A pamona grabber plugged into the DVM red socket, or a scope probe with ground wire disconnected, are much less likely to scoot around and short things. Scope local ground is only required for accurate radio frequency meaasurements. Audio amps are not radios. Scopes have a panel ground banana jack, which should be used with an alligator clip lead to analog ground of the product.
After an output transistor meltdown, resistors can be burned and capacitors can be opened or shorted. Resistor value should be less than printed on the label. Voltage across resistor should not cause a current that exceeds watt value. Experience helps you read watt values by size. Capacitors should have DC voltage across them.
Happy hunting.
Usually instead of taking transistors or diodes out, techs insert a dim bulb load series the amp AC input do limit damage from shorts. 60 w tungsten bulb series the AC input. Mine is in a grounded steel box with a circuit breaker, so if the wire pops off the edison socket it blows the breaker instead of setting a fire or burning my hand. The lamp should light up briefly the go dim, if the amp is okay. Steady bright lamp, the amp has a short.
Then the transistors & diodes can be tested with DVM. Vbe is +-0.6 v except relay and lamp drivers. Vce>1v except relay and lamp drivers.
Never test voltage with 2 hands on 2 probes. >25 v across your heart can stop it. Use an alligator clip lead to attach the black DVM probe to the analog ground, then take voltages with one hand and write them down. Voltage across a component is voltage 1 - voltage 2.
Never wear metal like a ring on hands, wrists or neck. 1 v at high current through a ring can burn your flesh to charcoal.
Wear safety glasses. Parts can explode, and solder can splash in the eye. Especially desoldering.
As you found out, a naked meter probe is dangerous to the product. A pamona grabber plugged into the DVM red socket, or a scope probe with ground wire disconnected, are much less likely to scoot around and short things. Scope local ground is only required for accurate radio frequency meaasurements. Audio amps are not radios. Scopes have a panel ground banana jack, which should be used with an alligator clip lead to analog ground of the product.
After an output transistor meltdown, resistors can be burned and capacitors can be opened or shorted. Resistor value should be less than printed on the label. Voltage across resistor should not cause a current that exceeds watt value. Experience helps you read watt values by size. Capacitors should have DC voltage across them.
Happy hunting.
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This is what's throwing me off. looking at this circuit board layout, it seems as if the resistor right next to (below) Q5, should be R9, which according to the parts list is a 475 ohm 1/4 watt resistor.
This is the resistor on mine that is a little scorched, and measuring 2.45k ohms. This is the area where I saw a small amount of smoke after the short. when I checked the same position on the other channel, that resistor is reading 2.75k ohm which according to the parts list, should be R12. This is how much the non-silkscreened board differs from the later versions. Hoping someone out there has this earlier special board layout. I can see now that it is uploaded that is is cracked!
For now, I am just comparing against the other channels values, and if I need to double check I do have the other 555 untouched as a reference, which happens to have the exact same board and layout.
This is the resistor on mine that is a little scorched, and measuring 2.45k ohms. This is the area where I saw a small amount of smoke after the short. when I checked the same position on the other channel, that resistor is reading 2.75k ohm which according to the parts list, should be R12. This is how much the non-silkscreened board differs from the later versions. Hoping someone out there has this earlier special board layout. I can see now that it is uploaded that is is cracked!
For now, I am just comparing against the other channels values, and if I need to double check I do have the other 555 untouched as a reference, which happens to have the exact same board and layout.
OK, I figured I would start labeling this old version to maybe help someone in the future and help myself right now. I ran into what appears to be a 1kohm +/- .25%. This is not called for in the parts list, and is the same on the untouched amp I have... any ideas?
under that lint I can see that it is connected to the V+ wire, but looking at the schemos I don't get it.
under that lint I can see that it is connected to the V+ wire, but looking at the schemos I don't get it.
Regarding post #6, my error. that is in fact R11 682 ohms. (newbie mistake, was reading the values upside down). The edit button is not there for me to delete post #6.
UPDATE: More testing, I found out r21 100 ohm totally blown, and also took out one of the Toshiba 2sd424 on the NPN side of the output. Checked Q4, 9 and 10 out of the board, and they are directly connected to R21, and they checked out ok on the mk-168 tester.
After seeing some of the cracking up close on the resistors r7,8,9,10,11,12 I ordered replacements out of caution.
Also finished the labeling in the pic below for anyone looking for the layout on the non-silk screened boards.
Thanks to all that chimed in so far, and kept my mind going...
I will update again once parts arrive.
After seeing some of the cracking up close on the resistors r7,8,9,10,11,12 I ordered replacements out of caution.
Also finished the labeling in the pic below for anyone looking for the layout on the non-silk screened boards.
Thanks to all that chimed in so far, and kept my mind going...
I will update again once parts arrive.
Hi wiz1,
If you short one output in a channel, you must replace all and also the drivers on that channel. If you have a dead driver, replace one stage back also and so forth. You probably also have damaged input transistors as one was reverse biased for a short period. It will work, but it's is no longer matched to its mate and may be noisy now. They must be matched.
Forget buying 2SD424 and 2SB554. Only use new, current parts from authorized distributors. I would highly recommend the MJ2119x series, fresh grease (not CPU grease!) and Mica insulators. Besides, I used to have to buy a bunch and select the ones that match in beta.
So why replace them all? Because the opposite bank was fighting a short to the other rail. They were operated well outside their safety maximums. The side that blew? Well, one finally failed first, they were all stressed. They can be damaged before total failure. I have to repair many amplifiers where the previous person only changed a couple parts that failed. I've even seen amps where the dead ones were removed and the amp was back in service. Nope, they didn't even put others in. If you had a shorted output, the opposing driver was also severely abused. If it fails, that's another complete output section. Not worth betting on a couple bucks. Never mind the labour and troubleshooting time.
Check all the base and emitter resistors. If they didn't go open, the value may have shifted. Often the resistance goes down, then up as it fails. Zero your lead resistance, or if you have a good bench meter it will have a four wire (Kelvin) mode for resistance.
This is the least expensive way to make a lasting repair.
If you short one output in a channel, you must replace all and also the drivers on that channel. If you have a dead driver, replace one stage back also and so forth. You probably also have damaged input transistors as one was reverse biased for a short period. It will work, but it's is no longer matched to its mate and may be noisy now. They must be matched.
Forget buying 2SD424 and 2SB554. Only use new, current parts from authorized distributors. I would highly recommend the MJ2119x series, fresh grease (not CPU grease!) and Mica insulators. Besides, I used to have to buy a bunch and select the ones that match in beta.
So why replace them all? Because the opposite bank was fighting a short to the other rail. They were operated well outside their safety maximums. The side that blew? Well, one finally failed first, they were all stressed. They can be damaged before total failure. I have to repair many amplifiers where the previous person only changed a couple parts that failed. I've even seen amps where the dead ones were removed and the amp was back in service. Nope, they didn't even put others in. If you had a shorted output, the opposing driver was also severely abused. If it fails, that's another complete output section. Not worth betting on a couple bucks. Never mind the labour and troubleshooting time.
Check all the base and emitter resistors. If they didn't go open, the value may have shifted. Often the resistance goes down, then up as it fails. Zero your lead resistance, or if you have a good bench meter it will have a four wire (Kelvin) mode for resistance.
This is the least expensive way to make a lasting repair.
If you want long term reliability, it's best to perform a complete rebuild.
No comments from the cheap seats about the "bespoke" parts! 😆
Dan
No comments from the cheap seats about the "bespoke" parts! 😆
Dan
Long term reliability doesn’t require that level of rebuild, just all new outputs and drivers. There was nothing wrong with the original D424/B554, other than the fact that if one was dead you need a new set and you can’t get them anymore. So MJ2119x it is. The original drivers were the weak link. C2344/A1011 we’re too weak to reliably drive four pairs on 80V rails. When they go they take outputs with them. Fine for two pair on 55V. Or as predrivers. The alternate D525/B595 were not rated for the voltage, so just putting in new ones was a roll of the dice. The devices to use back in the day were Toshiba C2238/A968 or the fullpack equivalents, but are now NLA except KEC’s new version. Lot of folks still on the fence about KEC - still not as field proven as UTC. MJE15032/3 have the necessary beef and are a safe alternative.
While waiting for parts to arrive, i decided to see how closely matched my Q1's and Q2's are. Measured with mk-168 tester, in circuit I got the following:
Left ( good channel) - Q1 B=6 vf=656 Q2 B=2 vf=661
Right - Q1 B=6 vf=660 Q2 B=6 vf=666
I'm not sure how much of a spread is acceptable.
Eventually on my other 555 Thinking about doing @Phloodpants input board and new on semis mj21193/4's. I just want to see if I can get this one going, I found myself wanting to come out here and mess around with this stuff today. I think I got the bug again!
Thanks @anatech @wg_ski and @indianajo for chiming in!
Left ( good channel) - Q1 B=6 vf=656 Q2 B=2 vf=661
Right - Q1 B=6 vf=660 Q2 B=6 vf=666
I'm not sure how much of a spread is acceptable.
Eventually on my other 555 Thinking about doing @Phloodpants input board and new on semis mj21193/4's. I just want to see if I can get this one going, I found myself wanting to come out here and mess around with this stuff today. I think I got the bug again!
Thanks @anatech @wg_ski and @indianajo for chiming in!
Hi wiz1,
Forward junction voltage drops do not indicate a match. Beta does.
A matching jig was designed decades ago. It placed two transistors in a differential arrangement using a constant current source for the tail current, and precision resistors on the collectors. You hung a meter between the collectors and looked for a null (0.00 mV) or close as you can get. Cheap meters do not have enough resolution to show a null. An FM centre tune meter would probably be much better. A good meter is the best solution. The transistors were in physical contact with a foam hood over them.
A few boards were designed for this tester. @Phloodpants designed one of them.
Closely matched transistors lower distortion and increase amplifier offset stability. With amplifiers that don't use a DC servo, matched parts will bring the DC offset to the designed value (some amps have inherent offset, you can calculate it). With well designed amplifiers the DC offset will tend to 0 mV.
Forward junction voltage drops do not indicate a match. Beta does.
- You must measure the transistor beta
- The two parts must be at exactly the same temperature and conducting the same current.
A matching jig was designed decades ago. It placed two transistors in a differential arrangement using a constant current source for the tail current, and precision resistors on the collectors. You hung a meter between the collectors and looked for a null (0.00 mV) or close as you can get. Cheap meters do not have enough resolution to show a null. An FM centre tune meter would probably be much better. A good meter is the best solution. The transistors were in physical contact with a foam hood over them.
A few boards were designed for this tester. @Phloodpants designed one of them.
Closely matched transistors lower distortion and increase amplifier offset stability. With amplifiers that don't use a DC servo, matched parts will bring the DC offset to the designed value (some amps have inherent offset, you can calculate it). With well designed amplifiers the DC offset will tend to 0 mV.
Wiz1, here's a snapshot of a 555 I board with the silkscreen. I believe it's identical to your non silkscreened board.
The issue where you poked the fuseholder and it popped... Makes me think about transistor insulator issues. Sometimes you can have an insulator that is partially bitten-through—perhaps from over-torquing, or a deformed mounting hole— and when you physically disturb it a bit, it will contact the heatsink below. Also you can get a piece of metal debris sandwiched underneath a transistor, and will make intermittent contact with the heatsink. I'd be re-doing all the insulators on the board that had the fault. (Micas are fine if they are undamaged, or change to sil-pads.)
The issue where you poked the fuseholder and it popped... Makes me think about transistor insulator issues. Sometimes you can have an insulator that is partially bitten-through—perhaps from over-torquing, or a deformed mounting hole— and when you physically disturb it a bit, it will contact the heatsink below. Also you can get a piece of metal debris sandwiched underneath a transistor, and will make intermittent contact with the heatsink. I'd be re-doing all the insulators on the board that had the fault. (Micas are fine if they are undamaged, or change to sil-pads.)
Hey Chris,
Do you have any matcher boards left, or can you post the schematic? This may help wiz1.
I had a hard drive crash, the head arm failed (Seagate) and I didn't get a lot of information back.
Do you have any matcher boards left, or can you post the schematic? This may help wiz1.
I had a hard drive crash, the head arm failed (Seagate) and I didn't get a lot of information back.
Sorry, I did not mean to convey that the fuse popped. The audio signal in the speaker was making popping noises. That turned out to be the fuse holder was not making good contact with the fuse. I tightened it up and sanded a little, as well as reflowed the solder on it, and it was way more solid.
Update: Dim bulb test FAILED!
After replacing all the resistors that were either cracked or a little heat stressed, and one Toshiba 2sd424 ( I know I'm supposed to replace all, but wanted to see if it would just come back to life first), I was very careful to notate the wiring as well as reference the other channel.
The 100w bulb starts bright then gets dim but still on.
I also replaced both of the 2sa1011 on the pnp output boards even though the OG's tested good as I heard this was a main failure point.
Not sure where else to go now.... I didn't replace any of the diodes or transistors on the input board, I pulled q4, 9 and 10 and they tested ok.
After replacing all the resistors that were either cracked or a little heat stressed, and one Toshiba 2sd424 ( I know I'm supposed to replace all, but wanted to see if it would just come back to life first), I was very careful to notate the wiring as well as reference the other channel.
The 100w bulb starts bright then gets dim but still on.
I also replaced both of the 2sa1011 on the pnp output boards even though the OG's tested good as I heard this was a main failure point.
Not sure where else to go now.... I didn't replace any of the diodes or transistors on the input board, I pulled q4, 9 and 10 and they tested ok.
Hi wiz1,
Basics here.
Where is the excess current going?
I don't understand why people try to use "dim bulb testers", except they are cheap. You need a VARIAC, or external variable bipolar DC power supply in order to troubleshoot. Increase the voltage until you begin getting higher current flow. Not a ton. Then you can measure at various points to determine the fault.
Using a dim bulb tester, after the initial surge it is possible to see full voltages, then something else can fail in that instant. Additionally, the circuit voltages will vary with with current draw. A variable you do not need and may well fool you.
As I mentioned earlier, components can be damaged and be abnormal without shorting or opening. That is why you replace the things I mentioned. Only one output or driver, bias or other parts fails and you can lose more expensive parts. I have seen people replace all the outputs and lose the lot because they were too cheap to replace the drivers and one failed. I don't work for distributors or semiconductor manufacturers. I have zero interest in you having to buy more. What I do care about is a properly working piece of equipment at minimal cost to the service person or owner. It's as simple as that.
I try to avoid having more service nightmares come in for me to fix. I'm not in your market, but the other good service techs don't want these as repairs either. It's also a shame when someone throws a perfectly good amplifier out because they didn't service it properly.
Basics here.
Where is the excess current going?
I don't understand why people try to use "dim bulb testers", except they are cheap. You need a VARIAC, or external variable bipolar DC power supply in order to troubleshoot. Increase the voltage until you begin getting higher current flow. Not a ton. Then you can measure at various points to determine the fault.
Using a dim bulb tester, after the initial surge it is possible to see full voltages, then something else can fail in that instant. Additionally, the circuit voltages will vary with with current draw. A variable you do not need and may well fool you.
As I mentioned earlier, components can be damaged and be abnormal without shorting or opening. That is why you replace the things I mentioned. Only one output or driver, bias or other parts fails and you can lose more expensive parts. I have seen people replace all the outputs and lose the lot because they were too cheap to replace the drivers and one failed. I don't work for distributors or semiconductor manufacturers. I have zero interest in you having to buy more. What I do care about is a properly working piece of equipment at minimal cost to the service person or owner. It's as simple as that.
I try to avoid having more service nightmares come in for me to fix. I'm not in your market, but the other good service techs don't want these as repairs either. It's also a shame when someone throws a perfectly good amplifier out because they didn't service it properly.