Model 3 Sound Amp Woes

[kuze]

So the sound from JAMMA boards in mono on my Blast City has gone dead quiet, if you turn it up to max volume you can sort of hear the game but there is an overpowering buzz to it.

I read this means that the amp itself is blown, so doing a cap kit won't resolve the problem?

As it happens, I had purchased a pair of Model 3 sound amps from eBay as "untested" previously so I figured I'd give one a try.

One of them has right angle connectors for CN4 and CN1 which don't fit the wiring in the Blast City PSU as well as some weird mods to CN3 and CN2, so I figured I would use the other one that's more of a drop in replacement.

However, on taking a look at it one of the resistors was broken in half. No problem, I swapped it from the other amp and gave it a try.

Hooking it up, I get much louder sound albeit a bit lower than what I'd call normal - however, there is still a buzz to the audio at any normal levels.

Would recapping the model 3 amp fix the buzz? Anyone know what could cause that issue?

 

[kuze]

On the left you can see the right angle connectors I'm talking about. Anyone know the part # for the ones on the right? (The 3 and 4 pin headers)

On the left you can see the mods I'm talking about re: CN3 and CN2. I'd be curious if anyone knows what they would do. I'm not using this one right now, but I'm thinking of giving it a try.
The pinout for them can be found here: https://wiki.arcadeotaku.com/w/Sega_Blast_City_PSU

My guess is that maybe they bypass the volume pot and force mono for some reason?

 

[skate323k137]

So the sound from JAMMA boards in mono on my Blast City has gone dead quiet, if you turn it up to max volume you can sort of hear the game but there is an overpowering buzz to it.

I read this means that the amp itself is blown, so doing a cap kit won't resolve the problem?

As it happens, I had purchased a pair of Model 3 sound amps from eBay as "untested" previously so I figured I'd give one a try.

One of them has right angle connectors for CN4 and CN1 which don't fit the wiring in the Blast City PSU as well as some weird mods to CN3 and CN2, so I figured I would use the other one that's more of a drop in replacement.

However, on taking a look at it one of the resistors was broken in half. No problem, I swapped it from the other amp and gave it a try.

Hooking it up, I get much louder sound albeit a bit lower than what I'd call normal - however, there is still a buzz to the audio at any normal levels.

Would recapping the model 3 amp fix the buzz? Anyone know what could cause that issue?

I cannot guarantee it would fix it but I did get rid of faint buzzing by recapping my blast amp. for what it's worth I recapped the PSU at the same time.

In your shoes I would start with a full recap of the one that still has the right pin headers.

 

[kuze]

So the sound from JAMMA boards in mono on my Blast City has gone dead quiet, if you turn it up to max volume you can sort of hear the game but there is an overpowering buzz to it.

I read this means that the amp itself is blown, so doing a cap kit won't resolve the problem?

As it happens, I had purchased a pair of Model 3 sound amps from eBay as "untested" previously so I figured I'd give one a try.

One of them has right angle connectors for CN4 and CN1 which don't fit the wiring in the Blast City PSU as well as some weird mods to CN3 and CN2, so I figured I would use the other one that's more of a drop in replacement.

However, on taking a look at it one of the resistors was broken in half. No problem, I swapped it from the other amp and gave it a try.

Hooking it up, I get much louder sound albeit a bit lower than what I'd call normal - however, there is still a buzz to the audio at any normal levels.

Would recapping the model 3 amp fix the buzz? Anyone know what could cause that issue?

I cannot guarantee it would fix it but I did get rid of faint buzzing by recapping my blast amp. for what it's worth I recapped the PSU at the same time.
In your shoes I would start with a full recap of the one that still has the right pin headers.

Right on, good to know. Yeah I'll probably recap the amp first, then if that doesn't resolve the issue I guess I could recap the PSU.

Do you happen to still have your Digikey or Mouser parts list/links from when you ordered caps?

Also did you replace the two huge 35v caps? If so, how did you get the white gunk off them - just a little ISO?

If I get antsy I might just swap the pin headers from one of the other Model 3 amps so I can give the other one a try before parts come in.

 

[skate323k137]

I always do the large filter caps too. I just slice the white gunk with a hobby knife prior to desoldering.

I left a cap list on the AO wiki. Shoot me an email [or pm me your email] and I can send you my digikey receipt with the part numbers.

The amp is very easy, single side plating. The PSU is thru plated so a hakko or something is pretty mandatory.

 

[kuze]

You're the man, skate!

Oh hey, that reminds me. I took this opportunity to swap the PSU fan, which was HELLA loud with the one that was listed on the AO wiki (Evercool PSU-REP-2PIN7MM), but guess what - it's even louder than the stock fan was!

If you've ever swapped fans before, got a recommendation on one that fits the bill? I'm not sure if it makes any difference, but my Blast PSU is the 220v version and there's an isolation transformer in the cab to make it work with 120v.

Otherwise I'll probably just look for a "silent" 12V 80mm PC case fan and remove the wiring and solder the 2-pin connector to it.

 

[skate323k137]

Yeah just hack one in... Keep the voltage as low as you can ideally, the blast only outputs a couple amps on 12v.

 

[kuze]

Yeah just hack one in... Keep the voltage as low as you can ideally, the blast only outputs a couple amps on 12v.

Will do! Any idea what type of resistor that is at R1?

If I had to guess, it looks like it's brown -> black -> black -> gold -> silver? So maybe that's a 10 ohm resistor with 10% tolerance? Something like this? https://www.digikey.com/product-detail/en/ohmite/OY100KE/OY100KE-ND/813585

 

[skate323k137]

I usually just take a meter to an intact one (resistor) and go with that measurement if the colors aren't absolutely clear. Resistors are usually better tolerance than caps to begin with and retain them.

 

[kuze]

I'll bust out the ol' meter & giver it a shot then.

 

[kuze]

My multimeter gave me a reading of 10 ohms for R1 on the amp I took out, so I guess I'm on the right track there. How would I know the right wattage or tolerance to choose though for replacements? My basic multimeter doesn't seem to give me that information, or maybe I just am not adept enough with it.

 

[skate323k137]

can't go wrong with tolerance... Lower percent the better of course.

Wattage wise for most resistors 1/4 watt is fine, but for power supplies you can need more like 1/2 watt or higher. As silly as it seems size is a good indicator, higher wattage resistors will be larger. Think of this like voltage rating for caps... Going larger wattage is ok, but not smaller.

 

[kuze]

can't go wrong with tolerance... Lower percent the better of course.

Wattage wise for most things 1/4 watt but for power supplies you can need more like 1/2 watt or higher. As silly as it seems size is a good indicator, higher wattage resistors will be larger.

Cool, I ordered some of those 2 watt 10 ohms +-10% resistors I linked above - might not even have to use them but it sounds like those should be a suitable replacement.

 

[kuze]

So I received the replacement caps today and replaced all of the electrolytics on my Model 3 sound amp.... Sounds fantastic now and plenty loud!

Mad thanks to @skate323k137 for sharing the Digikey part numbers for the caps.

As for the replacement fan, it's actually nice and quiet - just needed a bit of tweaking to make sure the wiring didn't get brushed up against by the blades of the fan after putting everything back together. So the Evercool PSU-REP-2PIN7MM is absolutely a fine fan replacement.