Yamaha BBT500H Digital Bass Amp Head - Missing Fan

[TomCarlos]

Team MEF -

I recently sold a Yamaha BBT500H Digital Bass Amp Head and need to resolve a problem for the buyer. The fan is missing. The fan is a generic type - 2410RL-05W-B50, 24vdc, .11amp.

I cannot tell from the schematic on page 56 if this fan is intended to run all the time or if it is controlled by some type of heat sensor. Can anyone tell? Look in the middle of the page, on the right, you will see the connector and surrounding electronics.

It will take a couple weeks to get a replacement fan and I am hoping to get the unit before then to turn it on and see if I measure 24 volts at the connection point. But just in case I do not, I would not have an easy way to determine if this were due to the amp not being hot enough to turn the fan on or if there was some other type of issue.

Service manual - yamaha_bbt500h_sm.pdf

And thanks in advance.

[Enzo]

The fan is missing, so replace it. Is there some reason to think it won't work right?

The fan runs on 24vDC, but note it runs on the -24v supply.

Got a heat gun or hair dryer? Aim it at the temp sensors and see if it clicks on.

[The Dude]

There are a couple of thermistors in the fan drive circuit PR102 & PR103, presumably mounted on or near a heat sink. When they get warm, the fan should run. If this weren't the case, why would you go through all the trouble to build that circuit? They'd just wire the fan directly to a supply.

[Jon Snell]

It is controled with a couple of thermistors. PR102 & PR103 and speeds up from standstill as it warms up to keep it cool.

[vintagekiki]

Fan (CN104) controls by positive thermistors PR102 and PR103 (position 6 F) on schematics page 56.
For Part No see page 52.

[J M Fahey]

Two weeks to get a fan?
Maybe here in Argentina but ... inside USA?

Just curious why would anybody remove a perfectly working fan.

Are you reselling a second hand find?

As mentioned above, a new fan "should" work, heat those thermistors up as suggested for functionality test, absolute worst case and if sales price does not warrant troubleshooting and repair simply leave it ON (straight to some 24V rail) all the time , no big deal.

PC supplies run their fans 24/7 for years.

My own 300W Bass amps use a 12V PC supply fan, running all the time, no problem after, say, 8 years, except lint buildup.

Of course, an instrument amp may get what? 40 hours use per Month?
A plain desktop PC can be ON 8 to 24 hours daily, all year long.

PC fans are designed for that kind of duty.

In bedroom use fan noise may be annoying but then WHY buy a **500W** amp is beyond me.

[TomCarlos]

Thanks guys....

This amp was part of my friend's estate. I will work with the buyer on this one. He ordered a fan from the far east. When it arrives, I will follow your instructions to see if we can get the replacement fan to engage when heating up the thermistors.

As always, I appreciate the replies.

[TomCarlos]

Ok.... success. I wanted to come back and close the loop on this one should we have future visitors to this thread.

I received the replacement fan. And of course, it did not have a connector on the end. Even if it did, there was no guarantee it would fit onto the connector at the PC board. So I hacked at the plastic and made room to connect wires attached to female pins (that I salvaged from a different connector). So some solder, heat shrink, connections made.

As for the Thermistors, I had to go hunting for PR102. I was thinking PR102 and PR103 would be close to each other. After all, they appear that way on the schematic (see photo and Jon Snell post #4). But as you can see in the photos, PR103 is near Q117 and Q118 (big heatsink). PR 102 is buried under the long bar on the other side of the amp - I call that Output row. This is where Q127 and Q130 are located. I spliced the full schematic together onto one image (see attached) - pages 24 and 25 of the service Manual.

I tested the unit (with the help of a heat gun) and the fan kicked on. I then went to idle (no bass, no heat) and the fan shut off.

The amp owner mentioned that the case ran a bit hot. I will have to test this out when he comes by to pick up the amp. My guess is that the combination of Class D, small enclosure, and low air flow will do that. But the thing we will test for this week is to make sure the amp does NOT shut down (under normal playing conditions).

Thanks again for your help with this one!!!

Tom

[Enzo]

Never assume things close together on the schematic will be physically close. They may or may not be. The schematic shows the electrical relation of the parts. A wiring diagram (or layout) shows the physical relationship.