Ampeg B-2 Fan Trouble

[abav]

Good afternoon. I have a b-2 rack mount bass head that came in with the problem..."it was working fine for most of the gig...then just quit"....he also noticed that the fan was not working.
I Noticed that c17 was blown apart...replaced that....also disconnected the fan and put a ps on it....works fine. Checked q18-19,D20,21,22...all tested good. I had IC-1 (lm-35 thermal sensor) in stock so i replaced it anyway.
I am getting 16v on the input of ic-1....also...i am getting 6.4 at TP17. I guess my question now is....Is the fan supposed to run all the time with a rpm change as the temp on the heatsink goes up??....or does it kick on at a certain temp? I had the unit on and blasted it with a hair dryer for quite sometime...and the fan never came on.

Any help or guidance would be greatly appreciated....

Thank you for your time.

Art

[pecorporation]

I believe the fan runs all the time on low and spikes high when the heat sink reaches a certain temp. I've had to use a heat gun on high aiming it through the rear vent to get a fan speed change.

You might want to verify that R54 & R55 are on-spec. Those use to be an issue on earlier models.

What voltages do you read on the +/- HT power rails? Should be +/- 64 VDC.

[Jazz P Bass]

Ampeg B2

Ampeg had two fan circuits on the B2.
The first one came on "too late"
The second one was biased on slow, faster/hotter.
I have attached a drawing that I made.
This info is direct fron Crate. (not Loud Tech)

[abav]

Thank you gentleman for both of your inputs. Yes....the board i have is the old revision...and the schematic i got has the second revision (fan always on slow at first)...i will do the fan update and the customer should be a happy camper!!!

Thank you guys again for your help!!!

Art

[Jazz P Bass]

Ampeg Fan circuit

You still want to verify that the fan changes speed when the amp warms up.
I have worked on the first edition circuit.
I had the fan running with the following set up.
150 mv 1KHz input.
Output into an 8ohm dummy load.
20Vac rms sine wave on the output.
That's 50 watts.
The fan came on within 2 minutes.

[abav]

Tomorrow when i get in I will run it the same way and let you know if the fan kicked on or not.

Thanks Again


Art

[abav]

Ok. Jazz P....i ran the amp like you suggested....it ran for about 5 minutes or so...the fan never kicked on....I stock all the other transistors except i am out of the j176 at this time....i will restock and swap it and see what happens....any other ideas please throw my way!!! Thanks..

Art

[Jazz P Bass]

Ampeb B2 fan circuit

Is this with the "old type" circuit?
Can you monitor the voltage at the junction of Q18 & R58?
The LM 35 should output a linear voltage rise with increasing temperature.
If all the parts around Q18 are good, Lm 35 is new, it "has" to work.
Unless Q19 is pulling down the LM35 output.
I am sorry but I did not note at what temperature the fan did turn on.
All I know is that it did.
I do recall the that I ran the amp upside down.
It got pretty toasty until the fan finally came on.

[abav]

Yes i have the "Old Circuit".....and I ran the amp again.....but longer....and the fan DID come on.....Boy, that's a little too warm for my taste.....Thank you for the mod. Now it will be more stable!!

Art

[Jazz P Bass]

Repost of the fan mod pdf.

Actually they had three different circuits.

Instead of R54 A/B being the correct 270 ohm value, they used a 680 & a 1K/ 5w resistor.
R55, the collector resistor was 220 ohms, instead of 470 ohms.