StageLine STA-162 power amplifier cooling fan circuit - Page 1
Forum section: electronics / repair

author: ascomm
author_rank: Contributor
timestamp: February 16, 2025, 10:59:57 am
content: Hi,I've been repairing couple of STA-162 Power amplifiers and have strange behaviour with one of them.Amp has active cooling fan control circuit and I don't really understand why the circuit is like it is.Attached is the schematic of the circuit. Could anyone clarify why there might be 48V supply for 24V DC cooling fan?The fan is box standard BLDC motor "PC" cooling fan (with 24V operating voltage).In one amplifier the Q50 transistor (TIP30C) is heating to over 110°C while the second amp's Q50 stays around room temperature. The voltages around the transistor are pretty much the same on the both amplifiers. On both devices the PCB is "cooked" under the transistors so they've been very hot on both units for some reason.

author: CaptDon
author_rank: Super Contributor
timestamp: February 16, 2025, 07:26:05 pm
content: Where in your schematic do you see 48VDC?

author: CaptDon
author_rank: Super Contributor
timestamp: February 16, 2025, 07:31:05 pm
content: It looks like there is a left side and a right side oscillator and they are using PWM to control fan speed so T50 should be full on or full off at the PWM rate. On the bad amplifier what does the drive signal to T50 look like compared to the good amplifier. For some reason it looks like T50 is running in the linear range on the bad unit. Very straight forward control circuit. Get your scope on it, problem should be evident. Bad or bound up motor??

author: ascomm
author_rank: Contributor
timestamp: February 16, 2025, 09:24:05 pm
content: Quote from: CaptDon on February 16, 2025, 07:26:05 pm+24V and -24V.

author: ascomm
author_rank: Contributor
timestamp: February 16, 2025, 09:26:04 pm
content: Quote from: CaptDon on February 16, 2025, 07:31:05 pmHave to check the signal on the Q50 base.Fans are ok.

author: CaptDon
author_rank: Super Contributor
timestamp: February 17, 2025, 01:22:03 am
content: I think I would add some kind of a resistor, maybe 2.2K or something around that range between the emitter and base of T50 to insure the transistor turns completely off when either PWM circuit is not pulling the base down through diode D39 / D40. Definately put the scope on it. Also check the frequency of the two oscillator circuits. They have an oscillator feeding a comparator to develop the pulse width of the PWM circuit. It does sound like either there is some leakage keeping T50 partially turned on, or the opposite scene where it isn't being fully turned on into near saturation. Cheers!! let me know what you find.

author: ascomm
author_rank: Contributor
timestamp: February 18, 2025, 07:38:57 pm
content: I measured all the resistors diodes and the Q50. All are ok. There are no any oscillations / PWM going on in that circuit. Just stable DC-voltages.Didn't find any faults.I also tested resistor between Q50's E and B but that didn't do anything.Measured voltages are written in the schematics if anyone wants to look.

author: CaptDon
author_rank: Super Contributor
timestamp: February 18, 2025, 09:32:55 pm
content: I'm sorry about the confusion. When I took a look at I.C.3 A and B I thought the circuit looked like an oscillator to generate PWM. How do the voltages compare to the good unit?

author: CaptDon
author_rank: Super Contributor
timestamp: February 18, 2025, 09:49:50 pm
content: Given the voltages you measured the transistor would be generating about 1 watt of heat and so I hope it has a heatsink. Also the apparent current in R123 appears to be 27 milliamps so I am not sure how you measure 43 milliamps at the fan? There appears to be 24 milliamps through R131 and the difference of 3 milliamps could absolutely account for base current. This circuit doesn't seem broken. From these observations the fan would be running at low speed since there appears to be about 5 volts across a fan you said is rated at 24vdc.

author: CaptDon
author_rank: Super Contributor
timestamp: February 18, 2025, 10:01:51 pm
content: All of the numbers seem to indicate a normally functioning circuit. The voltages and currents look reasonable. I can see 1 watt of heat being generated by the transistor. With .47vdc emitter to base the transistor is just beginning to conduct and therefore has a large C-E voltage drop with the downside of generating heat. My question isn't why is this unit running hot but why is the other unit running cold?? The voltages you show can not be the same for the good vs. bad amplifier! Ohms law doesn't lie and Trump hasn't repealed it. (Yet)

author: ascomm
author_rank: Contributor
timestamp: February 19, 2025, 05:50:18 am
content: Thanks!I'll check the other amplifier today and report back the voltages and currents in that.The Q50 does not have heatsink. As I mentioned in the first post both amps have browned PCB under the transistors so they have been hot for long time. Apparently that's how they are designed to work.BTW. I used El Cheapo DC current clamp to measure the fan current so it might not be very accurate.