This old audio amplifier temp rises by 16*C / 60*F with nothing connected. - Page 1
Forum section: electronics / projects

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 25, 2015, 05:49:35 pm
content: Hello there,I recently bought a used Sony TA-F5A 2x70W audio amplifier. Its about 40 years old. The output amplification stage has a Sony branded DIP-14 power audio amplifier driver, driving four TO220 of BJT transistors per channel.Whilst the amplifier is powered up, with no load, no input, nothing connected, the enormous heatsink at the back rises by about 16*C / 60*F above ambient temperature within about 10-20 minutes. I was surprised to find this. From what I can tell, there is nothing connected to heat sinks apart from the output transistors. Should I anticipate that something is broken on it? What could be going on? What would you advice me to check?Here is the service manual for the Sony TA-F5A:http://folk.uio.no/mariujon/hfe_sony_ta-f5a_service.pdfAnd here is the schematic of the output stage:Thank you for your time.Kind regards,Marius

author: dom0
author_rank: Super Contributor
timestamp: December 25, 2015, 05:52:54 pm
content: Most likely RT302 (quiescent current) not adjusted correctly / bad contact.Adjust it for maybe 5 mV or so across the output stage emitter resistors (~ 30 mA).

author: wraper
author_rank: Unknown
timestamp: December 25, 2015, 06:12:09 pm
content: Quote from: dom0 on December 25, 2015, 05:52:54 pmMaybe there is nothing to adjust. 30mA is about the minimum quiescent current which I would expect from such circuit. If it is approximately the same for both channels, just leave it alone.EDIT: in service manual it is written to adjust to 16.5mV -> 50mA per transistor, 100mA in total. So such heating seem to be a right thing to happen.

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 25, 2015, 06:16:00 pm
content: Hi guys,I measured 20mV and 12mV across R391 and R390 respectively. I found similar values for the other channel. What do you reckon?Thanks.EDIT: I edited the original post, there are actually 4 BJTs per channel.

author: wraper
author_rank: Unknown
timestamp: December 25, 2015, 06:20:58 pm
content: Quote from: king.oslo on December 25, 2015, 06:16:00 pmLook page 11 and adjust to value specified on test pins.

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 25, 2015, 06:35:33 pm
content: Ok, great.Do you reckon that the emitter currents I measured (50mA) through the four output resistors may be responsible for the 16*C/60*F temperature rise I measured? This is the size of the heat sink:Thank you for your time.Kind regards,Marius

author: dom0
author_rank: Super Contributor
timestamp: December 25, 2015, 06:47:20 pm
content: That heat sink has maybe 1 K/W or so, so 100 mA current total at maybe 100 V or more total supply voltage = 10 W, the number seems reasonable.Still, if RT302 is not an encapsulated trimmer I'd still replace it. These things are unreliable at best.

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 25, 2015, 06:53:21 pm
content: Blueskull, yes.Thank you for your time, everyone!

author: ciccio
author_rank: Frequent Contributor
timestamp: December 26, 2015, 12:08:33 pm
content: Quote from: blueskull on December 25, 2015, 06:48:59 pmQuote from: king.oslo on December 25, 2015, 06:53:21 pmI'm still recovering from family Christmas' dinner and lunch, but I believe there is an error in the conversion.While a 16°C increase is reasonable, and I would not mind to check for bias current, 60°F is way too much.Let assume ambient temperature is 20°C = 68°F, and the amplifier heats up to 20 + 16 = 36°C = 96.8 °F.That is a 16°C increase, or a 30.8°F increase.A 60°F increase fill end up with the fins at 68 + 60 = 128°F, that correspond to (128-32)/1.8 = 53.33 °C, that is a 33.3 °C increase, maybe too high for power transistor's safety. If this is the case, I'll check bias current.Best regards and happy new year.

author: dom0
author_rank: Super Contributor
timestamp: December 26, 2015, 03:56:20 pm
content: He's from Norway/Europa, it's not necessary to know strange and redundant units by heart

author: sarepairman2
author_rank: Frequent Contributor
timestamp: December 28, 2015, 04:19:30 pm
content: This is not weird. Some op-amps will run at like 60C with no load by design.

author: calexanian
author_rank: Super Contributor
timestamp: December 29, 2015, 03:51:48 pm
content: Generally theese circuits are biased in the imperical manner. Set bias for minimum current, apply a realtivley low signal to obtain a few Volts signal output, then increase bias to the point where you get rid of crossover distortion, then increase signal to full power to make sure crossover does not come back and then once that is done go check the emitter currents to see if they are anywhere close to spec. Don't forget to adjust do offset as well. you will need a clean signal generator and a scope. Older circuits like that do run hot. 16c is a little too hot though for such a large heat sink.

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 29, 2015, 03:55:33 pm
content: Quote from: dom0 on December 26, 2015, 03:56:20 pm

author: grumpydoc
author_rank: Super Contributor
timestamp: December 29, 2015, 04:02:14 pm
content: As to the amp warming up - is it oscillating or is it picking up hum and amplifying that?Anyway, it might get btter when you replace the output transistors

author: king.oslo
author_rank: Frequent Contributor
timestamp: December 29, 2015, 04:39:20 pm
content: Quote from: grumpydoc on December 29, 2015, 04:02:14 pmRight now its not doing much, its got its soft start resistors removed and exactly half its power transistors gone

author: Siwastaja
author_rank: Super Contributor
timestamp: December 29, 2015, 05:22:18 pm
content: Everyone seems to be missing the fact that such passive heatsink has surprisingly low power dissipation at low deltaT. It really needs some temperature difference before the air starts flowing. Smallish amount of power can cause 16 degC rise. For, say, 40-50 degC rise, a LOT more power would be needed.

author: rrinker
author_rank: Super Contributor
timestamp: December 29, 2015, 10:03:44 pm
content: And I wonder - what type of amp is it? The Technics that did me in college was totally backwards to the uninitiated - at low levels listening privately in my room, it got hot, very hot. But on those warm spring days when you pointed the speakers out an open window and ran the thing cranked to 11 - it was cool as a cucumber.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: December 31, 2015, 06:21:26 pm
content: Is this a Class A amplifier? Then the bias current could, perhaps, cause heat-up without small signal applied:http://www.learnabout-electronics.org/Amplifiers/amplifiers12.php

author: wraper
author_rank: Unknown
timestamp: December 31, 2015, 06:43:35 pm
content: It was working class AB amplifier. There was no need to destroy it.Quote from: John_ITIC on December 31, 2015, 06:21:26 pm2x70W class A would heat like crazy. That heatsink would be at least 5 times too small. On this one I would expect something like 150oC temp rise or even more.

author: RobertBG
author_rank: Regular Contributor
timestamp: January 01, 2016, 01:04:34 am
content: Quote from: rrinker on December 29, 2015, 10:03:44 pmIt's quite normal for this amp to get hot sitting idle,I'd recommend playing it before poking around too much inside.Although while you have it open the meters look quite nice with blue led'sI've had a few of them over the years and they are great units