Transistor gurus? Need help understanding if there is a fault. - Page 1
Forum section: electronics / repair

author: mkiijam
author_rank: Regular Contributor
timestamp: March 18, 2023, 05:25:58 am
content: This is a Realistic STA-204.Came to me with a shorted / failed TR405. I've replaced it (with a trusted part, even swapped for known good) and it seems to run hot. I find myself not really knowing enough about this circuit to fully understand if there is an issue or not. Here are the facts of the case so far.1. TR405 runs past 85 degrees C. I haven't let it run much further. The PCB is discolored like it runs pretty toasty, or at least has.2. TR405 E=13.3Vdc B=13.9Vdc C=24.5Vdc3. TR406 E=5.4Vdc B=6.0Vdc C=13.9Vdc4. R410 Measures 64 Ohms.5. R410 drops about 9 Volts. Meaning it's about 34.5Vdc on coming from D4026. TR406 E has about 350 Ohms to chassis. This reading is not super straight forward as it charges up a bit, then lowers, then finally stabilizes around that number, some caps charging up or something. However, even when I disconnect this part of the circuit(cut traces) the drop across R410 lowers to about 5Vdc, but the transistor still seems to run just as hot.A. Can anyone tell me if this appears normal for the voltages on this transistor?B. Surely, the 9 Volt drop across R410 is not right(over a Watt dissipation)C. I have replaced C412, actually all of the 10uf caps, as they were ESRing kinda high-ish.D. I have not replaced TR406 as the Voltages seemed fine.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2023, 08:07:48 am
content: The circuit shows a bunch of old style discrete buld voltage regulators. TR406 and TR405 are for some 13.1 V nominal. Getting 13.3 V is not that far off and should be well within the tolerances.It is normal that TR405 gets hot in a linear regulator. If at all the fault would be on the output side, with some parts consuming too much current. A mains supply higher than normal can also increase the heat dissipation. It is also not so uncommon to have old circuits to run some of the transistors quite hot, up to the point that over long time they turn the board brown or fail.One could think about adding a little more heat sink to TR406.The drop over R410 looks reasonable, it is an old days way to reduce the power loss in the transistor and have part of it in the resistor. The resistors also help measuring the current in case of a fault limit the current and R410 may be a type to fail open by design.Replacing C409 and C410 with slightly larger values could contribute to increasing the power loss.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: March 18, 2023, 01:07:06 pm
content: A few more things:1. Without knowing the package and installation details of TR405, it's difficult to tell what it is capable of. Do you have a picture of this part of the board?2. The current through R410 is 9V / 64ohms = 140mA. The power dissipated in R410 is 9V * 140mA = 1.25W. This is already above the 1W rating of the part.3. The power dissipated in TR405 is (24.5V - 13.3V) * 140mA = 1.56W.4. When it was made line power might have been 110V to 115V. What is it now? I can find up to 125V these days.It may be operating as designed, which may also be marginal. There could also be fault elsewhere. Maybe replace R410 with something like a 100 ohm, 3W resistor. If the current draw is the same, then this would drop 14V, dissipating 2W. Then TR405 would drop 6.2V for a dissipation of around 0.9W.

author: mkiijam
author_rank: Regular Contributor
timestamp: March 18, 2023, 04:52:31 pm
content: TR405 is a 2SD571.According to the schematic Voltage references, R410 should only drop about 3 volts. Yes, there is no way the 1.25W is acceptable.I'll check a few more things when I get back to the shop.Thanks All!

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2023, 05:08:09 pm
content: To much drop on R410 means to much current drawn by the part not shown in the part of the schematics.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 13, 2023, 11:30:18 pm
content: With my Fluke hooked up between the output of the regulator and the rest of the circuit, it pulls about 140mA.

author: M0HZH
author_rank: Regular Contributor
timestamp: April 14, 2023, 09:07:20 am
content: Circuit seems fine but it's not designed for 140mA, you probably have excess output load. Check further down the line, what's draining too much current.

author: mbz
author_rank: Contributor
timestamp: April 14, 2023, 09:12:02 am
content: Quote from: mkiijam on April 13, 2023, 11:30:18 pmPrevious calculations above predicted 140mV from 9V drop. As above clearly problem is downstream, find out where the 140mA is going

author: Greybeard
author_rank: Frequent Contributor
timestamp: April 14, 2023, 09:30:02 am
content: Something else getting hot?

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 14, 2023, 04:42:45 pm
content: What is the item this regulator board is in? Is it a stereo receiver or integrated amplifier? Please provide brand and model number.The 140mA load on 13V DC power supply rail is ~2x what it should be. I expect this to relate to an observable fault on one of the PC boards which receives the 13V. I recommend individually checking the mA flowing into each PC board which receives power from this 13V rail.Integrated amplifiers and receivers often contain speaker relays with 12V coils. DC sensing circuits and speaker relay sections are often powered from a 13V rail. Check the resistance of the relay coil and the circuitry in the DC sensing circuit. Perhaps there is excessive drain in this section?If the item is a stereo receiver, nearly all AM/FM sections are powered by 13V supply.Additionally there may be other boards or stages which also receive the 13V rail.If there are any digital logic chips or MCU in this unit, then it is typical to have another linear regulator which drops the 13V to 5V for the logic circuits. Check for a malfunction there. The entire load current of the 5V rail flows from the 13V rail.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 14, 2023, 06:40:29 pm
content: Have you access to a thermal imaging camera to test with, something else is probably getting hot as well. That circuit drafting style looks familiar to me, what is the device the problem is in?

author: DimitriP
author_rank: Super Contributor
timestamp: April 14, 2023, 07:46:12 pm
content: Quote

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 14, 2023, 08:36:40 pm
content: Quote from: DimitriP on April 14, 2023, 07:46:12 pmOops! I was reading the forum on my phone and scrolled too fast to read the very first line of the OP's post

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 14, 2023, 10:25:22 pm
content: QuoteQuoteSchematic shows +13V rail feedsonlytheAM/FM tunercircuitsandtheLED driverfor the AM/FM signal strength indicator. So... here's my list of questions:Does either AM or FM work?AM/FM section contains 2 IC chips which receive 13V power no matter what position the input selector switch is in. The main IC in the AM/FM tuner isIC201 (HA11211).It has series resistorR218 (22 ohms).Voltage drop across R218 should be <1V, which corresponds to~40mA. If anything is wrong with IC201 you won't hear FM at all, perhaps no AM either.If FM works:Does stereo indicator turn onwhen tuned to a strong FM station which is known to be transmitting in stereo? And does the stereo indicator turn off when not tuned to a station?The FM stereo decoder IC isIC202 (MC1310P or AN115). Series resistor isR268 (47 ohm). Voltage across R268 also should be <1V. This works out to~20mA.Does signal strength LED bargraph work?EDIT: Another eevblog user informed me the LB1405 provides ~5mA to each LEDIt's current drain depends onhow many LEDs are on at same time. Each LED takes~10-20mA.~5mA So,total current with 5 LED "on" will be ~30mA.When no LEDs are lit upthis board should draw very little current, likely <5mA. I haven't looked at the datasheet for IC801 (LB1405) yet but one of the resistors connected to it may control the LED current. So if these LEDs are brighter than they really need to be, then changing the correct resistor might dim them a bit. The human eye has a logarithmic response to brightness. Therefore reducing LED current by half makes them get only slightly dimmer to the human eye.There's a few mA that go elsewhere in the AM/FM tuner section, but adding this to what IC201 and IC 202 take suggests thetotal for the AM/FM section is ~70mAInteresting:Adding in the LED bargraph current (~30mA) for when all 5 LED are "on" gives a grand total of~100mA.To conclude, your measurement of 140mA isn't terribly out of bounds,but it certainly is higher than expectedPerhaps isolating one section at a time is a good plan:- Temporarily disconnect +13V rail from LED bargraph PC board. This terminal is marked 1 on schematic- Temporarily remove one end of R218 (22 ohms) and one end of R268 (47 ohms)-Now how much current is being taken from the +13V supply rail?There is a possibility that if AM, FM, FM stereo, and the LED bargraphare all functioning perfectly, then 140mA might be "normal" for this receiver. Back in the day they did press these linear regulator circuits hard. In my experience it is completely normal to see "darkened" PC board substrate in these linear regulators. And these regulator transistors like TR405 do run "hot to the touch."As to whether you wish to modify this regulator circuit to beef it up, one must ask"If it didn't break until it was >30 years old, then how much better does the circuit design really need to be?

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 14, 2023, 10:32:43 pm
content: Quote from: Xena E on April 14, 2023, 06:40:29 pm"Realistic"was the house brand for audio gear sold atRadio Shackstores in USA/Canada. To the best of my knowledge most of the Realistic home audio products were manufactured bySanyoorToshiba. In my opinion the "Realistic" brand is comparable to other mid-priced stereo components made from 1970-1990.

author: JeremyC
author_rank: Regular Contributor
timestamp: April 14, 2023, 11:06:57 pm
content: Quote from: elecdonia on April 14, 2023, 10:25:22 pmThe LB1405 (IC801) has internal ~5mA current source for each LED. I wouldn’t expect the current to be >30mA when all 5 LEDs are on.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 14, 2023, 11:51:18 pm
content: Quote from: JeremyC on April 14, 2023, 11:06:57 pmThanks! I'll edit my post to reflect this information. I wasn't aware that it was only 5mA per LED.

author: JeremyC
author_rank: Regular Contributor
timestamp: April 15, 2023, 12:04:24 am
content: Quote from: mkiijam on April 13, 2023, 11:30:18 pmMaybe some capacitors are leaky?Check:C802, 1/50 (Signal LED PCB)C803, 47/16 (Signal LED PCB)C263, 470/16 (tuner PCB)BTW, This amplituner is 35+ year old, I would replace all electrolytic caps.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 15, 2023, 12:11:07 am
content: Regarding electrolytic bypass capacitors from +13V rail to ground, if any have substantial internal leakagethey will get hot.I've rarely encountered "leaky" electrolytics. My experience is that they fail to "open circuit" or to "high ESR."But leaky enough to load down a power supply in solid-state gear? Unlikely in my experience.I have encountereda handful of ceramic and tantalum capacitors which had high leakage or even turned into dead shorts.QuoteI agree that it is a wise choice to test ESR of every electrolytic in old gear. I still use the legendary "Dick Smith" ESR meter which is able to check ESR with the capacitor still in-circuit, as long as power is off and all capacitors are discharged. If I find only 1 or 2 bad then I don't change them all. When the portion with high ESR gets over 25% then I go ahead with total replacement. Interesting: Large electrolytics fare better than tiny ones when it comes to developing high ESR. The ones with the biggest chance of high ESR or totally open circuit are <22uF in many cases.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 15, 2023, 11:47:48 pm
content: I'm away from my desk right now, but I thought I would post some more info that I have gathered since I posted about verify the current draw.Switching the SOURCE selector to FM (i.e. powering up the FM circuit) increases the draw from 70mA to 140mA. I went through ALL the resistors and measured the voltage drop across them and then added up the current using the schematic value. My math says there is about 65mA draw in everything OTHER than what is switched on with the FM/SOURCE switch.R218 (top center of schematic) has a 34mA draw through it with FM not selected. If I remember right, I think it went up a bit with FM ON.R268 has a 20mA draw. Same FM off.The rest of the resistors you see(other than what is switched on by the FM Selector switch) were 1-3mA draw.I had decided to just consider that it is "normal" and build a heatsink for the regulator. Or perhaps little leaks everywhere.Does the 34mA look okay?

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 16, 2023, 02:04:42 am
content: QuoteWe need to find out where that additional 70mA goes for FM position of selector switch. I think the mA for FM should increase by less than ~15mA. Try this:What is mA for AM ?How much does mA change for FM tuned to a strong local station vs. tuned to an empty spot on the dial?Is there any change for FM vs. FM stereo mode?Can you measure how much mA goes into the LED bar graph signal strength display?This requires 6 measurements, all LED off, then 1, 2, 3, 4, 5 LEDs on at same time.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 17, 2023, 11:58:00 pm
content: Quote from: elecdonia on April 16, 2023, 02:04:42 am80mA for AM.100mA FM tuned to nothing130mA FM tuned and stereo off /MONO95mA Tuned to a strong FM station and I pulled LINE 1 from connector "N" (the cable the feeds the LED driver circuit)After pulling this connection, even the AUX and PHONO inputs seem to pull less current.65mA AM with PIN 1 of "N" pulled.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 19, 2023, 01:36:22 am
content: Would you say that both AM and FM reception is working well?My thinking is thatseriousfaults in the AM/FM circuits would cause reception of some (or all) broadcasting stations to be weak or non-existent.If the performance of the AM/FM tuner is good then there isn’t much to worry about.It may be the case that the 140mA load on the 13V regulator is “normal” for this receiver.Perhaps attaching a larger heatsink to transistor TR405 is the best solution to its high operating temperature?

author: mkiijam
author_rank: Regular Contributor
timestamp: April 21, 2023, 02:14:34 am
content: Both AM and FM seem to work well.Any reason why i shouldn't just build a LM7812 circuit for it? or LM317 if it really needs to be 13Vdc. TR405 is just a tiny package. maybe finding a transistor that is just plain bigger, but I'm not versed enough in transistors subs to be helpful to myself. I'm about to make an order at JLCPCB, maybe I should throw in a little daughter card design...

author: jwet
author_rank: Frequent Contributor
timestamp: April 29, 2023, 08:05:56 pm
content: R410 is listed on the schematic as a 1W resistor, 145 mA through 68 ohms is about 1.3w. The transistor is burning even more power. R410 would have burned up long ago if it was pushing 1.3w from the start. I wouldn't expect that leg current to be much more than 60 mA max. probably about 40 mA. Something is taking an extra watt of power in that leg, that should create some real heat- you could use a damp finger test or some freeze spray and watch as it boils off. A 7812 isn't the answer- a 7812 dropping 17v with 145 mA would be 2.5W. A well heat sunk TO-220 7812 would only rise about 10 degrees at that but without a heatsink, can't dissipate much more that 1.5w. (theta 70K/w). Try to find the hog, something is wrong and if you don't find it, you'll just get it back in 6 months even with the linear regulator.