Hey everyone. I recently brought home a Teac AG-H300 Reference Series stereo receiver. It had the main transformers thermostat blocked in off state so it wasn`t receiving any power from the mains but that is fixed now.
The sound quality of the unit is amazing, however the units amp runs quite warm even when idle, at about 50-55 degrees celsius. The amp doesn`t have a pot trim to set the bias. It has 35w per channel into 8 ohms, and it`s powered by 42-0-42 VDC which in my oppinion for 35w is alot. The amp is stated to be capable to drive even 16 ohm speakers so maybe that is why the DC voltage is that high. The power transistors are D998 and B778
The sound quality of the unit is amazing, however the units amp runs quite warm even when idle, at about 50-55 degrees celsius. The amp doesn`t have a pot trim to set the bias. It has 35w per channel into 8 ohms, and it`s powered by 42-0-42 VDC which in my oppinion for 35w is alot. The amp is stated to be capable to drive even 16 ohm speakers so maybe that is why the DC voltage is that high. The power transistors are D998 and B778
These are very different products and you can't compare AB class amplifiers which may have any arbitrary bias level the designer wishes to use for the best (reference series) sound quality. The stereo amplifier could be running with considerably higher bias than the HT receiver. They are probably Chip-amplifiers with about one quarter of the bias current per channel of the stereo amplifier. Being a small case and enclosed heatsink, it may be quite normally hot, as Jon Snell thinks.
Since bias is fixed, it can't be easily messed with and that is good thing for safety and performance reasons with most curious owners. It also means you can't say it needs adjusting unless someone has already changed the resistors around Q517,8. Look carefully for signs of flux, resoldering, wrong value resistors etc. in that location.
I guess it is a possibility there has been an overheating problem if you found the transformer protection thermostat open circuit - a warning that something bad had happened in the past. Is the transformer also becoming hot now?
Since bias is fixed, it can't be easily messed with and that is good thing for safety and performance reasons with most curious owners. It also means you can't say it needs adjusting unless someone has already changed the resistors around Q517,8. Look carefully for signs of flux, resoldering, wrong value resistors etc. in that location.
I guess it is a possibility there has been an overheating problem if you found the transformer protection thermostat open circuit - a warning that something bad had happened in the past. Is the transformer also becoming hot now?
The transformer is also quite warm to the touch. I think that the reason why the thermostat was faulty is that the amp was used with a pair of 4 ohm speakers, or worst even two or more pairs. The type of people I got it from are famous for blowing amps because of improper usage, usually at home parties.
Probably the amp was running way above it`s limit therefore the transformer overheated and triggered the thermostat and it remained in the off state.
Note: The 0.5v and 1.1v voltages around the power transistor are ok
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That overload seems plausible, though I haven't met many people quite so foolish as to use a small, high quality amplifier for parties. The temperature rating of the 'thermostat' should be evident from its marking or part reference, since the manual only specifies the transformer, not its component parts or their ratings.
If you take great care not go near bare wiring, and you can keep your open hand in contact with the transformer for 10 seconds or so without heat injury, it will be operating safely below 60C, which otherwise, is as hot as you want anything in the case to be.
Trust me, there are people that foolish.
Unfortunately the thermostat is inside the transformers primary coil on top of which are the secondary coils and they are separated by a plastic former. I was lucky enough that I could bypass the thermostat as the lead that was soldered to the wire of the primary coil was reachable. I had to snap away some plastic bits tough to be able to solder a wire on that lead.
What about the amp, is it normal then to work this warm. I had a few other amps that ran quite warm when idle like a Yamaha A-500, Denon PMA-300V, Onkyo TX-SV545, but this Teac is the "hottest" of them all
Well, a heatsink that does not exceed 60C ( worst case, at about half the rated power, constantly) is likely the limit to safe operation. I use a class A amplifier that runs with enclosed heatsinks at 50-55C and that is quite safe, since the ventilation is good and the case is a full 19" rack size, meaning most of the case is cool inside.
Temperature is estimated by comparing with what we are familiar and as you say, this is hotter than other amplifiers. You can estimate the temperature simply, as safe by the method I described or get yourself a practical, surface measuring thermometer and be precise about it.
If you are still concerned, attach a 50 or 60C mains rated thermal switch centrally to the top of the heatsink. Wire that in series with the mains input to the power transformer. Take proper care with mains wiring and use suitably rated wire insulation and connectors. This is a low cost, reliable safety measure if it is a genuine, approved part.
Thermostats - NC
Temperature is estimated by comparing with what we are familiar and as you say, this is hotter than other amplifiers. You can estimate the temperature simply, as safe by the method I described or get yourself a practical, surface measuring thermometer and be precise about it.
If you are still concerned, attach a 50 or 60C mains rated thermal switch centrally to the top of the heatsink. Wire that in series with the mains input to the power transformer. Take proper care with mains wiring and use suitably rated wire insulation and connectors. This is a low cost, reliable safety measure if it is a genuine, approved part.
Thermostats - NC
I should have added in the above post that the amplifier already has comprehensive protection circuitry under microprocessor control which senses excessive current and DC faults at the output. That means the amplifier would not fail with abuse but the transformer protection did. Perhaps that is just as well.
In any case, you can measure the idle current to each amplifier by monitoring voltage drop across R533,534 or R535, 536 (take great care with meter probes that easily slip and will destroy your amplifier in a flash - use clips or solder leads to the resistors instead) and calculating via ohms law. It is always possible that fault current is being drawn by other parts of the receiver, so check that current to these is reasonable, assuming that nothing has yet been messed with, and bias in each amplifier is approximately equal so it is likely correct but of course as it is fixed by design, there will be no specification for this.
A complementary, Emitter-Follower design output pair like this, could require 100-125 mA for optimum, lowest THD bias but that is by no means the only bias current used by designers who often have different ideas about the value of distortion. Class AB is not a definite or optimum bias condition, just one that has some bias level that is considered enough for the application.
TEAC AG-H300 Manual - AM/FM Stereo Receiver - HiFi Engine
In any case, you can measure the idle current to each amplifier by monitoring voltage drop across R533,534 or R535, 536 (take great care with meter probes that easily slip and will destroy your amplifier in a flash - use clips or solder leads to the resistors instead) and calculating via ohms law. It is always possible that fault current is being drawn by other parts of the receiver, so check that current to these is reasonable, assuming that nothing has yet been messed with, and bias in each amplifier is approximately equal so it is likely correct but of course as it is fixed by design, there will be no specification for this.
A complementary, Emitter-Follower design output pair like this, could require 100-125 mA for optimum, lowest THD bias but that is by no means the only bias current used by designers who often have different ideas about the value of distortion. Class AB is not a definite or optimum bias condition, just one that has some bias level that is considered enough for the application.
TEAC AG-H300 Manual - AM/FM Stereo Receiver - HiFi Engine
Thanks for the info. I will disassemble the main amplifier board to see if it suffered any servicing. 2 screws in the back were already missing so who knows. Again thanks very much for the help, I appreciate it.
I need a little more help with this amp.
Now it`s starting to do wierd stuff. Sometimes while one the sound stops, the input led indicators start flashing from left to right in a very quick manner, and the FL display starts flashing also. Above all this sometimes the left channel crackels.
This goes away after I hit the top a little.
Please clarify whether you have speakers or inputs connected, whether you have checked the bias setting yet or done anything since your previous post. We need to be clear about test conditions to avoid making mistakes. There could be many causes and they need to narrowing down to decide the most likely to check.
It seems from your description that the protection circuit is cycling an error status repeatedly and this could even be caused by bad solder joints, capacitor failure or someone messing with it. Did you look for signs of resoldering or different parts yet? Make sure you don't connect the amplifier to any speakers or inputs whilst this problem remains.
I keep thinking this had a fault which caused the overheated transformer in the first place and the reason the amplifier was binned. In some cases the crackling can be solder joints or it could even be the power the transistors themselves which have failed. This type of fault has been posted here a few times to my knowledge, though I haven't seen it happen yet.
Make sure the amplifier is disconnected completely and remake the solder joints around the output transistors in the faulty channel. Be careful not to dwell on soldering and risk detaching the foil from the board - use a tiny addition of liquid flux or just use fine, cored 60/40 solder. Some old boards are delicate and the glue weakens with age.
Note: you don't need to quote complete posts to show who you are replying to. Just the part you want talk about makes it much easier to follow and for others to add to the thread, like normal discussions.
It seems from your description that the protection circuit is cycling an error status repeatedly and this could even be caused by bad solder joints, capacitor failure or someone messing with it. Did you look for signs of resoldering or different parts yet? Make sure you don't connect the amplifier to any speakers or inputs whilst this problem remains.
I keep thinking this had a fault which caused the overheated transformer in the first place and the reason the amplifier was binned. In some cases the crackling can be solder joints or it could even be the power the transistors themselves which have failed. This type of fault has been posted here a few times to my knowledge, though I haven't seen it happen yet.
Make sure the amplifier is disconnected completely and remake the solder joints around the output transistors in the faulty channel. Be careful not to dwell on soldering and risk detaching the foil from the board - use a tiny addition of liquid flux or just use fine, cored 60/40 solder. Some old boards are delicate and the glue weakens with age.
Note: you don't need to quote complete posts to show who you are replying to. Just the part you want talk about makes it much easier to follow and for others to add to the thread, like normal discussions.
OK, back to basics. A transformer does not like switched/arcing mains connection and will get hot, taking the thermal fuse out. Nothing more and nothing less. So I suspect a faulty mains supply caused the thermal issue in the transformer. The heat sinks naturally run between 50 and 60 depending on ambient temperature and ventilation. The output transistors run nicely at 70 and won't be destroyed until excess of 120 C.
Sorry 🙂
I have speakers connected, and no, I did not check the amp to see if it was fiddled with. The crackling sound was a loose 3 pin connector from the preamp board to he volume control.
However the other issue remains. I think it acts like the transformer is dropping in voltages as there is still sound for 2-3 seconds after it starts blinking, because the main filter caps don`t discharge immediately. But that is my opinion. Maybe that`s not the case
You need to test voltages and post them if you think they are at fault. The schematic shows the important voltages but guesses won't help, use an accurate meter.
Protection is cutting in and out again after the fault is cleared momentarily by its own switching action, then repeating. Is the speaker relay clicking when the lights flash or is the amplifier itself silent?
I repeat, disconnect the speakers and inputs completely to the receiver so that you don't risk further damage unnecessarily, then see what happens without the load interfering.
Given that a connector was loose already, I imagine you have been replugging other connectors too. That is probably a good idea but make sure they are cleaned if oxidised and refitted properly - when the power is off!
Protection is cutting in and out again after the fault is cleared momentarily by its own switching action, then repeating. Is the speaker relay clicking when the lights flash or is the amplifier itself silent?
I repeat, disconnect the speakers and inputs completely to the receiver so that you don't risk further damage unnecessarily, then see what happens without the load interfering.
Given that a connector was loose already, I imagine you have been replugging other connectors too. That is probably a good idea but make sure they are cleaned if oxidised and refitted properly - when the power is off!
Thanks for the info 🙂 I`ve heard somewhere that 70-80 degrees is still ok for an amp but I just wan`t to be sure
The amp does need a good cleaning, it`s full of dust, and maybe your right about the connections being oxidized, how knows where it was kept after it have been binned. Maybe a little WD40 on the connectors will do the trick.
The relay does not click on and off, the sound goes away gradually, like as the power to the main amp is cut and it still playes until the main filter caps are discharged. As I mentioned before the blinking is fast, maybe 3-4 times a second, and the input source indicators flash from left to right like when rotating the input selector fast.
fine, now what happens when you disconnect the speakers and inputs? 😉
WD residues can be a problem too - sure, use it as a cleaning agent but remove all traces before reassembly.
I did not try it yet, I forgot to mention this fault happens rarely, the next time it does this I will disconnect everything
OK. Since I cleaned the inner connectors the problem seems to be solved. However I found another problem. The tuners right channel sounds different then the left. The frequency response of the right channel is cut off at around 9-10kHz, its sounds almost the same when listening to a 64kbps mp3 song.
It`s not a huge problem but it might be an issue since I wan`t to sell the unit.
Your advice was very helpful so far, maybe you can help me with this too 🙂
I've worked on FM tuners and even built a fully synthesized microcontroller type but sorting channel balance could involve measurements with a CRO at least. Some people are lucky with just dirty connectors as you have been, but I don't have any easy advice to follow here.
Assuming that the AM balance is fine, it could be a simple misaligned stereo decoder chip component or the 19KHz notch filter before the output buffer opamps or even coupling caps in that area after the decoder. Unfortunately, writing a 5 min course in FM tuner design and repair here, is beyond my ability.
Assuming that the AM balance is fine, it could be a simple misaligned stereo decoder chip component or the 19KHz notch filter before the output buffer opamps or even coupling caps in that area after the decoder. Unfortunately, writing a 5 min course in FM tuner design and repair here, is beyond my ability.
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