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91
Amplifier Discussion / Re: Frontman 212R
« Last post by Enzo on June 02, 2020, 02:04:37 AM »
Q8 is NPN, with emitter -41.8 and base -40.5, makes the base 1.3v more positive than the emitter.  Sounds like should be ON to me.
92
Amplifier Discussion / Re: Frontman 212R
« Last post by dlbraly on June 01, 2020, 11:21:20 PM »
Q8 Emitter is -41.8, Collector 163m v, Base -40.5 so I think that is off.

Q7 Emitter -52m V, Base -360mV, Collector -40,3
Q6 Emitter -3.2mV, Base -223mV, collector -379mV

I still have my -16V on one side of R70, R72, and D19, The other sides are all around -360mV
by these readings, I guess my problem is not in this area.

Do I keep going back? I see a triangle that says "Mute" and another one in grid D8 by the input jack.


93
Amplifier Discussion / Re: Frontman 212R
« Last post by Enzo on June 01, 2020, 02:19:18 AM »
Think circuits, not parts.   Q12,13 are in a circuit that stretches from V+ to V-.  If Q12 is not turned on, then the voltage from Q13 will be sitting on its collector.   And if Q13 is ON, that will be V- or similar.

The emitters Q0,10 are not low.  They are around zero, yes, but they connect to V- through Q8.  What is happening around Q8 anyway?
94
The Newcomer's Forum / Re: Tda ICs Pentawatt PCB
« Last post by edvard on May 31, 2020, 11:48:32 PM »
The tab is connected to V-.

If you can avoid, or have no intention of, grounding the heat sink by attaching it to a metal chassis (most common)or other scheme, you can get away with mounting the chip directly to it with some thermal grease.  CAVEAT: Make absolutely sure that there isn't or won't be the opportunity for some wayward part to short the heatsink to ground or the V+ rail. 
Or just use an isolation pad.  For breadboarding, I wouldn't risk going without one.  For final assembly in a box, use some common sense (and a barrier between the heatsink and the rest of the circuit) and you can get away with it.

By the way, I've heard all the cool kids are using 1mil Kapton Tape for isolating their semiconductors nowadays.  Good thermal conductivity (not as good as Mica, better than Sil-Pads), pretty tough, and one 36 yard/~33 meter roll oughta last you a while...

P.S. All you ever wanted to know about attaching something to a heat sink:
https://www.eg.bucknell.edu/~dkelley/eceg351/FairchildHeatsinkMountingGuide.pdf
http://www.junradio.com/Discretes/TI/9701.pdf
95
Amplifier Discussion / Re: Frontman 212R
« Last post by dlbraly on May 31, 2020, 11:40:47 PM »
Hello
I did not understand the readings I was getting on Q12 PNP. Emitter 42.3, Collector -29.4, and base 41.9. Not sure how the Collector could be negative.
So I pulled Q12, Q14 and Q15 checked with a LCR-T4-H. Also checked C51, It said they were good so I put them back.
Put it all back together and still have the same readings at the output -29.4 VDC.

Q11 The emitter reads 41.9, Collector -40.2, base 41.8 VDC, So similar to Q12 readings - Figure it probably is ok.

Q13 emitter -41.6, Collector -29.5, and base is -41.4
Those reading seem right

Q9 Emitter 164m VDC, Collector 41.1 , Base 540m VDC
Q10 Emitter 164m VDC, Collector 42 , Base -2m VDC

Do these readings look right on Q9 and Q10. is the Emitter and base so low because I'm getting in to the preamp stage?

Kind of stuck, Can you give me a little push in the right direction?
Don't tell me is you know the full on fix, but maybe a hint or something I should read on.
96
The Newcomer's Forum / Re: Tda ICs Pentawatt PCB
« Last post by Enzo on May 29, 2020, 05:15:28 PM »
The mica insulator  (there are also silicone ones)  is there to prevent the rear surface of the transistor, which is usually the collector, from making electrical contact with the metal heat sink, which is usually grounded.  In the case of the five leg power amp ICs, the rear of the IC is usually one of the power rails, I forget whether V+ or V-.  In any of those cases, touching the metal back of teh part to groundeD heat sink will short out the power supply.
97
The Newcomer's Forum / Re: Tda ICs Pentawatt PCB
« Last post by Oooscar on May 29, 2020, 04:14:06 PM »
I see, thanks a lot... I have read about mica washers. The idea is to use a washer that transfers well the heat but not the electricity. Usually, go together both properties. The reason to do that is to avoid kind of antennae effects? I have not thought in those issues. Sorry... Too many questions...

About the heatsink, I have a piece of solid metal from a car radio, this piece is one the sides of the whole radio. Pretty impressive heatsink.
98
The Newcomer's Forum / Re: Tda ICs Pentawatt PCB
« Last post by joecool85 on May 29, 2020, 01:07:58 PM »
Thanks a lot...  :tu: About the heatsinks, do you think any piece/sheet of metal will do the job, or it is worth to use a proper one. Aluminum or denser... ?

Aluminum is the typical go-to for heatsinks.  Also, I hit edit by accident on your post, it's been a long day.  But I didn't actually edit the post.
99
The Newcomer's Forum / Re: Tda ICs Pentawatt PCB
« Last post by phatt on May 29, 2020, 08:17:35 AM »
Again Electronic junk comes cheap and I'm sure it won't take long to find a heat sink in any old computer, amplifier, dead battery charger or like gear that is thrown out.
Or just go to the hardware store and purchase a 2mmx50x50mm length of angle alloy (that's 2x2 inch for those in the land that still lives in imperial :-X)
Thickness, 2mm or thicker if you can find it. Cut at least 3 inches to give the heat somewhere to go. again more is better. And don't forget to isolate the chip with a non conductive washer.
Phil.
100
The Newcomer's Forum / Re: Tda ICs Pentawatt PCB
« Last post by Oooscar on May 29, 2020, 06:59:43 AM »
Thanks a lot...  :tu: About the heatsinks, do you think any piece/sheet of metal will do the job, or it is worth to use a proper one. Aluminum or denser... ?
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