hfe should read a number, other than zero and since most digital displays have a plus x count, meaning 1 or 2 digits as specified in the specification sheet, the hfe number should be higher than that.

The 2n3055 looks like it tested good till I asked about the probe polarities when you tested it. I wanted confirmation. Now, if you retest it, write the values down using the six points I mentioned. Something is amiss, whether it's a typographical error or other error.

What is the "resistance" of the MIG wire? Take one of them out and measure it on the lowest resistance scale or even the diode check function. Something isn't making sense to me.

The wire you use to connect to the hfe sockets should have zero resistance.

 

The wire seemed to measure about 0.8Ohm but in fact the meter gives that for a short circuit. I need to calibrate it but I'm not sure how right now.

I did all the measurements again:



I've really no idea how to read the diagram, know what's going on. Fuse F3 seems to me to be going directly to ground. Seems to stand between a tap in the middle of the transformer and ground at G and/or CT.

How the output from the Transistors can create excess current there I don't know and can't figure out.

 

The "ground" as your seeing is actually a "return". That fuse is in the common connection on the center tap transformer. It is the return for both the positive and negative 32 volts. You need a complete path for current flow.

Look at your work. Look how each NPN measured and each PNP measured. I will tell you right now that the 2N3055 now shows a good transistor. The 2SC1030 readings are screwy to say the least.

I could have "assumed" you erred but it was more important to you see how you erred.

Any #22 AWG solid wire could be used in that hfe connector. I noticed you had a paper stapler, as they to could have worked in the connector.

On the lowest resistance reading, when you short your two probes together, does it read 0.000?


If you are happy with the readings, I have one question. Do you have two 2SC1030's or equivalent on order?

 

I do have two 1030's ordered.

On the lowest scale - 200Ω - it reads 0.8Ω

I'm happy I've learned how to do it and now have a 'proper' set of readings.

So that's maybe the whole problem? Or something else caused both of them to die?

 

Ok. That is the same as you read on the MIG wire. So, I can think we can assume the MIG wire isn't introducing a false hfe reading caused by things like coating etc.

Something else causing this problem is certainly a possibility.

Reviewing your results with the 2SC1030, I have a strong suspicion that you have the emitter and base reversed on those readings. I will take the blame for that as the graphic I showed you from that testing page had the E and B reversed. Can you redo the hfe test with that in mind?

 

I don't think there's anything for you to take responsibility for, Joe. The pinouts you show there are the same as you showed before - only with the tranny turned over so's the 'short end' is on the right instead of the left.

And I think it is the correct one. The couple of datasheets I've found show it to be like that and the numbers I get when I test like that make sense I think.

So it was me doing it wrong. Which ain't unheard of with me.

Here's the numbers now:



Which looks like those transistors are alright? So now pull more, test more? Or put back in and do live testing for voltages? I still didn't get my el cheapo CRO but perhaps the dmm?

And for interest I was wondering could I breadboard something to test transistors like that? Or we need too much power? I've got a 15V bench psu is all. And anyway I wouldn't think breadboarding with mains voltages would be very safe for a goose like me. But just wondering what's done in the trade.

p.s. and another thing I wondered was what about putting a whole new amp in there?

An 'off-the-shelf' amp?

Just wondering. Could such a thing be done or it's impractical because the amp would have to be married to the inputs available and outputs required to such an extent that nothing off the shelf would work and it'd be a really expensive project to build one, might as well rebuild the existing.

Cars, you see. We do that with cars, don't we? Just chuck a new engine in there.

 

Put them in and do testing. The 2SC1030s were tested wrong, because of the diagram I first sent you. I should have sent you the one from the datasheet and not that tutorial.

That tells me your hfe tests were backwards, and I had asked you only to redo those hfe tests.

We can proceed to the voltage tests at any time. ONLY if the series lamp is not brightly lit.

When I ask for a voltage test at an element of the transistor, the ground should be on G or Ct.

When I ask for it acrossed a component, the negative and positive will be at the points I recommend.

You can test the voltages at the E, B, and C of the four transistors, TR4 through TR7.

 

Solder everything back in? Okay, Joe, thanks. I'll see if I can get onto it today.

p.s. yes, as I remember the series lamp was dimly lit. We'll see....

edit I've put the components back in the board but I've having trouble soldering them in.

This new 60W iron won't tin the tip.

Could I use this 'Baker's soldering paste' on it, you think?

It's a weird iron. It seems to get nearly too hot to hold but the tip is still cold.
Could it just be a piece of rubbish? Do they make rubbish irons like that?

p.s. Just saw the stuff about testing at B.C.E. on the transistors 4 - 7. I'll do that soon as I get them soldered in.

 

Right. Got some numbers:

 

You might have TR5 incorrect. Remember it's a PNP.

 

But those are voltages. What does PNP matter? I should swap the probes around - Positive on G or CT?

edit. Well, just been there and did it. Reversed polarity on the leads and checked TR5. Got the same readings but positive this time instead of neg.

 

I was hoping ....




You can set up the meter to measure the current by replacing the F3 fuse with the meter on the 20A scale. then disconnect the series lamp and energize the set.

Read the actual current being drawn.

 

That's going to be DC amps I guess, Joe, is it?

I've been doing all DC readings.

And this is where I've been doing them and what I think I've been connecting to:

P.S. Just did the current readings. With the DMM leads in the right holes.

On the 20A scale I got 0.03 on AC amps and 0.02 on DC amps.

p.p.s. I see the 'sim' readings. What are they? A simulator? You have a circuit simulator? I have been looking for one I can perhaps learn. Found 'partsim' but it's slow going and I don't know how to judge if it is really suitable.

 

You could have removed the fuse, and connected the meter leads to the wires on the fuse holder.

The 0.03 amps is a good number assuming the series lamp is not installed.

This leads me to thinking we need to measure the two electrolytic capacitors, specifically their ESR and capacitance. I don't know if you have an ESR meter. Your meter will not measure the 2200 uF capacitance of those capacitors.

Can you redo the current reading on the 20 ampere scale first and have the speaker connected. Then redo it after you de-energize the equipment and change to 2 amperes IF the reading remains 0.03.

That fuse is blowing for a reason. So far, there hasn't been a logical reason.

Sim is a simulator. There are many. I use one called Tina and Texas instruments has a free version called Tina-TI at their website. A number of users use LTSpice from linear technology, another free simulation software.

The readings were close enough in some cases to consider them the same, or at least not to cause the fuse failure as I am using part substitition in the simulator.

 

I don't have an ESR meter. If I were to look into getting one what feature, range, whatever, should I ensure that it has? DMM's to measure high capacitance get a bit pricey.

What about this dedicated machine, better price for me: https://www.ebay.com.au/itm/ESR-V5-...503966&hash=item3faad7a26b:g:jF0AAOSwsj5bk0uE



I had the speaker connected all the time. Should I have said that? Did you want readings without it?

I redid the readings on the 200mA scales and got the same: DC: 0.2 and AC: 0.3

You didn't say whether I should be using DC or AC.

Should I give the thing a try again in 'normal' mode? No series lamp? Burn another fuse I guess unless it is magically fixed...

 

I have one of these and they work remarkable well for the price.
https://www.ebay.com.au/itm/128-64-...h=item2869047c06:g:mMEAAOSwQt9brfmH:rk:6:pf:0



They test caps, resistors, diodes and transistors. Pretty clever software in them!
Look on Ebay, Amazon and other sites.

 

Thanks for that. I've ordered one. And I ordered one of those other things, too. That's my off-the-cuff impulse buying budget for the week.....

 

ok....you did well with the readings.

Once your tester comes in, check those two electrolytic capacitors. It was mentioned in post #2 about those being suspect. They also mentioned a test that you could do to check it. You can do that test if you wish.

I saw that ESR/diode/transistor/capacitor tester for under $30 at amazon.

 

Does the fuse blow even if you don't try to play, or do you get some sound for a few seconds then the fuse goes? Did the speaker make a loud "clunk" before the fuse blew that it didn't make in earlier life?

Going back to look at post #2 I saw this. I think I never responded.

The machine was left on all night by accident. It suddenly blew the next morning without anyone going near it. I think there was a sudden 'clunk' but can't swear to it no. It burst suddenly into loud, continuous 'white noise'.

I think I'll take those capacitors out and try that 'switch on without them' test.

Are they big enough to retain a charge that could hurt? Short across the terminals with an insulated screwdriver before I start?


p.s. I downloaded/installed Tina-ti Now to try learn how to use it. Is there a quick way to put this amp circuit in it?

 

First to the safety issue. Yes the capacitors can hold a charge. You can measure the voltage across the capacitor terminals till it drops to a safe level. Then you can short it out with an alligator clip prior to de-soldering the B+ and B- lines.

You can build it using the NPN and PNP models or I can send you a list of substitutes that I used. Either should give you a better understanding of the circuit.