Thanks for your input RB.

The OP is away for this week, so is unlikely to respond just yes.

However if you look back I think you will note his strategy is to replace the transistor set.

His facilities are limited to a DVM, which is why we are trying the voltage measurement approach.

However that will not help trying to power a 70 watt amp from a 5 volt rail.

Keep the suggestions coming.

 

RB… The resistors around the tube socket were replaced by someone else prior to the amp going down. Any discolored areas that you see like that are old damage. Someone has repaired this amp in the past.

The speaker output has - .140 vdc

I’m fairly exhausted tonight so I did not probe too deep into this but I wanted to do a few things and share what parts I replaced.


So here is what I have replaced so far:
DB1
IC 7, 8, 10
D1, 2, 4, 5
TR4, 5, 6, 7, 8, 9, 10, 11, 12
R73, 74, 96, 97, 115, 116, 117, 118

I just can’t seem to get my mind around it right now so I’m taking a break.

 

Check the power supplies as I indicated before doing anything else.

 

Thanks Studiot!
I am also considering just biting the bullet and go to ebay to get the exact ones so I know it will be right. Unfortunately obsolete items come at a premium.

The BC212/213/214 being the complements of bc182/183/184 - which in turn are roughly cheaper alternatives to BC107/108/109.

Not sure how reliable my memory is - but I think BC177/179/179 are complements to 107/108/109.

The straight 212/182 no suffix/A/B/C are E>B>C pinout, the L suffix is B>C>E like Japanese TO92 transistors.

Whatever is chosen - make the same change to the other channel in a stereo amp.

The current equivalents should be BC547 for NPN & BC557 for PNP.

In this case the A/B/C suffix indicates gain band.

 

What i have done today is check the transformer. Correct me if i have done this wrong. I removed the 2 white and 1 black wire from the transformer as you see on the left side of it. I placed my lack meter lead on the center one where the black wire was then read the other 2 lugs. Both read 36.5 vac. Then I unplugged the power and read resistance at the same locations. Both read 1 ohm.

The voltage seems awfully low...........am I wrong?

 

I was going to say I am expecting an alternating voltage between 30 and 36 volts so yes that's great. The 1Ω measured with an ohmeter is not a useful measurment, except to confirm continuity.

Your transformer is therefore 36-0-36 (I expect the extra 0.5 volts is due to your mains).

This will yield DC rails of 50 - 0 - 50, as expected.

Are you quite sure that there is only ±5.3 volts DC at points 1 and 2 on the diagram?

If this is the case then either

1) Something is drawing so much current that the supply is dragged right down. I doubt this since the supplies are symmetrical.

2) The bridge rectifier and or capacitors are faulty. You can test the bridge rectifier with your ohmeter. Test each diode in turn both way (with the power off and the capacitors discharged). Remove C62 and C61 and measure the DC ouput of the bridge (it will not harm the following circuitry).

Oh and by the way, Ian -thank you for you substitution offers. Unfortunately they will not do in this case as they could be subject to nearly the full voltage rail and are not rated for this. This was why the designers chose the 212/182 over for instance the 183 or 184, which are also short on voltage rating.

 

Well I pulled both the bridge rectifier and C61 & 62. The bridge rectifier tested good. I found the thread on here concerning "how to check a capacitor" and by following what VincinChristmas posted it would appear that they are both "open". I placed the DMM on ohms and when I connected to the cap (correct polarity) it did register at all. Unless there is some other way to test with my DMM I'm hoping this is it. It's getting late so i will check the current on the bridge rectifier tomorrow as you suggested.

 

So, it appears the bridge is OK, and the capacitors did not check shorted. Even if the caps are open, the voltage at points 1 and 2 from studiots drawing should be much higher. Other possibility's are burned connection points or an open trace on the PCB.

 

Before you put the bridge back.

Connect the transformer secondary (72 volts) to the ac input of the bridge with flying leads on the bench.
Measure the output of the isolated bridge, both AV and DC volts. (This will not harm anything).

Next reconnect the transformer leads, still leaving the bridge out of circuit.

Check that you have 72 AC volts at the bridge input pads.

 

Checked the secondaries on the transformer and have 73.1 vac. Removed the bridge rectifier out of the circuit and hooked it to the secondaries and read 32vac and 65.9 vdc across the + and - on the BR. I am stopping there for the evening and will resume soon.

 

Before you put the bridge back.

Connect the transformer secondary (72 volts) to the ac input of the bridge with flying leads on the bench.
Measure the output of the isolated bridge, both AV and DC volts. (This will not harm anything).

Next reconnect the transformer leads, still leaving the bridge out of circuit.

Check that you have 72 AC volts at the bridge input pads.

I'm going to reconnect the secondaries to the pcb. Both whites and the 1 black correct? Then check the voltage as you mentioned.

 

If you reconnect the transformer leads before putting the bridge back you can check the transformer voltage is reaching the input pads on the pcb to the bridge by measuring the AC voltage there.

Then put the bridge back, but take the caps out of circuit.

What DC voltage do you now read at point 1 and 2?

 

Wow.....ok well here is where I am at. With the bridge rectifier and C61 & 62 out of circuit I read 73 VAC at the input pads of the BR. Put the BR back in and read 0 vdc at the "+" and -5.3 at the "-" plus I got a puff of smoke. When I flipped the pcb over to see what had smoke checked R96 and R118 were extremely hot and so was TR8 and TR10.

Everything was hooked up carefully and proper. Now I'm even more confused.

 

Right so we are back to 5.3 volt supply rails.

Looking at your previous list of voltages more closely it implies that the current through R96 and R118 is around 11 amps, dissipating a power of around 40 watts.

By comparison, at full belt your 80watt amplifier should deliver 3 amps rms, 4.5 amps peak with a voltage swing of plus and minus 36 volts, into 8 ohms.

The attached spreadsheet should make interesting reading for some.

No wonder they are getting warm.

The output transistors are therefore turned on as hard as they will go and dragging down the power supply from 50 volts to 5 volts.

I really thought it would be a power supply issue but obviously not, it was my other option so we must now go on a hunt for whatever is overdriving the output.

I am sorry I have been dealing with a cyber attack elsewhere over the last few days so I will not have time until Thursday to study the circuit and measured voltages till then.

There are plenty of other knowledgable folks here so there is no reason for them not to chip in with suggestions.

 

Right so we are back to 5.3 volt supply rails.

Looking at your previous list of voltages more closely it implies that the current through R96 and R118 is around 11 amps, dissipating a power of around 40 watts.

By comparison, at full belt your 80watt amplifier should deliver 3 amps rms, 4.5 amps peak with a voltage swing of plus and minus 36 volts, into 8 ohms.

The attached spreadsheet should make interesting reading for some.

No wonder they are getting warm.

The output transistors are therefore turned on as hard as they will go and dragging down the power supply from 50 volts to 5 volts.

I really thought it would be a power supply issue but obviously not, it was my other option so we must now go on a hunt for whatever is overdriving the output.

I am sorry I have been dealing with a cyber attack elsewhere over the last few days so I will not have time until Thursday to study the circuit and measured voltages till then.

There are plenty of other knowledgable folks here so there is no reason for them not to chip in with suggestions.

Thanks for your help. I understand you have things to do. I'm going to coast for a few days to take care of some things then come back to it. I hope maybe others have some direction but if not that's cool.

 

Of all of the idiotic, careless, moronic, bungling, simple errors to have made!!!!! I feel like I have simply wasted the time of those that have posted on this thread in an effort to help me. First I apologize to all of you. What has happened is when I replaced TR8 and TR10 the originals were BDV65 & BDV64. My search lead me to a TIP142 & TIP147 which are correct.........as long as you put them in the right place.......arrrrrrrrrrrrrrrrrrgh. I had them switched. As careful as I was with everything else I overlooked this thinking I had them correct. Luckily I ordered extras in case so I put new ones in the correct place and what do you know...........no short! However I'm the kind of person that believes that things happen for a reason. Had I not switched them I would not have done the search and found this website. I fully intend to look at all of the study material and lessons on this site to better educate myself in electronics. I did learn a few things from this ordeal. Thank you to studiot, Ramussons, Bertus, The RB, ian field, and tubeguy for your input.

 

OK with the transistors in the right position the amp powered on OK and I was able to play thru it for about 30 seconds when there came a small "pop" thru the speaker and once again the fuse has blown. So I'm better off than I was however things are not quite perfect yet. Going to bed and letting this rest for a day or two.

 

Did you also replace R96 and R118 - the emitter resistors this time around. They are very likely to have been fried. Checking 0.33 ohms is not reliable with an ordinary meter. Try it, with the resistors in circuit (and the power off) and see what you get.

Take heart, every true audio engineer cuts his teeth on blown output transitors.

 

Did you also replace R96 and R118 - the emitter resistors this time around. They are very likely to have been fried. Checking 0.33 ohms is not reliable with an ordinary meter. Try it, with the resistors in circuit (and the power off) and see what you get.

Take heart, every true audio engineer cuts his teeth on blown output transitors.

0.33 Ohm resistors are probably less than the test leads on a typical DMM - but you can usually short the prods to measure the leads and subtract that from whatever the reading comes to.

Those resistors probably won't go high in value, they're most likely either good or completely open - depending on the construction I'd probably expect some form of visible burn mark.

 

0.33 Ohm resistors are probably less than the test leads on a typical DMM - but you can usually short the prods to measure the leads and subtract that from whatever the reading comes to.

Those resistors probably won't go high in value, they're most likely either good or completely open - depending on the construction I'd probably expect some form of visible burn mark.

Valuable thoughts, Ian.

I have seen emitter resistors burn out in power amps, though I must admit It's a long time and I can't rememebr whether they went low or high.