Armstrong 421 output transistors

Jump to Latest
#21
I hope the OP does not mind an observation.
He has A14 boards, there are a number of comments that they can be updated / converted to drive a pair of silicon outputs, which is surely correct.
Problem is that there are no suggestions as to what to change on the A14 board to achieve it. (Unlike the excellent job Trevor [latala] did for the A15 boards.) For simple people like myself to copy.
Perhaps some one who has done it made notes and that could be shared? (As mentioned before, previous threads on various forums, I can find, end before any detail is given...)
Or point to a thread where details of 'how to' are included?
Alan
 
Last edited:
#22
mrpaulfrazer said:
Excuse my extreme ignorance here but how does 271 affect the bias? Now for an even dafter question - would I increase or decrease the values of the resistors to get the increased bias for the Si transistors? Would variable resistors be a good idea? You can see I am an absolute novice regarding solid state circuits!
Click to expand...
It doesn't, it was a typing error.
its 281 and 278

basically the other side of the voltage divider
and just leaving the thermal tracking side alone.

30 to 50 ma of idle current is fine
Far as I understand this amplifier is already biased very high around
150 ma

could confirm by measuring voltage drop across emitter resistor on final transistors.
on working channel. TR25, TR27

hopefully dont make typing error again. emitter resistor numbers look
like 285 , 284
 
Last edited:
#23
Alan4411 said:
I hope the OP does not mind an observation.
He has A14 boards, there are a number of comments that they can be updated / converted to drive a pair of silicon outputs, which is surely correct.
Problem is that there are no suggestions as to what to change on the A14 board to achieve it. (Unlike the excellent job Trevor [latala] did for the A15 boards.) For simple people like myself to copy.
Perhaps some one who has done it made notes and that could be shared? (As mentioned before, previous threads on various forums, I can find, end before any detail is given...)
Or point to a thread where details of 'how to' are included?
Alan
Click to expand...
When you change over to silicon
your just looking to achieve the same idle current as the original circuit.

Far as the A15 board emitter resistors are .47 ohms on the final power transistors
and idle current is said to be around 150 ma or around .07 voltage drop across 285 or 284

resistor 281 and 278 are 18K on A15 board, likely the same on A14

basically been said that lowering resistor 281, 278 from 18K to around 12 to 13K
will get a silicon replacement to idle around 150ma

you could either change 281, 278 to 12 or 15K and see what bias is.

or something easier like I mentioned in post #12

you can add parallel resistors to the existing resistors to change bias.

18K + 33K parallel = 11.6K
18K + 36K parallel = 12K
18K + 38K parallel = 12.3K
18k + 43K parallel = 12.6K

so as you can see you have better control of resistance
by using parallel resistors.

And in the OP's case he can just measure the voltage drop across the working channel.
And when adding silicon outputs. you just going to match the same idle current.
or get same voltage drop across emitter resistors.
 
Last edited:
#25
Apologies for not responding earlier but for some reason I stopped getting emails to say there were new posts in this thread. I was about to post basically saying what Alan has said in post 21. Since then I see there are several helpful suggestions. I am on my way out just now so cannot digest them properly but will come back to look at them as soon as possible.

Thanks all

Paul
 
#27
About 30 years ago I repaired a 424 receiver at that time AL102 were hard and expensive to obtain
I took out the bias chain and replaced the top resistors with 22 k the thermistor and bottom resistor were replaced by 2 off IN1448 diodes the junction of the top resistor and the in4148 were then connected to the base of the driver transistor were then connected via a 4.7 k resistor
the transistors were then replaced with BD140 and 2n2955 silicon
This worked easily for 20 years +
When setting up turn down the power supply voltage and increase the voltage while monitoring the bias across the emitter resistors on the outputs
I am sorry that i do not have the ability / facility tp post a drawing at the moment
Trev
 
  • Like
Reactions: JMFahey
#29
The MJ2955s arrived in the week so I tried substituting them for the blown units. I tried it as it is through a 60W lamp limiter and the channel works. I couldn't actually find R285 but 284 is 0.82 ohms. I couldn't get a DC voltage drop across this (is that right?) but on the AC range there was a 250mV drop. I only left it on briefly as the lack of a DC drop confused me (easily done!). Can anyone advise please?

Another problem that has now arisen is that if I use the A14 board that was originally with the faulty channel the sound comes through briefly but after a few seconds it drops low. I have tested the transistors on that board and they all test OK. I have replaced all the electrolytics on that board. I don't know whether it distorts because my "source" at the moment is just the hiss from the FM radio section. Again, can anyone help please?
 
#30
mrpaulfrazer said:
The MJ2955s arrived in the week so I tried substituting them for the blown units. I tried it as it is through a 60W lamp limiter and the channel works. I couldn't actually find R285 but 284 is 0.82 ohms. I couldn't get a DC voltage drop across this (is that right?) but on the AC range there was a 250mV drop. I only left it on briefly as the lack of a DC drop confused me (easily done!). Can anyone advise please?

Another problem that has now arisen is that if I use the A14 board that was originally with the faulty channel on either channel the sound comes through briefly but after a few seconds it drops low. I have tested the transistors on that board and they all test OK. I have replaced all the electrolytics on that board. I don't know whether it distorts because my "source" at the moment is just the hiss from the FM radio section. Again, can anyone help please?
Click to expand...
 
#32
The Armstrong's of that era have a number of known problems ! The electrolytic capacitors are prone to fail but they are now 60 years old !
The edge connectors are a known weakness they do go intermittent difficult to check
The thermistor should not be a problem I have known them to have defective finish for years and still be ok
Did you change the drivers also ?
Trev
 
#33
I cleaned the edge connectors and have swapped the boards around several times and the fault follows the board. I have replaced all the electrolytics but none of the transistors. They all checked ok on my tester but one or more could be failing. I haven't gone into the types of transistors fitted. Are replacements reasonably easy to obtain? I wondered about the thermistor as it is ok for a short while but then fades out to a lover volume level. There is so little on the board to go wrong really.
 
#38
Errr something not right there.
Can I ask what voltage you measure at F1 and F2? Then again the voltage at the negative terminal of the output cap (286?)?

If you get similar results are the new output transistors very hot in a short while?
 
#39
The power supply board fitted is a Z10 which is a little different from the Z17. The voltages on the fuses are the same as on the collectors as they are directly connected to them. However if I remove the fuse on the channel where the Si transistors are then the voltage on the remaining Ge ones increases to 48 and 24v. I haven't done any modifications to the board on the Si channel to change the quiescent current as the fading out on the other driver board took over my attention. With the fuses removed and hence the higher voltages the fade out is much quicker.

I have only had it running for a few seconds so I don't know whether the new transistors get hot, vut in view of the above I expect that they would
 
#40
PS The voltage on 286A is 24.9v and on B is 22.4v. A is the channel with the Si transistors.

PPS ! I posted the above without thinking about it, but the voltage must be the same as on the collector of the Tr27s. Testing the voltages again I now get around 48v on the collectors of the Tr25s. I cannot explain why they have increased so much but I imagine that seems much more reasonable.

One of the A14 boards is still fading out, though

PPPs I removed the power supply board to check the number on it. Maybe the bad connections suggested by Trevor would explain the odd voltages.
 
Last edited:
You must log in or register to reply here.
Top Bottom