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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 11th Jan 2020, 5:57 pm   #21
Vakito227
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

Would you people recommend I replace the drivers with 2N2955s on both channels?

Also just while we're on the subject, what would be a suitable modern replacement for a 2N2869? Just in case any of the output transistors give up

Thanks
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Old 13th Jan 2020, 1:01 am   #22
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

I am no expert on transistor output bias circuitry but using silicon would require significant changes. Even changing the type of germanium transistor may cause problems as the gain and frequency response will be different. I suggest you concentrate on getting the driver stage working first.
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Old 13th Jan 2020, 7:14 pm   #23
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

Hi Peter. I have an ex equipment ( used good)2N2148 you can have for the price of a pint if it would be any good to you.
I might have some NOS 2N1305/CV7353 as well
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Old 14th Jan 2020, 10:27 am   #24
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

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Originally Posted by PJL View Post
I am no expert on transistor output bias circuitry but using silicon would require significant changes. Even changing the type of germanium transistor may cause problems as the gain and frequency response will be different. I suggest you concentrate on getting the driver stage working first.
I did have a quick read up on this, something about altering resistor values for the bias.

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Originally Posted by Tim View Post
Hi Peter. I have an ex equipment ( used good)2N2148 you can have for the price of a pint if it would be any good to you.
I might have some NOS 2N1305/CV7353 as well
Hi Tim, that sounds very tempting! The 2N2148 and 2N2147 have very slight voltage and frequency differences, will this be a problem?

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Old 14th Jan 2020, 2:54 pm   #25
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

The bias is a bit more complicated than resistor value changes due to the Vbe of silicon being much higher and temperature dependent. This usually involves adding some forward biased silicon diodes in the bias chain. I would avoid this unless the germanium output transistors prove to be faulty.
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Old 15th Jan 2020, 12:47 am   #26
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

As suggested earlier this would be my suggestion:
1. Disconnect all of the 4 output transistors.
2. Replace the dead 2N2147.
3. Clip the DMM set for volts across the collector and emitter of the 2N2147.
4. Switch on just for maybe 5 seconds and note the voltage reading.
5. If the measurement is around 10V, repeat the test on the other channel.

If both channels are OK then you can move on to the output stage.
1. Test the 2 2N2069 output transistors on the other channel.
2. Check the 4 bulbs all show continuity
3. Set all 4 bias pots to maximum resistance (350 ohms)
4. Replace the output transistors on one channel
5. Clip your DMM set for volts between 0V and the -ve end of the output decoupling capacitor
6. Switch on just for maybe 5 seconds and note the voltage reading.
7. If it reads approximately -22.5V then you can move on to set the bias current.
8. To set the bias current you really need 2 DMM's, one to monitor the voltage as above, and one to monitor the current by inserting the meter in series with the collector of one of the transistors. The two bias controls will interact and must be changed a little at a time to maintain the -22.5V whilst setting the current to 33mA.

PS: There is a slight risk the change in transistor types will cause instability (the amp will oscillate). If you get high bias current readings or erratic current readings as you attempt to adjust the bias then switch off.

Last edited by PJL; 15th Jan 2020 at 12:55 am.
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Old 25th Jan 2020, 6:53 pm   #27
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

I now have a 2N2148 to replace the dead 2N2147, Many thanks to Tim for this!
I'm not too familiar with transistors but one thing has flagged up which I am concerned about. On the diagram there is 35v going into the collector and 45v coming out of the emitter. The maximum collector-emitter voltage on a 2N2148 is 40V compared with 50V for a 2N2147. Will this be a problem?

PJL, I will attempt to follow your guide although I don't want to risk replacing the driver transistor and switching it on, even for 5 seconds and risk it destroying this 2N2148 I now have. Are there any tests without the driver transistor fitted that I can perform first just to guarantee it will not destroy it?

Thanks
Peter
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Old 25th Jan 2020, 10:01 pm   #28
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

Under normal operating conditions it should be OK as the Vce is only 10V but fault conditions such as instability could be a problem. The mains transformer shows a 117V primary with no means of adjustment so I guess you are using a stepdown transformer that may deliver a higher voltage under low load conditions and therefore increase the -45V. Ideally, you would use a variac to keep the voltages under control but a simple lamp limiter would help.
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Old 27th Jan 2020, 5:58 pm   #29
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

What wattage incandescent lamp would be ideal for a limiter on a circuit like this?
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Old 1st Feb 2020, 5:24 pm   #30
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Default Re: Rock-Ola 40218-1-A Transistor Amplifier Problems

Quote:
Originally Posted by PJL View Post
As suggested earlier this would be my suggestion:
1. Disconnect all of the 4 output transistors.
2. Replace the dead 2N2147.
3. Clip the DMM set for volts across the collector and emitter of the 2N2147.
4. Switch on just for maybe 5 seconds and note the voltage reading.
5. If the measurement is around 10V, repeat the test on the other channel
I'm getting 6v between the collector and emitter on the left channel (2N2148)
and 12v between the collector and emitter on the right channel (2N2147)
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