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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

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Old 13th Jan 2020, 1:01 am   #22
PJL
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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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Last edited by Tim; 13th Jan 2020 at 7:37 pm.
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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

Quote:
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.

Quote:
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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