Hi,
I'm working on an elderly Akai reel to reel for a friend. Resolved most of the expected issues (contacts, pots, dry caps), but I am left with a record issue.
Initially, I could not erase, or record. Traced this to a faulty 2SC971 in the bais oscillator. Replaced with a BRY51. Got a good recording, everything was fine ... See attached schematic of the oscillator.
After this one recording I can erase, but not record (just silence).
Checked out the basics and signal is getting to the PB/Record head, but it is being swamped by the signal on the bias head. Somehow, the amplitude has gone crazy high. Should be around 65volts rms, I measured (and felt!!) several thousand volts PtoP with a scope connected to the bias head. Checked bias head and both DC resistance and inductance match specification. Oscillator frequency is bang on spec at 65khz.
Suspected that the serial loading capacitor on the coil output (C402 on the attached) was the problem, but desoldered and tested with a C meter and again, bang on spec, although the C meter is of the low voltage type....?
1) Is the BFY51 an ok sub for the 2SC971?
2) What on earth can generate such high voltages, but still function at the design frequency.
Going mad with something that was a favour for someone, and suffering the after effects of what may only be described as a stun gun experience to the hand !!
Any pointers really (really) welcome.
Thanks
Darren
I'm working on an elderly Akai reel to reel for a friend. Resolved most of the expected issues (contacts, pots, dry caps), but I am left with a record issue.
Initially, I could not erase, or record. Traced this to a faulty 2SC971 in the bais oscillator. Replaced with a BRY51. Got a good recording, everything was fine ... See attached schematic of the oscillator.
After this one recording I can erase, but not record (just silence).
Checked out the basics and signal is getting to the PB/Record head, but it is being swamped by the signal on the bias head. Somehow, the amplitude has gone crazy high. Should be around 65volts rms, I measured (and felt!!) several thousand volts PtoP with a scope connected to the bias head. Checked bias head and both DC resistance and inductance match specification. Oscillator frequency is bang on spec at 65khz.
Suspected that the serial loading capacitor on the coil output (C402 on the attached) was the problem, but desoldered and tested with a C meter and again, bang on spec, although the C meter is of the low voltage type....?
1) Is the BFY51 an ok sub for the 2SC971?
2) What on earth can generate such high voltages, but still function at the design frequency.
Going mad with something that was a favour for someone, and suffering the after effects of what may only be described as a stun gun experience to the hand !!
Any pointers really (really) welcome.
Thanks
Darren
The 2SC971 is classed as an RF BJT in my equivalent book while the BFY51 is an audio BJT but I see some class the 2SC971 as "audio " elsewhere .
Its got a metal casing common in old BJT,s this can form a type of screening , if you cant get hold of an old BJT then try an RF variety of a modern type.
In any case replace the one you installed to see if it makes a difference .
Yes inductance's and capacitors energized can can be "shocking " .
Its got a metal casing common in old BJT,s this can form a type of screening , if you cant get hold of an old BJT then try an RF variety of a modern type.
In any case replace the one you installed to see if it makes a difference .
Yes inductance's and capacitors energized can can be "shocking " .
Have you checked the pinout? BFY51 is EBC, 2SC971 supposedly also is but with Japanese transistors I wouldn't be sure. You could try a BD139 if it does turn out to be an ECB, otherwise BC337 or 2N2222 should be good enough at least for testing.
By the looks of it the circuit is uncritical RF wise (10 nF across B-C) but may show some variation depending on beta.
By the looks of it the circuit is uncritical RF wise (10 nF across B-C) but may show some variation depending on beta.
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