Under Construction! Blog!
The Project: I have about 20 reel to reel tapes from the late 50’s and then the 60’s s that belonged to my Dad who died in 1994, and about 10 of my own from the 1970s and 80s, but nothing to play them back on.
Enter the local Hospice Thrift and Gift shop and a slightly battered 1964 reel to reel tape recorder for $15.00 .
Though I used to service tube powered stuff years ago, any spares would have been a big deal as would the voltages involved.
(90+ DC!) But This was all transistors (“Solid State”!) Yea!
I located at least three separate sets of voltages used by the deck:
~22v V DC for the transistor record & playback amplifier,
~155 V DC to trip a large solenoid used to reverse tape direction
6V AC to power pilot lamps.
This 1964 Panasonic reel to reel tape recorder has several separate power supplies for its amplifier, relay electronics and more. I wanted to be sure its ancient transistor amplifier got a decent smooth DC voltage. I measured and monitored all voltages, including what was coming out of the AC transformer
Lower right is the removed inch square selenium bridge rectifier to convert 16V AC to 22 v DC for the amplifier;,
It had been screwed to the chassis to dissipate potentially (lethal!) Heat and discourage thermal runaway (the peak of the AC is 1.414.times higher than the RMS “Average” value the meter reports as it sees ‘power’ not ‘peaks’)
these have questionable leakage characteristics and the pre
sent danger that if they overheat, noxious reeking selenium fumes would result.
No Thanks!
Upper right are four 1N4004 modern silicon rectifiers (1c/piece!) I am arranging in a square ‘bridge’ which is a full wave rectifier to replace the large selenium one. They can take an amp of current and withstand up to 400v reverse current. They drop 0.6v each so dissipate little power unlike selenium’s 1-3v.
Should do nicely as a current check showed circuit barely used 250mA- a quarter the rating
Lets solder those four together. AC leads from transformer are to the left- Heat shrink tubing keeps it all isolated. Silicon rectifiers seem pretty heat resistant yet I used retractors (lower right) to sink the soldering heat away from hurting the plastic / ceramic packaging.
Old versus New- about 1/20th the size! I had used retractors to sink the heat on the leads during soldering, Capacitors have liquid juice in them that can get damaged by excess heat.
Post Mortem- the old capacitor. Signs of corrosion on its electrodes and Not Much Electrolyte Juice left in it. It still worked but had obviously lost a lot of its ‘capacity’ which does not bode well for stopping Hum in audio circuits! Newer capacitors can have different issues where they can even burst long before their 50 years is up!
A similar process was repeated for a much higher voltage circuit: This one is used to actuate a solenoid which flips the tape direction mechanism.
This time, 110v AC line current is rectified and smoothed to deliver 155v DC. This time, an 8 Ohm power resistor was present before the solenoid to mitigate the sudden current and back EMF from the solenoid that could damage the capacitor that powered it through a relay .
It had blown and was also replaced.
THIS IS LINE VOLTAGE, No Isolating Transformer!
and I maintained and replaced the shielding and insulation for this section better than the original. this tape unit, interestingly, had no Ground lead of its own, Grounding sockets only started becoming common after 1967 in the USA.
That’s It. I reworked the original metal bracket with pliers and insulating tape to ensure nothing would be exposed or short out.
Early tests showed the amplifier was much quieter than before.
I used its line output to my Linux PC and kept the volume down to minimize distortion, relying on far more modern post amplification by the PC- Yet the machine’s own speakers and amp actually worked fine. I could see little or no 60Hz. noise with halted tape and at full volume using a Spectrograph App.!
Excellent. Don’t ask me about dB’s here-
Basically, I hear no Hum,.
I demagetised all the tape heads and swabbed ’em with Q tips and alcohol. No obvious signs of wear and they seem well aligned.
But that’s Another Blog.
Ah, the Nostalgia!
== Paule