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Old 19th Mar 2019, 2:28 am   #10
Chris55000
Octode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 1,425
Default Re: Nearly silent output from vantone reel to reel machine

Hi!

What fault finding equipment do you have?

You'll need a good quality digital multimeter and a small a.f. oscillator to diagnose this type of fault – you can get little kits/modules of a.f. sources for as little as 99p (free postage!) online!

You also need one of those little Chinese LCR T4 Testers for testing the passive components, as well.

Very low volume on this type of amplifier is commonly caused by degradation failure of the germanium amplifier transistors, particularly if somebody has accidentally reversed the polarity of the battery supply in the past – this will destroy them, but not always immediately, and power supply reversal can cause failure of a peculiar type where d.c. voltage readings test very close to normal but the transistors are incapable of correctly amplifying – I have personally had cases of this!

The first thing to do is to try and obtain the service–sheet or circuit diagram of the player's amplifier board – it's a standard transformer–coupled class B design about 250mW output with two additional transistors for tape head preamplification/frequency correction

If you can manage to read off the numbers on the metal transistor cans I'll give you a few typical voltage readings to expect!

Chris Williams
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