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Old 7th Aug 2017, 10:06 pm   #1
Mike1948
Retired Dormant Member
 
Join Date: Aug 2017
Location: Chippenham, Wiltshire, UK.
Posts: 5
Default SRP51/SRP52 problem

Hi, I am very much an amateur with electronics, but have some knowledge! I have a 1970's vintage Bush record player with add-on amplifier to provide stereo. Both have developed the same fault of intially working well, but after a minute or so, the sound cuts to a much quieter distorted sound. I have found the circuit for the SRP51/52, and from reading a number of posts about these circuits had reasoned that the problem was almost certainly the output pair of transistors (AC128 & AC176). Being easiest to get at, I started with the SRP52. I have replaced the output pair and also the two 2.2 ohm resistors between them as there were signs of overheating. When switching on, the voltages measured across C10 give the expected 24.5v. Other measurments shown on the diagram appear to be broadly within limits, including the voltages around VT2 VT3 & VT4. Ater a minute or so, the quiet hum changes to a loader note, and the voltage across C10 drops to around 11v. All other voltages around the circuit drop too. The voltage on TR1 seconday remain at 12v ac throughout. I appreciate that this is a big ask, but if anyone can suggest where I look for the problem I would be very grateful!
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