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!

Re: SRP51/SRP52 problem
 

Remove the 50 Ohms variable resistor in the base bias of the output transistors and fit a good quality fixed resistor of between 18 to 22 Ohms. This is a common fault that causes the output transistors too draw to much current with increased hum and overheating.

Re: SRP51/SRP52 problem
 

Thanks Geoff, I will try this today. I found the post last night regarding checking the half rail voltage and quiescent current setting, and found that the latter was way out of spec. Adjusting this pot stabilised things, but the sound was rather distorted and I noticed the quiescent current was leaping around wildly. So I will follow your suggestion today.
Thanks. Mike

Re: SRP51/SRP52 problem
 

OK an update to this. I've replaced the AC176 & AC128. Also replaced R8 & R9 both of which had become out of spec. R13 & R14 both replaced with new 2.2 ohm resistors. Removed RV5 and replaced it with a fixed resistor. This required a surprising low value to produce a reasonable quiscent current. 12 ohms produced a current of 8.8µa measured as per test procedure. Half rail voltage set up as per test procedure. (24.5 input giving 12.2). All voltages on diagram generally a little under that specified, (0.9v is 0.8v, 12v is 11.8).
So now the circuit is no longer failing as described, in fact seems to now have one problem - fairly pronounced distortion, mostly noticeable on lower notes
Any suggestions please? Very grateful for anyone's help.
Thanks,
Mike

Re: SRP51/SRP52 problem
 

The output bias current measured between + rail point 11 and TR4 collector should be between 6 to 7 milliAmp (mA) for AD161/AD162 transistors, will be similar for AC176/AC128. The bass distortion could be mechanical due to the stylus not tracking properly.

Re: SRP51/SRP52 problem
 

Milliamps, Mike, not micro-amps. Such a low idling current will cause severe crossover distortion.

Re: SRP51/SRP52 problem
 

Hi Geoff & Andrew,
Thanks for both your replies. Ah, this issue of measuring the quiescent current has cropped up in many earlier posts. The recommended method to measure it in this circuit according to the service book is to measure across the two resistors (whilst in circuit with power on and volume control at minimum), and aim for the indicated current to be between 9.25 & 12µa. I have 8.8µa. Someone posted the service manual page during the earlier debate about this. Others had suggested the measurement to be done via the + rail and TR4 collector, so I will check that too.
It’s the SRP52 (add-on amp) that I’m working on, and to simplify testing I’m using a cd player to test. I’m wondering if there are problems with the smoothing capacitor? If I replace many more components the only original left will be the circuit board!

Re: SRP51/SRP52 problem
 

How are you measuring the current?

You have to either interrupt the circuit so the current bring measured flows through the meter, or measure the voltage across a suitable resistor and calculate the current using Ohm's Law. Connecting a meter on a current range across a resistor is not going to work, and could cause damage.

Re: SRP51/SRP52 problem
 

That is exactly what the service sheet suggests should be done. Probably OK with a analogue meter, but how the reading relates to actual current flowing will depend on the meter's sensitivity.

20k Ohms per Volt is suggested.

Re: SRP51/SRP52 problem
 

Using the two lower current values (6mA & 9.25uA) I make the meter resistance at the probes approx. 2.869k.

Lawrence.

Re: SRP51/SRP52 problem
 

Well, I seem to have got there in the end. One faulty AC128 I believe, as although I replaced it and the AC176 to no avail, it appears that the AC176 replacement was not good (old one turned out to be OK), and the first of the AC128's I tried was also not good. However, a second AC128 coupled with the original AC176 worked OK after checking the quiescent current....
Maybe cheap transistors not a good buy (they were very cheap). I wonder if they were untested? Perhaps I need to check them before using them. Any tips on that?
Thanks for all your help and suggestions, and your patience dealing with a novice!
Mike