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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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7th Aug 2017, 10:06 pm | #1 |
Retired Dormant Member
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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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7th Aug 2017, 11:05 pm | #2 |
Heptode
Join Date: Mar 2016
Location: Essex, UK.
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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.
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Geoff |
8th Aug 2017, 10:14 am | #3 |
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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 |
9th Aug 2017, 4:56 pm | #4 |
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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 |
10th Aug 2017, 12:19 am | #5 |
Heptode
Join Date: Mar 2016
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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.
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Geoff |
10th Aug 2017, 9:00 am | #6 |
Nonode
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Re: SRP51/SRP52 problem
Milliamps, Mike, not micro-amps. Such a low idling current will cause severe crossover distortion.
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Andy G1HBE. |
10th Aug 2017, 10:19 am | #7 |
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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! |
10th Aug 2017, 11:07 am | #8 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
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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.
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If I have seen further than others, it is because I was standing on a pile of failed experiments. |
10th Aug 2017, 11:59 am | #9 | |
Moderator
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Re: SRP51/SRP52 problem
Quote:
20k Ohms per Volt is suggested.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
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10th Aug 2017, 12:29 pm | #10 |
Dekatron
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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. |
12th Aug 2017, 11:06 pm | #11 |
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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 |