Old Preamp Troubleshooting Help?

Thread Starter

swen

Joined Aug 1, 2014
40
I have an old Shure M64 stereo preamplifier that is acting up. I'm hoping that what it's doing will be a pretty good indication of what the problem is. When I power it up, it produces normal output (stereo sound that goes into an auxiliary input to an amplifier [usually for a turntable]). But within 5 seconds, the output (sound from the stereo system) begins fading, and within 20 seconds, the output is down to zero. If I switch it off and back on, the cycle repeats. The preamp is from the 1970s and utilizes transistors, I assume. Would appreciate any help troubleshooting it. The circuit diagram for it is available here: https://pubs.shure.com/view/guide/M64/en-US.pdf

Thanks for any help!!

M64a.jpg M64b.jpg M64c.jpg
 

Hymie

Joined Mar 30, 2018
881
I have an old Shure M64 stereo preamplifier that is acting up. I'm hoping that what it's doing will be a pretty good indication of what the problem is. When I power it up, it produces normal output (stereo sound that goes into an auxiliary input to an amplifier [usually for a turntable]). But within 5 seconds, the output (sound from the stereo system) begins fading, and within 20 seconds, the output is down to zero. If I switch it off and back on, the cycle repeats. The preamp is from the 1970s and utilizes transistors, I assume. Would appreciate any help troubleshooting it. The circuit diagram for it is available here: https://pubs.shure.com/view/guide/M64/en-US.pdf

Thanks for any help!!

View attachment 152117 View attachment 152118 View attachment 152119
A colleague of mine was a dab-hand at fixing hi-fi systems.

Despite not having a circuit diagram or being particularly highly qualified in the field of electronics – he had an exceptionally high success rate.

His technique was to closely observe all electrolytic capacitors, and replace any that showed signs of failure, through their enclosure bulging due to internal pressure.
Measuring each capacitor value (while in circuit) would give a good indication whether all was well.

If he wanted to give a really good service – he would replace all electrolytic capacitors within the unit.

Given the fault you describe – I would not be at all surprised if it was due to a failed/failing capacitor.
 

BR-549

Joined Sep 22, 2013
4,938
Have you worked with electronics before? Have you any tools and equipment?

Monitor the voltage at the 30 VDC output. Do you know how to test C21, C22 and C1?

And as long as you have it apart......replace all electrolytics as suggested.

Has someone already replaced C21 with that white nytronics cap? I can't make out the value.

There should be C21 at the diodes-R1 junction. I don't see it. Can you get a better picture of the nytronics cap. If that was a replacement for C21.....it might be in the wrong place. It is possible that it's for the on-off switch and C1 is located where I can't see. But no switch cap on print.

Any clarity?
 

Thread Starter

swen

Joined Aug 1, 2014
40
Thanks for the responses. I know I need to be careful around capacitors, but that's about all I know. It looks like there are 5 or 6 large ones in there. I've posted more pics. The Nytronics cap says "600V." Is there a way to test them without removing them? None of them look seriously distressed.
Equipment: I just have a multimeter and a DC power source (and soldering iron).
The 30V DC is actually an alternate power input, and I just use the switch between 30V DC and 120VAC as an on/off switch.
What do you mean by "clarity"?
M64d.jpg M64e.jpg M64f.jpg
 

ebp

Joined Feb 8, 2018
2,332
If both channels behave the same way, look for parts of the circuitry that are common to both. There is some chance that something might be wrong with one or more components that are not common, but I would not expect the effect to be nearly identical for both channels - the bad component on one channel is likely to be either better or worse than the corresponding component on the other side.

The power supply is common, but I would expect failed capacitors to make a mighty buzz at two times line frequency, rather than causing the signal to quietly fade away. C1 is a possible suspect - it filters a bias voltage common to both sides. Excessive shift in the bias voltage could cause the behavior described.

Something causing overloading of the power supply would conceivably cause the behavior. R1 is remarkably high resistance, and an overload would possibly pull the supply voltage down dramatically. According to voltages marked on the schematic, the circuit only requires about 3 milliamps. Fading that is temporarily reversed by power-down isn't really the sort of thing I would expect from a failing capacitor. How long must the power be off to "restore" operation?

I suspect the Nytronics cap is across the transformer primary. A 600 volt cap, and only one, at that, would be something of an oddity in a 30 volt circuit.
 

BR-549

Joined Sep 22, 2013
4,938
Thank you. The clarity was for me.

Because of the dis-assembly difficulty on that unit.....which will be needed to test caps........most people would replace all electrolytics while it's open.

I can not find the nytronics cap value on print.....but it shouldn't need replacing.

Can you find the caps on print and match on board? Have you a clear idea of what to replace?

Do you think you could safely live test unit...BEFORE...re-assembly. For further troubleshooting. It might not be a faulty cap.

Are you using DC power supply or AC line for preamp?
 

ebp

Joined Feb 8, 2018
2,332
One more thought: A flaky power switch might account for the behavior and slide switches are not paragons of reliability. If the switch makes good contact while moving from the off to on positions, but settles at excessive resistance in the on position (as might happen from lubricant "going bad" or contact oxidation) the supply voltage might start out OK but rapidly decay. There would be virtually no "reset" time associated with this - cycling the switch would be enough to achieve restart.

Anything to do with loss of supply voltage is very easily verified with a voltmeter. The general rule for all troubleshooting is to start by checking the power supply voltages.
 

Thread Starter

swen

Joined Aug 1, 2014
40
If both channels behave the same way, look for parts of the circuitry that are common to both. There is some chance that something might be wrong with one or more components that are not common, but I would not expect the effect to be nearly identical for both channels - the bad component on one channel is likely to be either better or worse than the corresponding component on the other side.

The power supply is common, but I would expect failed capacitors to make a mighty buzz at two times line frequency, rather than causing the signal to quietly fade away. C1 is a possible suspect - it filters a bias voltage common to both sides. Excessive shift in the bias voltage could cause the behavior described.

Something causing overloading of the power supply would conceivably cause the behavior. R1 is remarkably high resistance, and an overload would possibly pull the supply voltage down dramatically. According to voltages marked on the schematic, the circuit only requires about 3 milliamps. Fading that is temporarily reversed by power-down isn't really the sort of thing I would expect from a failing capacitor. How long must the power be off to "restore" operation?

I suspect the Nytronics cap is across the transformer primary. A 600 volt cap, and only one, at that, would be something of an oddity in a 30 volt circuit.
Yes, both channels react the same. If I power off and instantly power back on, I get the same response. Which one is C1? I guess I could remove it and test it with my multimeter....
 

Thread Starter

swen

Joined Aug 1, 2014
40
Thank you. The clarity was for me.

Because of the dis-assembly difficulty on that unit.....which will be needed to test caps........most people would replace all electrolytics while it's open.

I can not find the nytronics cap value on print.....but it shouldn't need replacing.

Can you find the caps on print and match on board? Have you a clear idea of what to replace?

Do you think you could safely live test unit...BEFORE...re-assembly. For further troubleshooting. It might not be a faulty cap.

Are you using DC power supply or AC line for preamp?
Am using AC power.
I'm sure I could trace out the caps in the schematic and match them to the board. For sure, I would test any new parts before I button it all back up.
I am a terrible solderer, though, and would prefer to just fix the one bad cap, for now.
 

Thread Starter

swen

Joined Aug 1, 2014
40
One more thought: A flaky power switch might account for the behavior and slide switches are not paragons of reliability. If the switch makes good contact while moving from the off to on positions, but settles at excessive resistance in the on position (as might happen from lubricant "going bad" or contact oxidation) the supply voltage might start out OK but rapidly decay. There would be virtually no "reset" time associated with this - cycling the switch would be enough to achieve restart.

Anything to do with loss of supply voltage is very easily verified with a voltmeter. The general rule for all troubleshooting is to start by checking the power supply voltages.
I had thought about the switch, and experimented with holding it in position with a little pressure. No change. I also cleaned the reachable interior contacts of the switch with alcohol. Again, no change.
 

BR-549

Joined Sep 22, 2013
4,938
Ok swen.......I don't think you should try to repair it. Do they have repair shops where you live? Can you get a repair estimate?

Have you priced a new preamp?
 

ebp

Joined Feb 8, 2018
2,332
Measure the voltage between the input jacks for the 30 volt DC power supply when the AC power is turned on. It should be about 30 volts DC, but anything in the range of about 27 to 33 should be OK.

If that voltage is OK and you are confident you can make the measurement without probes slipping or fingers in the wrong place:
Check the voltage on the big blue capacitor on the opposite end of the terminal strip from the big white capacitor (careful ! - line voltage present on that white cap and some of the terminals on the strip, if the white cap is used where I think it is). The blue capacitor is C22 in the schematic and should have nominally 23.9 volts DC across it - anything in the range of about 20 to 27 volts should be OK. Again, be careful because there is line voltage nearby.

I can't tell which cap is C1 from the photos. You probably can't get at it without pulling the circuit board out of the box.
 

pfofit

Joined Nov 29, 2006
57
C1 is in the middle of the board as is R25
However, R3 is at the top. One end is easily accessible, the other may require finesse for a novice.
Not sure which end is at C1 connection point or base of Q1. Q1 is just to the left(edit: I meant right) of R3 but you never know with tracing.
Cheers
M64c.jpg
 
Last edited:

Ylli

Joined Nov 13, 2015
986
To follow up on pfofit's post, using your multimeter set to DC volts, connect the black (neg) lead to ground (the shell of any of the RCA connectors is ground). With power on, probe the accessible lead on R3. Post the result. Now poer the unit off, wait, then power it back on while you monitor the voltage on R3. Does it start at a higher voltage and then drop down?
 

Thread Starter

swen

Joined Aug 1, 2014
40
I will do as y'all say, as soon as I get a chance. May I assume the recommendation to be careful is regarding the 120 VAC and/or the caps? I will read up on dealing with caps before I proceed. Will post results. Also, I don't have any problem with pulling the circuit board. I've dealt with vastly more modern and complicated boards before (though just to tweak a trimpot, and the like).
R-549: I don't need the pre-amp; I have a receiver. But this particular pre-amp is somewhat desirable in the audio community, probably because it's a little retro and will fit in well with audiophiles' old stereo systems. I want to fix this one for the satisfaction of it, and hopefully to learn.
 

Thread Starter

swen

Joined Aug 1, 2014
40
"Measure the voltage between the input jacks for the 30 volt DC power supply when the AC power is turned on. It should be about 30 volts DC, but anything in the range of about 27 to 33 should be OK."

I get 28 V

"If that voltage is OK and you are confident you can make the measurement without probes slipping or fingers in the wrong place:
Check the voltage on the big blue capacitor on the opposite end of the terminal strip from the big white capacitor (careful ! - line voltage present on that white cap and some of the terminals on the strip, if the white cap is used where I think it is). The blue capacitor is C22 in the schematic and should have nominally 23.9 volts DC across it - anything in the range of about 20 to 27 volts should be OK. Again, be careful because there is line voltage nearby."


Big blue cap (220 uF): Shows 21 V when DC is "on" (that is, AC supposedly "off"). When I turn DC "off" (therefore using 120 VAC) the big blue cap drops from about 17 V when initially checked, down to <1 V within about 20 seconds. Which seems to mimic my problem.

"To follow up on pfofit's post, using your multimeter set to DC volts, connect the black (neg) lead to ground (the shell of any of the RCA connectors is ground). With power on, probe the accessible lead on R3. Post the result. Now poer the unit off, wait, then power it back on while you monitor the voltage on R3. Does it start at a higher voltage and then drop down?"

With "battery" on (therefore supposedly with power off, since no battery is connected) and AC plugged in: the silver-band side of R3 indicates 14.89 V. With "battery" off, R3 voltages drops from about 7 V (as soon as I can get leads on it), down to about 1 V within approximately 1 minute.

I found C1 (IEC capacitor) on the board easily enough, since it's labeled on the board:
20180512_145139_resized.jpg

If I need to, I can pull the board to get to C2, C21, and C22 or whatever. "Emitters" Q1 and Q3 are visible on the top (accessible) part of the board. They look like transformers. Same thing?

Before I started bothering you guys, I thought that a capacitor had to be removed before it could be tested. I can access C1 safely in situ with multimeter probes if that will help. And, if necessary, I can remove the board to get at things more easily.

Are the "supply" cap voltages the "+" side? What leg of an emitter is the "supply" leg?
 

Thread Starter

swen

Joined Aug 1, 2014
40
I got no comments regarding my testing posted above, so am not sure if the big blue cap is the problem (based on the testing), or whether I should open it up more and test voltages across C21, C22, and C1. Any input?
 
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