Hello,
Here I am again. It took longer than I thought to get to it because a lot of things have happened since my last post.
The preamp has it's tone control not working, no matter how the "tone defeat" switch is set.
Today I've opened the preamp and checked some things, starting with the power values :
At the bridge I've 45.2 V between + and -.
At S1 : 24.80V and -21.02V at S2
Then 18.18V at S3 and -18.42V at S4
At S5 15.86V and S6 -16.13V
on the headphone amp, S7 is at 15.2V and S8 -15.45V
And 17.5 /-15.83 V at the phono preamp.
I would like to get your advice on these voltages ?
Many thanks in advance for your help and answer.
Here I am again. It took longer than I thought to get to it because a lot of things have happened since my last post.
The preamp has it's tone control not working, no matter how the "tone defeat" switch is set.
Today I've opened the preamp and checked some things, starting with the power values :
At the bridge I've 45.2 V between + and -.
At S1 : 24.80V and -21.02V at S2
Then 18.18V at S3 and -18.42V at S4
At S5 15.86V and S6 -16.13V
on the headphone amp, S7 is at 15.2V and S8 -15.45V
And 17.5 /-15.83 V at the phono preamp.
I would like to get your advice on these voltages ?
Many thanks in advance for your help and answer.
Well, I had an Apt back in the day. It was highly regarded but I don't know why. 741 Op Amps. I guess the good phone stage. I replaced it with a first generation Hafler and it was much cleaner.
I would:
Re-cap it. Replace all electrolytic caps.
Replace the op amps with modern ones. Even the Signetics is a huge step above, but the newer, is it a TI?, that is really, really good.
Transformer if fine. Voltages are "nominal"
The newer Linear Tech regulators are cleaner than the old LM series.
-21.02 is low, but still enough to drive the regulators. Replacing the power supply filter caps might resolve it.
Good! The 78/79xx series regulators have +/- 4% tolerance, so could be as high as 18.72 or as low as 17.28. You are well w/in tolerance, and fairly balanced at only .24V differential.
Good! These are derived from the 18v regulators via 3 diode drops, so should be 3.2-3.3v less than regulated voltage. Yours are 3.32 & 3.29 respectively.
Good!
NOT so good, the pos. side is a bit high. These are also derived from the 18v regulators, via additional filter transistor regulation. They should be @1.5 LESS than 18v. Replace the 100uF caps first (C19/C20), if that doesn't resolve it, check the pos. transistor (Q8-2N3904) & resistor (3.3k).
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Are you sure, maybe early ones? As I was so disappointed with the sound, I opened it up and got the "well that explains that" The TI was already obsolete by then, but it was a big step up.
Again the Linear Tech regulators are quite a step up from the old 78/7900 series. Among other features, lower dropout. Add decent modern film bypass caps and you supply is about as good as it gets. I am not convinced faster rectifiers are audible.
Newer chips like the OPA 1611 and others are better than most can do with discrete. Lots of possibilities. Better than a JC-1? Well, I bet I can't tell. Maybe some can.
Again the Linear Tech regulators are quite a step up from the old 78/7900 series. Among other features, lower dropout. Add decent modern film bypass caps and you supply is about as good as it gets. I am not convinced faster rectifiers are audible.
Newer chips like the OPA 1611 and others are better than most can do with discrete. Lots of possibilities. Better than a JC-1? Well, I bet I can't tell. Maybe some can.
Most were TLO72, others were RC4558 (only headphone jack amp).
APT Corporation Holman Stereo Pre Amplifier Manual | HiFi Engine
APT Corporation Holman Stereo Pre Amplifier Manual | HiFi Engine
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Thank you all for your insight !
Since my last post, I've started to swap capacitors. New 1000µF caps and a lot of 10µF/35V in place, it still miss a couple, though. Then switch to the other values. I plan to end the swapping this week end. If everything else run as planned...
Then I will check voltages again and check to find why my left phono channel is not working, solve the tone control problem and assemble the SMD moving coil preamp we discussed in another thread.
I think you'll read something from me soon ;-)
Thanks anyway !
Since my last post, I've started to swap capacitors. New 1000µF caps and a lot of 10µF/35V in place, it still miss a couple, though. Then switch to the other values. I plan to end the swapping this week end. If everything else run as planned...
Then I will check voltages again and check to find why my left phono channel is not working, solve the tone control problem and assemble the SMD moving coil preamp we discussed in another thread.
I think you'll read something from me soon ;-)
Thanks anyway !
Hello,
I'm back at it. I could not do anything to it before because my father in law passed away and it was very difficult du to the circumstances of it's passing. Anyway, I swapped all electrolytic capacitors except for C43 because I can't count to six... So I ordered only five axial and I needed 6... Dumb.
The voltages are now :
transformer secondary still at 43.5 V
S1 : 25.67V, S2 : -21.85V
S3 : 18.17V S4 : -18.41V
S5 : 15.84V S6: -16.09V
S7 : 15.17V S8 : -15.41V
S9 : 16.86 V S10 : -17.33 V
So it seems the capacitors have an effect after all !
Then I started chasing why the left phone channel is dead. I check ed all the voltages on Q2,Q3, Q4, Q5, Q6 and Q7 and compared them to their cousins Q102, Q103, 0and so on. Everything is fine. So I checked Q1 and found it dead. So I put one 2N5458 I had bought...
I have to test with a signal to check if I've corrected the flaw, but it is too ate for this now. See tomorrow..
I have a couple of questions though.
Should I be concerned by the voltage difference between the positive and negative rails ? Should I change the regulators and try to get closer values between the positive and negative ?
Should I change Q101 because I've changed Q1 ?
and last but not least, how would you check the tone control circuit to find why it is not working ?
Thank you in advance for your help. I miss my music a lot in these difficult time !
I'm back at it. I could not do anything to it before because my father in law passed away and it was very difficult du to the circumstances of it's passing. Anyway, I swapped all electrolytic capacitors except for C43 because I can't count to six... So I ordered only five axial and I needed 6... Dumb.
The voltages are now :
transformer secondary still at 43.5 V
S1 : 25.67V, S2 : -21.85V
S3 : 18.17V S4 : -18.41V
S5 : 15.84V S6: -16.09V
S7 : 15.17V S8 : -15.41V
S9 : 16.86 V S10 : -17.33 V
So it seems the capacitors have an effect after all !
Then I started chasing why the left phone channel is dead. I check ed all the voltages on Q2,Q3, Q4, Q5, Q6 and Q7 and compared them to their cousins Q102, Q103, 0and so on. Everything is fine. So I checked Q1 and found it dead. So I put one 2N5458 I had bought...
I have to test with a signal to check if I've corrected the flaw, but it is too ate for this now. See tomorrow..
I have a couple of questions though.
Should I be concerned by the voltage difference between the positive and negative rails ? Should I change the regulators and try to get closer values between the positive and negative ?
Should I change Q101 because I've changed Q1 ?
and last but not least, how would you check the tone control circuit to find why it is not working ?
Thank you in advance for your help. I miss my music a lot in these difficult time !
I would look into why the unregulated negative supply is only -21V instead of -25V.
This lower than normal input voltage could degrade the operation of the 7918 regulator.
The 7918 itself would not be a cause of the low voltage. Perhaps C62 is bad, or else the
relay control circuit could have a problem. Does operating the mute switch affect the -21V?
This lower than normal input voltage could degrade the operation of the 7918 regulator.
The 7918 itself would not be a cause of the low voltage. Perhaps C62 is bad, or else the
relay control circuit could have a problem. Does operating the mute switch affect the -21V?
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Hello Rayma,
First of all, both C62 and C63 and C64 are brand new. C62 is a 105°C Panasonic reading 987.9 µF Vloss .8% and ESR at 0.07 Ohm. So, IMHO good for service.
So I got an idea. What is the voltage if nothing draw current after C62 ?
I removed the 7918 regulator, C63 and C64, R93 (reading 1.5k actual) and R84 (9.7k) and R87 (4.2M) and the link to the headphone preamp.
Without anything consuming power, voltage at C62 is 31.7 V. And I see a ripple of about 0.6 V alternating at that point (positive supply rail at the C59 cap is less than that (about 0.2 V (but with all the wiring present).
So I wonder if something is amiss on the rectifier bridge or if the negative voltage regulator input is somewhat bad ?
I must add that the copper traces around C59 and C62 show traces of a previous intervention (some part of the copper has disappeared from too much heat and I bet it was not done on initial soldering at the factory. ) So something was wrong at some point and needed looking at it. But that was before my time.
Thank for your help and input. I'm somewhat lost here.
First of all, both C62 and C63 and C64 are brand new. C62 is a 105°C Panasonic reading 987.9 µF Vloss .8% and ESR at 0.07 Ohm. So, IMHO good for service.
So I got an idea. What is the voltage if nothing draw current after C62 ?
I removed the 7918 regulator, C63 and C64, R93 (reading 1.5k actual) and R84 (9.7k) and R87 (4.2M) and the link to the headphone preamp.
Without anything consuming power, voltage at C62 is 31.7 V. And I see a ripple of about 0.6 V alternating at that point (positive supply rail at the C59 cap is less than that (about 0.2 V (but with all the wiring present).
So I wonder if something is amiss on the rectifier bridge or if the negative voltage regulator input is somewhat bad ?
I must add that the copper traces around C59 and C62 show traces of a previous intervention (some part of the copper has disappeared from too much heat and I bet it was not done on initial soldering at the factory. ) So something was wrong at some point and needed looking at it. But that was before my time.
Thank for your help and input. I'm somewhat lost here.
I reread the post above and saw I forgot to say that playing with the mute switch does not change the voltage a bit at C62.
So,
With a new negative regulator, the voltage raised a bit at 25V. (*) And with a new rectifier bridge, the ripple went down to 0.03V at C62. Now I have the same voltage at C59 and C62 +25.4V and -25.70V. Then I went to the phono amplifier voltage. The capacitor was new, so I checked the resistor 3279 Ohm but browned a bit. I put a new one and had still too much voltage. A new 2N3906 and everything is fine ! (-16.90 V from the negative -18.40V so exactly 1.5V less)
Today, i check the left channel phono preamp to see if my fix is good and fine and try to find the fault in the tone circuit.
* : I wonder what fault has the old one ? It did not become hot, nor stop regulating but draw too much power from the input and made the voltage too low ? Same question about the bridge. Seeme the diodes where not functioning as specified and gave a high ripple ....
With a new negative regulator, the voltage raised a bit at 25V. (*) And with a new rectifier bridge, the ripple went down to 0.03V at C62. Now I have the same voltage at C59 and C62 +25.4V and -25.70V. Then I went to the phono amplifier voltage. The capacitor was new, so I checked the resistor 3279 Ohm but browned a bit. I put a new one and had still too much voltage. A new 2N3906 and everything is fine ! (-16.90 V from the negative -18.40V so exactly 1.5V less)
Today, i check the left channel phono preamp to see if my fix is good and fine and try to find the fault in the tone circuit.
* : I wonder what fault has the old one ? It did not become hot, nor stop regulating but draw too much power from the input and made the voltage too low ? Same question about the bridge. Seeme the diodes where not functioning as specified and gave a high ripple ....
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