Refurbishing Denon Direct Drive TTs
flavio81
Turntable technician
I wanted to say...
... what Radiokom said. You are wasting your time and potentially making things worse if you think this can be repaired by trial-and-error or replace-this-and-pray.
You need to understand how the TT works, and assuming you know electronics, get the schematic and start to diagnose. @Pio1980 , an illustrious forum member and owner of some of the finest Sony DD turntables, already resumed how the TT electronics work.
Most importantly if the motor runs at crazy fast speeds, this means that the main control transistor (the one that is mounted on the heat sink) is not receiving a signal to regulate(reduce) the amplitude of the AC signal that drives the motor. If the transistor is not broken then the signal is not being received thus the whole speed control board needs to be diagnosed.
More or less I agree with Radiokom. DON'T GET A TURNTABLE TECH because most of them know little about analog electronics.
Your best bet is to get an electronics engineer who is willing to take on this job. This would be the ideal
flavio81
Turntable technician
Pio said:
The head picks up an AC signal prerecorded onto the inner platter rim surface, its then integrated to a DC voltage level of strength related to platter rotation rate and amplified to drive an AC passive platter motor throttle consisting of a Graetz diode bridge in series with the AC platter spindle motor, the bridge having power transistor across the points where dc output would normally appear. This throttle arrangement allows a DC only power gain device to control AC voltage level to the motor.
Ingenious! SONY uses a similar arrangement on their AC DD turntables.
Denon DP-1200 service manual is here:
www.nodevice.com
Block diagram:
It is eactly the same strategy as the early Sony direct drive turntables.
The "motor drive amp" has as critical part the transistor that is on the heat sink. As i mentioned, if it receives no signal or full signal, the speed will get crazy (or stop).
If the Low pass filter (LPF) is not working right, the speed will not get stable, it will "hunt" around the correct speed.
The service manual has a full explanation of how the circuit works, because Denon engineers are nice people.
Tr4 is the critical part i was alluding.
If you want to go the "trial and error " route, first make sure all the switches there are completely clean and working correctly (test them with a multimeter). Including the speed adjust pot and VR1.
Then try testing Tr4, Tr3, the diode D3, also testing the resistors around Tr3 and Tr4. Even more important, check D4 zener diode, it should give a regulated voltage, if D4 is toast then nothing would work since it sets the operating voltage for the control electronics. Obviously, replace the ones that are faulty. Tr4 is critical because it operates at high voltages (at least 100V), it can't be replaced by a general-purpose power transistor. Make sure you are getting a suitable replacement.
If this doesn't get you reasonable (not crazy fast) speed, then test Tr1 and Tr2.
The probabiltity that the IC is at fault is small, this should be the last component you mess with.
If D4 was faulty giving crazy high rail voltages, or Tr3 was fried and sending out the collector current to the base, then there might be the possibility that IC was damaged. But this would be the last thing to replace. And it would be best to test the IC with an oscilloscope instead. The service manual show the shape of all the signals the IC works with.
Don't go replacing capacitors as if this was the first thing to do.
Side note, i see the IC (TCA955) is more or less readily available. @Pio1980 this enables creating an alternate speed control board for our Sonys, by modification of the Denon schematic.
The head picks up an AC signal prerecorded onto the inner platter rim surface, its then integrated to a DC voltage level of strength related to platter rotation rate and amplified to drive an AC passive platter motor throttle consisting of a Graetz diode bridge in series with the AC platter spindle motor, the bridge having power transistor across the points where dc output would normally appear. This throttle arrangement allows a DC only power gain device to control AC voltage level to the motor.
Ingenious! SONY uses a similar arrangement on their AC DD turntables.
Denon DP-1200 service manual is here:
Denon DP-1200 Service Manual — download free
Denon DP-1200 Service Manual PDF file download free. Denon DP-1200 Service Manual will help to repair the device and fix errors.
www.nodevice.com
Block diagram:
It is eactly the same strategy as the early Sony direct drive turntables.
The "motor drive amp" has as critical part the transistor that is on the heat sink. As i mentioned, if it receives no signal or full signal, the speed will get crazy (or stop).
If the Low pass filter (LPF) is not working right, the speed will not get stable, it will "hunt" around the correct speed.
The service manual has a full explanation of how the circuit works, because Denon engineers are nice people.
Tr4 is the critical part i was alluding.
If you want to go the "trial and error " route, first make sure all the switches there are completely clean and working correctly (test them with a multimeter). Including the speed adjust pot and VR1.
Then try testing Tr4, Tr3, the diode D3, also testing the resistors around Tr3 and Tr4. Even more important, check D4 zener diode, it should give a regulated voltage, if D4 is toast then nothing would work since it sets the operating voltage for the control electronics. Obviously, replace the ones that are faulty. Tr4 is critical because it operates at high voltages (at least 100V), it can't be replaced by a general-purpose power transistor. Make sure you are getting a suitable replacement.
If this doesn't get you reasonable (not crazy fast) speed, then test Tr1 and Tr2.
The probabiltity that the IC is at fault is small, this should be the last component you mess with.
If D4 was faulty giving crazy high rail voltages, or Tr3 was fried and sending out the collector current to the base, then there might be the possibility that IC was damaged. But this would be the last thing to replace. And it would be best to test the IC with an oscilloscope instead. The service manual show the shape of all the signals the IC works with.
Don't go replacing capacitors as if this was the first thing to do.
Side note, i see the IC (TCA955) is more or less readily available. @Pio1980 this enables creating an alternate speed control board for our Sonys, by modification of the Denon schematic.
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flavio81
Turntable technician
@Squirrel! i edited my post above.
Good luck, these Denons deserve to get a full electronic service and then replinthed and paired with a really good tonearm. They're part of turntable royalty and have little to envy to other turntables, including my beloved Sonys.
Good luck, these Denons deserve to get a full electronic service and then replinthed and paired with a really good tonearm. They're part of turntable royalty and have little to envy to other turntables, including my beloved Sonys.
Semanon
Super Member
I’m not sure finding an electronics engineer who wants to repair a turntable will be any easier than finding a good tech, but that’s just me.
No matter how you call it engineer or tech, he do not need to know anything about turntables. If he can read schematics and take measurements he will fix it without pain within hour. Really not a rocket science. Not near close to Space X
cafe latte
Addicted Member
Not necessarily. Many won't touch turntables and many may be fine with TV's but not say a valve amp.No matter how you call it engineer or tech, he do not need to know anything about turntables. If he can read schematics and take measurements he will fix it without pain within hour. Really not a rocket science. Not near close to Space X
My friend who has now passed away gave me his Conrad Premier 4 with thr premier 3 pre before he died. He gave it to me as he blew it up and gave it to 4 repair shops. I when on holiday in the UK fixed the melted tracks and replaced the components and found the issue that was I think puzzling the techs.
The OP amps were controlling the current to thr valves. It is likely the Techs did not realise the importance of the OP amps.
Anyway I fixed it. He blew it up again when the cancer had set in and he was slef medicating with special tobacco and he decided to bias the valves using a metal screw driver and slipped, bang!!!
I fixed it again and I connect it when in thr mood for sweet valves.
Chris
Semanon
Super Member
You missed my point. There are few lots of people capable of fixing turntables, but few people who are INTERESTED in repairing them.No matter how you call it engineer or tech, he do not need to know anything about turntables. If he can read schematics and take measurements he will fix it without pain within hour. Really not a rocket science. Not near close to Space X
I am fully capable of troubleshooting and fixing a toaster, but I have zero interest, therefore I make a poor toaster repair person, even if there is a market. I am likely capable of fixing that Denon, since I have repaired other Denons (amongst others TTs), but I have no interest or time, nor would it be more financially rewarding than remaining in my chosen career.
It is at the intersection between reliability, availability (which includes interest), and capability that a good candidate for this repair lies. I’ve seen such a person is hard to find, and if you settle for less than three out of three, the outcome becomes less predictable.
No, it’s not rocket science, and it won’t explode when it reaches orbit, like the recent Space X. But it requires skills most people do not have, and those who do put their skills to use elsewhere.
Squirrel!
Member
Well, thanks everyone for the lively discussion.
I got this off the 'bay 3 months ago not working. So psyched with this monster of a turntable. A 100 volt step-down transformer, new feet from Music Hall to replace the hardened/deformed original feet, resetting/cleaning the magnetic tape head that controls the corvette wheel looking 4.5lb platter speed (BRILLIANT!), a new (used) speed control board along with the Ortofon 2M Red (going Blue when the tweaks are finished), and a good cleaning (still needs polishing), the sound is very sublime. This is the epitome of why I got back into HiFi, 8 + months ago. This is the true vintage sound, I, and all of the others are searching for. In my opinion, this DP-1200 bests the Wiim Pro (Amazon Music Unlimited) and my DCD-900ne in terms of sheer 70's/80's sound quality.
It is an emotionally engaging experience listening to this behemoth. I am completely enamored with the build simplicity, and of the build quality that is second to none. This particular Denon DP-1200 is a MUCH better table than can be had today for anything under a grand, IMHO, and was well worth all of the headaches to get it to this point.
First Record played? Why, Linda Rondstadt of course! But the second record played? Linda Rondstadt, you silly goose!!!
I got this off the 'bay 3 months ago not working. So psyched with this monster of a turntable. A 100 volt step-down transformer, new feet from Music Hall to replace the hardened/deformed original feet, resetting/cleaning the magnetic tape head that controls the corvette wheel looking 4.5lb platter speed (BRILLIANT!), a new (used) speed control board along with the Ortofon 2M Red (going Blue when the tweaks are finished), and a good cleaning (still needs polishing), the sound is very sublime. This is the epitome of why I got back into HiFi, 8 + months ago. This is the true vintage sound, I, and all of the others are searching for. In my opinion, this DP-1200 bests the Wiim Pro (Amazon Music Unlimited) and my DCD-900ne in terms of sheer 70's/80's sound quality.
It is an emotionally engaging experience listening to this behemoth. I am completely enamored with the build simplicity, and of the build quality that is second to none. This particular Denon DP-1200 is a MUCH better table than can be had today for anything under a grand, IMHO, and was well worth all of the headaches to get it to this point.
First Record played? Why, Linda Rondstadt of course! But the second record played? Linda Rondstadt, you silly goose!!!
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flavio81
Turntable technician
Nice, very nice!
Glad to see you brought it back to life.
Now perhaps you can search for a really top tonearm to be fitted there on the future, along with a custom plinth !