I made measurements as indicated above. Everything was as expected except the 27 v and - 27 v measurements were closer to 30 v and -30 v respectively. I also made the measurements indicated in the previous post. It seems like so far we have not identified a problem. (other than me being a nervous nelly
)I made measurements as indicated above. Everything was as expected except the 27 v and - 27 v measurements were closer to 30 v and -30 v respectively. I also made the measurements indicated in the previous post. It seems like so far we have not identified a problem. (other than me being a nervous nelly
The power supply appears to be functioning normally, which is good news. You could check that the +/- 18V arrives at the main board at pins 10, 11 & 12 as well as checking the ground (0V) connections.
When you said you experienced distortion after a period of time of non-use, under which conditions were you using the pre-amp? I.e. type of source, which input was used and was this distortion present on both L & R channels?
Well let me be as forthcoming as my memory permits. As I reflect on this, it seems to me that the last time I used the preamp, it worked fine (great sound) and then over a period of time (couple days maybe) It seemed to have an audible problem. I can not give specific detail as my memory will not permit it. Recently (2 or 3 days ago), I pulled it out to try it with my system to see if it would work for my sons use. I wasn't really thinking about the last time I used it I guess. It has been awhile. Anyway the system I hooked it up to was output from my computer (main audio out) to the preamp (aux, or tape ins) and out to my DH200. As soon as I hooked it up the improvement in sound over my tascam US-122 interface was quite evident. I was very pleased. Unfortunately, it was not long before I started to hear some odd, non musical sounds. It started as high pitched staticy sound. It wasn't just the same but seemed to "flow". It was a kind of washy sound that was not so present as to totally distort the music. Just a quiet background bother. I checked one of the RCA connections and accidentally pulled it out creating a loud distortion. In fact, it apparently was enough to blow the 2.5 amp fuse in the DH200. I thought at the time that I had blown the speakers. I actually went through the process of removing the speakers and checking them and the crossovers. (at this point I had returned the tascam to the system and some distortion was still present but different sound) The speakers were fine. I figured out it was the DH200 and replaced the fuse. I hooked everything back up (again with the DH 101) and I was presented with the sound I have described initially in this thread. I hooked up the DH 101 to a different set of speakers and power amp and confirmed the problem was in the preamp. The distortion is present on both channels and over most of the audio spectrum, although the mid vocal range is the most egregious. It is not a subtle sound, yet the music is recognizable. It does not seem to matter which input is used. There is a fuse (the original one) in the preamp. Is it possible that blowing it could produce similar distortion that the blown DH200 fuse created?
You said "You could check that the +/- 18V arrives at the main board at pins 10, 11 & 12 as well as checking the ground (0V) connections."
I don't understand how I would measure that.
The blown fuse in your DH200 was most likely caused by the partial removal of the RCA connector(s). This action would have disconnected the ground connection between the pre-amplifier and the amplifier before the signal link was broken and induced a high level of 60Hz mains voltage at the amplifier's input. Neither your pre-amplifier nor amplifier have a mains earth connection to mitigate this, so it's never wise to unplug a signal lead while the equipment is powered up.
I suggested you next check that the +/- 18V supply from the PSU arrives at the main pre-amplifier PCB, the largest board in your pictures. Observe the safety recommendations identified previously. Using your meter set to the 25V DC range, switch on the pre-amplifier, place the black probe on point '10' and the red on point '12'. These points should be marked on the largest PCB. You should measure 18V. Then place the red lead on point '10' and the black on point '11'. Again this should read 18V. Switch the pre-amplifier off.
Next, check the ground continuity as follows:
Perform these tests with the unit powered OFF. Select the resistance range on the meter, ohms X1. Touch the two meter probes firmly together and verify the meter needle pointer swings to the right and aligns with the 0 on the ohms scale. You may need to adjust the 'zero' control knob on the meter to achieve an exact zero. The greater the resistance measured, the further the meter needle moves to the left. The polarity of the probes doesn't matter for these tests.
Place one probe on the power supply board point 11 and the other on the main metal chassis (preferably at one of the cover screw mounting holes). The meter should indicate less than one or two ohms.
Perform the same test between point 10 on the largest PCB and the chassis.
Perform the same test between point 2 on the largest PCB and the chassis.
Take further readings between each RCA connector outer ground ring and the chassis.
Please record any positions where the reading is greater than a couple of ohms.
*Update
First while trying to align ohm meter to zero to check for continuity, I was getting strange readings. I checked the battery and it was almost dead. I changed the battery and rechecked all the previous settings. The only thing that changed was that this:
was now measuring more like -35V DC and 35V DC respectively. I rechecked both the positions you carefully described in previous posts on the PS and the large board and the volts were + or - 35 volts in both places.
All continuity tests (I think that's what we were testing with meter at X1) were showing zero. I should just double check that by RCA jack outer ground ring you mean the exposed outer metal circular portion that one normally makes a connection to or from on the back of the unit?
I don't want to forget to thank you for your guidance as this process continues
. Thanks!Edit: I forgot to mention that on your last set of instructions (RE: LARGE BOARD) I could not locate a number 2. There is a 2R and a 2 L.
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You've now verified that the grounding is correct and yes, I meant the outer metal circular portion. Sorry if I wasn't clear.
I'm pleased you find my instructions usefulI don't want to forget to thank you for your guidance as this process continues
That would make sense as I was unable to determine from my copy of the manual how these are labelled. Please measure on both the L and R pin suffixes I identified earlier. Presumably there is a Q9R and a Q9L transistor on this board?
It looks from the results so far that you have some damaged transistors in both channels of the pre-amplifier. Could you take some more measurements (powered on, volume control to minimum, no source input) please:
Set meter to 25V DC range. Black probe to 10L. Red probe to junction of R27L / R25L. If the meter needle moves backwards, reverse the probes (reading is now negative). Record voltage and polarity. Repeat above for the R channel suffixes.
Set meter to 10V DC range. Red probe to point 12L, black probe to junction R21L / Q9L. Repeat for R channel.
Set meter to 10V DC range. Black probe to point 11L, red probe to junction R22L / Q10L. Repeat for R channel.
Not sure if this was ever resolved, but I occasionally have a distortion problem on my DH-101. It happened before, and I fixed it--one of the power supply capacitors was erratic. In essence, the preamp was only half turned on; even the LED was lit only partially. Turning it off and back on will get the capacitor to begin working. Since it happened on one side of the power supply previously (can't recall if it was on the positive or negative side), it likely is the cap on the opposing side.
This is why I am going to start my recapping with the power supply. I may even go with larger voltage capacitors to help avoid this in the future (provided they'll fit).
This is why I am going to start my recapping with the power supply. I may even go with larger voltage capacitors to help avoid this in the future (provided they'll fit).
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