Hello, I’m Bob, and as a newbie, I’m sure some of these questions have been asked before. If so, please direct me to the right place. I bought this DCC900 new and it worked normally for a while, then began having audio dropouts. (The vendor wouldn’t service it.) With more time now, I decided to have another look at it. SMD caps replaced per the helpful video. It played erratically again, but now no audio. This is using a pre-recorded tape. I found the service manual, such as it is. There is a signal at “RD MUX”, although I’m not sure what it’s supposed to look like. “RD CLK” is 3.07MHz, vs manual’s callout of 960kHz. (3.07 is 24.57/8, which makes more sense to me.) There is data at pin 23 of Q423, at least. In various places I’ve had to clean off corrosion that seems worse than capacitor leakage. This unit may be beyond practical repair.
Hi and Welcome to the forum.
Did you recap both boards?
Did you chick on the read-write board the side A-B connections on the board, like explained in this video?
Yes, both boards. (I had not seen this video, but the other one was very helpful.)
However…out of curiosity I metered the bias (voltage) on the heads and found that the flex-cable connections to three of the digital play heads, and the bias coil, were open. This may have been partly due to insufficient care taken when I put the FFC back in the connector, and partly due to the end of the cable having delaminated, possibly because of age and/or cap leakage/moisture. This isn’t going to be easy top repair…
If the flat cable (assuming the one attached to the head) is damaged, it is for sure going to be very hard.
(BTW, thanks for the welcome to the forum; I was trying to keep comments to the point.)
Yes, the FFC to the head. But, we’ll see. I’ve trimmed the insulation back to where the copper tracks are good, and soldered thin wires to each one. Got continuity through the PB heads. Will try the following: A small PCB will be glued to the bottom of the FFC, and the thin wires will solder to tracks, and a FF connector will also be soldered. Another short FFC will go from there to the R/W board.
An update. I fabbed the board and connector as mentioned above. Got continuity through all the heads. The unit played when I hooked it up! But after a few minutes, dropouts began, which is probably the original problem. Heads were all biased, but bias voltage didn’t agree with calculation of Q102-Q104 area. Found that the collector of Q102 was unsoldered, apparently due to leakage corrosion. Replacing Q102 corrected the voltage. Still assessing whether this has affected the dropouts.
How did you measure the continuity of the heads?
Can you share?
Sointenly! Two ways. When I was checking the cable, I just used the DVM-ohmmeter. In operation, each head is biased with a 560-ohm resistor from about 6V, and each is about 50 ohms, so the current is around 10mA. The DVM sources considerably less current than that, so should be safe. (I’ve heard that older meter-type ohmmeters shouldn’t be used on analog heads due to the current they provide, esp on the lower ranges.) Once the thing is hooked up, the bias will cause about 0.6VDC to appear across each head. Those can be metered on the board, either on the head-side of the 560-ohm resistors, or the FF connector. (If you see around 6V, that path is open.)
The PB bias coil is driven by 560 ohms too, but develops a slightly lower voltage.
I used the schematic to make a sketch of the head connections and their pin numbers on the cable, to make the ohmmeter check go a little faster.
Pin 13 - common, digital PB
Pins 3, 5, 7, 9, 11, 15, 17, 19, 21 - head connections, digital PB
Pins 23 & 25 - analog PB feedback coil
Pins 26 & 29 - analog PB feedback coil
Pin 27 - common, analog PB
Pins 24 & 28, analog PB L & R
Pins 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 - digital record heads
Pins 1 & 22 - digital PB bias coil
Hi,
Since we have some heads that came from Philips, How could we measure them without connecting them to a player? I know how to measure for the playback part but do not believe it can be done for recording?
I assume you have the head assy’s with the FFC cable attached, as used in the DCC900. I checked the recording heads with an ohmmeter. Checking while connected to the deck might be difficult, since constantly-changing voltages will be applied to them. (If you hold the FFC with the fingers up and pointing toward you, pin 1 is on the right.) There is apparently a record head connected to pin 2, one to pin 4, etc, up through pin 20. The other sides of all the heads are connected to an internal common. So, ohming between pins 2 and 4 will measure two heads, between 4 and 6 will measure two heads, etc. I think those heads were measuring less than 50 ohms, but as long as they are consistent, I assume they are OK. Does this help?
Hi, Yes I understand this now much better. The reason I am not getting any reading… the head is dead.
With a new working head on the play side I measure around 50 ohms. On the recording side 3 ohm. heads.
Ralf
Uh oh.
Seems like I measured 22 ohms for something, that might have been the PB bias coil (pins 1 & 22). The recording heads may have been lower, but they were all about the same. I hope they are good, have not tried recording yet.
I could make a suggestion…maybe you have the same problem I did. The heads themselves were OK, the copper tracks on the FFC were broken just above where the insulation ends and the gold fingers are exposed. I used the Dremel tool to sand away the insulation to expose the copper tracks above the breaks. This has to be done carefully or the tracks will be sanded too. Measure there for continuity, if OK, you can decide what to do next. (I tried this because I didn’t have anything to lose.)
If your heads are really dead, I wonder what knocked them out. Service manual warns that they will only withstand 150VDC from static, etc.
The copper/plastic is very delicate in that area and you need to get a little practice removing the cable and assemble it back in the connector. I will try, but I am almost positive it is not the case.
What can knock them out? Several things. Static is one, People using demagnetizing tapes, not knowing you can kill the head, but also the leaking capacitors shorting the TDA’s.
Ralf
They are listed with limited lifetime in documents, even shorter when using abrasive analog tapes, for that purpose there is an extra protective layer that may get damaged when cleaning with alcohol.
All very helpful to know! I will be interested to hear if there are breaks in the copper of your FFC’s.
That is one thing I’ve learned the hard way - the FFC must be reassembled carefully. When removing it, pull the little locking tabs on the connector out. When putting it back in, make sure the tabs are pulled out fully, so that the cable will go in with low force. Hold it fully seated and square, and try to push the tabs back to the locked position. The videos don’t seem to mention this in detail. In my case, I don’t think I had the tabs open all the way, and when I pressed the cable in, the delaminated end allowed the copper/Kapton layer to scrunch back. (I’ve had experience with FFC’s in industry, but never seen one start to come apart like this.)
Gotta say…my experience is similar to Video99’s.
I agree, the 68uF caps are bypasses and should be able to go to 100uF’s. I replaced the nonpolar caps with MLCC’s (ceramic chip caps).
If the capstan is servo’d to match the data output from the heads, maybe after 25 yrs, I should replace the belt.
Some progress, anyway.
I replaced the belt with the nearest I could find, although the original is in surprisingly good cond for being 25 yrs old.
Found that I had spazzed, had gotten a FFC cable and conn with tin plating, which doesn’t match the original’s gold. Removed my conn, installed gold part, plugged in gold FFC, checked for shorts & continuity, got everything hooked back up and it plays! And records!
However, it will be going along and the pinch roller will just stop. I can nudge it with a little screwdriver and it will turn some more, maybe stop, maybe run for a while. I think the capstans are still turning; the start-stop behavior is too rapid for them to be stopping. I cleaned the rollers and capstans with an alcohol wipe, dried them, let it sit overnight, but the behavior is unchanged. This is don’t understand.
What next? Perhaps these tapes have SSS, as another post suggests. I have a couple of 3M-Scotch blank tapes, a Philips pre-recorded demo tape, and a box of BASF blanks. The first two types play unreliably with the pinch rollers stopping erratically. I can often hear a faint but ominous squeal. The BASF’s seem to run OK. If the tapes are afflicted, they probably shouldn’t be used, so that the tape path won’t load up and need to be cleaned.
Have you replaced the pinch Rollers yet?
Ralf
Have not done that. Where on earth would we get new ones?
The BASF blanks squeak too. But I did find that if the tape case can be displaced slightly outward (away from the heads), it won’t squeak, and the tape travel is fine. (If there is squeak, record and/or playback will fail.) I wonder if there is any adjustment for how far in the tray settles. The service manual doesn’t show any mech adjustments.