Uher 4000 Report Monitor Amplifier Component Layout

[DMcMahon]

Rechecked why PSU not powering the 4000 and found out that I had wired the +ve wire to the incorrect pin on the 8 pin DIN !!!!

PSU now working OK, 4000 current drain around 300 - 600mA depending upon operating mode, PSU is only rated 500mA max, but seems to function well at 600mA.

Continued investigation into why play not working mechanically unless the tape tension lever pushed and then the playback sound is far too fast.

Both reel table clutch set-ups checked OK.

Then found by moving the heads assembly back out of the way that the pinch wheel is not making contact with the capstan shaft when Play selected. Found that the nut that secures the pinch wheel is very loose.

There is a metal triangular plate above the pinch wheel which has a hole in one of its 3 corners. Above the plate there are 3 little green loctited screws, 2 of which I can see have sharp cup pointed ends which would make contact with the top of the triangular plate. I cannot see the end of the third screw.

I assume that the pointed screws are some sort of alignment/tilt adjustment to get the pinch roller to the correct angle to the capstan shaft, but I cannot find information on this adjustment in any of the relevant service manuals.

I am thinking at the moment that the hole in the plate must align with the screw that I cannot see its end, otherwise if the hole aligned with one of the other 2 pointed screws, then that particular screw end would go into the plate hole and serve no function ?

[DMcMahon]

Managed to rotate the metal piece and saw that there were was actually a hole in 2 of the 3 corners and with a large magnifying glass and bright light just about able to see that the third screw had a flat end (not cup pointed) so deduced that the 2 cup pointed screws must align/go into the 2 holes.

Correctly re-aligned the metal piece and tightened down the loose securing nut for the Pinch Roller. There were 5 fibre washers under the nut, some of them looked slightly concave shaped so not sure how tight to tighten the nut, got it pretty tight and no sign of the Pinch Roller rotation not being free.

Mechanical Play now working very well. Once I have proven Playback and Record I plan to lock the nut using Blue Loctite 243 Threadlocker.

My friends tape that came with the 4000 is in a bad way, large chucks of tape falling off the backing, the tape is not sticky or dry oxide dusty, I think it is just an old cheap thin tape.

Will put some decent tape onto his spool and check out Recording & Playback. One aspect of Recording my friend was concerned about is that automatic recording using the ALC (Automatic Level Control) switch setting gives too strong recording level with the VU meter going hard over. I advised stick to manual recording.

[Uncle Bulgaria]

Excellent! Good deduction. The CR 240 has instructions for adjusting the ALC - I can't remember if you have the service manual for yours. They could be similar - shall I dig out the relevant page?

[DMcMahon]

Yes thank you, I do have the Service Manual. Way back in post # 27 I referenced that the ALC board had some modifications including an extra pot, which I deduced from the Service manual was R819 for ALC adjustment. At the time I said R819 was not shown on my schematic (Hungarian downloaded one) but I can now see R819 on the large schematic I brought from SDS.

The ALC set-up looks a little involved, still need to find my signal generator before I could attempt it properly. Will first do some normal recordings and then ALC recordings to quantify exactly what the situation is.

[Uncle Bulgaria]

Ah, SDS to the rescue! Mine had different components from the PDF I found online as well, then the SDS manual made everything clear.

[DMcMahon]

Loaded some good condition decent tape (TDK LX 35/90) onto the spool and did some music recordings from the phono DIN connection.

The record level signal is very strong indeed, in manual can easily get a 0dB VU reading at record level of no higher than 1. In the ALC modes the record level goes hard over on the VU meter and sounds distorted. This is with 470kohm resistors in the Phono to DIN cable record lines to attenuate the strong phono signal.

Playback of a manual recording is good and playback level on VU meter is very similar to record set up levels.

While checking playback the tape transport suddenly stopped and nothing works now apart from power on LED is still good.

So the 4000 is still not giving in !

[DMcMahon]

Could not get it going, so stripped it apart again with nothing obviously found wrong, re-assembled and now back working, not sure what was happening ??

After studying the schematic again to try and work out why the DIN record signal is so strong, I realised that the 4000 series is a little different to what I am used to, in that the 5 pin Radio/Phono (the name should have been a give away ) DIN connector has 2 sets of inputs, namely "Radio" on pins 1 & 4 and "Phono" on pins 3 & 5.

Normally (at least on older R2Rs and I think the Uher Royal de Luxe series) the standard audio 5 pin DIN has inputs on pins 1 & 4 and outputs on pins 3 & 5.

So the Phono to DIN cable I was using (a standard wired cable with the inline 470kohm signal attenuating resistors) has the record signals connected to pins 1 & 4, which on the 4000 is the Radio input which is more sensitive input than the Phono input on pins 3 & 5.

So I will retry with the cable rewired to pins 3 & 5.

For the record signal I am currently using my Marantz CD player audio output phono connectors and am now wondering if these provide a stronger output than typical amplifier tape out phono's, so will check that out as well.

Of course all this will make no difference to the microphone very strong record levels.

[DMcMahon]

Retried DIN recording with the cable wired to the phono input at pins 3 & 5 (without any attenuating resistors in the cable) and got the same results as above. This is as expected, as the Phono input pins connect to the Radio input pins via an internal 470kohms resistor.

Tried a couple of old amplifiers to provide the tape out phono signal, both were slightly lower amplitude than the Marantz CD Player audio out signal, but basically got the same results, i.e.very strong record signal in manual and in automatic ALC mode the record level again is hard over on the VU meter.

As the manual record signal is so strong, at this stage I do not necessarily think that the ALC setup is bad.

Tried microphone recording and got the same results as above both in manual and ALC.

While doing all the above discovered that there is a very noticeable hum issue in manual and ALC modes when set up to record, when there is no audio signal present. Traced this to the fact that there are no earth connections on the CD player, the amplifiers (s) and the 4000. Adding a temporary earth to any them greatly reduces the hum. Hum also present if the 4000 (in record) is running on its batteries and nothing else is earthed.

Next I will try increasing the value of the attenuation resistors in the DIN cable.

[Uncle Bulgaria]

It sounds like there's not enough attenuation before the recording circuits. Have you checked the signal path for anomalies? One of the things I love about these Uhers is their great variety of inputs and outputs in such a small box! They can be connected up in so many different configurations.

I suppose the earth connection is supposed to be made on one of the dedicated pins on the DIN connections. I've not had this problem myself on the CR 240 which is also un-earthed on mains operation, but I can't think if I've used it without an earth path.

[DMcMahon]

No not yet, but there is not a lot in terms of attenuation, apart from the 470k R1 which from test results looks OK.

The 4000 DIN earth/0V pin 2 connection does not connect (at least resistively) to the 4000 metalwork.

If I can find my signal generator (or use an app via laptop) I want to check the VU meter 0dB calibration R136 adjustment.

[Uncle Bulgaria]

Ah, of course - considered in reverse the meter could be the culprit. Does it start clipping when the recording level knob is unexpectedly low?

You've intrigued me now. After I've knuckled down to some work I'll check the arrangement in the CR 240. It would surprise me if it were wildly different, but I seem to remember some lugs grounding the signal to the chassis.

[DMcMahon]

I have not really examined the record or playback signals for this issue but by ear they sound good so I doubt if clipping.

[DMcMahon]

Found signal generator and connected up for testing the meter calibration as per the service manual.

For the 10mV signal into the DIN (Radio pin input) got the required 775mV at the Monitor DIN at around 0db on the VU meter with Record level to 1, this co-incidentally is the same low setting that typically I was adjusting to for DIN music and microphone recordings. So all looked good apart from what appears to be a very low record level setting.

Switching the Record Mode switch to either of the automatic settings ALC1/ALC2 gave a much larger signal waveform on the monitor socket with clipped peaks , so distorted/overloaded. So then decided to look at the ALC sensitivity pot R814 adjustment, as per procedure in the manual.

Opened up the 4000 and accessed the ALC board, access to pot R814 and also pot R813 (ALC Control time) is through holes on the ALC board, these holes were sealed over. I carefully opened up the holes, removing the thin membrane of sealant material using a small instrument screwdriver, I am sure I did not disturb the pot adjustments.

Directly after this everything was different for the better, now to get 775mV and 0dB you have to adjust the record level to 6, which seems a much more
correct area. Also when switching to ALC1/ALC2 the signal level stayed the same and the VU meter stayed at 0dB, so all looked good.

Tested music recording and then had to set record level to 3 for 0dB and ALC1/ALC2 when selected stayed around 0dB.

Tested microphone recording and it was exactly the same as originally, i.e. record level 1 for very strong manual recording and overloaded/VU off scale for automatic recordings.

Rechecked DIN recording, you guessed it, the fault had returned !!

So need to go back in and see if can get it to toggle back into the good state.

Looking at the ALC circuitry difficult to work out how it is affecting the monitored signal, easy to see how it affects the record amp signal that goes to the record head. I think somehow it must be affecting the record level control which is connected into the ALC board but nothing obvious to do with R813/R814 circuitry.

[DMcMahon]

Went back inside the 4000 and it toggled back into the good condition. So later I will do some probing/taping around the ALC board and surrounding area to hopefully manually toggle between good/bad and bad/good and then home into the root cause.

[DMcMahon]

Problem found and fixed, traced to bad miniature screened cable connection at the amplifier board, identified as connection # 172 on the SDS schematic. By gently pressing the end of the cable could easily get it to toggle good/bad.

Using a magnifying glass could see that the live inner core had no insulation over it and it and was probably making intermittent contact with the cable screen (which was not insulated/sleeved). Measuring resistance between the screen and the live wire proved that the live wire was shorting down to the screen when vey light pressure applied to the cable. The screen connection goes to the -ve side of the batteries/power supply so effectively the wire was being shorted to 0V.

Unsoldered the connections and cut a bit of outer insulation off the cable to get a short length of insulated live wire, reconnected and all good now.

Possibly this was an original build defect caused by bad cable termination ?

Looking at the schematic I find it difficult to fully understand the functionality of the screened cable connection. It connects back to the ALC board (output of FET T1) and at the Amplifier board connects to the junction of C86/R151. R151 is a 130 ohm resistor that connects to one end of the Record Level control pot, so with the cable shorting down it would connect the short via the 130 ohm to the record level control and this probably explains why the control was so extra sensitive in the fault condition. Why the ALC operation would overload in the fault condition is not so easy to understand.

Did various DIN (Radio & Phono inputs) & microphone recordings all good. Typical manual record level being around 3 to 4 (for 0B average) depending upon loudness of music. With a range of smaller and larger input signals the automatic ALC1/2 operation performed very well tracking/maintaining the automatic record level to around 0db.

To finish off, applied a little blue Threadlock to the previous loose pinch wheel nut (ref post 81/82) and a couple of small drops of Superglue to the Start switch sprung contact arm (ref post 74). Tested the operation of the Start switch OK.

The Superglue repair subsequently turned out to be a big mistake !

Later when rechecking Playback found that Playback no longer worked and traced it to the Start switch. The Superglue fix once it had set fully, had changed the flexibility of the sprung arm and the switch when actuated by the Start lever would not toggle over.

Had to remove the switch and get all the Superglue off (which was not easy at all) all OK now thank goodness.

Will return the 4000 to my friend the owner this afternoon at another South Coast Blues Open Mic/Jam event, where can give it a road test.