Uher 4000 Report Monitor Amplifier Component Layout

[DMcMahon]

As initial investigation into recording not working, last night I tested the Bias in Record mode. Measured at the amp board at screened cable that connects out to the Erase head (also checked continuity of the cables from the Record & Erase heads back to the amplifier board) got a good clean stable 100kHz (10uS) sine wave Bias waveform of around 64 volts peak to peak amplitude.

At the Record head connection got same good waveform at around 28 volts peak to peak. Do not know what they should be amplitude wise on the 4000 but they look good, the Record head bias being lower amplitude than the Erase head is normal.

In the Service manual on page 33 para 4.3.2.1 there is a setup procedure for the Bias, it states to adjust R145 for 25 volts / 100kHz at TP III (Test Point 3) using AC voltmeter.

It does not state what type/model of voltmeter, depending on voltmeter used the measurement/displayed result could be Average, Peak or RMS, by calculation 64 volts peak to peak would be around the very low 20s both for Average and RMS.

TPIII is not shown on the schematic, it is shown on Fig 21 in the Service manual but there is no obvious way of knowing where TPIII is actually on the board (circuit wise), no obvious marking on the board itself showing TPIII, so no real way of knowing where on the Bias Oscillator circuitry to connect the volt meter for the 25 volts adjustment.

Hopefully the 4000 documentation I have ordered (including component layout) will identify TP III. But anyway happy at this stage that Bias is good.

Doing a recording and monitoring the Bias signal going to the Record head there is little if any sign of audio signal present. I know from doing this on other recorders that it can be difficult sometimes and subjective to quantify if the record signal going to the record head is good, due to the fact that the audio signal is superimposed (modulated) on to the much larger amplitude and higher frequency Bias waveform. But I think from a quick test, the lack of audio signal looks very suspect which ties up with the non recording results.

[ms660]

So far as I can make out TPIII is the output from the oscillator, the oscillator output is connected to board 800 from the main board via a screened lead, it's the junction of two of the osc coil windings on the schematic.

Usually AC voltages are given in RMS unless otherwise stated.

Lawrence.

[DMcMahon]

That is very interesting thank you Lawrence. I may be missing something, but on my schematics I cannot easily see any connection between the Bias Oscillator circuit and the 800 board (ALC/Automatic Level Control) ? I can see the output of the ALC (designated 3) which I think may be a screened cable connecting to what I think is the Record amp circuitry.

Is the junction of the 2 oscillator coils you reference the same junction as C84/C85 I still cannot see TP III on my circuit, so does yours show TP III at this point ?

[ms660]

The schematic I have access to is the one on cryptomuseum (schematic link B):

https://www.cryptomuseum.com/covert/...port/index.htm

It's not the first set of schematics in link B but the second set of schematics, the component numbers might be different to yours but it shows TPIII on that schematic.

Lawrence.

[DMcMahon]

I can see it now in 2 places in that document, one in the 4200 Report Monitor schematic and the schematic above (the one I think you reference) which is difficult to determine the 4xxx model from the schematic (due to the way they are cut up) but definitely not the 4000 Report Monitor. I also see in both these schematics that board 800 is not the ALC board like it is in my 4000 Report Monitor.

The name on the schematic for your 800 board is betriebsarten-wähler, which translates as "Mode Selector", so presumably there is a board called this in the 4200/4400 and maybe also the 4000 Stereo L which is also referenced in this Servicing Instructions Supplement document.

The TPIII connection shown connects directly (via some sort of selector switch) to the 4200 stereo head, so effectively identical to where I measured the Bias signal as 64 volts peak peak in my 4000.

So in conclusion the TPIII identification is missing from the 4000 Report Monitor schematic, at least the one I am looking at.

Sometime out of interest I will connect my Valve Voltmeter to TPIII to see what the RMS voltage reads, normal DVMs, Avo meters etc will be no good as 100kHz is too high for them to give an accurate reading.

Thank you Lawrence.

[DMcMahon]

Tonight checked out the Record amp to see why no recordings were being obtained when previously checked using microphone. Not yet found my Signal Generator so connected CD player to the Radio/phono DIN input as a signal source (much easier than using a microphone).

Using scope checked through the Record amp circuitry (really looking forward to receiving my order from SDS of the components layout drawing, as I am still finding it hard to locate the correct components on the actual amplifier board), everything checked OK.

Signal very messy around the output transistors T 38 / T 39 due to the Bias signal breakthrough (must check the Bias Trap adjustment sometime, which I think is the “Bias Compensation” procedure in the service manual).

Final signal to the recording head could just about be seen) modulated onto the large Bias signal (fairly typical in my general R2R experience).

Did recording but nothing obtained on Playback on the 4000. Played back the recording on a Grundig TK 23 and got excellent playback, did this a number of times and every time got very good results when played back on the TK 23 but nothing at all on the 4000.

So now recording is working very well, need to do microphone recording again, no reason why this should not work now but previously did not.

Also need to revisit the Playback amp and find out what is wrong there.

[DMcMahon]

As an aside while checking things out I looked at the Automatic Level Control (ALC) board (board # 800 in the 4000 Remote Monitor). I see that it has an extra (it has 2 normally) pre-set potentiometer (pot) and other components added like a modification.

These components are not shown on the schematic I have.

But believe the pot is R 819 because in the Service manual for ALC setup it references adjusting R 819 saying "from unit no. 1 426 03121", so I guess this most likely was a factory modification from that unit # onwards.

[Uncle Bulgaria]

In my CR 240 repair (posts passim) I found numerous components that were different in the diagram from what I had on the board. I wish I'd found SDS sooner, as they sent me a new set of diagrams with the same board revision I had. They are not only clearly coloured, but are also clean and sharp originals rather than the dodgy PDF I was working from!

I think you'll be well pleased when the diagrams arrive.

I had success using my 'phone or PC as a signal generator through the amplifier via the tapehead connections - there are numerous apps or websites where a wave and frequency of choice can be continuously fed out.

[DMcMahon]

Yes I am really looking forward to receiving my documentation order from SDS, it will make life a lot easier to locate the relevant components. Thank you for the input on the Signal Generator apps, I have used a couple before but then mainly for burning test frequencies to CD to then play the CD into R2R line inputs, will have to look at some of these apps.

[DMcMahon]

Update on Playback problem. Recorded a long stretch of music (Cream - Greatest hits) to use as a Playback music source and checked the recording was good on my TK 23. Tested playback on the 4000 using this recorded tape.

As before just get a short duration sound burst of good music, the end of the music does not suddenly cut off, it decays quite quickly (and then have to power down and up again to get a second burst of sound etc), so it has reverted back to the short sound burst symptom after previously ending up with no playback sound.

I noticed again (as briefly previously reported) that I get the playback sound burst not only with the Monitor switch switched to T (Tape) but also when switched to S (Source).

To my mind there should be no playback signal when switched to Source so got sidetracked and decided to investigate further in case it is relevant to the playback problem.

Rechecked the voltage operation of the Monitor switch (at board # 200, do not know its official name, but it is the logic/control board I would say) and noticed that there is a strange cyclic voltage charging/discharging effect on the opposite side of the Monitor switch to the switched live side.

There are two 0.47uF Tantalum bead (C8/C9) capacitors (maybe for de-coupling), one on each side of the switch that connect out through 100kohm resistors (R18/R20) to the Playback amp to switch the appropriate FET transistor (T19 /T20) to select Source or Tape Monitor operation, so there maybe normally some charging effect when switching voltage, but it looks strange to me.

I measured the 2 capacitors and C9 (I think) was very low at around 100nF (in and out of circuit), did not have a Tantalum to hand, so replaced it with an ordinary (Aluminium) 0.47uF electrolytic.

But still got the same symptoms then discovered that there appears to be a problem with the Start switch. On the schematic it show Start switch K4 connecting to the 200 board into resistor R21. If I am interpreting the circuit correctly then with the Start switch Off there should be the +4.9V supply switched through to R21 and when the Start switch is On ground/0V should be switched through to R21 (or vice versa for the On/Off conditions). R21 being in one of the switched lines for Source or Tape.

Measuring at R21 input it appears to be effectively floating, i.e. not getting either +4.9V or 0V from the Start switch. So I need to locate the Start switch to check it out further, I assume it will be near the Start button area. There is a white wire that connects to R21 on board 200 and this wire disappears into the cable harness so need to try and see where it subsequently comes out of the harness.

Do not currently understand what the Start switch has to do with this circuit, it may all be a red herring in terms of the Playback problem, but it definitely looks incorrect.

[DMcMahon]

Very easily found the Play Switch, it is above the motor, below right corner of the Drive Wheel, virtually in front of my nose at times when probing the amplifier and Logic boards.

Both its NC (Normally Closed) and NO (Normally closed) pair of contacts are not making mechanically or electrically, both when the switch is not actuated and also when the switch is actuated (see photo images), so this 100% agrees with the voltage measurement results at the Logic board, i.e. the Play switch is not switching the + 4.9V supply and 0V through to the Logic board.

Had a little play with the switch, difficult to see what adjustment needs to be done, not just a simple fine tweak by the looks of it.

[ms660]

Looks like the toggle spring has gone walkies....(for illustration):

https://uk.rs-online.com/web/p/microswitches/9074502/

Lawrence.

[DMcMahon]

Thank you Lawrence, I came to same conclusion, I found it was impossible to get either contact of the switch to make in any position. Found a good French video that covered the servicing and from that could see that, as you say there is a sprung arm missing on my switch.

Looks like it must have fallen off (no sign of it in the unit) and looks like it just sits/hooks tightly between the middle contact and the contact arm that is pushed down when the Play button is pushed.

It looks very similar, if not identical to the cam operated 3 switches (these are riveted together) for the 3 speeds, I found an RS part number for them (1 individual switch) namely 741-0928 but it is discontinued.

So great that you have an alternative RS part number, a nuisance that RS only sells them in packs of 10 at £16.50 !

SDS do not list the part in their price list, but I will ask them about it.

David

[DMcMahon]

This morning temporarily linked out the normally open contact of the Play switch, this restores very good playback sound, so looks like once a new switch is fitted that Record & Playback are sorted (still need to recheck microphone recording) then that will leave the noisy drive to try and improve.

I am awaiting reply from Germany if they stock the switch and if so the cost. Have seen one on eBay in Italy, of course do not know its condition, from the 1 photo it looks tarnished but that is not a problem, around £8 in total, at that price may be better to get the set of 10 from RS at £16.50 plus postage (may be able to resell the spare ones ?). Also looks like might be able to get them cheaper from a few sites, particularly USA, need to check more.

[DMcMahon]

Forgot to mention in last post that also Playback works correctly only in the Tape position of the Monitor switch.

[barretter]

I've got several of those switches. Send me a personal message with your address and I can post one to you.

[barretter]

I mean a private message. They're all brand new, unsoldered, from RS.

[DMcMahon]

Thank you, that is very good of you, I do not expect to get it FOC though, will discuss shortly in PM.

SDS came back to me saying K2, K4 & K7 (I do not know if they are all the same type) are sold together under order # 1030032 (named micro switch diverse), price EUR 1,19 each. Tracked shipment is 8,98 EUR, untracked 4,81 EUR.

[DMcMahon]

Tried to send PM (twice) but it is either very slow or not working ?

[barretter]

Yes, I just tried but it doesn't seem to work. Can't think of a solution at the moment.