Schaub Lorenz Music Center 5001

[Termorilis]

Hallo. I'm italian.
Hi I have this beautiful device. Some time ago I started the restoration. Only unfortunately now I have to stop because I have discovered that the erase head is shorted. In fact, it records strongly distorted and does not erase previous recordings. I need this replacement. Unfortunately, the head cannot be repaired. If anyone knows who I can turn to, I would be very grateful. Thank you.
In the photo, the head is partially hidden in brown.

[jamesperrett]

I don't know this unit (although others on here do) but your symptoms are typical of the bias oscillator not working. The same oscillator is often used to drive the erase head so the head may actually be working but just not receiving any signal.

[TIMTAPE]

I second what James says. It's unlikely the erase head has shorted but usually erase heads in transistorised machines have a DC resistance measured with a DMM of 1 ohm or so but an impedance at the erase/bias frequency much higher.

[DMcMahon]

If it is recording (albeit distorted) then the bias oscillator must be working to some extent which makes it more complex to diagnose.

David

[Termorilis]

Indeed you are right. I took the cartridge apart and saw that the coil is good. There are few turns. The thread is thick. The value around 1 ohm is justified.

[ricard]

On the other hand, if there is no bias at all, the resulting recording will actually be severely distorted, although the resulting signal when played back will be at a fairly low level as well.

[jamesperrett]

Originally Posted by DMcMahon

If it is recording (albeit distorted) then the bias oscillator must be working to some extent which makes it more complex to diagnose.

In my experience you'll still get some kind of recording with no bias - just very distorted. The symptoms sound exactly like a failed bias oscillator to me.

[DMcMahon]

Termorilis do you have access to an oscilloscope ? this would be the best way to check/monitor the bias oscillator signals.

David

[ms660]

If it's nothing simple like a wire off or a poor connection I would check the voltages on the erase/bias oscillator transistor.

Lawrence.

[DMcMahon]

The 5001 technical manual lists the erase frequency as 54kHz and states it should measure 7.5 volts with a VTVM connected in parallel with/across the Erase head (in Record mode). It references connection points 63 and 62 (which are shown on the schematic), I do not know if these connection points are physically identified in the 5001.

The 7.5 volts will be an r.m.s. value which equates to around 21 volts peak to peak value as seen on an oscilloscope.

The VTVM is a Vacuum Tube Volt Meter which has high input impedance and can measure accurately AC voltage at high frequencies. An ordinary / basic analogue or digital multimeter/voltmeter would not measure a voltage at 54kHz accurately if at all.

For high frequency voltage measurement a VTVM or FET VOM or up market electronic multimeter is required or better still an oscilloscope.

The Record voltage is listed as 14 volts (equates to around 39V peak to peak) measured in parallel with the Playback/Record head, once again in Record mode. Connection points are listed as 57 & 58.

David

[DMcMahon]

Originally Posted by ms660

If it's nothing simple like a wire off or a poor connection I would check the voltages on the erase/bias oscillator transistor.

Lawrence.

First page of attached lists some of the static voltage measurements in Record.

First and second pages detail the Erase and Premagnetization voltages (Record Bias) measurements and third page shows relevant section of schematic.

David

[Termorilis]

Then. I have checked the erase circuit components and they are ok. I also checked the capacitors with capacitor and digital meter. The capacitors are ok. I plugged in the oscilloscope while recording and if I disconnect the erase head from the circuit, I have a sine wave. If instead I connect it to the circuit I have a straight line

[bronco.]

Hi! First post here.

It could be a short in the erase head wire. I'd check it if wire is coaxial type.

[Termorilis]

The cable is coaxial and is ok. I have checked the cancellation circuit components, but they are ok. I checked the capacitors. All ok. Their values ​​are also accurate. The only electrolytic installed in the cancellation circuit is the 25micro one. The capacimeter measures about 26 micrograms. I checked the insulation and it's ok.
I found the 50 ohm trimmer connected to the emitter of the AC128 transistor which shows me a resistance greater than 100 ohm.

I changed the trimmer from 50 ohm to show a value less than 50 ohm ... and the situation has improved a lot!
Now the head erases the previous tracks! Now it no longer feels distorted but a little noisy compared to the radio. (background rustle) but it is slight. I should connect an external source to judge the quality of the recording.
It must also be said that when I disassembled the erase head, I found it slightly misaligned with the recording head. In fact I found the adjustment screw spaced from the seal mark.
I then proceeded to align the cancellation circuit. With the oscilloscope I reach a voltage of about 7 v which corresponds to about 21 volts peak to peak. At 20 volts so I have maximum efficiency. However the situation has now improved a lot. Now I proceed to the alignment of the recording circuit. We update!

[Radio_Dave]

I've got three of these machines and two of them had a dry solder joint inside the erase head oscillating coil, which tested ok with a multimeter but were visibly poorly connected when I looked inside.

It's also worth checking/replacing the mercury wetted relay, which switches from record to play and the Philips/Valvo electrolytic capacitors.

David

[DMcMahon]

Well done, you are progressing well.

David

[TIMTAPE]

Originally Posted by Termorilis

It must also be said that when I disassembled the erase head, I found it slightly misaligned with the recording head. In fact I found the adjustment screw spaced from the seal mark...

Just looking at your photo of the head block, the two head gaps do seem to be misaligned re height. Misaligned like that, the erase head will probably not fully erase the track it is supposed to erase, but may also erase part of the wanted adjacent tracks.

Well done on spotting the faulty 50 ohm trimmer in the oscillator circuit.

[Termorilis]

I solved. I adjusted the pre-magnetization trimmer and the cancellation trimmer with the oscilloscope. There was no need to replace anything. Now it records very well. Another problem to be solved is that of the functioning of the pilot tone at the end of the recording. Does not work. Some advice?

[DMcMahon]

I know nothing about the pilot tone operation. Having had a quick read about it in the manual I do not really understand its function.

There is quite a lot of technical information about it in the manual, do you have the manual ?

David

[DMcMahon]

One old Forum link with some information.

https://www.google.com/url?client=in...tFJt7NnR0VEQ4d