Ferrograph Re-capping

[ms660]

I would replace it too, its resistance affects the voltage supply for V2,V3,V4 & V7 heaters and the supply to the solenoid circuit.

Lawrence.

[DMcMahon]

I agree, the resistance is a long way out, but have you checked what the meter reads with just the meter leads connected together? what ever that resistance is, it will be added to the actual resistor resistance.

I am not really sure of the function of R6, not sure about the smoothing reference.

[ms660]

Surge limiter.

Lawrence.

[DMcMahon]

Have you measured the DC supply at C10 to see how close it is to the 11V shown on the schematic ?

Do you know what this solenoid function is, is it to do with EOT (End Of Tape) detection and stopping the drive ?

David

[ms660]

The solenoid holds the start control in position when the control is set to start, apart from switching off the mains it's de-energized by the stop button switch or the auto stop switch, these switches short out the solenoid, hence the 100 Ohm series resistor.

Lawrence.

[untune87]

Originally Posted by DMcMahon

I agree, the resistance is a long way out, but have you checked what the meter reads with just the meter leads connected together?

Good point, but only 0.06R from the leads so not enough to shift it far.

I will order and swap out a few resistors since R76 also appears closer to 2K2 than 1K8. Worth putting carbon comps back in here or would it make more sense to go metal film or wirewound?

Also, in place of R1 (100R) there appears to be a pair of 100R resistors in parallel. Possible repair?

Thankyou for everyone's contributions so far

[Sideband]

Originally Posted by untune87

This morning I have pulled a few caps out and measured them to see if they had drifted.

In the PSU, the 16uF + 16uF can is reading ~22uF + 22uF (one a little under, one a little over.)

The 8uF reads 9.35uF.

The two 0.1uF caps across the switch on the deck read 150nF and 142nF.

Any of these enough to be causing issues with the function of the solenoid holding or the motors having issues?

A spot of advice when measuring paper capacitors on a DMM, don't rely too much on the reading. If the capacitor is only slightly leaky, the DMM (most of them anyway) will generally give a reading that is higher than the actual capacitor value, sometimes by quite a lot. It is down to the way the DMM actually measures the capacitance. The two .1uF caps you mention above may well be OK but in order to check for leakage, the capacitor should be checked at something near its voltage rating. There is no way a DMM will do that. In any case 150nF and 142nF are not enough away from the nominal value to matter anyway.

With electrolytics, the tolerance is so great + or - 25 or 30% is quite normal.

[DMcMahon]

I do find that other digital capacitance reading meters (non DVM type) generally give similar readings to the DVM reading.

[untune87]

Thanks for the advice regarding the readings, I'll take that into account. At the moment I'm just waiting on delivery of a couple of bits then I can put the PSU back together. I couldn't source any 25mm diameter 16+16 cans so I went with a 35mm and a new clip - it will not be centered but there's just enough give that the pins won't touch the chassis. I'm going to insulate them anyway to be on the safe side. There is actually an unusued 35mm hole cut in the chassis, perfectly suited to the bigger can, but it would mean trying to unpick some wires from the bundles to relocate them.

I aim to swap out the 16+16 can and the 8uF on the power supply, along with the 2R2 on the 4700uF filter cap, the 1K8 (which might be fine but I thought I may as well) and the 100R near the solenoid. All using modern carbon films. Once that's done I'll re-evaluate and then look at the amp section!

[untune87]

Quick question - schematic says there should only be a 100 ohm resistor here. Is the chunky one also a resistor?

[DMcMahon]

Yes definitely a 100 ohm resistor, larger power (wattage) rated than the smaller one.

[untune87]

Originally Posted by DMcMahon

Yes definitely a 100 ohm resistor, larger power (wattage) rated than the smaller one.

Thanks David, I thought so, just wasn't sure if it might be some kind of axial cap or something. Unsure why it's in there, should only be a 1W resistor. Going to pull them both out and swap them for a single 1W 100R, as per the diagram

[DMcMahon]

If they are in parallel first try measuring their combined resistance, should be around 50 ohms if both are intact.

[untune87]

Originally Posted by DMcMahon

If they are in parallel first try measuring their combined resistance, should be around 50 ohms if both are intact.

Out of circuit they individually test around 125R, I replaced them anyway while I was doing it.

Put everything back together and fired it up. The solenoid seems to be holding now, but even though it does, it doesn't always seem to keep the capstan motor running. Sometimes it does, other times you have to keep the lever held hard (even though the solenoid is holding it) to shift it what feels like a millimeter, then you hear the motor start whirring again. This seems to go for play, FF and RW in all speeds. Not sure if perhaps some mechanical adjustment is therefore necessary? Or maybe just dirty contacts?

Despite all this, the reel tables (correct term?) don't revolve; It has just wound a lot of tape around the capstan because the take up reel didn't move. I'm guessing that points to duff motors which is probably a bit beyond my remit unfortunately

[whiskas]

I would leave the high voltage 16+16uf cap in place, unless it's leaking electrolyte, getting hot during use, etc. They often measure up to +50%, so ~22uf measurements are normal.
Same goes for the 8uF cap, ~9uf measurement is absolutely fine.
** Remember to always wait for these caps to fully discharge before taking measurements!! **

[Sideband]

Originally Posted by untune87

Thanks David, I thought so, just wasn't sure if it might be some kind of axial cap or something. Unsure why it's in there, should only be a 1W resistor. Going to pull them both out and swap them for a single 1W 100R, as per the diagram

Looks original and could have been a later modification by the manufacturer (after the original circuit was drawn). This often happens during production and is why amendment sheets were issued to the trade. Perhaps it was found that something didn't always operate correctly (like a solenoid not always latching properly) perhaps caused by slightly low mains in some areas and by reducing the value of the resistor, it cured the trouble. It was probably quicker to just add a resistor in parallel rather than remove the old and replace with a 47 ohm.

If it was me, I would replace with either a single 47 ohm 1 watt or two 100 ohms in parallel. It's just the sort of thing that might cause the solenoid latching problem you are experiencing (depending where the resistor is in the circuit of course).

[untune87]

Originally Posted by whiskas

I would leave the high voltage 16+16uf cap in place, unless it's leaking electrolyte, getting hot during use, etc. They often measure up to +50%, so ~22uf measurements are normal.
Same goes for the 8uF cap, ~9uf measurement is absolutely fine.
** Remember to always wait for these caps to fully discharge before taking measurements!! **

You are probably right there but I chose to replace them anyway due to their age and given that I had the power supply out.

Originally Posted by Sideband

Looks original and could have been a later modification by the manufacturer (after the original circuit was drawn). This often happens during production and is why amendment sheets were issued to the trade. Perhaps it was found that something didn't always operate correctly (like a solenoid not always latching properly) perhaps caused by slightly low mains in some areas and by reducing the value of the resistor, it cured the trouble. It was probably quicker to just add a resistor in parallel rather than remove the old and replace with a 47 ohm.

If it was me, I would replace with either a single 47 ohm 1 watt or two 100 ohms in parallel. It's just the sort of thing that might cause the solenoid latching problem you are experiencing (depending where the resistor is in the circuit of course).

Given the way that the smaller of the two was wrapped around the post I'd say it was original. Now the interesting thing is that their combined resistance was about 60R, I've replaced it with a single, new 100R 1W (very close tolerance too) and the solenoid is holding again, although like I mentioend in my previous update, it seems that despite it staying in place like it should, it still needs a manual pull sometimes to engage the motor. Given that I have replaced a couple of other resistors, plus the PSU caps, it's hard to say what has influenced the fact that it is at least partially working again. When fitting the new resistor I intentionally left the legs longer as it was in a difficult place to reach without unscrewing the board and freeing some of the wires. It may be worth me temporarily clipping one of the original 100R (~120R actually) back across the new one and seeing if the drop to nearer 50R affects it - what do you think?

Also it seems like the motors are the issue. Capstan seems to be fine, but the wind back and wind on don't move. In FF and RW, the hubs each spin freely by hand so I'm guessing it's not a lubrication or bearing problem? Can anyone offer advice on where to look here?

[ms660]

According to the schematic that's been posted the supply and take up motors in FF or Rew are supplied from the same point as the capstan motor which is the 245 volts supply on the mains transformer via switches S1a,b & c, S3 and plug and socket P3 & SK3, the relevant pin/socket numbers for the FF and Rew supply on P3-SK3 are 2 & 7.

On playback or record the supply motor is out of circuit and the take up motor is connected to the 150 volts supply on the mains transformer again via the same switches and plug and socket but this time the relevant pin/socket numbers on P3 & SK3 are 10 & 11.

I would check the switches and P3/SK3 contacts

If that helps.

Lawrence.

[untune87]

Morning all,

A development! I've spent some time today following continuity from the connectors and through the tagboard, to see where each wire goes to in relation to the wind on/back motors to make sure it wasn't a bad connection (thanks Lawrence!) Through this I've discovered the culprit - one half of the big switch that puts the motors into circuit (with the pair of 0.1uF caps across it) didn't appear to be making any contact when the play lever was engaged. Other half seems fine, so I wondered if perhaps it had just come loose or deformed with age and needed some help. Clipped my DMM to the contacts, pulled the lever closed and reached in to give the long, brass strip a little nudge - at which point it twisted at an angle and stayed there. Looks like there is a fracture in the brass strip halfway down, where it bends, and I've just helped it along by moving it. It would make perfect sense that the contact responsible for the winding motors is broken, and since the other isn't - the capstan motor still runs.

So! Looks like all I need to do is swap out the strip (although I may as well do both) and I'm good. I may be able to source a replacement switch, and I guess at a push it's not too difficult to make a new strip if I can get the contact point on there and match the angle etc. Going to remove it so I can take a closer look.

[DMcMahon]

Well done ! and good luck with sourcing replacement switch/repairing, may not be easy.

David