Akai 1800 Check Out

[DMcMahon]

Yesterday to get the motor started I must have been turning the motor shaft clockwise which results in the motor then spinning up clockwise which is the incorrect direction.

This morning found that by turning the motor shaft anti-clockwise then the motor spins up anti-clockwise which is the correct operational direction.

Traced problem to broken motor wire, the yellow wire in first photo. this wire connects to the large 450 Ohm green resistor (current limit ?), the other end of the resistor connecting to the junction of the 2 motor run capacitors, so the yellow motor wire must be the auxiliary winding.

Reconnected the wire and the motor now starts up OK and runs in correct anti-clockwise direction. Tape play, rewind and fast forward all work well on 7" tape.

Before I reconnected the wire, I retested the tape operation after manually getting the motor spinning up in correct direction and somewhat surprisingly all tape transport functions appeared to work normally (apart from having to manually assist motor spin up) even though the run capacitors were not connected to the auxiliary winding.

Measured the 2 run capacitors, the 2uF was around 2.4uF with good ESR and good insulation resistance on megger check (so low leakage), the 0.5uF measured high at around 1.3uF with good ESR & low insulation resistance (so probably high leakage, which may have made the meter reading of capacitance higher than it actually is). These 2 capacitors are paralleled when switched to 50Hz mains frequency.

[DMcMahon]

Originally Posted by 8 Tracker

Wrap a cable tie tightly around the top of those rubber reel retainers. It gives them a decent amount of grip again.

Mark.

Mark,

Was the idea to do this temporarily for a period to compress the hole diameter or was the idea to leave the cable tie on permanently ?

David

[8 Tracker]

David,

It's a permanent fixture. If you use black cable ties, you hardly notice them.

Mark.

[DMcMahon]

Have got it rigged up to progress the 8 track faults. 8 track unit swung out and resting on support, has already toppled off once and its capacitor sparking to nearby metalwork. Due to tight connecting cables it is awkward to work on properly.

Now have one of the track indicating lamps working and by measurement the other three should work once I get the head lift mechanism working properly.

[DMcMahon]

Re-assembled Akai GX-4000D cam donor machine, need to later source replacement cams for it.

After several iterations believe now that have got the 8 track change mechanism working reliably. Found that the shaft of the sprung loaded top plate was catching/binding up in elongated hole in the lower plate when head assembly raised for track changing. By slackening the 3 top plate securing screws could get it to work OK but unable to get it OK with all 3 screws tightened down (tried all sorts of tightening sequences including loosening/trying to minutely reposition the mounting pillars that the securing screws screwed into).

In the end had to file out the elongated hole a little bit. All looking good now but need to re-assemble the 8 track unit back to the tape transport chassis and make sure still working when the 8 track unit is vertical. Fairy confident this should be OK.

After that will refit the valves to the amplifier now that I have a replacement 6AR5 and check out the amplifier, starting with the DC supplies.

[DMcMahon]

The 6AR5 replacement I have is a 6AQ5.

One earlier post indicated a small wiring change may be required to use a 6AQ5 in lieu of a 6AR5 and another post indicated may be not, so I need to double check that out before powering up the amplifier with valves fitted.

[DMcMahon]

Looks like the 6AQ5 will be fine.

Both the 6AR5 & 6AQ5 have the G1 Control grid connection going to pin 1.

In the 6AQ5 the G1 Control grid is also connected out to pin 7. So I need to make sure that nothing is connected to pin 7, as sometimes unused valve pins are used as connection points for components.

[DMcMahon]

Only practical way to have the 8 track unit in vertical position and operate the track change switch was to put both chassis' s temporarily back into the case, quite a struggle getting the 18 way edge connector/cable in correct position to be able to connect it to the 8 track unit when both chassis's back in the case.

The track change is still working very well (mechanically).

[DMcMahon]

Received my Roberts Service manuals (copies) that I was hoping would have legible schematics to use instead of the out of focus 1800L schematic.

One manual was for the Roberts 1725-8L, its A4 schematic was good quality but all valve (different valves as well) no transistors like the 3 per channel in the 1800, so not much good. It also had a separate A4 schematic for the 8 track unit but totally different to the 8 track in the 1800

The other manual was for the Roberts 1725-8L III, this one is basically the same as the 1800L schematic, not great quality but useable. It has the complete schematic including the 8 track across two A4 pages . Being USA, mains input and power supply circuitry is only for 110 volts.

These schematics like the Akai 1800L have zero information on DC voltages at the power supply and valve electrodes.

[DMcMahon]

Struggling somewhat to check if the 6AR5 output pentode valves have their unused pin 7 connected to anything. The valves are covered with a bundle of tight wires and very difficult to see/work out what pin is what.

The components that connect to the valves (that normally would help to work out which pin is which) are not visible or accessible as they are on the underside of the board above the valves and even with the board three securing screws removed the board will not move as held down by numerous very tight wiring connections. Certainly not built for serviceability/fault finding.

There is a third 6AR5 valve, used for the Bias Oscillator, this one I can see easily and its pin 7 is being used to connect components to, so do need to see if the output pentodes have their pin 7 connected before plugging in the 6AQ5 which connects its Control Grid out to pin 7.

[ms660]

Originally Posted by DMcMahon

Struggling somewhat to check if the 6AR5 output pentode valves have their unused pin 7 connected to anything. The valves are covered with a bundle of tight wires and very difficult to see/work out what pin is what.

You might be able to check from the valve side with a meter set to measure high value resistance WRT chassis.

Lawrence.

[DMcMahon]

Thank you Lawrence, that is a very good idea which I will do shortly.

David

[DMcMahon]

Checked out pin 7 resistance of the output valve bases (topside) to chassis, both were open circuit. As a double check managed to prise apart the various wires and could also visually see that nothing was connected to the pin 7's. So OK to use the 6AQ5 replacement.

Fitted all the valves & powered up slowly all OK, all valves light up.

Measured the various DC voltages.

V5 6AC4 rectifier output to C5 reservoir = +303V

HT A (C4 filter) output transformers and V2/V3 output valves anode supply = +274V

HT B (C3 filter) V2/V3 output valves screen grid supply = +256V

HT C (C2 filter) V1A/V1B 12AT7 driver valve anodes supply = +235V

LT D (C1 filter) transistors supply = +31V

LT E (D401) 8 track solenoid supply = +21V

Have no voltage data to compare to, but the values to me look fairly typical/not abnormal.

Not yet checked ripple levels or valve electrodes voltages.

[DMcMahon]

Checked the grid coupling capacitors by measuring the DC voltage arriving at the grids.

For the 12AT7 both grids are good at 0V. The feed to these coupling capacitors are at lower voltage as from transistor circuitry.

The 6AR5 & 6AQ5 grids have between +5 to +10V (fed from the 12AT7 anodes via coupling capacitors) so look like coupling capacitor leakage, checked it also with those valves removed ((had to back off mains voltage to stop the HT going above the 350WV (working voltage) of the large electrolytics due to lower load)). Got similar results, not terrible but not good.

So need to check out the those 2 coupling capacitors, not overly keen to do this as looks difficult access and much wiring to be disconnected.

[DMcMahon]

Got access to the two 0.02uF 400V coupling capacitors (C302). Did not test that bad, on megger at 500 volts were around 10MOhm, replaced them with new 0.022uF 600V Polypropylene.

Also on the board were four 100uF electrolytics (cathode by-pass) for the valves, removed and tested. The two larger 25V ones were OK capacitance wise but somewhat high ESR. The two smaller 3V ones each were > 400uF, replaced all four 100uF with NOS parts.

Now to reconnect the wiring to the board.

[DMcMahon]

During very early stage of refitting the various wires to the board, one of the wires connected to pin 6 of the 12AT7 valve, when positioning the wire, the valve pin snapped off.

That is a nuisance, have no spare valve bases, have ordered one.

The valve base securing fittings seems strange, one screw and one rivet.

[DMcMahon]

After drilling out securing rivet and disconnecting some more wires, have removed the 12AT7 valve socket, in preparation for arrival of new (NOS) replacement B9A socket.

[Cobaltblue]

Originally Posted by DMcMahon

The valve base securing fittings seems strange, one screw and one rivet.

On the underside you can see that the screw has a corresponding pillar so this is just an example of value engineering.

You need a screw to hold the pillar and a hole, save a hole and a rivet reduce the number of manufacturing actions.

We are still doing it today, our mechanical design department cost everything up by the number of cuts, punches and actions.

Cheers

Mike T

[ms660]

The one in the photo also acts as a grounding point so far as I can see.

Lawrence.

[DMcMahon]

Originally Posted by Cobaltblue

On the underside you can see that the screw has a corresponding pillar so this is just an example of value engineering.

You need a screw to hold the pillar and a hole, save a hole and a rivet reduce the number of manufacturing actions.

We are still doing it today, our mechanical design department cost everything up by the number of cuts, punches and actions.

Cheers

Mike T

Thank you Mike, there is still a hole required for the rivet.

2 of the other valve bases have rivets, one has a rivet and a screw (the screw again securing a pillar on the other side) and another has 2 screws (1 self tapping into the chassis and the other a machine screw with nut), so they were not 100% consistent with their value engineering.

David