National Panasonic RS-260USD Cassette Deck Checkout

[DMcMahon]

This is ongoing with no progress, fuse is still not blowing but both VU meters still going very dim. Replaced the 10 Ohm current limit resistor R134 (wondered if its value was changing when warm) that is in series with the lamps but no change.

Also disconnected rectifier diodes D1 and D2 to isolate all external connections/load but no change.

As getting no where with the dim lamps, will park this for a while and leave just 1 lamp connected and start looking at the other problems of hard over VU meters, loud hum and original problem of poor recording results on the left channel.

David

[DMcMahon]

Last night resolved the dim VU meter lamps, then discovering/realising that this particular fault symptom was a phantom fault induced by myself.

When the 315mA fuse originally blew I replaced it with another 315mA, which subsequently blew again later. As my supply of 315mA fuses was getting low I decided to try running with a lower value fuse, I tried several values and settled on 100mA which I have loads of. At this time the measured current drain was around 120mA, the 100mA fuses are time delay and do not blow at 120mA unless left running for some time. At this time the VU meter lamps would have been dim but because I was working on the PCB and had the unit upside down, would not have seen the dim lamps until later.

Thinking about it more last night I wondered if the fuse value was having any effect, so fitted the correct 315mA fuse and low and behold the VU meter lamps then were back to normal brightness. The current more than doubled from the previous 120mA to 250mA, the voltage across the paralleled lamps being 4.8V

Measuring the resistance of the fuses gave around 6 Ohms for the 100mA and < 1 Ohm for the 315mA, the 6 ohms in series with the 10 Ohm current limit resistor was limiting the available current to the lamps.

I knew that resistance of fuses decreases with amperage value but did not realise the extent of the difference.

None of this of course explains the intermittent blowing of the 315mA fuses, hopefully this fault will not return.

The VU meters fault of going hard over currently has gone away so next will do a test recording to see if the left channel still has the original fault.

David

[DMcMahon]

Test recording playback shows that the right channel is still bad, I previously said it was the left channel but discovered that the left and right speaker connections were swapped over at the amplifier.

Was getting ready to investigate and found that the unit would not power up, the 315mA fuse has blown again ! with no fault found to explain it

This motor fuse also feeds one side of the VU meter lamps but the lamps are not connected now because I removed the lamps mounting plate to replace the foam support for the meters that had turned to dust.

David

[DMcMahon]

Located and fixed the intermittent blowing of the 315mA fuse.

A track on the board that the motor switch connects to (grey wire) was intermittently shorting to a screw (one of the 3 screws that secures the board), a small amount of the insulated copper track had become damaged exposing the bare copper to the edge of the screw head which was right next to the track. See photo 1.

The screw screws done into metal support post that connects to the metal chassis so directly grounding the track.

During much of the previous investigations the 3 screws were not fitted as needed to get access to the underside of the board (component side) so explains why the fuse only blew occasionally (only after the board had been resecured).

Cut away a section of the track and rejoined using a red wire link as per photo 2.

David

[DMcMahon]

After fixing the fuse blowing issue recorded the current drain at the fuse.

Stop = 250mA (VU meter lamps load only).
Play, Fast Forward and Rewind = 180mA (VU meter lamps + motor load).

Interesting that the current drops 70mA when motor is powered.

Cleaned the heads again and as expected no change to the bad right channel recordings.

Cleaned the Play/Record switch again and both channels again now working fine, need to re-assemble everything and recheck.

David

[DMcMahon]

Re-assembled everything and the bad recording results on the right channel is back again ! i.e. low volume and distorted.

As before Playback (of known good recordings) is good on both channels and recording levels set up fine on the VU meters and erase works well on both channels.

Measured all the Playback/Record switch contacts all good both in Playback and Record modes.

With headphones connected the real time record signal sounds very good, the record signal is taken from a tap on the headphones transformer primary out to the record head via a few components.

Next plan to check out the record bias set-up and record signal to the record head.

David

[Nickthedentist]

Well done on finding the fuse blowing fault. I suspect that screw would originally have had a red fibre washer under its head.

[DMcMahon]

Yes quite possible, bad design though having the track right next to the screw head.

David

[DMcMahon]

Had forgotten that there is an insulated panel that sits on top of the board and the 3 screws screw down through the panel so the screw heads will be insulated from touching the board, the shaft of the screw in question may still get fairly close to the track.

I had removed the panel early on and had forgotten all about it until now !

David