Magnavox 620S Record player

[Michael Maurice]

This record player had been sent to me by another forum member, it had been partially dismantled by another member.

All the valves were out and in another box, wires were left hanging off, the cartridge was in bits and the switch was bridged because one leg was o/c.

The knobs and rear inside cover are with the original owner

The record player is fitted with the Magnavox version of the Collaro Studio turntable. This was stripped and serviced and featured in a thread of its own

After the deck was serviced, attention was paid to the cartridge, the originals were NLA and hadn't been for years so with the help of Musonic a suitable cartridge was obtained.

The old cartridge was removed from the bracket and the its bracket used as a mount for the new one

[Michael Maurice]

Next the bracket of the old cartridge was used to mount the bracket from the new cartridge, after carefully cleaning both brackets, they were soldered together. The new cartridge was fitted to the bracket and the whole assembly fitted into the pickup head.

[Michael Maurice]

Now attention is turned to the amplifier. This uses two UCL86's and a UY85. The chassis is connected to one side of the mains.

Each UCL82 drives one channel, the idea being that the internal speaker is driven by one channel and an external speaker is connected to the other hannel, the circuit is arranged so that the external speaker is electrically isolated from the chassis.

If you look at the photo's you'll see a large 470 ohm resistor connected to the middle valveholder, originally a capacitor was mounted right next to it, in fact it was mounted so close that it had melted!

These capacitors and the two waxies were replaced together with some resistors.

The volume control was US, one side of its switch was open circuit and so was bridged. These dual gang 1M ohm pot with switch are not available from the usual sources but I found a this company who were able to manufacture a new one. it isn't cheap but it means the repair is safe.

[Michael Maurice]

One problem I did come across was a low frequency motorboating on both channels. After much head scratching I found that both 330K resistors which form the anode load on each section of the UCL86's triode section had gone very high in value.

I'll take a couple of pictures of the finished later but the record player can now be returned to its owner.

[paolo]

Well done on progress to date. These old decks can present a considerable challenge. How does the deck sound? The reason I ask it that the "angle of attack" of the stylus on the new cartridge must be quite severe. Is there any way of reducing the angle?

Paul

[Michael Maurice]

Paul, it sounds quite good considering. it was never intended to be Hi Fi.

[paolo]

A point well made, Michael

[Michael Maurice]

A hitch.

I got a phone call this morning from a rather disappointing owner who tells me that there was sparking in one of the valves, no sound and smoke from under the chassis!

He decided to swap valves around to see if he could fix it, not a good idea

The resistor in question is a 390R in the cathode circuit of the UCL82's pentode.

I presume the sparking means that some electrodes are shorted out and the resistor took full current and burnt out, but I have another theory:

The balance control is used as the grid bias bleed resistor on the pentode of the UCL82, this means the bias resistor is anything from 0 - 2M ohm. Suppose this went open circuit even temporarily, this could be the cause of the heavy current.

I'm thinking of fitting 2 1M ohm resistors between the slider of the pot and each of the grids. Even this might be high, maybe 560K would be more suitable, to ensure the total resistance didn't exceed about 500K.

I've ordered 2 x UCL82's and an UY85.

Your thoughts would be appreciated.

[Herald1360]

Are the valves UCL82 or UCL86? Posts 3&4 differ from the last one.

[Michael Maurice]

Chris, its UCL82, I have corrected post 4.

Thanks for letting me know

[Michael Maurice]

An update:

Success: I obtained new valves and upon refitting them, I got sound but it had a loud background hum on one channel only.

Now I carried out lots of tests, one of which was to modify how the balance control worked.

Originally the balance control was connected between the two pentodes grids with the slider connected to ground. The balance control providing the grid leak resistor.

The problem with this arrangement is that if the control became noisy, it would upset the grids operating conditions.

I thus wired a new balance control to operate in conjunction with the volume control and fitted 1M resistors between the grids and ground.

But the hum was still there.

According to the data the UY85 can handle a reservoir capacitor of 100uF, as the original was 32uF, a 47uF capacitor was connected in parallel to give approx 80uF capacitance.

But although the ripple on the HT was much reduced, the hum remained.

Eventually I did what I should have done and swapped valves over, the hum then followed to the other channel. One of the new UCL82's was faulty.

I managed to obtain NOS UCL82's and upon fitting them, the record player worked well.