Playback covers - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

TIMTAPE · 6th Jul 2021, 5:55 pm

Thanks, good idea.

ricard · 7th Jul 2021, 5:19 am

Something that has amazed me is how head shields seemed to be absolutely necessary on early machines, i.e. from the 1950s, whereas on later machines they only seem necessary to get those last dBs of SNR when it comes to hum.

Case in point: 1950s Tandbergs have a large head shield which when in place almost completely covers the metal shield around the head, with the tape entering and exiting through the resulting slots. Moving the shield away manually results in a very obvious increase in hum level.

Queue the 1960s, and machines like the Tandberg 8 and 15 do away with the head shield completely, however it makes it's comeback on HiFi oriented machines such as the 9100X which has a fairly puny head shield, more of a head shade than a head shield.

Is this difference due to some change in head design, making later designs less susceptible to hum pickup?

Radio Wrangler · 7th Jul 2021, 11:56 am

Changes in head design, changes in wiring, and also in how mains transformers are mounted. Mount them the wrong way and they can induce circulating currents in the metalwork.

Magnetic screening is a right so-and-so. You never get the improvement you seek in doing one thing, you need to win a bit here, a bit there and get everything right to get hum minimised.

David

TIMTAPE · 7th Jul 2021, 3:10 pm

Small DC motors helped too. Teac/ Tascam made a line of open reel machines with such a capstan motor mounted in a mumetal case and the direct drive DC reel motors also well shielded. The play head is mounted in a mumetal case but with no shield on the front or rear face.

I've found some Nakamichi cassette deck heads picked up some mains transformer hum. I was able to minimise residual by rotating the transformer through 90 deg and mounting it to the case backplate.

The Tascam 122 MKII can have its hum reduced to MkIII level by the same treatment.

Maybe the need for cooling meant some AC motors couldnt be easily magnetically shielded.

Another machine with very high hum was a 1960ish Philips dictation machine running two 3" tape reels in a clear perspex cassette. The machine was so compactly built that high hum induction from the AC motor would have been very hard to tame I guess. The follow up machines which used cassette width tape in two separate perspex cassettes had similar high levels of hum, moderated by aggressive high pass filtering.

Some of the older Ferrograph series up to 6 allowed various methods to optimise hum reduction including a mains transformer which could be rotated in its mount for minimum hum, as well as rotating the playback head transformer, and even slight adjustments of the angle on the the metal flap on the playback head.

I've found it an enjoyable challenge to reduce hum and other interference induction in tape machines. The expense in parts was minimal. I enjoyed the detective work in locating the exact sources of interference and then devising ways to reduce the noise sources one by one for a greater net improvement in S/N.

6th Jul 2021, 5:55 pm	#21
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Playback covers Thanks, good idea.

7th Jul 2021, 5:19 am	#22
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Playback covers Something that has amazed me is how head shields seemed to be absolutely necessary on early machines, i.e. from the 1950s, whereas on later machines they only seem necessary to get those last dBs of SNR when it comes to hum. Case in point: 1950s Tandbergs have a large head shield which when in place almost completely covers the metal shield around the head, with the tape entering and exiting through the resulting slots. Moving the shield away manually results in a very obvious increase in hum level. Queue the 1960s, and machines like the Tandberg 8 and 15 do away with the head shield completely, however it makes it's comeback on HiFi oriented machines such as the 9100X which has a fairly puny head shield, more of a head shade than a head shield. Is this difference due to some change in head design, making later designs less susceptible to hum pickup?

7th Jul 2021, 11:56 am	#23
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Playback covers Changes in head design, changes in wiring, and also in how mains transformers are mounted. Mount them the wrong way and they can induce circulating currents in the metalwork. Magnetic screening is a right so-and-so. You never get the improvement you seek in doing one thing, you need to win a bit here, a bit there and get everything right to get hum minimised. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

7th Jul 2021, 3:10 pm	#24
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Playback covers Small DC motors helped too. Teac/ Tascam made a line of open reel machines with such a capstan motor mounted in a mumetal case and the direct drive DC reel motors also well shielded. The play head is mounted in a mumetal case but with no shield on the front or rear face. I've found some Nakamichi cassette deck heads picked up some mains transformer hum. I was able to minimise residual by rotating the transformer through 90 deg and mounting it to the case backplate. The Tascam 122 MKII can have its hum reduced to MkIII level by the same treatment. Maybe the need for cooling meant some AC motors couldnt be easily magnetically shielded. Another machine with very high hum was a 1960ish Philips dictation machine running two 3" tape reels in a clear perspex cassette. The machine was so compactly built that high hum induction from the AC motor would have been very hard to tame I guess. The follow up machines which used cassette width tape in two separate perspex cassettes had similar high levels of hum, moderated by aggressive high pass filtering. Some of the older Ferrograph series up to 6 allowed various methods to optimise hum reduction including a mains transformer which could be rotated in its mount for minimum hum, as well as rotating the playback head transformer, and even slight adjustments of the angle on the the metal flap on the playback head. I've found it an enjoyable challenge to reduce hum and other interference induction in tape machines. The expense in parts was minimal. I enjoyed the detective work in locating the exact sources of interference and then devising ways to reduce the noise sources one by one for a greater net improvement in S/N.