Kenwood KA7300 Transformer upgrade

[Ken Overit]

Hi everyone,

Just after some input on transformer replacement on a Kenwood KA7300.

Original unit has two 100v primary : 33v secondary center tapped transformers. (Japan import)

I would like to replace the transformers with a 240v primary, but I am having trouble finding one with 33v secondary.
Managed to find a few toroidals with 30v CT and 35v CT that I can make fit.
The 7300 power boards tend to run hot as they are factory, so I don't want to push the friendship.

Are there any better options out there ?
Other thoughts ?

Ken

 

[restorer-john]

Why not just pickup a quality Japanese step down transformer. Most quality JDM stepdowns also have a step up capability, often with several taps (100/110/120 and 120/220/240) and can of course be used in either direction.

But if you want to replace the TXFs, it's a good candidate as there are luckily only the one secondary CT winding in each transformer.

Where are you located? In Australia, you might find (ebay search?) a pair of Ferguson PF4361/1 200W conventional transformers which have 33-0-33 secondaries. They were used extensively by hobbyists in the early 1980s who built thousands of ETI-477 (series 5000) MOSFET power amps from kits. Ferguson still had the transformers cataloged up until the late 1980s.

Here's a thread showing the amps and the transformers. You may even find an old Series 5000 power amp come up on eBay and just buy it for the transformers.

Series 5000 MOSFET stereo power amplifier - Upgrades

www.stereonet.com

 

[elvi]

I would like to respectfully point out, 35 v is within 5% of 33v.

 

[restorer-john]

I would like to respectfully point out, 35 v is within 5% of 33v.

Absolutely, but it depends on where the OP lives. In Australia, traditional mains 240V (the KA-7300 was made in the 240V days, unless it has a multi 100/110/120/220/240V selector. Some Pioneer I recall even had and an additional 127V primary option (saudi-arabia?) is now a nominal 230V with a wider tolerance. Nothing in the S/M suggests such flexibility.

The EU has also nudged their range from 220V to nominal 230V and the UK which was 240V is now more nominally 230V. If you combine that with a modern transformer (likely 230V rated) and you put it on 240V+ in a regional area, with an already higher nominal secondary voltage, you'll end up potentially at the upper voltage limit of the filter caps in the PSU.

All I can say is in Australia, they've changed little other than localized taps on transformers and only if people are complaining on solar feed-in or higher than normal voltages. I complained sevedral years ago about lower than normal voltages after the power company dropped a tap in out street. They monitored it (pole, drop and dist board) for a few weeks and agreed with me, put the tap back to where it was 400M down the road, and everything has been great since.

Right now, at night (no solar feed-in) it's 245.1V RMS at 1900hrs.

 

[Ken Overit]

Thanks for the replies, and yes I am in Australia.

Wanted to avoid a step down if possible btw.
Will have a look at the Ferguson option.
Wasn't too sure on the acceptable tolerance on the secondary voltage. My power does vary between 230-240v according to my UPS units

 

[restorer-john]

Thanks for the replies, and yes I am in Australia.

Wanted to avoid a step down if possible btw.
Will have a look at the Ferguson option.
Wasn't too sure on the acceptable tolerance on the secondary voltage. My power does vary between 230-240v according to my UPS units

Figured you were, Ken.

They are old now, but they pop up on eBay all the time. Keep an eye out on scumtree/fleabay for the ETI amp mentioned above and you may get lucky. Toroids are no fun, in an amp like that.

 

[mbz]

Reconsider the 100V step down, means you can buy other goddies from Japan, eg, sansui alphas. Picked up a nice denon DP-57L cheaply, and I don't play vinyl, only for test.