KA-7300 restore

[hhaammiisshh]

Picked up a KA-7300 and about to start cleaning and restoring.
Unit is a Japanese model so branded Trio, runs on 110V and is missing the presence switch and DIN connector on the back. Also no A+B speaker select.

Unit is in very good condition cosmetically and the inside is a little dusty. It appears to have been cleaned a bit but some dust trapped in corners under the boards and in more difficult to reach places.

So far just taken all the covers off and made a cap shopping list. The service manual I have is for the US version I think so the control amp board layout is different. Planning to order through Mouser they seem to have a everything in stock.

Will deoxit all the switches and pots and recap the entire unit.
When tested it seem to lack a bit of power. I see an old post on here mentioning something similar. But I'll reassess this after cleaning.

Was hoping that there was a new volume/balance pot option but my research tells me that's not the case. Not that this one is particularly bad but would have preferred a new replacement.

Was also thinking to replace the relay whilst I was at it.

Some pictures of the unit attached (I know the toggle switches need to be rotated 90 deg, fortunately only one of the is glued in this orientation).

Any other advice for things to look out for on this unit gratefully received.

Planing to pair it with an Audio Technica turntable and maybe Klipsch RP-600's but not decided on these at the moment.
I'm new to this HiFi world so any suggestions also welcomed.

 

[turboyam]

If it sounds ''quiet'', it might be a commun to both channels, switch or pot that's dirty (not fully conducting)... What are your test speakers? their efficiency?.

 

[gort69]

Common failure points are the relay driver (Qk2), Ck7 and the solder pads for DZk1,2 and Rk1,2 all on the power supply board which runs very hot. And agree - it could use 100 or so more WPC.

 

[hhaammiisshh]

Thanks will definitely check these places in the power supply board. Order some replacements if required.

Have tested it just with the rubbish LG home theater speakers I have and with headphones. Problems with volume & balance present on both.
Was demoed with some Martinlogans from the guy I purchased it from. Again it didn't really seem to drive them that well.

But I'm not going to draw any conclusions until it's all cleaned and at it's best.

If I could just find the dial to turn up the WPC should be all good

 

[leesonic]

I always preferred the Trio name to the Kenwood name. Probably because I was born in England.

That lookd like a nice amp, good luck with the restore.

Lee.

 

[hopjohn]

Working on one of these currently. Some tips for you, takem or leavem. If further detail is required just ask. I or someone else will follow-up.

• When recapping the control board it's probably best to unsolder the six jumpers wires to the left side board first (it's divided into two halves) in order to do the best quality work and get the flux off the boards etc.
• The amp boards have TO-92L at Qe3 and Qe4 (2SC1885 / 2SA912) that can be upgraded to TO-126 KSC2690AYS / KSA1220AY
• The power supply has the issues described by @gort69 that will undoubtedly need attention and likely repair. A simple freshening up may not be enough as a long term solution. Off boarding the dropping resistors Rk1 and 2 is my recommendation, but it takes real effort to do so.
  
• The output modules have way too much compound beneath them. I'd suggest an application of an appropriate amount of fresh compound to each.
• The relay driver Qk2 (to a KSC2690) and associated protection circuit caps are a must to replace. Maintain a low-leak cap (Nichicon UKL series) at Ck9. I'd replace the relay as well (MY4-02-DC24).
• The power switch contacts should be bridged at minimum, and at the very least introduce a Quencharc device (or make your own with a 100ohm 1/2W resistor in series with a .01uf X2 safety cap) across the switch. Other options are more advanced. These include a triac or soft-start relay to well protect the power switch.

 

[hhaammiisshh]

Thanks @hopjohn you suggestions are very much valued. Will definitely apply these fixes.

As for the power board it "looks" to be in good condition and I don't see any signs of heat damage. But that probably won't be obvious until I get the board out.
Might just go ahead and replace the zener diodes and resistors mentioned, leave the off boarding to some future project.

 

[MaartenVB]

it got that nice toasted look, but at least the cap didnt drop his pants

 

[hhaammiisshh]

Recap is finished.
Thanks for the help and suggestions
the sound is a lot bigger and clearer now.
There is still a bit of background hum that can be easily heard if you crank the volume with no signal.
Any suggestions to reduce this?
For now I will probably listen to it as is, as it is not noticeable to me (non-audiophile) when music is playing.
Several applications of contact cleaner and deoxit to volume/balance and other controls have left them all sounding clean with no pops or crackles when adjusted.
Someone had put a capacitor across the power switch previously so hopefully it is in good condition. I replaced it with the quencharc as I had it for that purpose.
I used CPU thermal paste under the amplifier modules as I had it to hand. Seems to work well as the top of the heat sink gets nice and warm. Warmer than the face of the amp module.
Now I just need to find some suitable speakers!