Stumped by a hum

[samgraysound]

Hi All,

I've been repairing a KLH 24. It's a turntable that somehow has a radio and power amp crammed into the case along with it.

One channel of the power amp was dead so I rebuilt it. Replaced all the transistors and electrolytics, checked every resistor and replaced everything out of tolerance.

It used germanium TO3 transistors for the output pair and a transformer as the phase inverter. I used silicon transistors for replacement and upped the bias resistors a touch to get rid of the crossover distortion. It's basically the same circuit as the Fender Rhodes Peterson Amp.

The channel now works but has a hum on it. The other channel does not have the hum. Please help me troubleshoot. Schematic is attached.

 

[Bo Deadly]

Although it is very possible to replace the Ge with Si, in practice it's probably quite tricky. The Vbe drop of Ge is ~0.2V but Si is 0.6. So that's a 3x difference. And Ge power transistors probably leak like crazy. You would have to tweak all 4 of those resistors setting the bias but the only way to know what to do would be to measure and test and measure and test ...

If you post the voltages at the 5 points between C and D for both the good channel and the rebuilt one we might be able to advise. Try to get a stable reading within at least a tenth of a volt.

But honestly, I personally would just put Germs back in. There are piles of them on Ebay for a song. Otherwise, if you hack it to use Si, is it a KLH 24 anymore?

 

[samgraysound]

But honestly, I personally would just put Germs back in. There are piles of them on Ebay for a song. Otherwise, if you hack it to use Si, is it a KLH 24 anymore?

One of the germs was dead. I didn't feel like trying to find an NOS one, but maybe I should?

I'll be back with voltages.

Sam

 

[Bo Deadly]

One of the germs was dead. I didn't feel like trying to find an NOS one, but maybe I should?

Unless someone here pops up and tells you exactly what to do to make the conversion, I would not only get Germanium but try to match the part. Even though the circuit might seem simple, you don't REALLY know that it will work correctly and safely. It was designed for a certain parts and presumably the engineers did know how to make it work correctly and safely. So if you don't use the right part, all of that engineering is basically out-the-window.

And are you going to also convert the other channel? If not, they could definitely sound different. At the very least the power could be different which is to say one channel will be noticeably louder than the other.

If I were doing this I would actually find out all about the particular transistor (what's the part number?) and get 10, create a small jig, test them for Vbe and gain and then replace all 4 with NOS.

UPDATE: although apparently I was wr-wr-wrong about Germ power transistors being cheap because searching Ebay for "germanium to3" doesn't turn up that much and what there is is kinda expensive actually. So maybe just get the one you need, pop it in and move on.

 

[synthiaks]

I would listen to Squarewave!

i myself know much less than him, but i would think like this:

its a difference between Ve=Vb-0.6 and Ve=Vb-0.2 And a lot of the calculated resistor values in this circuit are based upon the latter.

Maybe the 60s transistors were less good than the ones today but the circuit designers were not.

if you really want to get dirty anyway, i would suggest a good old signal backtrace. Where is the hum introduced in the signal chain?


S

 

[PRR]

Properly fixed-up Si won't hum more than Ge.

Hum comes From The Power Line.

You have not said what knobs affect the hum. Your diagram cut-off all connections from the amp-core to the power supply. I only dimly remember the little brother of that KLH. Look, listen, diddle, and *observe*.

 

[samgraysound]

Properly fixed-up Si won't hum more than Ge.

Hum comes From The Power Line.

You have not said what knobs affect the hum. Your diagram cut-off all connections from the amp-core to the power supply. I only dimly remember the little brother of that KLH. Look, listen, diddle, and *observe*.

No knobs affect the hum, and it is on one channel of the power amp only. I have recapped the power supply, but I will upload that part of schematic tomorrow.

 

[samgraysound]

Your diagram cut-off all connections from the amp-core to the power supply. I only dimly remember the little brother of that KLH. Look, listen, diddle, and *observe*.

Here's the power supply with the power amp. It's not that I haven't done my own troubleshooting (diddling and observing) already. I just have stopped putting that information in threads where I ask for help because people seem to want to start from scratch regardless.

 

[abbey road d enfer]

Here's the power supply with the power amp. It's not that I haven't done my own troubleshooting (diddling and observing) already. I just have stopped putting that information in threads where I ask for help because people seem to want to start from scratch regardless.

I don't see how changing from Ge to Si can induce hum; however, are you sure that the gain is the same?
Another possibility is the Si circuit oscillates; have you cheched the waveforms and idle current?

 

[samgraysound]

Alright gang. It's another classic case of the fuckups. I read the schematic as saying the base biasing resistor was 62 ohms and upped it to 75 ohms, but it was actually 6.2 ohms.

Landed on 27ohms to eliminate crossover distortion with MJ15004G transistors. Hum is gone and it is running a lot cooler.