MC275 Rebuild Part III ?

[rkic]

Got back to NM Sunday afternoon. Let amp lie while I replaced caps in speakers (EV TRX's). Glued caps into place. Will Hook up tonight.
New thermistor from audio classics waiting for me so replaced that yesterday. Reads 14.6 ohms - replaced thermistor reads over 60 ohms.
Ran all tubes through TV-7 D/U tube tester. All tubes tested strong. KT88s are Genalex Gold Lions - all tested at 68 right on the dot. The rest of the peanut tubes are McIntosh branded Mullards. Some variation between types/sides but all well above the minimum.
I will run power up test tonight with all tubes except output tubes and check voltages as I go.
Worked on adapting console yesterday also. Added place for computer grade surge suppressor, modified to move tuner and preamp from directly over power amp to console portion next door. Added extra shelf over power amp section.
Sliced single section top into 3 section top on table saw - used new Freud thin kerf combination blade to minimize chipping on off side. Used natural oak stain on exposed ends.
So far so good.

 

[CarlV]

Genalex output tubes too, huh? Wish I could be there when you do get it all together and fire it up. It will certainly be a vintage dream setup to be proud of. :boink:

Carl

 

[rkic]

Ran voltages with small tubes only. Here are the results for the KT88s:
KT88 1 2 3 4
Pin 3 450 449 450 448 448
Pin 4 450 450 449 449 449
Pin 5 -57 -55.5 -56 -56.5 -55.2
Pin 8 .6 0 0 0 0

Tonight will be the hook up the speakers, preamp and source and plug in the output tubes...
Wish me luck.

 

[ron-c]

Rkic,

Your bias should be around -60 or more without the outputs especially if the selenium was replaced by a diode on the small board. Was this with electrolytic or the polys in? It might work fine but if the bias drops much more you may see red plate glow on the outputs. We are using -61 volts in current production amps which seems to work well with the EH and SED KT88s.

Ron-C

 

[rkic]

Originally posted by ron-c
Rkic,

Your bias should be around -60 or more without the outputs especially if the selenium was replaced by a diode on the small board. Was this with electrolytic or the polys in? It might work fine but if the bias drops much more you may see red plate glow on the outputs.

Ron-C

Well, in that case I'll use my brand-X KT88s tonight!

 

[ron-c]

Rkic,

That is a good idea!

Ron-C

 

[rkic]

Checked the brand-X tubes - unknown manufacturer - I bought these from Sonic Frontiers several years ago with the idea that I would run them in my Cary SLA-70B but never got around to it.
they are white boxed with "Made in Slovakia" on the box. So I suppose from the JJ Tesla factory? They all test strong at 67 -68 with one tube at 65.
Fired up amp and promptly fried 1 amp fast blow fuse. Shut down amp and put in 2 amp fuse - fried it next. Put in recommended 5 amp slow blo and amp ran.
LOUD hum - 120 hertz perhaps? Source did play through the hum but with the volume cranked up. Hum stays constant loudness level which source can over come.
Any ideas on the source of the hum and the remedy?
Sigh... it's never simple is it?

 

[ron-c]

Rkic,

Did you pull the Solen caps and put in the polarized electrolytics? If not there is your hum. A polarized cap acts as a one way diode you know to control the AC voltage.

Ron-C

 

[rkic]

Originally posted by ron-c
Rkic,

Did you pull the Solen caps and put in the polarized electrolytics? If not there is your hum. A polarized cap acts as a one way diode you know to control the AC voltage.

Ron-C

I haven't ordered them yet - but will. I will be going to Tempe weekend after next so I will pickup the parts then.

Any suggestions on what items to check/replace on the C-22 preamp? Might as well combine orders if I can.

 

[ron-c]

The last C22 I rebuilt had a fried power supply, like burnt up wires. After that got fixed I replaced the caps in the signal path and it did sound very good. On the old pre amps the leaky signal caps cause DC on the controls which makes them noisy and slowly destroys them.
The main problem with these is the worn out volume control due to above. If you are lucky it may be in good shape. Orange drop caps work fine in the signal path for that classic sound..
Always check high value resistors, the mega ohm stuff. They tend to be out after all these years. The voltages on a C22 should be spot on to the schematic when everything is correct.

Ron-C