38-4 restoration need advice/ help - Printable Version
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38-4 restoration need advice/ help - conroyp - 02-12-2013
Hi everyone, I am a new member with two 38-4 consoles. One is working fine with a recent recap (not by myself), and the other I have just recapped myself, and now have many questions so here goes... After replacing the paper tubulars and the elctrolytics, I reused the Mica caps, i tested socket voltages per bulletin #281, and noticed the DC readings were high. The power transformer is taller than my working set, and according to the part number stamped on it I have a 115v 25/40 hertz xfmr. Should I stop here and locate a 60hertz transformer? I do get recption on both bands, the volume level is low compared to the working set as well.
RE: 38-4 restoration need advice/ help - Radioroslyn - 02-12-2013
25cy Transformer is ok. Sounds like you have other issues. Weak tubes or a bad resistor can give you those symptoms. Check voltages around the tubes to see where the problem is.
Terry
ps Where in MD?? Grew up in Sandy Spring.
RE: 38-4 restoration need advice/ help - conroyp - 02-14-2013
Tubes test OK on a heathkit TC2 ( not sure if thats a decent tester or not??) and play OK, in the working set. I have a order of resistors on the way from just radios, and I have to admit I only ohmed out about half of them when I did the caps. What tolerance should the resistors be within?
Also have a signal generator coming in the near future.
Terry, im down rt.108 in clarksville.
RE: 38-4 restoration need advice/ help - codefox1 - 02-14-2013
Most older carbon resistors will have drifted way up in value or will behave strangely when under power. Nominal tolerence was 20% for most application. New ones with 5% or better toleranece are available for pennies apiece so it just makes sense to replace 'em all when doing a recap job. No foolproof to measure old resistors or capacitors.
RE: 38-4 restoration need advice/ help - Arran - 02-15-2013
See if the voltage chart mentions anything about Ohms per volt with their readings. The resistors are worth checking even without taking voltage readings into account, especially on any resistor with a body end dot code.
Regards
Arran
RE: 38-4 restoration need advice/ help - conroyp - 02-15-2013
thanks, theres definatley some resistor problems, One which i tested last week is now open.... so ill dig in once they arrive. Anybody have preferrences on solder? I have some rosin core lead free, 96%tin, 4%silver. Is that OK?
RE: 38-4 restoration need advice/ help - morzh - 02-15-2013
Lead-tin eutectic solder's the best. Rosin core is good. I don't like leadless solder.
RE: 38-4 restoration need advice/ help - conroyp - 03-03-2013
The resistors have arrived, ill dig back into it this week.
I also got the rubber isolators for the tuning cap. Anybody know how to get the two in the front into place? Can they be installed without taking the tuning cap off the chassis?
RE: 38-4 restoration need advice/ help - conroyp - 03-21-2013
Been busy lately, All new resistors and caps. I aligned it, tuning and reception is good on both bands, but I need some help, the volume is low, and it seems like there is too much bass.
I also have some high voltages. These problems were present before, I figured the off spec. components were to blame.
the DC tube socket voltages are around 20-30 volts high, at 115vac input.
heater voltages are ok. Any recommendations?
[Image: http://i818.photobucket.com/albums/zz105/5X8Rider/38-4schematic_zps79ae2f7f.jpg]
[Image: http://i818.photobucket.com/albums/zz105/5X8Rider/38-4socketvoltages_zpsb990451c.jpg]
RE: 38-4 restoration need advice/ help - morzh - 03-21-2013
The DC tube voltages will be higher if you use today's Mains (120V vs 110V). You probably also use today's DMM and not the old type Voltmeter that would lower the voltage due to lower input impedance.
As for the volume low - Bass high,
1) Have you changed ALL capacitors and ALL resistors?
2) if not, start probing around the Switch #40 for all connected caps' and resistors' values.
3) If you haven't change #58/59 caps (likely to be micas not films) check them.
RE: 38-4 restoration need advice/ help - conroyp - 03-21-2013
Morzh, thanks,
I replaced all the resistors and all but the mica caps,
I used a variac to set input test voltage to the same voltage the schematic quoted for their readings (115v), line voltage just makes it worse.
Ill check over #40 with a meter, it does seem to work though.
#58-59 are new, Ill check them again.
Anything else???
RE: 38-4 restoration need advice/ help - morzh - 03-21-2013
All parts 29-33 (caps-resistors). Especially 29.
Also....remote, but what if the output transformer is coming down with something?