40-155 first look - Printable Version

40-155 first look - Printable Version

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40-155 first look - bridkarl - 09-24-2025

Intact. Even the back is there in good shape.

Usual dust and previous lazy/bad repair jobs. Looks like a couplet?? A few caps where someone just added a cap in parallel with the old cap rather than actually replace the part. A burned out resistor - 150 ohm 1 watt I think. Only one 1 watt in the unit so that must be it.

After a few hours work - I wish I was better at this - I have one 16mf electrolytic neatly hot glued to the chassis. The other just hanging in space as I’m not seeing a good attachment point. I think I can make a little U-bracket for it to rest on. The schematic says a 12 and a 16 mf, but with higher voltages today I think two 16 mf will work.

Replaced or heat shrinked some bad rubber wiring.
A few caps hiding under the tuning deck. I think if I carefully unhook that I can get to them?
Gradually replacing the other caps.

Nothing too awful YET

Today I rebuilt the bakelite block. The wax seemed not to want to melt, but eventually it all came out.

Also replaced the caps under the tuning deck, cleaned switches and tube sockets. While I wait for remaining capacitors and resistors I will rebuild the pilot lamp -
Cleaned and exercised the switches - they all seem to be functioning well.

The wiring for the lamp has just crumbled to dust. I’ve ordered some nylon washers.

I’ll need to order some rubber chassis mounts and buttons.

Finished the recapping, added a power cord. Fired it up with no tubes. No issues. Added tubes. Again fires up with no issues. Realized the 1232/7G7 tube is missing. Replacement on order.

RE: 40-155 first look - RGK20m3 - 10-14-2025

Looks like a dead short solder bridge on the Bakelite block, doesn’t it?

RE: 40-155 first look - bridkarl - 10-14-2025

Oh. Let me look at that. Thanks. That one picture is old so I may have corrected that already.

RE: 40-155 update - bridkarl - 11-01-2025

Been going through this. Replaced volume control.
Redid bakelite condensor completely. Checked continuity on audio transformer, power transformer, and speaker - all good. Zero volume.
I checked and saw zero voltage across the filter caps so I’m thinking the rectifer 84 tube has gone bad. New one on order. I needed a spare anyway.

RE: 40-155 first look - MrFixr55 - 11-01-2025

Hi Bridkarl,

What looks like a solder bridge may not be; verify that there is no solder bridge or other short. Looking at the green resistor soldered to the nut holding the capacitor #61, that resistor looks very burned and likely open. BTW, the case of that cap is NOT at ground potential; it should be insulated from the chassis. The case of Capacitor #58 should be at ground potential.

With the 84 out of the socket, set meter to ohms and measure resistance from the cathode of the 84 to the center tap of the power transformer. There should be a relatively low resistance at start but the resistance should steadily climb to tens of kilohms as the filter caps charge. If there is a short, fix it. in addition, check the resistance of the filter choke and resistors 59 and 60 (I suspect that there is an open resistor here. If any of these are open, there will be no B+ to the radio. Resistors 59 and 60 complete the B+ circuit and grid bias voltages are developed across it. Next, perform the following steps in sequence very carefully.
With power on and the rectifier and output tubes in their sockets, and with the speaker plugged in:

With the meter on 700VAC, measure between the junction of the HV secondary center tap and resistor #60, and each of the legs of the secondary at the 84 socket. there should be somewhere around 300VAC give or take.

With the meter on a range higher than 300VDC, measure between the junction of the HV secondary center tap and resistor #60, and the junction of the cathode of the 84 and the speaker field coil. There should be approximately 270V. If much higher, then there is no load. if 0V then the 84 is bad. However, other than a bad solder joint in the pins or the filament (heater) being open, there are few failures that should cause the 84 to be so dead that there is no output. I do not believe that there is anything akin to a fusible link between the cathode of the 84 and the cathode pin. If the output of the 84 was shorted to ground, there would be some fireworks in the tube or power transformer. Perform the next step.

Measure between the junction of resistor #59 and chassis ground, and the junction of the cathode of the 84 and the speaker field coil. The voltage should be about 18 volts or so less than the previous voltage reading. If 0V or close to it, then Resistor #59 0r 60 are open. Move the negative lead to the chassis. The reading should be the same value as that obtained at the junction between the resistor and chassis ground.
Measure between chassis ground and the junction of capacitor 58, the speaker field and B+ to the rest of the radio. There should be approximately 180VDC at this junction. If 0V, then the speaker field is open or mis-wired. If much higher than 180V, then there is no load. Check wiring to the output transformer and measure the junction of the output transformer and the plate of each 41 output tube. The value should be close to that at the junction of the speaker field and capacitor #58.

If all the measurements are good but there is absolutely no sound, not even a slight hum, then suspect the speaker voice coil.

Hope all of this helps.

RE: 40-155 first look - bridkarl - 11-02-2025

Bless you. I’ll go through all of this as suggested.

The resistors are on order as I saw they looked bad. I’ll replace them and neaten all this up. I redid the condensor block today so it seems fine.
I checked the wiring on the speaker today against an original factory chassis side by side on the bench. I now see what you meant about the difference in the cap grounds. Originally there was a cardboard sleeve on the positive side doing that. I didn’t realize that until I got my example chassis out.

This is extremely helpful. I plan to add this to my repair notes as standard operating procedure. This is why I have a wooden bench and lots of clip leads. I use my isolation transformer as the power switch in such instances.

RE: 40-155 first look - morzh - 11-03-2025

I think it is the angle of the photo, not a short.

RE: 40-155 first look - bridkarl - 11-03-2025

That’s been totally reworked now. No shorts, no issues. Now I’m just waiting on some resistors so I can continue working on this. Helpfully I now have an untouched chassis side by side so that is making this easier.
Put in a correct volume control. Someone had subbed a 500k pot - I guess that worked - sort of.

RE: 40-155 first look - bridkarl - 11-15-2025

Another roadblock. I decided to check the wiring on the speaker and realized the four stranded wire from the speaker plug was bad as was the plug so I used the plug from my parts unit. The colors are so faded I can’t tell which one goes to the positive of the 16mfd cap. They all look yellow. I traced out the wires but can’t figure out that one. Then it’s easy because there are just two wires on the bakelite block.
I’ve tried to neaten all this up. So some progress. 12 mfd by the audio transformer, 16 mfd connected to the chassis, replaced resistors (both burned) to terminal strip.

RE: 40-155 first look - RodB - 11-16-2025

You can use an ohmmeter to measure the plug pins. Two of the pins should have about 1100 ohms; that's the power supply field coil across the filter caps (12 and 16 mfd). The other two pins are the output transformer primary wires and should measure about 450 ohms. If you trace the socket pins in the chassis, each of the field coil pins should run to the power supply 12 and 16 mfd caps. Each of the opt pins should run to an output tube plate pin.