KF7LZE's Blog

Update: The series is complete. For more articles and information, visit these other posts:

Part 1: Identification and Task List
Part 2: Intake Checks
Part 3: Capacitor Replacement
Part 4: Resistors and Controls
Part 5: First Power-Up
Part 6: Socket Replacement and First Alignment
Part 6.5: Diagnosing an RF Intermittent
Part 7: Conclusion

I’m continuing to work on the Grunow 5-U chassis that’s on my work bench for my customer. This is part 4 of the series, where we’ll be checking the resistors, addressing the control situation, and adding an isolated line input to the radio. Check out a previous entry in the series to catch up on anything you’ve missed.

Part 1: Identification and Task List
Part 2: Intake Checks
Part 3: Replacing Capacitors

We left off after Part 3 with all the radio’s capacitors having been replaced.

Now it’s time to move on to the next phase of the repair to check and replace as necessary drifting resistors, to repair or replace the volume control, and retrofit an isolated line input to the radio to let music be played from an external source.

The copy of the schematic I’ve been using so far has been pretty decent for replacing the capacitors, but the parts list is very difficult to make out. A couple of the resistors in the radio have color markings that seem incorrect and their entries on the parts list aren’t legible. Nostalgia Air schematics vary pretty widely in scan quality. Fortunately, I located a copy of the original source material:

the Perpetual Troubleshooter’s Manual, volume 9. Published from 1933 into the ’60s, these impressively thick (5″) volumes contain original hard copies of schematics for the majority of radios sold in that model year. The parts list is much more legible on this copy. It’s small print but perfectly clear:

Now it’s time for the real work. We’re going to deal with the volume control first, because it has the greatest number of connections to circuits. It’s involved in the power on-off switch, a tone tap, and the volume. If it’s bad, it would be inefficient to have replaced the components touching the volume control only to have to go back and re-work later.

The lock ring unscrews from the front, and with the connections removed the control pulls out of the back. The control’s case is held on by tabs which are crimped onto the phenolic back board.

I used the exacto knife to pry the tabs up and release the backing:

The volume control is now separated into the potentiometer, and the on-off switch on the back. Turns out the volume control wasn’t correctly seated and the switch was making bad contact, reseating the internals quickly fixed that problem.  Inside the volume control pot itself, there’s a carbon resistance strip connected to a metal wiper. The further along the strip, the more the resistance is to the tap. The terminals function as a voltage divider on the signal to control the volume.

In this case, there was built up grime inside the control and the wiper was making poor contact with the strip. A good shot of control cleaner, working the control back and forth many times, and using a q-tip cleaned it right up and it makes good contact throughout its range of rotation now.

Since the volume control has been fixed, now it’s possible to add an auxiliary iPod input. The center tap of the volume control goes through a DC-blocking capacitor, and then to the grid (input) of the first audio tube. The signal wire from an RCA cable can be connected to the same tap, and whenever the radio is tuned away from a station, the auxiliary will be audible. Because the design of these old radios could introduce a voltage onto  the audio cable itself, I’m also fitting an isolation transformer which will allow the AC audio signal to pass, but won’t allow DC voltage through which could damage the device it’s attached to.

Before I can wire in anything, however, I need to reinstall the volume control. While it’s out, this is a good time to check the resistors surrounding it.

I measured the resistors directly with the multimeter; for resistors with capacitors in the circuit where you can’t get a clean reading, it helps to reverse the leads. It turns out, surprisingly, that most of the carbon resistors around the tube sockets were within their tolerances. The resistors around the volume control needed to be replaced, though, as several were spec’d at 10% tolerance and had drifted by as much at 25%.

I’m using 1W metal oxide resistors to replace the 1/3 and 1/2W resistors. All my replacement resistors are specified at 5% or better to provide many years of consistent resistance.

And with the replacements complete:

During the process of reinstalling the volume control, the grid cap lead to the 1st Audio tube snapped internally. I had to peel back the shield, solder an extension to match the length of the previous wire, fix heat shrink tubing to prevent the solder connection from grounding to the shield, then pull the shield back out to its original length.

With the resistors replaced, it’s now time to finalize hooking up the iPod input. Connecting the hot lead of the signal cable to the center tap (wiper) and the signal return to the grounded terminal of the volume control, the aux input will now be active and audible when the radio is tuned away from a station. Here’s the completed isolation transformer assembly outside of the radio:

The isolation transformer is designed to be attached between two jacks, so has a male and a female end. I used a coupler to provide two female ends. Because of the serious danger of removing the isolation from the audio input, I used a piece of heat-shrink tubing slid over the entire chain of connectors to seal it in form-fitted plastic. This safely ensures the connections will be maintained – and also will keep dirt from getting into the connectors and causing reduced performance.

We’re making good progress towards completion, and was lucky that the band switch also just required an internal cleaning to restore proper operation instead of a complicated replacement operation.

• Replace all wax and electrolytic capacitors.
• Check resistors for drift and replace as necessary.
• Clean or replace band switch.
• Clean or replace power switch and volume control.
• Replace power cord with polarized power cord for safety, and add interference-suppression to the power line input.
• Add inline fuse on the power transformer primary for safety.
• Adding an isolated auxiliary input (RCA jack) for playing an iPod or other music device.
• Replace bias cell with lithium battery or eliminate and replace with 5 Megohm resistor.
• Replacing rubber chassis and tuner washers to ensure proper alignment.

Update: The series is complete. For more articles and information, visit these other posts:

Part 1: Identification and Task List
Part 2: Intake Checks
Part 3: Capacitor Replacement
Part 4: Resistors and Controls
Part 5: First Power-Up
Part 6: Socket Replacement and First Alignment
Part 6.5: Diagnosing an RF Intermittent
Part 7: Conclusion

Continuing in the series Repairing Antique Radio Electrics from Start to Finish, we’re moving on to Step 2: Intake Checks. Feel free to revisit Step 1 for a general overview of the parts of an antique radio. Picking up from where we left off, the radio was on the bench waiting for attention:

We’ve seen that the radio looks like it’s in decent physical condition. A can of compressed air like you’d use to clean a computer keyboard was useful in cleaning 74 years of accumulated dust from between the plates and coils; wet wipes and a damp sponge helped to clean up some of the surface grime. Now it’s time to start initial tests. We’ll begin with the tubes.

From left to right, the radio came to me with the 6A7 (modulator/oscillator), 6D6 (IF amplifier) and 75 (first audio) tubes (shown in the top row.) From my own parts stock, I’ve found a type 42 output tube and a type 80 rectifier tube pictured on the bottom row.

Vacuum tubes are high-voltage and high-temperature devices, and years of operation under those harsh condition can cause the emission surfaces to wear out, increase in resistance, or even short or go open like a lightbulb filament breaking. Inserting damaged tubes into a radio can cause permanent damage, so it’s important to test tubes for any professional restoration. I use my Precision model 10-40 tube tester to check the tubes condition:

The Precision 10-40 is a “Dynamic Conductance” tube tester, which partially approximates actual operating conditions in the radio. It’s not a perfect test, but is useful for quickly sorting good from bad tubes. The controls on this unit allow you to set the operating parameters for the tubes (filament/heater, plate, grid and bias voltages) and set their connections. It checks for bad tube elements, as well as overall emission. Shown here testing the 6A7, which was good. I repeated this process for all 5 tubes, and found that 4 test solidly good and only 1 tests marginal.

The only marginal tube is the 75 First Audio, which just barely falls on the “replace” side of the weak line. Even significantly weak tubes can continue to perform okay, but at a certain point the audio starts to lose volume. This one will be fine for testing purposes, and probably fine for another decade of listening, but will end up being replaced at the end.

This radio is in great shape for its age and there are no immediately obvious defects. I’ve seen radios with mashed in tuning capacitors, tubes snapped off in the sockets, dangling wires and such and this radio has none of those. If the passive components in the radio (components, coils and transformers) are in good condition or are repaired, the radio will work. Now it’s time to check the durable passive components: the antenna coil, oscillator coil, both IF transformers, power transformer, speaker output transformer, field coil and voice coil.

This set was sent to me identified as a Grunow model 586. When I started to trace out the circuit, however, I noticed differences in the coil and bandswitch arrangements. This led me back to the original chassis tag which identified the chassis as type 5-U, not the type 5-W chassis installed in the model 586. The Grunow model 589 (or possibly the 587 or 599) is the correct identification for this model radio, depending on the cabinet and speaker options installed.

Shown in this snip of the schematic is the input stage of the radio from the antenna and antenna coil on the left, oscillator coil in the center and the 6A7 modulator/oscillator tube. A tube serving as both modulator and oscillator is also frequently called the “converter”, and for brevity’s sake I will be using that term going forward. It’s called the converter because it converts the incoming radio waves to the intermediate frequency in one step using one tube where earlier designs needed to use two tubes. The top schematic is the correct one; the bottom schematic is the incorrect schematic, just for comparison.

On the left is the antenna coil. The primary goes from the Antenna input down the winding to the chassis ground as does the Doublet (dipole antenna) input. Each of the two secondaries is switched depending on which band is selected using the band switch knob. The oscillator coil is similar, towards the center. In the 586, the oscillator coil secondary is center-tapped and the switch bypasses part of the coil; in the 589, the broadcast and shortwave oscillator coils are wound separately.

That’s about the only difference between the two chassis. It’s a slightly more complex coil arrangement, and might yield slightly better performance, but it’s really a very minor difference. The limiting factor in this set’s performance will be the fact that it has only five tubes.

Checking the antenna coil’s integrity is easy, just use the multimeter. Shown here the antenna coil primary measuring 23.5 Ohms.

Looks good. I tested the secondaries through their entire signal path (from the grid cap on top of the 6A7 through the band switch in both positions) and found them to all be good, so moved on to the oscillator coil. The primary windings of the oscillator coil are switched from Pin 4 of the 6A7 converter through the band switch to the junction of a pair of resistors that feed the 6D6 IF amplifier.

Unfortunately, during this test it turned out the band switch has a short inside the switch body. Both sides of the switch segment that switch the oscillator coil primary have continuity at the same time. This shouldn’t happen, and it means there’s probably some crud stuck inside the switch. I’ll have to clean it thoroughly later. The secondaries are switched from another set of terminals on the band switch; from the center pole to ground on one winding and to the edge of a trimmer on the other. Both primaries and secondaries are intact. This is the band switch that will need repair:

Next up is to check the IF transformers. There are two transformers, each with two windings, so a total of four tests. Selecting the tie points for the leads from the schematic, I test continuity the same as the antenna and oscillator coils and find that all IF transformers are intact. And so is the power transformer!

Unfortunately, though, another discovery: the on-off-volume switch is either completely clogged, or dead. The primary winding, 5V and 6.3V filament and heater windings, and B+ are all intact however. That just leaves the speaker coils (transformer, field and voice):

All those coils check out as well. In addition to these pictured checks, I tested the tuning capacitor for shorts by attaching one lead to the stator, and one to each rotor segment, and moving the plates through their full range of motion. The plates showed no connection throughout their full range of travel, so there should be no dead spots on the tuning dial and it should track properly.

Based on this initial assessment, this Grunow 5-U chassis is going to need these repairs:

• Replace all wax and electrolytic capacitors.
• Check resistors for drift and replace as necessary.
• Clean or replace band switch.
• Clean or replace power switch and volume control.