I hadn't turned on the 6-HF-1 in a few days because I had been listening to the Akai reel to reel through my mid 90s Pioneer stereo system. I turned it on night before last, and I noticed distortion in listening to music on FM radio. I turned the tuner so the tuning eye was on dead center of the frequency, and there was still distortion. It sounded like the FM station was way off frequency. Then, I played a record that has great fidelity and it sounded simply horrible... real distorted sounding. I didn't play the Magnecord reel to reel as it is on my work bench undergoing some adjustments.

I had had a strange sound as the set turned on, and a pop in the speakers both when the set is turned on and turned off. Now, the strange noise is gone and so is the pop. Could it be a bad tube? I decided to just go ahead and spring for all new tubes for the tuner and power amp. I already had new 6V6GT output tubes, so I didn't order any of those. Everything else, except for the rectifier tube in the power amp, was ordered.

My 6-HF-1 is sick! Any help here would be appreciated.

Time to do some serious troubleshooting with a low-distortion audio generator and an oscilloscope.

Before troubleshooting the signal path I would check the DC voltages against the values in the SAMS. I would start with the voltage at the cathode (pin 8) of the 6V6s. I think that is the most critical voltage in the set. Larry, could you pls post the schematic of the amplifier section?

David and Brian, I found the problem. There is a badly shorted 6AU6 tube in the tuner. I pulled both the amp and tuner and ohm'd out all the resistors, checked for cold solder joints, and found no problems. But that 6AU6 lit up my short light on the tube tester like a Christmas tree!!!!

To be sure that the tube is the problem, until you can get a new one, you could simply pull the bad tube and the amplifier should come alive. Records should sound normal.

-David

I had a 6AU6 in another set, so I pulled it and tried it in the Mark I tuner. The tube made a big difference on FM radio, but there's still distortion when playing records. I noticed some singers on FM radio have a "lispey" sound. I pulled that tube out like you suggested, David, and it made some improvement but records don't sound sharp and crisp like they did a week or so ago.

Obviously I've got some other issue in the tuner. I'm going to check speakers and crossover capacitors too to be sure there are no problems there. The guy I got it from cut the three-wire speaker wire, so I had to rewire that. Can't remember if I soldered each of those wires or just twisted them together and taped them. I'll check that out too.

Something could be pulling the B+ voltage down. You'd better take some voltage readings before you BBQ the power transformer.

Ken D.

There are two 6AU6s in the tuner, and two more of them in the power amplifier. One in the power amplifier lit up the short light briefly in my tube checker when first powered up. It powered on up and tested just fine... I tried it again and the short light didn't light up again.

I have all new tubes ordered for both the tuner and power amplifier, except for the 4 output tubes in the amp and the rectifier tube.

Larry please measure the DC voltages at pin 3 (plate), pin 4 (screen), pin 5 (grid), and pin 8 (cathode) of the 6V6s and let us know. Please do this before operating the set again. When performing the test, try to make the measurements as quickly as possible after the set has warmed up, and then shut the set down. We want to minimize the time that it is powered-up until we can be sure that nothing is being damaged.

Be very careful when making the voltage measurements. For the set to operate correctly there should be over 300 volts on the plate and screen of each 6V6. Be careful not to short any terminal to ground or to any other terminal, and keep one hand in your pocket.

-David

I think one hand is already doing something wretched.

I don't think there's any problem in the power amp. I believe the problem is in the tuner, which is where all the switches and controls are located. I'm not going to get too carried away with more testing until the new tubes arrive and I get them installed. If there still is a problem, I may have to take it to a close friend of mine who has a signal generator and oscilloscope and is experienced at troubleshooting radio problems. I don't have all this fancy equipment.

You may be right that there is no problem in the power amplifier. However, there is a possibility that a fault in the power amplifier is pulling down the B+ and affecting the tuner, which I think is powered off the main power supply. If this is the case it could become disastrous very quickly. It would be a shame if you were to have to hunt down a rare power transformer or output transformer for such a valuable set, or even worse, suffer a "combustion event."

See, Larry, there is so much power handling capacity in the power supply and output stages that any fault can result in really excessive current that can do a lot of damage quickly. On the other hand, the small-signal stages (tuner, first audio, etc.) carry very low current. About the worst that could happen is that you'd smoke a 1/2 watt carbon resistor -- no big deal.

That is why I so strongly recommend that whenever an amplifier is misbehaving, the DC voltages at the power stage be checked first. Just to make sure the stage is operating correcting and that the malfunction could not result in serious damage. We are not talking about signal tracing using any fancy equipment. No oscilloscope, no audio generator, just a simple DMM or even a cheap analog VOM will do fine.

This is one of those problems that a simple recapping won't fix. Blindly changing tubes may or may not solve the problem either. It's time to do some basic troubleshooting, and we always start with DC measurements. Whenever I repair an amplifier, it's not done until I've checked the voltages against the schematic and confirmed that they are in the ballpark.

-David

It's called Dynamic Troubleshooting, something us *real* techs are trained to do.

"Static" troubeshooting can only do so much... checking resistors, etc.
Any nimwit can change a capacitor.

Some nimwits can't.

Not to 'jack Larry's thread but I appreciate the frank answers by some of the more experienced members of this forum. I’ve worked in back room shop environments and we all tend to use coarse language and an occasional naughty word, we even laugh at dirty jokes and call each other condescending names but it’s all in fun.
It’s been 30 year since I’ve earned my living as a service tech, I’m lo longer at the top of my form but I do know sound advice when a professional gives it, no mater how tart the delivery.

You can say THAT again!
Some of the language at my shop is enough to peel paint off the walls.
I have a filthy mouth anyway.

Did you check your needle? It might have some lint stuck on it.

Yes, I cleaned the needle with a soft toothbrush, also removed the headshell and checked all connections, and everything looks fine. I'm doing some resistance readings, and checking capacitors that I installed a few months ago, and I found one cap right off the rotating switch that won't even register on my capacitor checker. All other caps check just fine. I guess I need to replace that one... but it's a .22 and I am out of that value but could sub a .33 for it.

David, here are the voltage readings:

V13 (6V6GT)--
pin 3-- 332V
pin 4- 340V
pin 5- 11.6MV (0 volts)
pin 8-- 23V

V14 (6V6GT)--
pin 3-- 332V
pin 4-- 341V
pin 5-- 44.5MV (0 volts)
pin 8-- 23V

V15 (6V6GT)--
pin 3-- 331V
pin 4-- 341V
pin 5-- 55MV (0 volts)
pin 8-- 23V

V16 (6V6GT)--
pin 3-- 330V
pin 4-- 340V
pin 5-- 73MV (0 volts)
pin 8-- 23V

David, I think this is about as close to identical voltages readings as you could get on the 4 output tubes. I don't see anything here that could smoke the power transformer or start a fire.

Voltages all look reasonable to me. Plenty of cathode bias, cathode currents are reasonable and equal. I'm glad you checked -- now you can troubleshoot at leisure without being concerned about burning up something.

If you feel ambitious you might measure the voltages on all the other tubes and compare against the values in the schematic.

I hope you will perform these measurements on every set you rebuild. It's the only way to make certain that everything is working properly.

Good work!

-David