Stereo 70 Bad Right Channel

I am new to working on audio amps and to the Stereo 70.  I scored a ST 70 and PAS3 a few days ago and am excited to get it working.  I was told it had one bad channel and it has sat unused for years.

I have been doing a lot of reading on this unit.  It was evident this amp had most if not all the caps updated as well as the selenium rectifier.

After inspection, cleaning, testing tubes, and checking for issues I was ready to turn it on.  I adjusted the bias on both channels.  All tubes are operating.  With my ear to each speaker I can hear a faint hiss on the left speaker, nothing on the right speaker.  

Next I hooked the unit up to a bulb limiter, which is connected to a variac.  

I recorded the following plate voltages.  I am guessing these are lower than expected because I am going through the bulb limiter.  

All 4 output tubes were around 163 VDC.

The left 7199 had 128 VDC at pin 1 and 47 VDC at pin 2.  
The right 7199 had higher readings; 160 VDC at pin 1 and 80 VDC at pin 2.

When taking these 7199 readings I heard a pop in the speakers when probing each pin except pin 2 of the right side.

I compared every resistor on the driver board.  The only discrepancy I found was on the two large 330K.  Left side was around 350K, right side was 142K.
So I now suspect that 330K resistor, which I do not have a replacement for at this time.  I do have some 1/2 watt at this value.

At this point I am wondering what else I should check and if this resistor could be the problem.

Thank you!  Mark

BTW, you can view pictures of the amp at noalibirocks.com/na/pics/pics.php?mode=Stereo70/before

After writing this I went back and took more readings. This time without the bulb limiter.

On both 7199 I have 359 VDC at pin 1, 308 VDC at pin 2.

All output tube plates around 365 VDC.

I am not hearing any hiss from either speaker now. I need to inspect the PAS3, get it hooked up to the ST70 and see what happens.

Thank you

A couple thoughts:
-FYI: If there is a Guitar Center nearby they stock SOVTEK 5AR4 (not cheap but convenient)
-If you have a Sucrets (or Altoids) tin full of 1N4005/6/7 you could do the "yellow sheet" mod posted above. You could also bypass the 5AR4 altogether with forward 1n4005/6/7 to see if that gets you in business.
-You might try pulling the left side EL-34s to see if your B+ rises. Repeat after replacing the left side tubes then pulling the right side EL-34s. It is best to have at least one set of outputs in place whenever you power on with the rectifier inserted. Powering up the B+ with no EL-34s is bad juju.
-Never met a quad cap that I liked... I keep a drawer full of 47µF, 450V Panasonic caps on hand so I can bypass each segment of a quad cap, one at a time, for debugging.
-No sound from one speaker... at all? I might probe the silent outputs to see if there is any measurable DC or AC present. I have worked on just a few Dynaco/Dynakit/VTA amps but have encountered outrageous hookup errors and open circuits a couple times. Especially in amps that have been "updated". I am assuming that the speaker does not (now) have an open circuit.
...J

Here is the latest, before reading or trying any of the posted suggestions yet.

Those readings are while connected to the variac (isolation transformer) with output at 117V.

I inspected, tested and hooked up the PAS3 and am playing a CD connected to the SPARE.  I have no sound at either speaker of the ST70.  I can detect audio at pin 7 of each 7199, but no audio at any other pin.  I would expect amplified audio from pin 1 of each 7199?

No audio detected at any of the EL34 pins. Makes sense as I have nothing coming out of the 7199.

I do have a spare 5AR4, and I do have plenty of 1N4005 and 1N4007.

I have yet to test the amp directly to mains, still using the variac.

Thoughts?  Thank you

.

Mark, for what it is worth, if you have a working IPAD/IPOD with some music on it and a cable with a miniature stereo plug at one end (for the IPAD/IPOD) and 2 RCA plugs at the other end (for the ST70) then you can drive the ST70 without using the PAS.  The volume control on the IPAD/IPOD can be used to raise or lower the output signal level going to the ST70.

Use your Variac, but keep the voltage at or below 75 volts, and you should be able to hear some music coming thru the speakers.

Then you can put your effort into getting the ST70 working. It is a lot easier to get one unit working than 2 units, since you could have problems in either.

Once the ST70 is working to your satisfaction, pull the RCA plugs out of the ST70 and add the PAS. Plug the IPOD Cable's RCAs into any of the Line Level Inputs on the PAS (either of the FM jacks, or the SPARE jack), set the Selector Knob accordingly and see if you can hear any music.

As I said .... for what it is worth,
Stew

A bunch of things. And why I dislike unmetered Variacs so much!

First tests with the unit turned OFF and all caps discharged and the tubes OUT.

a) Measure resistance from each tab of the quad-cap to the chassis. It should be substantial-to-infinite in every case. If it is not, one/another/more sections of the cap are shorting. This cap *WILL NOT REFORM*, it needs to be replaced.

b) Measure resistance across each component on the PC board. Are there any differences one-to-the-other channel? If so, start there.

NEXT: You mentioned that you tested the tubes. How, and with what, did you do this? If an emissions-tester, does it have tests for "Shorts" and "Gas"? If so, did each tube test good? Same question if a GM tester? And if a GM tester, how did you pair the output tubes at re-installation?

a) Install all the tubes, not including the 5AR4. Measure the filament voltages at each tube. Measure at the rectifier socket. They should be at/about 6.3 and 5 V respectively, with a 117V input +/- a few fractions of a volt.

b) Measure the bias voltage at the front sockets - it should be somewhere between 0.9 and 2.3V or so. This is separately rectified via the selenium rectifier under the chassis. Twiddle the adjustment pot - is there a response on each channel? If all is good, so far, shut off and cool down.

c) NOW. Do you have access to an ammeter and do you know how to use it? If so, set it up to measure the developed load as B+ ramps up in the amp as the rectifier starts to pass DC. You should be somewhere about 1.3A when the amp is fully warmed up. Too little/too much are problems. Too little, 80%+ of the time is a bad rectifier. 15% of the time, an open winding on the transformer. 5% of the time, something else. Too much is 90% of the time a shorting section in the quad-cap, 10% of the time, something else such as a partially shorting winding or tube.

d) WITH GREAT CARE AND CAUTION - as I expect you do not have or use an isolation transformer (separate rant) - and with the amp on its side so you can reach the quad-cap tabs, install the rectifier and turn on the amp. Keeping a sharp eye on the ammeter, measure the voltage-to-chassis at each tab of the quad-cap. Those voltages will be listed in the manual (readily available on-line) but the 'first' tab should be nearly/a bit over 450VDC. I use shrink-tubing on my tester probes when working at these voltages to prevent inadvertent shorts. Just the tiniest bit of tip exposed.

If the tubes are free of shorts or gas, and good (via a reliable tester, not just a VOM on the filament pins), and all the voltages are present and correct, I suspect something mechanical. If you have the old phenolic (brown) board, check all the traces for cracks or breaks. If your amp is wired with solid wire, check _EVERY_ connection to make sure that it is intact and that the insulation is not holding the connection in place thereby fooling you.  Check the "stereo/mono" switch. A good many of these have gone "open" over the years - invisibly. Check each socket and make sure nothing is loose.

If there are discrepancies - start with those. The 5AR4 comes first to mind. If you have a known-good 5U4 sitting about, use that. It de-rates the amp by about 20%, but is perfectly adequate to drive it for testing purpose, and even permanently if you do not mind the de-rate - and set the bias properly.

That is about all I can suggest from 20,000 feet and without the amp in front of me. If you were nearby this could go further, but I have no idea where you are.

I will be out all day today so will not be able to get back to this until a week night.

I have an isolation transformer/variac with a meter (V and A) and I am reading 1.3 amps with the amp warmed up and running.

I am confident the PAS is working because I can probe and hear the audio at the input jacks and at pin7 of each 7199 in the ST70.

If I probe pin 1 of each 7199 I hear a pop at each speaker. Makes me think that my OTs and output tubes are good.

I have a basic tube tester. I think it is a Mercury. It tests for stength and shorts. All tubes were strong.

I recall reading 30 ohms resistance on one side of the ac cord, so I need to check my vac coming in at the amp at the primary and see if I am really at 117 vac.

I did read 6.4 vac on the heaters at the front panel pins. Will check the 5 vac at the rectifier tube.

I did adjust the bias on each channel and trim pots worked on both sides.

I think that the audio signal is not getting past pin 7 of each 7199 so I have more probing to do on that driver board.

Appreciate all the support! Mark

To me it seems that the 7199 is at fault ( it pops in the speakers when touching pin 7199-pin7

That both 7199 is faulty is not likley.
Is it really 7199 tubes ? Are the board modified in any way ( this was done to be able to use
6U8 or simular tubes that had another pinout) ?
Whats the voltage on pin 1 2 3 6 7 8 9 on the 7199 when powered on ? Is the filament visually
glowing ?

Pin 7 is the first pentode grid. Putting a load-to-ground on it will disrupt its function on a momentary basis. *POP*. And a VOM -will- put a very small load on it.

Otherwise, you are entirely correct inasmuch as both 7199s being bad is unlikely.

I still suspect something mechanical if all voltages and the quad-cap are good - connection/switch/trace/cold-solder or similar. And given my several experiences with the mono-stereo switch, I would start there.

I made a checklist and did not get too far without running into a question.

I pulled all tubes, drained the caps.  Measured each leg of the can cap to ground.  Every leg was > 100K resistance.  

I started comparing all resistors on the driver board.  I have two pairs coming up with way off values.

The 2 large 1K in the center both read 47 ohms.  I am a bit color blind but I think the colors are brown, black, red, brown.

The two 620 ohm resistors are reading around 630K ohms.  Color band is blue, red, brown, brown.

Could something else in the circuit throw these numbers off?

Pictures can be seen at noalibirocks.com/na/pics/pics.php?mode=Stereo70/before

Thank you

Just to make sure... resistor values (in general) can't be measured in circuit. You would have to lift one end to measure a resistor's value. Comparing Left channel to Right channel node voltages can be done without lifting components.

Making progress and the good news is the amp is now working!  

I think it came down to increasing my VAC coming into the amp.  
With all tubes installed I had to have 125.5 VAC on my variac to read 117 VAC at the AC cord leads inside the amp.
I am guessing this is because one side of the ac cord has about 30 ohms resistance, not sure how to calculate the drop that may cause.

Looks like I should replace that ac cord.  Wondering if I should install a 3 prong cord with grounding?

I think some of my voltages are still too low.  EL34 pins 3 are 392 VDC, pins 4 are 396 VDC.

Rectifier pins 4 and 6 are 355 VAC.

Just realized I have yet to redo my quad cap readings to see how much they rose.

The 7199 pins left and right follow.  There are some differences.  I am suspicious of the right side 330K 3 watt resistor.  Had some low readings earlier on that one, around 160K. Also a big difference at pins 4 and 5 VDC, but they are both around 6.0 VAC.

7199 left (these are all VDC)

1 - 245
2 - 97
3 - 32
4 - 101
5 - 101
6 - 1.15
7 - 101 mVDC
8 - 105
9 - 97

7199 right (these are all VDC)

1 - 256
2 - 88
3 - 31
4 - 9
5 - 9
6 - 1.2
7 - 132 mVDC
8 - 94
9 - 87

Another note of concern is the amp is now drawing 1.5 amps.

I have yet to try the amp on regular house mains.

I may be good!!!???

I started over, plugged directly into mains which are reading 119.x VAC.  I have no loss on the AC cord and that full voltage is at the PT primary.  No heat on the AC cord and no resistance measured.
Also verified a more accurate reading on current and the unit is running at 1.35 amps.

Rectifier

2 : 433
4 : 365 VAC (manual says 360)
6 : 365 VAC (manual says 360)
8 : 433

Left output tubes (both are identical within 1%)

1 : 1.54
2 : 6.6 VAC
3 : 422 (manual says 410)
4 : 426 (manual says 415)
5 : -33
6 : -33
7 : 6.6 VAC
8 : 1.54

Right output tubes (both are identical within 1%)

1 : 1.54
2 : 6.5 VAC
3 : 421 (manual says 410)
4 : 426 (manual says 415)
5 : -32
6 : -32
7 : 6.5 VAC
8 : 1.54

Not sure what the 7199 readings should be (not all pins shown in manual).

7199 left (these are all VDC)

1 : 262
2 : 104
3 : 34.7
4 : 6.5 VAC
5 : 6.5 VAC
6 : 1.23
7 : 107 mVDC
8 : 113
9 : 104

7199 right (these are all VDC)

1 : 274
2 : 93
3 : 32.9
4 : 6.6 VAC
5 : 6.6 VAC
6 : 1.29
7 : 144 mVDC
8 : 101
9 : 94

Quad Cap research and actual readings;

Section A (Designated by a Square): 305 VDC  actual : 298
Section B (Designated by a Triangle): 375 VDC  actual : 376
Section C (There is No Designator): 415 VDC  actual : 427
Section D (Designated by a Semi Circle): 435 VDC   actual : 443

From all the data, it does look like you are good. Now, keep a very sharp eye on the bias for the next several days - and if it remains steady, you should be good for a long time.

Good luck with it!

I will likely order those 330K 3 watt resistors as one was off and replace the pair.

Thank you to everyone for all the help! Mark