Phonoland

Would like advice concerning some voltage readings that were out of specification. Before readings all caps were replaced in amp (mra5-L6), and several resistors that were out of spec were replaced. Voltages were taken from reference points on issue 3 of schematic. Switches were set to minimum, zero signal input, test line voltage was 123 volts (about 5% higher than spec line voltage).

Voltages at T1, J5, 5U4, 5879, 6SN7, both 6L6's, caps C1-C3 were all at or within stated voltages.
Voltages out of spec were as follows (spec/actual):
6SK7 pin 5 (35/50), pin 6 (70/84), pin 8 (260/89)
6SL7 pin 3 (0.5/41.2), pin 5 (260/204) verified with a second tube with same readings.
12AX7 pin 6 (195/178)
Voltage at R60/R61 (90/73.4)
Voltage at R25/R57 (290/204)

Tubes were tested 'good' on a Hicock tube tester by supplier. Assuming the tubes are good what else should I look for to determine why some voltages are well out of spec? Notably the 6SK7 pin 8, 6SL7 pins 3 and 5, voltage at R25/R57.
Thanks

Hi Steve,
1.What "type" meter are you using ?
2. Why don't you axk your tube supplier ??
Ron Rich

Steve, I'll see if I can help here,

Voltage at R60/R61 (90/73.4)
Voltage at R25/R57 (290/204)

Given where it is located I'd check for about 380-390v into the first filter C2. The 5U4 may be getting weak. Check this voltage --first--.

6SK7: What is the voltage on the other side of R44 -in a non-conducting state it should be about 260. If voltage is true, check for at/near zero volts on pin 4 (grid 1) on SK7. That kind of drop indicated heavy conduction. Look for possible leaky C21 and remember that if the mech mute-squelch plug is not present and in play mode a heavy squelch voltage is applied to that tube so jump pin 3 on the m-s plug to ground to kill squelch. Also, remove the SK7 and check for grid 1 voltage.

6SL7 pin 3 (0.5/41.2), pin 5 (260/204) verified with a second tube with same readings.

In a quiescent state there should be little-no voltage on pin3 -it is connected as a diode. Again, how is the amp being tested? Is it out of the machine and is pin 3 of the mute-squelch grounded? This will cause the low readings in this section.

12AX7 pin 6 (195/178)

Not radically faulty. just check volts on both sides of the plate resistor R50.

I am using this schematic: http://www.verntisdale.com/schem/mra5-l6.jpg

Rob

Using the same schematic as you, Rob. I'll follow up with your suggestions and post results. Thanks.

Hi Rob,
Thanks so much for the help. I guess I missed the footnote on the schematic that required grounding pin 3 on J4.
Here are the new results. All the other voltages on the schematic were tested and found to be within specs.
5u4 pin 2, 438 volts
C1 420
C2 431
C3a 160
C3b 282
C3c 315
C3d 331
R44 voltages 239/288, both sides of resistor
6sk7 pin 4, 0.15 volts; pin 8, 239 volts
6sl7 pin 3, 0.76 volts; pin 5, 204 volts (pin 5 voltage is still low, spec is 260)
R50 voltages 331/182, both sides of resistor

What would be the cause of the low voltage on 6sl7? Is it significant? Two different tubes tested the same.
Thanks again,
Steve

Good time to check values on resistors associated with those tubes.

6sl7 pin 3, 0.76 volts; pin 5, 204 volts (pin 5 voltage is still low, spec is 260)
R50 voltages 331/182, both sides of resistor
What would be the cause of the low voltage on 6sl7? Is it significant?

What is the voltage on the other side of R25...and what is the voltage @ pin 6? If it is higher than spec'ed (4VDC). Check also @ pin 4 and R59. With M-S plug pin 3 grounded there should be no voltage at pin 4 SL7. This section of the tube amplifies a sample of the audio which is then rectified in the other section and applied to the reactance tube (SK7). The SL7 does not directly affect audio to the speaker -only loudness.

Rob

Matt:
All resistor values were verified when all caps were replaced. Several resistors out of spec were replaced as well, including the ones at R25 and R59. But I'll check the values again to be sure.
Thanks.
Rob:
There is a Note #7 on the schematic but I don't see the reference on the diagram like the one for Note #8 (located between R57 and R59). Is Note #7 referring to any of the voltages that are being discussed, and where is the reference on the schematic? Looks like that testing is done with m-s pin 3 un-grounded and AVC at #1.

I'll check the 6sl7 pins and resistors as you suggest and post back today.
Thanks.

Rob:
Resistors R25 and R59 were retested and are within 2% of spec.

Voltage on both sides of R25 read 208/336. Spec on schematic is 260/290.
6sl7 pin 6 voltage reads 4.8; Spec is 4.0. Pin 4 voltage reads zero.
R59 voltage on both sides of resistor read 0/0 with m-s pin 3 grounded.
R59 voltage on both sides of resistor read 0/40.9 with m-s pin 3 un-grounded.

The comment to "see note 8" which is between R57 and R59 appears to be an error, and I believe should say
"see note 7". The voltage measured there (at R59) is 40.9 (un-grounded); the spec is 35 volts. Do you agree it may be an error?

Also, is it possible that the voltage stated at Pin 5 is an error and should be 206 instead of 260? I checked that voltage using 3 different brands of 6sl7 tubes known to be good and obtained the same reading of 208.

Would it be possible for you to confirm the 6sl7, pin 5 voltage reading of 260 volts as stated on the schematic by an actual measurement?
Thanks Rob,
Steve

Steve,
Once again--what type of meter are you using ??? If a newer "digital" type, your readings may be OK, as the older ones varied more then today's--Yes--that should read "7", not "8"--it is penciled in on my schematic--errors are ALWAYS "possible" !! Ron Rich

Voltage on both sides of R25 read 208/336. Spec on schematic is 260/290.
6sl7 pin 6 voltage reads 4.8; Spec is 4.0. Pin 4 voltage reads zero.
R59 voltage on both sides of resistor read 0/0 with m-s pin 3 grounded.
R59 voltage on both sides of resistor read 0/40.9 with m-s pin 3 un-grounded.

With M-S pin 3 at ground and AGC sw at min, zero volts at the grid is correct. The cathode shows conduction along with the plate voltage. At this point all I can think of is either a gassy tube, leaky C 18 or error in the schematic.

Again, this stage does not contain any signal fed to the speaker. If your pickup&styli has lower output than what was originally provided, the AGC will have little or no effect on the level.


If you are interested, take a look at the HFMA-1 (V-VL): http://www.verntisdale.com/schem/hfma1-l6.jpg There are some different voltages and values in that chassis. To be honest, I never paid attention to the notes on these schematics -mainly because they didn't reproduce well enough to read on my xerox copies from 1978. I just looked the circuits and used an RCA tube manual along with bench tests to get an idea of what was good, and what needed to be modded.

Would it be possible for you to confirm the 6sl7, pin 5 voltage reading of 260 volts as stated on the schematic by an actual measurement?

It will, but I'll have to find a rebuilt amp here (I know I have at least one, somewhere) or wait till I visit one of my VL locations.

Rob

Thank you for all the help. Even without fully resolving the one voltage issue many of my questions were answered, including verifying a schematic typo.
Steve