开云体育

added pictures of waveforms of 2n3055 and 2N5881. At least they are now labeled. the earlier ones numbers are all FPJ13007 with the poor thermal mounting per that picture. that is no more, just not enough thermal mass or xfer. still signals really have not changed.
looking at them together I am thinking the issue must be prior the Q1174/84 chopper/switch.

Adjusting the Osc never manages to give me a leading edge just changes the overall Freq.

The waveforms in the book are all at correct supply (aprox 28VDC) so not a whole lot of help while running at 11.5 to 12VDC input. wish the book had a few waveforms on the other side of T1171 to verify operation at 11.5V

I may try to put the good original 2n1899 back in circuit (original location Q1184) and pair it with a TO-3 and see what happens.....when I have time later.
搁别苍é别

glad to hear the DC/DC converter is working that should be a relief.
so why is the Ac-Dc linear causing an issue?? bad rectifier? or one/both of the filter caps bad?

it is 9am here, had a chance to try a pair 2n5881, 2n5039, 2n3055, and Mj10000 earlier this mourning.
11.5VDC input and looked at the waveforms junction D1176/77
I had attached sockets and left them dangle off the side for testing...now where did i put that heatsink??
the MJ10000 (Darlington) would not give stable waveform- kinda expected it may not work.
it does not seem to matter much between the rest. The venerable 2N3055 drew the least current (3.5A) and was stable, 2n5039 a bit more current and stable same with the 2N5881. not having heat sink limited the "on" time. Conclusion- The Chopper xsistor is not too important which were my original thoughts prior to over thinking a replacement. It appears the venerable 2N3055 should be fine. I wonder why the used the 2n1899??...I may find out later when i get the rest of this darned thing working.
I still do not understand why I do not have a leading edge anywhere close to "per the book" . I think the "on" time is too long. I may have to go back prior the choppers....I only have the PDF of the AC/DC supply and the waveforms are really hard to see...they were in blue in the original and obviously did not xfer well.

I also did look at the waveforms with and without scope attached same waveform just 0.5A more current draw.
nice square wave at the Base of choppers.
I will post some pictures later today of the 3055 after I get them off the tablet thing. lots of other stuff that needs be done prior my being gone for the next 10 days (performances/rehearsals/lots of driving- hate the last item)
搁别苍é别

What I have found so far on the 465B trigger problem:
Serial number B061387, service manual says B060000 and up.
Removed case and did a very brief physical inspection. Very clean, no dust. No obvious burned parts so far, no burn smell. Most transistors seem to be socketed.

Voltages: input selector is set to 104-126. I know the power company sometimes keeps switching between 120 and 125 because my UPS complains about that.

Used an old Radio Shack (yeah I know, can't afford a Fluke) DVM with true RMS capability.

Voltage test point, reading, ripple, comment
+110 unreg 164.7 Vdc .425 Vac dc voltage seems high?
+110 111.9 2.8 mV
+55 54.8 1.3 mV
+15 14.95 1.4 mV
+5 5.00 1.4 mV
-8 -7.98 1.4 mV

So that all seems nominal.

Schematic 6, A and B Trigger Generator board
Set the front panel controls for DC measurement per the schematic.
Test Point 43, which is not marked on the board layout picture, but is marked on the schematic:
Should read zero volts, reads -.1mV so I would call that zero (input to Q7324A and B).

U7331
Pin, expected Vdc, actual
1. -0.8, -1.1mV
2. 0, .761 V
3. 0, .761 V
4. 4.9 actual (tied to +5 through 1k resistor)
5. -8, -7.95
6. -1.0, -.959 (close enough)
7. ground, -.5mV (effectively zero)
8. 10.6, 10.72
9, 10.6, 9.73
10. 0, -39.7 mV
11. 10.5, 10.68
12. -0.9, -.852
13. 7.1, 7.18
14, 0, .7 mV
15. 0, .7 mV
16. -0.8, -204.6 mV

I'm thinking based on these numbers the answer must be staring me in the face. Pins 2, 1, and 16 aren't correct, so I'm thinking either a U7331 problem or possibly the A trigger view circuit which hangs off of 1 and 16 may have a problem. I'll check the voltages in the trigger view circuit next opportunity, and maybe pop out Q7424 and Q7422 to isolate the chip from the trigger view circuit and see if anything changes on 2, 1 and 16.

I didn't write down the numbers but a quick probe of some of U7131 on the B trigger side showed expected voltages, especially on 2, 1, and 16.

You guys are an amazing resource! I knew I came to the right place. I also hope this thread turns out to be useful to someone else.

Lorn
KK4KRI

What is like gold? There is nothing from the prior to know what your referring too
Dave
NR1DX

--
Dave
Manuals@...
www.ArtekManuals.com

Suhweet! Thank you so much! This is like gold!

Lorn
KK4KRI

It's 11:00 AM here and I just hooked it up. It is now running off the unfiltered battery charger into its own filters. It's got about 15 volts going in, best I can do without the variac. However, I think it will be enough to detect heat buildup in a few hours. It used to shut down in less than 2 hours. I am injecting the power right before the thermal cutout so if it does get hot it shouldn't let the smoke out. I can't sit here and watch it all day.

If there is no heat buildup in a few hours, I'm going to disconnect it and also disconnect the secondary of the AC power transformer and just let that run with no load. At that point I can be sure the DC/DC convertor is running properly and we can trust the waveforms.

Right now, the ripple is 3 volts P-P while it was only 1 volt with the 10,000 uF.

We'll see.

Hello Group,
Moving on to the next issue (this one is minor... but annoying) on my recently acquired 7623A.
When I select any scale (either from Vertical amplifier or from Time Base) that requires 2 zeroes, it displays only one zero while the scope is still cold.
After warming up for - say - 5 to 10 minutes - it becomes normal.
While it's still cold, also, if I press IDENTIFY, the word identify doesn't show and instead, it shows (IIRC) a u symbol, in place of one of the zeros that are currently on display.
I narrowed down the temperature sensitivity to U2232, the "zeroes logic & memory" IC.
However, I`m not sure if the problem is in it, or if one of the signals coming into it is at a marginal level, and maybe when U2232 is cold, it's just misinterpreting the inputs which are marginal.
Fiddling with the "Column Match", or "Row Match" adjustments doesn't have any impact on the problem... it still "swallows" the second zero while cold, no matter what position those 2 adjustments are.
So, my questions to the group are:
1. Is there any Tek semiconductor manual available that shows what it has inside? The "Tek_Made_sm.pdf" available at TekWiki doesn't show it.

2. Does any of you guys have a broken readout board and would be willing to part this IC with me? I know that Sphere and Qservice have them, so I already have that as a last resort.

3. On my suspicion that the problem may *not* be at U2232, looking at the Column decoded signals that enters to it (and which are ultimately sampled at Time Slot 1 and memorized, to define whether it should add one zero, or two, or none), I find a little bit odd that the signals there don't really resemble "logic" levels, each (C1, C2, C3, C4 and C10) are sort of different from each other, and some of them really look much more analog signals then a digital signal with clearly distinct discrete levels.
I will try to take pictures from each one probed on my other scope and post it.
From what I understood, although the original Row and Columns signals are analog (OK. Discrete 10 levels analog signals)... after they are processed by the Row and Column decoders, those signals should have been translated into 10 different "logic" outputs, with only one output showing an "active" level, and all the other ones showing an "inactive" level (I don't say high or low because I`m not really sure if they're active high or active low).
Still, those "Column Decoded" digital signals (that don't look like digital) are the same ones going to the character generator IC matrix... And there doesn't seem to be a problem for the character generation circuitry, and all the displayed characters are correct, leading me to conclude that despite the Column signals don't look like what I expect... they might be correct anyway.
So my 3rd question: Can any of you take a look at the column decoded signals (the ones entering at pins 10,11,12,13 and 14 of U2232) and tell me if they look like digital signals, with clearly distinct "active" and "inactive" levels?

Thank you in advance for any input...

Rgrds,

Fabio

Hi chuck,
Thanks for the reply, as far as I Know nothing has been changed, I have owned
The Scope since about 2002, Before that it was in the Navy.
Thanks............Peter

Tony, did you do the Power Supply test on page 5-8 of the manual : Adjust
Inverter Control R1293 for a meter reading on TP1625 of +40V? Maybe your
operating point for the Inverter Controller is way off. Mine was way off in
the negative range until I adjusted it.

Best,
John

On Tue, Apr 24, 2018 at 10:52 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:

Hi John :
It is overall power handling issue for the PSU , U1275 is the controller
for the shut down action, 5V light is NOT only output, there is feedback
to R1304 to controller too !
On the other hand the total current sampling from T1235 to U1275.
I did check C1316, C1317 and C1318 are normal .
My PSU was damaged by miss-align one pin of SENS input on LV Board ( P1483
) :
pin 1 ------- Beam I Sens input on LV board NO Connection.
pin 2 ------- GND Sens input on LV board connected to P83 pin 1
-------Beam I sens. --------------- Beam I sens short to GND.
pin 3 ------- +5V Sens input on LV board connected to P83 pin 2 -------
GND sens, -------------means +5V short to GND !
pin 4 ------- -50V Sens input on LV board connected to P83 pin 3 --------
+5V sens, ------------- short -50V to +5V !
pin 5 ------- +50V Sens input on LV board connected to P83 pin 4 --------
-50V sens -------------- short +50V to -50V
pin 6 -------- -15V Sens input on LV board connected to P83 pin 5 --------
+50V sens ------------- short -15V to +50V
pin 7 ------- +15V Sens input on LV board connected to P83 pin 6 -------
-15V sens ------------ short +15V to -15V

The results is all ( +5V, -50V, +50V, -50V, -15V, +5V and Beam I sens
short to GND ). may be serious damage for the PSU or main unit ?
Anyway it is under repair for the damage !
RegardTony CheungAPR 25 2018



From: JJ <jajustin@...>
To: [email protected]
Sent: Wednesday, April 25, 2018 5:26 AM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Tony,
If all you can draw is 100ma on 5v lights, you could have a problem with
that power supply's pi section filter - I had to change C1318 to get my PSU
up. It measured 40uf instead of 100uf. That cap may be right on the edge of
failure at certain loads. There are also 2 other caps in that filter -
C1316, C1317. Note that 5v lights is not regulated on the LV regulator
board - it just goes through the board to the connectors - so the problem
is localized to the cap rectifier board.

I would first change C1318 with any 100uf cap (doesn't have to be tantalum
to just test). And, do the same sequentially for the other two - swap out
then test. They're pretty easy to change.

Best,
John



On Tue, Apr 24, 2018 at 2:35 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:

Hi John :
Yes, I did put it back to 7904 , but find something NOT working !
1. Graticule Light always on ----------- I did check NO -15V at R1451,

but

PSU was tested with full loading pass .2. Front panel push button's light
not function.3. For Horizontal Mode Switch ALT , CHOP will shut down the
PSU. A or B is OK !
Regard
Tony Cheung


From: JJ <jajustin@...>
To: [email protected]
Sent: Wednesday, April 25, 2018 1:21 AM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Tony, have you tried putting the PSU back into the mainframe to see if it
works in its normal load and interaction environment? You may be chasing
the proverbial wild goose! :)

Best,
John

On Tue, Apr 24, 2018 at 12:16 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:

Hi:
It is a good idea ! But I leave my workshop already ! May check day

after

tomorrow. It is midnight in Hong Kong !
One thing I find at least, I disconnect-15 V loads, it become normal.

So

that my next step to check the resistor!
Regard
Tony Cheung


從我的 iPhone 傳送

dadhills@... 於 2018年4月24日 下午11:40 寫道：

Have you checked R1286, 1K, 5%? It is a carbon composition resistor

and

if it has drifted high in value, it will lower the "total" load current
limit threshold.

Dave

On Tue, Apr 24, 2018 at 07:59 am, Yiu On Tony C wrote:


Hi Harvey :
My PSU are test alone and connected to Loading resister and " sens "

return to

LV board.
For the current limiter was controlled by U1275 , and I find the

zener

VR1297

---- 9V are opened , I replace it and now always shut down again ,

even

I open

the zener again , still shut down ! It means cannot return as

before ,

I did

check all the diodes on A12 , but no defective !
There may be something wrong again !
RegardTony CheungAPR 24 2018

From: Harvey White <madyn@...>
To: [email protected]
Sent: Tuesday, April 24, 2018 9:41 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

On Tue, 24 Apr 2018 13:27:01 +0000 (UTC), you wrote:

Hi :
My 7904 still have problem for PSU !
+/-50V, +/-15V, +5V , +130V are normal with loading power on !
Once I connect the loading to +5V light , it cannot load even 0.1A

!

it will

shut down PSU. ( If normal , It can output up to 3.5A ! )

The diode for +5V lights are defective ( shorted ) and replaced by

fast

recovery diode FR607 ( junction Cap 80pF )or HER208 ( junction cap

14pF ).

Sounds as if the supply is working normally for regulation, but the
shutdown circuitry is too sensitive. There's a current sense

resistor

in there, and some associated parts. Could the current sense

resistor

be too high in value? That will certainly cause this supply to shut
down on a lower current. I'd also check the amplifier for that

sense,

to make sure it is working properly.

Harvey

Electrolytic capacitors that are failing
tend to fail when cold, but improve as they
warm up.

You might want to consider looking at the
bevy of electrolytic capacitors in the switching
power supply. If they haven't been replaced
already, they are past due.

-Chuck Harris

cartronics via Groups.Io wrote:

Hi All my 2445A has developed a fault with the horizontal trace on all channels,
There are no error codes,
On switch on from cold the horizontal trace appears wider than normal, the readouts on the graticule are are ok
After about ten minutes warm up it then appears correct ,
Just wondering if any one else has had this fault and hopefully rectified it?
Thanks.......Peter

Type 130 L,C possibly? Wasn't a lot of Tek stuff with meters. 105 square wave gen., and the 190 A and B constant amplitude signal gen.

I was just watching season three, episode eight of The Magicians, and spotted a Tek box at 36:17.? It's in? 60's era cabinet, and there appears to be a meter in the center.? I first thought of the early time mark gen, but that's obviously not it.? A power supply maybe? The funny thing is that it was located in a room called "Priceless Artifact Storage"
-Dave

It just ran off DC for a whole day, literally 24 hours. Stayed cool as a cucumber. The supply was about 16 volts with 1 volt of ripple, mainly sawtooth.

Next I go one of two ways, either inject the DC onto where they put it off the transformer, like at the thermal cutout, without external filtering, or to disconnect the load from the transformer and plug it in and see if it gets hot. I think the voltage was a bit low, but I attributed that to low line voltage. Maybe not. I might have a bad transformer.

I think internally is has 2 X 1,300 uF for filtering, I'll leave the external 10,000 uF out of the circuit and see how much ripple I get. It should be the same and put that question to rest.

On yours I have been thinking, a better mounting arrangement can be had. there are holes for the base and emitter leads, or wires pigtailed off them, and the bolt will take care of the collector. I would put put them on the metal side, which I think is flat and should have more room than the PC board side. I'll look at mine one more time and figure out a good replacement. Now I am thinking maybe the ft is maybe not that critical, with it switching at such a low frequency, 3 MHz might be fine, not sure.

That driver transformer might only show 3 volts DC going to the primary but there is obviously a pulse involved. Since I'm sure mine switches right and I again have the bench space I can get some relevant waveforms. So figure out which ones you want or think you will need and I'll add in the ones I think will help and I'll get them fairly soon. I now have my own camera again so I don't have to borrow one anymore. It is actually not as good a camera as the one I had been borrowing but it seems to have more lens range in the way of going wide and the macro function. It's only 5.1 megapixels but I downsize pictures anyway so they don't take forever to load. It will do.

I think I am going to modify my bench power supply to go up to 3 amps instead of 2, this thing pulls about 2.3 amps, sometimes less. It's a damn shame not to be able to have a nice ripple free supply because of 300 stinkin' mA.

Hi John :
It is overall power handling issue for the PSU , U1275 is the controller for the shut down action, 5V light is NOT only output,? there is feedback to R1304 to controller too !?
On the other hand the total current sampling from T1235 to U1275.
I did check C1316, C1317 and C1318 are normal .?
My PSU was damaged by miss-align one pin of SENS input on LV Board ( P1483 ) :
pin 1 ------- Beam I Sens? input on LV board? NO Connection.
pin 2 ------- GND Sens input on LV board connected to P83 pin 1 -------Beam I sens. --------------- Beam I sens short to GND.
pin 3 ------- +5V Sens input on LV board?connected to P83 pin 2 ------- GND sens, -------------means +5V short to GND !
pin 4 ------- -50V Sens input on LV board?connected to P83 pin 3 -------- ? +5V sens, ------------- short -50V to +5V !
pin 5 ------- +50V Sens input on LV board?connected to P83 pin 4 --------? -50V sens? -------------- short +50V to -50V
pin 6 -------- -15V Sens input on LV board connected to P83 pin 5 -------- +50V sens ------------- short -15V to +50V
pin 7 ------- +15V Sens input on LV board connected to P83 pin 6 -------? -15V sens? ------------ short +15V to -15V

The results is all ( +5V, -50V, +50V, -50V, -15V, +5V and Beam I sens? short to GND ). may be serious damage for the PSU or main unit ?
Anyway it is under repair for the damage !
RegardTony CheungAPR 25 2018



From: JJ <jajustin@...>
To: [email protected]
Sent: Wednesday, April 25, 2018 5:26 AM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Tony,
If all you can draw is 100ma on 5v lights, you could have a problem with
that power supply's pi section filter - I had to change C1318 to get my PSU
up. It measured 40uf instead of 100uf. That cap may be right on the edge of
failure at certain loads. There are also 2 other caps in that filter -
C1316, C1317. Note that 5v lights is not regulated on the LV regulator
board - it just goes through the board to the connectors - so the problem
is localized to the cap rectifier board.

I would first change C1318 with any 100uf cap (doesn't have to be tantalum
to just test). And, do the same sequentially for the other two - swap out
then test. They're pretty easy to change.

Best,
John



On Tue, Apr 24, 2018 at 2:35 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:

Hi John :
Yes, I did put it back to 7904 , but find something NOT working !
1. Graticule Light always on ----------- I did check NO -15V at R1451, but
PSU was tested with full loading pass .2. Front panel push button's light
not function.3. For Horizontal Mode Switch ALT , CHOP will shut down the
PSU. A or B is OK !
Regard
Tony Cheung


? ? ? From: JJ <jajustin@...>
? To: [email protected]
? Sent: Wednesday, April 25, 2018 1:21 AM
? Subject: Re: [TekScopes] 7904 Mainframe damaged

Tony, have you tried putting the PSU back into the mainframe to see if it
works in its normal load and interaction environment? You may be chasing
the proverbial wild goose! :)

Best,
John

On Tue, Apr 24, 2018 at 12:16 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:

Hi:
It is a good idea ! But I leave my workshop already ! May check day after
tomorrow. It is midnight in Hong Kong !
One thing I find at least, I disconnect-15 V loads, it become normal. So
that my next step to check the resistor!
Regard
Tony Cheung


從我的 iPhone 傳送

dadhills@... 於 2018年4月24日 下午11:40 寫道：

Have you checked R1286, 1K, 5%?? It is a carbon composition resistor

and

if it has drifted high in value, it will lower the "total" load current
limit threshold.

Dave

On Tue, Apr 24, 2018 at 07:59 am, Yiu On Tony C wrote:


Hi Harvey :
My PSU are test alone and connected to Loading resister and " sens "

return to

LV board.
For the current limiter was controlled by U1275 , and I find the zener

VR1297

---- 9V are opened , I replace it and now always shut down again ,

even

I open

the zener again , still shut down ! It means cannot return as before ,

I did

check all the diodes on A12 , but no defective !
There may be something wrong again !
RegardTony CheungAPR 24 2018

? ? ? From: Harvey White <madyn@...>
To: [email protected]
Sent: Tuesday, April 24, 2018 9:41 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

On Tue, 24 Apr 2018 13:27:01 +0000 (UTC), you wrote:

Hi :
My 7904 still have problem for PSU !
+/-50V, +/-15V, +5V , +130V are normal with loading power on !
Once I connect the loading to +5V light , it cannot load even 0.1A !

it will

shut down PSU. (? If normal , It can output up to 3.5A ! )

The diode for +5V lights are defective ( shorted ) and replaced by

fast

recovery diode FR607? ( junction Cap 80pF )or HER208 ( junction cap

14pF ).

Sounds as if the supply is working normally for regulation, but the
shutdown circuitry is too sensitive.? There's a current sense resistor
in there, and some associated parts.? Could the current sense resistor
be too high in value?? That will certainly cause this supply to shut
down on a lower current.? I'd also check the amplifier for that sense,
to make sure it is working properly.

Harvey

Tony,
If all you can draw is 100ma on 5v lights, you could have a problem with that power supply's pi
section
filter - I had to change C1318 to get my PSU up. It measured 40uf instead of 100uf. That cap may be
right on the edge of failure at certain loads. There are also 2 other caps in that filter - C1316,
C1317.
Note that 5v lights is not regulated on the LV regulator board - it just goes through the board to
the
connectors - so the problem is localized to the cap rectifier board.

I would first change C1318 with any 100uf cap (doesn't have to be tantalum to just test). And, do
the
same sequentially for the other two - swap out then test. They're pretty easy to change.

OK - I've been on vacation in Australia for nearly 3 weeks, and this thread was running before I left!
So I deleted the zillions of mails on this topic, and my comment might well have been already covered.

The 5V lights is sensed by the Inverter Control chip, U1275. It is added to other signals via R1304.
If resistors around that area have changed value, the 5V lights sensing could be trigger happy.

This was probably identified as a problem area, because two different values are in the parts list -
9k31 for pre B181294 and 8k45 after.

Craig

Is your 453 a mod 703H?
Does anyone have a 453 mod 703H manual?
Does anyone know what was done for mod 703H?
The obvious is larger CRT, side controls moved to front and solid state vertical amplifiers from nuvistors.


Thanks
Glenn

--
-----------------------------------------------------------------------
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI LM NRA LM SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"

Actually there are two different manuals for the 465B. The early 465B were actually "engineering models" pretty much made by hand (the better of the two types many believe) and followed by the "production 465B.
If I'm not mistaken , one of the great things about the early 465B was that the xistors "plugged in" to the circuits using neoprene sockets while the later production models had them soldered in to the circuits with the 'plugables" being obviously much easier to troubleshoot

One manual covers S/N B059999 and down with other covering S/N B060000 and up. I have CD of both,
There are some significant differences between the two so unless you have the right manual many things don't match...

I repaired/ refurbished all 400 series scopes for quite a while. Must say that of all of them my favorite far and away is the 465B.. a really GREAT scope....well worth putting in top shape .... On your trigger problem.... there are tunnel diodes used in the heart of the trigger ckt... they are "weird " little devices that have unique electrical characteristics that are perfect for use in a trigger ckt... could be your problem... just a guess

Doug is talking about this: "Troubleshooting your Oscilloscope". You should
read through this - it tells you how to use the scope itself to narrow the
problem down. I recommend it highly. It's designed for technicians to
narrow down Tektronix scope problems quickly through the front panel
controls. The only way to do that is to use the scope to debug itself
according to them.

I don't think you need the exact serial number for the 465B to use the
service manual - they are all pretty much the same.



Best,
John

There is an old manual out there on the internet about how to troubleshoot Tek scopes.? A Google search should uncover it quickly.? I have used it to troubleshoot scopes and it is quite amazing how much information you can get about your problem without any test equipment at all.? Most of it is done from the front panel controls on the scope itself.? Good luck.


From: blackholenulled <lorn@...>
To: [email protected]
Sent: Monday, April 23, 2018 6:51 PM
Subject: [TekScopes] 465B won't trigger

Have a 465B in pretty good shape.

I can get a trace and waveform on the display, but it doesn't seem to trigger. The waveform will just roll sideways.

I have a paper repair manual, although I need to check if the serial number range matches.

I haven't cracked the case yet. My first thought is to check the supply voltages. Although I should probably do a more thorough operational check which should isolate the problem.

I don't have any calibration equipment. Just a DMM and the built in calibrator on the front. So I'm not inclined to make any internal adjustments unless absolutely necessary.

Any thoughts where to start?