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Old 21st Apr 2019, 12:44 pm   #1
OldTechFan96
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Default Hameg HM307-3 Cleaning and Repair

I have been interested in buying a mini CRT oscilloscope after seeing various Heathkit examples online. When the opportunity arose to buy a little Hameg I jumped at it! I thought that it would be good companion to my larger Hameg 'scope.

The example I bought was sold as being untested and very filthy. It needed to be completely stripped down and cleaned. The insides were covered in dirt and dust. Washing the mainboard will be the only way to get it clean.

Before the big strip down I applied power via my lamp limiter and it fired right up. The raster could be moved about but it was thicker than it should have been. The 'scope reacted half-heartedly to the other controls and not at all to any signal input. All of the controls felt gritty and sticky.

Thankfully, all of the voltages from the PSU were all spot on.

Yesterday the 'scope was striped down and all of the controls were cleaned with IPA and switch cleaner.

This morning the main board was washed in cold water with a bit of Fairy Liquid and rinsed thoroughly. A hairdryer set to cold was used to get any stray water off the mainboard. It is now drying in the sun.

Before the mainboard was washed I removed five ICs using a chip extractor. One IC suffered a few broken legs so it will need to be replaced unless it still makes contact with the IC socket.

Once the Mainboard is dry I will give it a thorough inspection.

I will upload pictures of this work in progress later.
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Old 26th Apr 2019, 9:41 pm   #2
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Default Re: Hameg HM307-3 Cleaning and Repair

Here are a few before and after photographs. I'll get it all back together tomorrow.
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Old 30th Apr 2019, 4:40 pm   #3
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Default Re: Hameg HM307-3 Cleaning and Repair

The CRT base is wobbly so I am going to reglue it. After I removed the base I realised that there is ten wires coming from the CRT and the base has twelve pins. The CRT is a 3RP1A.

Which base pins go without a connection from the CRT?

Thanks

Edit: Pins 5+12 go without a connection?
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Old 30th Apr 2019, 7:08 pm   #4
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Default Re: Hameg HM307-3 Cleaning and Repair

Pins 5 and 11 are internal connections according to Radiomuseum (12 is heater).
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Old 11th May 2019, 6:31 pm   #5
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Default Re: Hameg HM307-3 Cleaning and Repair

The CRT base has been reglued this afternoon.

A SN74LS74AN has has to be replaced due to its broken pins. It looks to be readily available online. I'll buy a new one.

A TL710CN has a few broken pins but they are all unused in the data sheet. It is expensive online. Hopefully it works.

I have started to partly reassemble the 'scope.
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Old 12th May 2019, 10:23 am   #6
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg 307 tube.
I attach the data sheet for the 3RP1 tube, which confirms your base connections. wme_bill
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Old 19th May 2019, 9:55 pm   #7
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Default Re: Hameg HM307-3 Cleaning and Repair

The scope is now all back together. The SN74LS74AN has been replaced. Another casualty of fragile legs was T001 and T002. They are marked TIS69 and are silicon N-channel FETs used in Y-preamp.

What would be a suitable replacement for this FET? I once found German forum which suggested a suitable replacement but I cannot find it!

The first switch on was uneventful. All voltages from the PSU were fine. Nothing shows up on the CRT. R197 gets quite hot and looks to have done so in the past. Is it a big issue at the moment that the FETs are missing?

The hot resistor is the next thing I will investigate. I'll have a look around the Y final amplifier.

Any ideas?
Thanks!
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Old 19th May 2019, 10:06 pm   #8
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
What would be a suitable replacement for this FET? I once found German forum which suggested a suitable replacement but I cannot find it!
Possibly this - translated by google:-
https://translate.google.co.uk/trans...77&prev=search
Where a couple of BF245As are suggested (I think)...
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Old 19th May 2019, 11:31 pm   #9
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Default Re: Hameg HM307-3 Cleaning and Repair

That's the one! A BF245 looks to have been used successfully. A BF256 is also suggested.
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Old 20th May 2019, 11:16 am   #10
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg 307. input FET TIS69.
This is a low Idss N channel FET which is supplied as a pair matched to 10mv Vds and 10% Idss current. Texas 1971/2.
BFS21 is a Philips matched pair, very similar.
Otherwise use a couple of low current FETs and select in circuit for similar Idss and Vp. I have used 2N5245 and BFW12. Also try BF245, BF256A, BFW61, 2N3822, though you may need a wider range to make your selection.
As a manufacturer, it is cheaper to use tightly specified components and pay the extra purchase cost than have to select or adjust on the workbench. But for us just repairing one machine, a bit of adjustment is really the simplest, and vastly cheaper. I have found that I can get two pairs to match well from a selection of 5 or 6 from the same batch.
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Old 20th May 2019, 7:43 pm   #11
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks wme_bill. I will try a pair of BF256s as they are available locally.

I've been measuring the voltages on the BF458s in the final Y amp and they are all wrong!

What they should be:

E- 14.5V. C- 87.5V. B- 15V.

What they are:

T123: E- 20V. C- 47V. B- 21V
T124: E- 17V. C- 136V. B- 8V

I've pulled them and tested them with my DMM and don't show up as faulty.

Any ideas as what to do next?
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Old 20th May 2019, 8:31 pm   #12
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Thanks wme_bill. I will try a pair of BF256s as they are available locally.
A point worth mentioning is that BF256 pin designations seem not always to conform to the datasheets. When I built a 'Mini-Mod pantry transmitter' a while back, which used a BF256, initially the Min-Mod wouldn't work. I suspected the BF256 connections were wrong, and looking into it further, it became more of a conundrum.

At pics 1 & 2 below, I've attached a couple of bits of info that I drew up at the time.

A transistor tester won't come to your rescue - leastways, not if it's a Peak Atlas DCA55.

The displays which mine showed are below at pics 3,4,&5:

1) 'It's a JFET'. Well I knew that already.
2) 'Drain and Source not identified'. Huh?
3) 'Gate identified'. (Allegedly).

Ho hum...

Good luck with the 'scope refurb.
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Old 21st May 2019, 1:05 pm   #13
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
I've been measuring the voltages on the BF458s in the final Y amp and they are all wrong!

What they should be:

E- 14.5V. C- 87.5V. B- 15V.

What they are:

T123: E- 20V. C- 47V. B- 21V
T124: E- 17V. C- 136V. B- 8V

I've pulled them and tested them with my DMM and don't show up as faulty.

Any ideas as what to do next?
Assuming that TR123 and TR124 are not leaky to base, then the voltages at the bases seem to suggest an imbalance lower down the chain. If you disconnect the final Y-amp stage at EY1 and EY2 then what are the voltages now at these two points, i.e. at the output of T005 and T006? While disconnected, what are the voltages at the bases of T123 and T124?

Assuming zero volts is now measured at the bases of TR123 and TR124 while disconnected at EY1 and EY2, then if you apply the same voltage (i.e. 15v) to the input side of both R183 and R184 simultaneously, what are the voltages around T123 and T124 now? Are they now balanced? Anything on the CRT?

If the voltages at EY1 and EY2 are still unbalanced with the final stage disconnected, then check voltages around all of the BF199s in the pre-amp stage. Check also for o/c resistors in that part of the circuit.

Probably enough to be going on with for now...
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Old 21st May 2019, 7:30 pm   #14
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for that WaveyDipole, here is how I got on:

With the four pin connector to the Y-preamp board removed 0V was measured on the base of both TR123 and TR124.

I applied 15V to the base of TR123 and TR124. I've only got a single output bench PSU so I improvised to apply voltage to both transistors.

Collector of TR124- 89V
Collector of TR123- 93V

Emitter of TR124- 14.9V
Emitter of TR123- 15.5V

The voltages are not quite balanced but are close. Nothing appears on the CRT.
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Old 22nd May 2019, 8:52 am   #15
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Default Re: Hameg HM307-3 Cleaning and Repair

That looks much more sensible. The slight differences might be on account of values of resistors drifting over time so you might want to check the values of R190, R191, R197 and R198. You could also try disconnecting pin 6 and pin 7 from the tube to see whether the CRT is affecting the reading.

However, it seems you may have a bigger problem lower down the line. What voltages did you get at EY1 and EY2? If they are not even then follow it back down BF199 (T003 - T006) stages to pins 7 and 8 of the IC.
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Old 22nd May 2019, 9:58 am   #16
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Default Re: Hameg HM307-3 Cleaning and Repair

BTW, forgot to mention that if you are satisfied that the resistors are OK and that the CRT is not influencing the reading, then you might be able to balance out the amplifier with EY1 and EY2 still disconnected and 15v applied so that both sides have an equal reading by using R223, but first check that the pot and R196 are both OK as well.

What voltages do you have at pin 1 and 14 of IC001?
Did you remove the broken FETs completely from the circuit?
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Old 22nd May 2019, 9:24 pm   #17
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Default Re: Hameg HM307-3 Cleaning and Repair

Hello WaveyDipole. I have been following your instructions from post 15. I feel like we are getting somewhere!

R197 and R196 measure correctly. R190 and R191 (both 150Ω) measure lower at 135Ω.

When pins 6+7 are removed from the CRT the voltages on TR123 and TR124 remain similar to what they were with pins 6+7 connected. I think this rules out the CRT having an affect on the circuit.

Voltages on EY1 and EY2 (with the connector attached to the Y-preamp):

EY1: 21.7V
EY2: 7.8V

Voltages on BF199s T003-T006 (All wrong!)

I believe they should be: B-2.7V, E-2V, C-15.6V

T003: B-5.9V, E-3.6V, C-8.2V
T004: B-5.9V, E-5.1V, C-22.6V
T005: B-22V, E-21V, C-22V
T006: B-8.9V, E-21V, C-23V

T005 looks to have had it. I pulled it and tested it with my component tester and it showed a diode between C and E. Using diode check on my DMM there is a 0.7V forward voltage between C and E. I believe it should read O/L. I decided to pull the rest and test them.

Three of the four BF199s give these strange readings and show a forward voltage between C and E. All defective, no doubt.

The forth one is open and tests as a diode on my component tester.

All of the above will need replacing. Is this amp DC coupled? I heard that if you miss a malfunctioning part it will destroy any replaced parts upon being turned on.
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Old 22nd May 2019, 11:45 pm   #18
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
R197 and R196 measure correctly. R190 and R191 (both 150Ω) measure lower at 135Ω.
What about the pot R223? Is that Ok?
Those resistors are about 10% off their nominal value. Even if their tolerance is 10% then the measurement (which I am assuming was made out of circuit?) is borderline and they are probably best replaced.

Quote:
Originally Posted by OldTechFan96 View Post
When pins 6+7 are removed from the CRT the voltages on TR123 and TR124 remain similar to what they were with pins 6+7 connected. I think this rules out the CRT having an affect on the circuit.
Good. No leakage via the CRT plates then.

Quote:
Originally Posted by OldTechFan96 View Post
Voltages on EY1 and EY2 (with the connector attached to the Y-preamp):

EY1: 21.7V
EY2: 7.8V

Voltages on BF199s T003-T006 (All wrong!)

I believe they should be: B-2.7V, E-2V, C-15.6V

T003: B-5.9V, E-3.6V, C-8.2V
T004: B-5.9V, E-5.1V, C-22.6V
T005: B-22V, E-21V, C-22V
T006: B-8.9V, E-21V, C-23V
T003 and T004 should have different voltages around it than T005 and T006:

T003 & T004: B~+2.7v E=+2v C=15.6v
T005 & T006: B=+15.6v, E=+15v, C= <+24v

The base voltages of T003 and T004 will depend on the output from the IC. I am a bit puzzled by your reading though. For example, T003 collector is connected directly to T005 base and should have the same voltage reading. You show 8.2v on one and 22v on the other? That cannot be unless the track is broken?

While the voltage at the bases of TR003 and TR004 looks a little high, it is exactly the same on both outputs from the IC which is a good sign. What's the voltage at the inputs, i.e. pins 1 and 14?

Quote:
Originally Posted by OldTechFan96 View Post
T005 looks to have had it. I pulled it and tested it with my component tester and it showed a diode between C and E. Using diode check on my DMM there is a 0.7V forward voltage between C and E. I believe it should read O/L. I decided to pull the rest and test them.

Three of the four BF199s give these strange readings and show a forward voltage between C and E. All defective, no doubt.

The forth one is open and tests as a diode on my component tester.
All of the above will need replacing.
Although one sometimes gets odd readings from those cheap testers, these are "ordinary" NPN transistors so should measure as such. Those anomalies, as you surmise and which appear to be backed up by your DMM readings, would suggest that these transistors are at least leaky and you have identified at least one that appears to have one junction that is o/c. I would agree they need replacing. Check the low value resistors such as R027, R028, R025, R026 as well. There should not be any significant current across R023 and R024 so I think they should be OK. While you are checking, also have a look at T007 and R036. These will probably be OK but it doesn't hurt to check while investigating this part of the circuit.

Quote:
Originally Posted by OldTechFan96 View Post
Is this amp DC coupled? I heard that if you miss a malfunctioning part it will destroy any replaced parts upon being turned on.
Looking at the circuit, there are no capacitors decoupling the various stages of the amplifier. Each stage is indeed DC coupled to the next either directly or via various bias resistors. A leakage or short within a component will usually cause an unexpected current flow somewhere. If that current flow exceeds design limitations, it can indeed damage adjacent components along its path within the circuit, sometimes in a cascade fashion. The amount of damage will depend on which component fails, its mode of failure and how well the circuit is designed and built among other factors. Perhaps someone much cleverer than me can comment in more detail.

Last edited by WaveyDipole; 23rd May 2019 at 12:05 am.
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Old 24th May 2019, 7:04 pm   #19
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Default Re: Hameg HM307-3 Cleaning and Repair

More progress:

Those four BF199 have been replaced. This brought the voltages at EY1+2 to 15V. Right where they should be.

The voltages on TR123 and TR124 are correct, apart from the collector voltages. One sits at 87.4V and the other 92.8V.

Voltages at IC output:

7- 6V
8- 6V

Voltages at IC input:

1- -11.8V
14- -11.8V

The voltages are too high. Is the IC OK?

The above measurements were made without the FETs in place.

Next I will work out the pinouts for FET sockets. I bought some BF256 for this.

The pinouts in the datasheet differ from what the component tester says. What should I trust?
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Old 24th May 2019, 10:27 pm   #20
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Default Re: Hameg HM307-3 Cleaning and Repair

Looking through the service manual of my Hameg HM203-4 I noticed that it used the same ICs in the Y preamp. I swapped the IC from the HM307-3 to the HM203-4 and it worked fine.

I guess that the IC is not the problem.

Attached is what I think is the pinout for the FET sockets.
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