Fluke 845 Germanium Transistors - Page 1
Forum section: electronics / general

author: LaserEng
author_rank: Regular Contributor
timestamp: October 22, 2021, 03:39:22 pm
content: I have obtained a Fluke 845 and in the process of repairing it I have found that the power supply is not working and the 4 Germanium transistors on the power supply card have been swapped by a previous owner to Silicon ones but with no change to the biasing componentsand the 2 PNP's that drive the transformer have shorted C-E.I couldn't find any info anywhere on the 2 transistors Fluke used GA2817 and GA2875 and certainly have not found any available. I was wondering is anyone here has had to replace these transistors and if you did, which transistors did you use to get it working?Might have to end up buying whatever Germanium transistors I can find and trying them one by one until I find something that works

author: Kean
author_rank: Unknown
timestamp: October 22, 2021, 04:17:30 pm
content: I'm not familiar with the Fluke 845 or its variations (A/AB), but did you look at TiN's repair log?Seehttps://xdevs.com/fix/f845ab/Q201 appears to be an MPS404A in one of the photos. Not sure if you are talking about CR201-CR206 but they appears to be 1N5397.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 22, 2021, 04:28:27 pm
content: Q201 , Q202 in the power supply should be OK to use silicon transistors. Depending on the transitor gain one may want to change the frequency adjust trimmer a little, e.g. to allow a higher resistance if the transistors have more gain.Q113, Q114, Q203, Q204 are a bit more tricky. These are used as chopper demodulation and may need high low CE saturation and maybe higher BE reverse voltage.edit: The MPS404 looks like a special chopper transistor, maybe OK for Q113 and so on. For Q201, Q202 it could be more normal ones, maybe a little higher current ones like BC636.

author: LaserEng
author_rank: Regular Contributor
timestamp: October 23, 2021, 09:15:11 am
content: Thanks Kean and Kleinstein, you both seem to be correct with MPS404 in particular MPS404A as after googling the Fluke PN 4805-182600 it lead me to the Talon Electronics websitehttps://talonelectronics.com/shop/item.aspx?itemid=2824and it says to use MPS404A instead of the original Ge transistor. Seems that the transistors for Q201/202 need a high Veb as MPS404A has Veb of 25V.Also by googling the fluke PN for Q113, Q203/204 I found that for the same Fluke PN in the Fluke 760A calibrator that they use 2N1302 instead, luckily local electronics shop had these, wonder how many decades they had sat on the shelfSo most likely 2N1303 would be fine for Q113.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 23, 2021, 10:51:34 am
content: Q201 and Q202 don't need special transistors, just standard PNPs (e.g. 2N2905 or similar) should work. A 40 V rating is normally no problem to get when you don't need extra low saturation. The other 4 transistors are those where more special types like the MPS404 make sense.

author: TimFox
author_rank: Super Contributor
timestamp: October 23, 2021, 11:00:32 pm
content: The emitter-base breakdown voltage is one of the few advantages that Ge transistors have over Si BJTs.Modern Si BJTs have base-emitter breakdown voltages around 6 V, even if the collector-emitter rating is, say, 200 V.I remember this being a problem when I first learned about astable multivibrators and Eccles-Jordan bistable multivibrators (flip-flops) around 1968: the vacuum-tube original circuits would send maybe -100 V to the grid of the shut-off triode, which was OK. One could use essentially the same circuit with a Ge BJT at lower collector voltage, but extra diodes often were needed with Si BJTs to avoid large reverse bias on the shut-off transistor.

author: LaserEng
author_rank: Regular Contributor
timestamp: November 01, 2021, 02:18:13 pm
content: What would be a good modern equivalent of the input protection diodes or if someone has data on the diodes used would be great?The ones in my unit are Microsemi CD55105 and I have not been able to find any data on them to see what would be an acceptable modern equivqlent .They are available on Microsemi's website at $15 each but couldn't find any data on the Microsemi site.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2021, 11:05:59 am
content: I would consider BAV199 diodes, as todways standard low leakage diode. They usually come as duals. The input is no that high in impedance anyway.

author: serg-el
author_rank: Regular Contributor
timestamp: November 02, 2021, 01:50:37 pm
content: Quote from: LaserEng on November 01, 2021, 02:18:13 pmJudging by some reports, this is a 90 V zener diode with a maximum current of 75 mA.https://www.datasheetarchive.com/pdf/download.php?id=66d9d9c47daadfc8453a6a97bde2e729790ce1&type=M&term=cd55https://nationalstocknumber.org/nsn/5961-01-110-6856